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jPart 5 of The Popular Science Educator will be on sale next Thursday October 31st

T HE success of THE POPULARSCIENCE EDUCATOR is largelydue to its fine series of explan atory drawings, which make clear manyof the great principles of science, andat the same time explain difficult processes and devices. Even a roughsketch will often convey more to themind th an a long description, andwhen the diagrams are as carefullyth ought out and as well executed asthose which are appearing in THEPOPULAR SCIENCE EDUCATOR theyhave a va lue which is incalculable.That is why so many schoolmastersand mistresses are advising theirscholars to take this book.

t is not intended, as has alreadybeen expla ined, as a substitute forexisting text-books ; but as a supplement to these and to science lecturesit is proving of th e greatest va lue. Those who have not had the advantage of a science education are findingthat the book supplies them withexactly what they want. Most sciencetext-books presuppose that the readerknows, at any rate, the elementaryprinciples of the subject he is studying.But those who come to THE POPULARSCIE NCE EDUCATOR with a desire toget some sort of working knowledgeof a ll the sciences find that they canfollow the story week by week and

chapter by chapter without having torefer to other books. For example,technical terms when used are fullyexpla ined, and though they may lookformidab le, th eir derivation, as ex-

TWO FASCINATING BOOKSThe Editor has produced twofascinating books, one for boysfrom eleven to seventeen, and theother for children of both sexes upto seven or eight. The former iscalled THE BOY'S BOOK OFWONDER AND INVENTION. tis issued at 6s. and contains hundredsof pictures with experiments, storiesof famous inventions, explanatorydrawings, and so on. The other isTHE NURSERY RHYME OMNIBUS,the biggest collection of nurseryrhymes and the best illustratedbook of its kind ever issued. tis published at 3s. 6d. Both bookscan be obtained from any bookstallor bookshop, or ordered at anynewsagent s.plained, makes it far mo re easy toremember them .One of the most popular sectionsof the book is that dealing withBiology. Some readers have COljlfessed

that they thought this was a drysubject, but as the story of .life isunfolded they find that it is as fascinating as a novel.n all th e sections the latest information is embodied. so that those whoread the book will find that they havean up-to-date knowledge of sciencein all its branches.

The present part is particularly richin ex planatory illustrations. We see, forexample, how the amoeba, the simplestform of animal life, consisting of asingle cell, lives its life, eating andmoving and multiplying in a way thatseems like a fairy ta le.We see on page gr the carbon cycleby which this important -element isabsorbed by p l n t s in the form ofcarbon dioxide gas, extracted from thegas, and the oxygen passed out into theair for animals to breathe.Vve see how astonishing recordscould be made for the high and long

jumps if athletes could be transferredto the Moon. We are shown thedifferent ways in which fire is made byman. We see how the inclined planeis used in modern life, and how a greatcity gets an ample supp ly of pure waterfor drinking and other purposes.Altogether, this part is well up to theinterest of its predecessors.HERE RE SOME OF THE PRINCIP L FE TURES IN P RT

THE SUCCESSION OF LIFE EXPERIMENTS IN BALANCINGAn interesting account of how scientists believe thatliving forms have evolved from lowly one-celled plantsand animals to highly developed forms . Illustrated.

THE REMARKABLE LIKENESS OF TWINSA full page of photographs showing the remarkablesimilarities in the appearance of twins and tr ipl ets,who, as science explains, are the only people who arereally alike.

THE SIMILARITIES AND DISSIMILARITIES OFMAN AND BEASTTwo full pages of illustrations showing how the skeletonof man is similar in principle to those of variousan imals, and yet at the same ti i ie differs widely.

BALANCE AND THE CENTRE OF GRAVITYA chapter explaining the meaning of centre of gravity,how it vvorks, and why some things balance andothers fall over. Fully illustrated.

SJ'ABLE AND UNSTABLE EQUILIBRIUMA page of drawings showing the three forms of equilibr ium and ho\.v if a body is to be in stable equilibrium,its centre of gravit y must be low.HOW CENTRE OF GRAVITY AFFECTS LIFE

A full page of drawings showing people in variouspositions carrying out different tasks, with an explanation of why they take up the attitudes they do.

A full page of drawings showing various experimentsthat can be carried out with ordinary objects.THE TOWER THAT DOES NOT FALLA full-page illustration of the famous Leaning Tower

of Pisa, with an explanation of why in its curiousposition it is still perfectly safe.HOW HEAT IS TRANSMITTEDA chapter descrihing the three ways in which heat tspa ssed on, with an explanation of the miner s safetylamp. the reason why dark clotheq are warmer thanlight, and so on. Illustrated.HOW A HOT-WATER SYSTEM WORKSA full-page drawing illustrating central heating andthe hot-water supply t o the bath, etc.NITROGEN WHICH IS NECESSARY TO LIFE

The story of nitrogen gas and how it enters into th ecomposition of all living bodies. Illustrated .THE NITROGEN CYCLE

A full-page drawing showing how plants and animalsobtain their supplies of n itrogen.HOW AMMONIA IS MADE FOR COMMERCIAL USEA double-page drawing showing the whole process ofmaking ammonia, necessary to so many industries.THE THREE NEAREST PLANETS

An interesting and graphic account of Mercury, Venusand Mars, the th ree p lan ets ne arest to the Earth.

Ask Your Newsagent to Send You The Popular Science Educator Every Week J


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THE M RVEL OF COLOUR IN THE NIM L WORLDc w

pahW e expec t to have co lour among t he fl ower s but t her e a r e equa ll y bril l iant hu es to be fo u nd in th e anima l wor ld. A s can beseen by th is plate t he br ight co lours are foun d among ma mma ls birds rept i les fi shes in sects she l ls an d such low ly cr eaturesas t he sea anemones. Of course th e examp les given here are m ere ly t ypes of th e ri ch co lours w h ich are to be found in a llclasses of anima ls verteb rate and in vertebrate. Th e opah is one of t he m ost br i l l ian t ly co loured of Br i tish fishes and theanem ones show n are fo und on t he sout hern coast s o f Eng land . The edible frog is common on t he continent of Europe thP.Orni thop tera bu tte rf l y w it h i ts m agni ficent iridescent co lou rs due t o th e li ght bei ng b rok en up in to the co lours of t he spectrumby the t i ny li nes on it s w i ngs i found in th e So lomon Islands. Th e beauti ful Am pu ll ari a shell is common in Lak e Nyasaand t he m acaw is o nl y one of m any ri chl y co loured par ro ts found in South Am erica whi ch are t o be see n at the London Z oo.Th e mandri l l with its queer b lu e and red nose is a strikin g exam ple of bri gh t colour in a m amm al


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Story of Life and Living Things and how these are constantly being affectej by their surroundings ,PHYSIOLOGY, ZOOLOGY, EMBRYOLOGY and BOTANY

.BIQLQGYi The-., I _.. ;9

THE SIMPLEST ANIMALS ND PLANTSIn both the animal and plant worlds th_ere are tiny creatures which are made up ofonly one cell, and here we read some interesting facts about these lowly forms of life

LL animals and plants, as we have.f-\.. seen, are made up of minuteparts or units known as cells,and in the highe{forms of life there are .millions of these. cells, some unitingtogether to form one kind of tissueand -others uniting to form other tissue,each group h'aving its own particularkind of work to do in the animal orplant. ..But there are some very lowly::tnimals and plants which consist of onlyone cell and this has to do all thework, moving about, catching anddigesting food, and producing newcreatures to car:rY on the race.Among the animals one of the mostinteresting is the amoeba. There areseveral different species, but they areall very much alike. They are foundalmost everywhere where there is anydegree of moisture, in ditches andponds, in the damp soil, on the surface

of mud, and; even in salt water and asparasites .in the digestive organs ofmen and. higher animals. In thislatter case they are the cause of thatunpleasant complaint dysentery.Most of. the amoebae need a microscope to make them visible, but thereis one speeie large enough to be seenwith the n ~ } r n eye, and that is aboutone hundredth of an inch in diameter.Now the name amoeba means change,a n d it was given to this simple animalbecause, unlike ourselves .and thehorses and dogs and birds and allhigher animals, it does not keep thesame form. t is always changing itsshave.

t is just a mass of protoplasm, andit moves abou:t in a curious way. tkeeps on bulging out first in one placeand then in another and throwing Ot\twhat are something like feet or toes.When it puts, out a projection from

one part it _draws in at another.i;: andso by throwmg out these pseudopods,as scientists call them, or false feet,the amoeba is able to move about fromplace to place.But the projections or false feet arenot merely limbs for moving, they arefingers for grasping, and as it. goeshere and there the amoeba is with itsfingers seizing particles of food; l'heseconsist of other one-cel}ed living things-minute plants like bacteria ancl onecelled animals something like theamoeba itself.t likes a varied diet, as we do, butall its food consists of organic matter.

t is no more able to take :iniheral foodand digest it than we are, and thisproves that the amoeba is an animaland not a plant. Only plants can takemineral matter and change it iritofood. '. Food can be taken into the amoeba's

Here we see a o n e ~ c e l l e d plant enormously magnified. The r r ~ ~ n t and centre of its body are colourless and sunlight can e n t ~ rso as to act upon the chlorophyll, or green colouring matter, in the restofthe plant to form organic substances from the mineralsand salts absorbed through the cell wall. The part of the plant that does this is called the pyrenoid, and the part with the ~ r e e n colouring matter is known as the ct\loroplast. The front of the plant throws out whip-like parts which are waved aboutand enable it to move. This single-celled plant multiplies, as shown in the lower part of the drawing. The nucleus divides, the protoplasm follows suit by grouping round the nuclei, and then two new plants burst out from their parent

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THE SIMPLEST ANIMALS AND PLANTSbody at any point, the semi-fluidprotoplasm simply rolling over andengulfing the food material.ust as .the protoplasm of theamoeba's single cell has to move thecreature about, so it has to providemouth and stomach for the consumption and digesting of the food. Theamoeba has no definite mouth, and sowhen it comes across something suitable for food it throws out two pseudopods, embraces the morsel, and then,closing the arms round it, unites theseand the food is i'liside the amoeba,being digested in a little cavity whichit has made for the purpose. This iscalled a vacuole, which simply means;:i n empty place, and is really a tem-porary stomach. As soon as the morsel is imprisonedin the cavity a digestive juice coversit and begins to dissolve it, and thesingle cell of the amoeba is then doingthe work that inour more complexbodies is done bythe gland-cells.Good Digestion

more complex than the amoeba. Theyare classed together as the Protozoa, aword which means first livinganimals, and a single one is called aprotozoan. Some of these, like the bell animalculefound in.stagnant water, have a mouthand little cilia or threads which waveabout and drive food into the mouth.The mouth then closes. At the otherend the animalcule is extended into akind of tail, and in this the protoplasmacts as in our muscles and enables thecreature to grip and hold on to thewater-weeds.Many of the protozoa have cilia orthreads all over their exteriors andswim about by moving these. Someseize the chalk or lime in the water,and with it build a skeleton or shell.The globigerina ooze which is forminga bed of .chalk on the bottom of theAtlantic Ocean today consists of

Scientists call the method binaryfission, which simply means splittingor dividing into two equal portions.The strange thing about this method ofreproduction is that the parent disappears when two children are thusborn, and so the amoeba, as has beensaid, never knows old age ; it haslearnt the secret of eternal youth.A still stranger thing sometimeshappens. Two amoebae come togetherand become one creature, their twonuclei uniting. But not long afterwards the nucleus assumes the dumbbell shape and divides and there onceagain are two amoebae.The protozoa exist in enormousnumbers in the ocean and in lakes andrivers, and form the food of manyhigher creatures. Many fish, especiallythose that travel in schools, liveon such food. Other fish feed on ratherlarger animals which in their turn liveupon the protozoa.The fish that feedin this way haveon each s ide of

their gills a kind ofrake, called a gillraker. The rakersare used by the fishto catch the tinycreatures that formtheir food as thewater that containsthem passes overtheir gills.Sea ammal

Sometimes t h efood captured is at iny one-celledplant called a diatom, and then theamoeba h s not r o u b 1 e , for thediatom puts up nofight. But at othertimes the prey maybe a one -celledanimal with cilia orhair -like threadsthat are constantlylashing about, andt h i s c r e t u r ethough so lowlyfights for its life.The amoeba thenwastes no energy

The food of the- Greenland,.whale, consists largely of erotozoa, or tiny one-celled creatures thatlive in the sea. It takes my,riads of these into its moU .th with the water, and then strains outthe w ater through a great strainer made of hanging plates of whalebone, shoWn in the picture

The whale, whichis, of course, not afish but a ma:inmalliving in the sea,s t r ins protozoaand other srriallanimals and plantsout of the water bya somewhat similarstrainer consistingin putting out or withdrawing pseudopods. t gives all its attention to itscapture. For some hours it remainsunchanged in shape while the captured.animal hurls itself about in a vaineffort to break out of its prison. yetthough the protoplasm is so soft andjelly-like it does not yield to thisprolonged attack, but resists as thoughits walls were of steel. At last thevictim becomes still from exhaustion,and at once the protoplasm -of. theamoeba begins to digest its victim.

Like all other animals, the amoeba,though consisting of a single simple cell,has to get rid of its refuse matter aftereating, and this it does in the followingmanner. Near the surface of itsbody is a little spherical space filledwith water and the dissolved waste,and from time to time the protoplasmin the amoeba's body c l o ~ e s iri on thisspace and, squeezing it, squirts thewaste out through the surface. Thelittle cavity in which the waste matteraccumulates and from which it isejected i,s called the contractile vacuole.Other single-celled animals are rather

myriads of shells m


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THE C RBON CYCLE Y WHICH LIFE IS KEPT OIN

ttt\lants take carbon dioxide from thalf. lind after seizinq the carbon tobuild liP. t i s s g ~ ~ l e a v \ qive offI Oxqqen from plants taken in bi/animals combines with carbon t if

food to form carbon dioxide whichis. th n qiven out for use blJ plants

OXl tjen from p/antstaken in blJ amma swith the air theqbrea_the. to burn upfood eaten blf th m

Fires and all kinds of living animals are constantly pouring carbon dioxide gas Into the atmosphere. The green plants take incarbon dioxide from the air, and their green colouring matter, under the influence of sunlight, extracts the carbon from the gasand then returns the oxygen to the atmosphere. Oxygen is necessary to the iife of animals, which breathe it in, and in theirbodies it burns or oxidises the food materials they have taken in from the plants, which consist largely of carbon. .The oxygenin ari animal s body combines with the carbon, and thus forms carbon dioxide, which is breathe.d out for the use of the plants.Thus a regular cycle goes on which men of science describe as the carbon cycle; It must be remembered that the atmosphereand the soil are the primary sources of.all food. The atmosphere contains carbon in the form of carbon dioxide gas togetherwith water vapour, while the soil contains water and nitrogen in the form of nitrates. We s in another part of this book thatthere is a regular nitrogen cycle just as there is the carbon cycle shown here. Animals are unable to make use of carbon andnitrogen in the simple forms indicated, but plants, aided by the light and heat of the Sun, can and they build up the carbon of thecarbon dioxide gas into Carbo-hydrates which are eaten by animals and split up by them releasing carbon dioxide gas and waterl


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THE SIMPLEST ANIMALS AND PLANTSwhich we shall learn more later. Thecentral and front parts of the cell areclear.The protoplasm that is colouredgreen by the chlorophyll is generallycalled the chloroplast, a word meaningpale-green form. In the centre ofthe colourless protoplasm is a sphericalnucleus, and projecting through holes inthe front and sharper end of the cell aretwo cilia, whip-like projections that arelashed about and drive the little plantthrough the water. There a.re also inthe front pa.rt of the cell two vacuoles,or cavities, which can contract and a.reprobably for the purpose of squirtingwater from the celLMore noticeable in this pa.rt of theplant is a red-eyespot, which is believedto render the plant sensitive to light.In the chloroplast towards the backof the cell is an object about the samesize or a little larger thanthe nucleus. Its outer layerconsists of starch, as provedby the fact that it turns ab l u e b l a c k colour whentreated with iodine. In thecentre is protein matter,the most complex of allchemical substances known,which helps to build up thebody of a living creature.

bursting out of the pa.rent cell, in whichthere may be four daughters floatingabout. In some of the plants this iscarried still farther and as manyas sixty-four little cells have beencounted within a parent cell, ready tobreak out and begin iife on their ownaccount.A.lthough when we a.re examining aspecimen of Chlamydomonas under themicroscope it appears to swim a.bout atrandom, it does not do this vvhen innatural surroundings. Its' movementsthen are definitely directed to bring itinto conditions where it is able to live anormal and healthy life.This may be proved by a very simpleexperiment. We must take from anopen rain-tank some water with thegreen tinge that is evidence of thepresence of Chla.mydomonas, and placeit in a tumbler near a window.

dust y the wind and wake up, as itwere, in a new pool. Then they divideinto a number of sections which ea.ehgrows into a new Chla.mydomona.s. Insuch drought conditions ordinary members of the family would perish.t is interesting to know that boththe one-celled animals and the onecelled plants sometimes link themselves

together with their kind into colonies.Those slimy masses of floating greenthreads tangled together that we seefloating on ponds and ditches in thesummer a.re examples of one-celledplants united end to end. Among theanimals some protozoa live attached toone another, forming a kind of clusterof tiny shells, but living independentlives.The remarkable thing is that theselowly plants and animals have thefundamental characteristics of theirloftier relations. The one-celledanimals cannot take minet'a.lmatter and from i t build uporganic matter as can the onecelled plants. t s the functionof all plants, however lowly inthe scale, to change theorganic matter into organic andmake i t f it food for animals.

A Constant Carbon CycleThe carbon in our bodiescame from .the proteins, fats,and carbohydrates which wetook as food, and these weobtained either from plants orfrom other animals that hadconsumed plants, such as oxen,sheep, pigs, rabbits and soon.The chalk cliffs of Dover and the Downswere built up millions of years ago by tiny

o n e ~ e l l e d animals_that lived n prehistoricsea and made small shells to live in Whenthey died the shells fell to the bottom, andeveritua ly the sea bed was raised to form

Building a WallThis object is calledthe pyrenoid, whichmeans a fruit - stone,probably because of itsappearance. t is thispa.rt of the single-celledplant which is of verygreat importance. t

builds up the celluloseof the c e - w a andwhen water containingcarbon diox ide andw e a k s o u t i o n s of

the cliffs and downs. The inset shows some of these shells greatly magnified. They are much like tiny shells being formed in the Atlantic today

The animals cannot get theircarbon direct, but the plantstake i t from the carbon dioxidein the air, or, in the case ofwater-plants, from the carbondioxide dissolved in the water.

mineral salts passes through the wallinto the cell the pyrenoid takes theirelements and builds up organic compounds just as successfully as does thetallest and biggest tree. The energy todo this is provided by the light actingon the chlorophyll. Oxygen is givenoff by the lowly plant just as i t is bymore important members of the plantworld.These plants a.re members of theAlgae order to which the seaweedsbelong. Like the amoeba, they multiplyby fission, but in rather a differentway.When the time for division comesthe cell withdraws its flagella or whips,the nucleus divides into two the protoplasm gathers round the two parts.and soon a cellulose wall is formedround ea.eh. Flagella are shot out andafter swimming round inside the parentfor a time the cells burst the wall andpass out. They are thus real daughtersof their parent, which is left a mereempty and broken cell-wall.Sometimes one of the daughter cellswill itself divide into two before

After a very short time it will befound that the side of the glass nearestthe window has become covered with agreen deposit. This is due to the factthat the lowly plants, by means of theircilia, have swum a.cross to the light,which is so necessary for their welfare.I t is interesting to know that theCbla.mydomona.s makes provision fortimes of drought which. would, in theordinary way, be fatal to its continuedexistence. The delicate cells are damaged by excessive heat or cold, butdrought is their' chief danger. Yetliving as they often do in small poolsand streams that are likely to dry upin warm weather their risk is great.Now when large numbers of newplants a.re formed in the pa.rent as theresult of splitting up, these when theyescape are not able as are smallernumbers to reproduce in their turnby splitting up. Two of them have tocome together, and after their cilia a.reentangled they fuse together and thengo into a state of rest. In this statethey can resist drough t and i f their poolbe dried up they may be carried like

Where does this gas comefrom ? Well, certain rocks like limestone yield a small quantity when theydecompose, but the bulk of it comesfrom the large quantities that arethrown off by fires as they burn, andby living creatures as they breathe out.There is thus a c01ista.nt carbon cyclegoing on. The plants breathe in carbondioxide and by the aid of sunl.ightacting on the chlorophyll, or greencolouring matter, break up the carbondioxide, ta.king the carbon to maketheir leaves and other parts, andthrowing out into the atmosphere theoxygen.The animals eat the plants andbreathe in the oxygen of the air, whichthereupon burns or oxidises the organicmatter in their bodies and enables themto give out carbon dioxide, whichprovides food for new plants.There is thus a rongh balance so longas the cycle continues moving, but ifthe a.mount of animal life in the worldshould diminish very greatly therewould in time be a shortage of carbondioxide, and plant life would necessarilydecrease also.


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n this picture we h ~ v a graphic representat-ion of an amoeba the lowliest of all living animals magnified many thousandsof times. It is a little mass of protoplasm enclosed in a mel .Tlbrane, and has no definite form but changes from moment tomoment. By stretching out projections known as pseudopods, or false feet, it is able to move about and these feet alsoseize particles of food. In the lower part of the drawing we s how the amoeba multiplies to carry on its race9


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IF ENGL ND WERE SITU TED ON TH OON

The wonde-rful 100-inch telescope at Mount Wilson Observatory, linked with modern photography, enables us to take photo-graphs of the Moon so fine in detail that we can see exactly what a lunar landscape is like. Here is part of such a photograph,showing those chiiracteristic features of the Moon s surface, the lunar craters. Some of these are so big that if placed in themiddle of England, as shown in the inset one of them would fill the whole of the Midlands. f England were situatedon the Moon it would appear in size as shown, for the outline drawing is made to the scale of the Moon and its craters.Scientists are not agreed as to how these craters were formed. Some think they are extinct volcanoes others that theywere caused by meteorites striking the Moon and others that they are burst bubbles on the Moon s crust9


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PHYSIOGR PHY A description of the Physical Universe with the Daily Monthlyand Yearly Happenings in Earth and SkyASTRONOMY GEOLOGY PHYSICAL GEOGRAPHY and METEOROLOGY

THE ROM NTIC STORY OF THE MOONThe Moon is the nearest of all the heavenly bodies to our Earth, and, as weread here, man has discovered a great deal about its nature and conditionA FTER the Sun the Moon is the bestknown object in the heavens,and though it is not alwaysvisible, yet because of the light it doesgive when it shines in the night it hasalways been an object of great attention.In the old days men were. very muchafraid of darkness, just as some childrenare nowadays, and the Moon, becauseon many nights it lighted up the Earthin the dreaded period of darkness, wasregarded as a friend.The full Moon always appears to usabout the same size, from which it maybe supposed that its distance from theEarth does not vary a great deal. Thisis the case, for while at its greatestdistance it is 252,970 miles away, at itsnearest i t is 221,610 miles. Anotherauthority gives the figures 252,7rn and221,462. The distance cannot be determined exactly within a few miles.

t is clear from the fact thatthe distance is not always thesame that the orbit in whichthe Moon moves round the Earthis not a circle but an ellipse.t travels in this orbit at a speedof 2,287 miles an hour, or about

3,350 feet per second.Men always noticed that therewere markings on the Moon sface, but till the invention of thetelescope it was not possible toknow what these markings were.But when 1609 the greatGalileo turned his telescope uponthe Moon and examined it, heannounqirl at . once that i t was

p. world like our own, with seas andmountains.The mountafos could be distinctlyseen,,. though not quite as clearly as inthe wonderful photographs taken bythe giant telescopes of today. Thedark patches now known to be vastplains were supposed to be seas, andwere given such picturesque and poeticnames as the Sea of Serenity, the Seaof Tranquillity, and so on.Unlike the Sun, the Moon, thoughappearing very bright a t night, does notshine by its own light. t merelycatches and reflects the sunshine, andit would take 600,000 full Moons togive us as much light as we receivefrom the Sun. Of course, the visiblesky could no.t contain so many Moons.f the whole sky as we see it werepacked with full Moons we shouldreceive only about one-eighth of thelight which comes to us from the Sun.

t may seem curious, but the haHMoon does not give anything like halfthe light of the full Moon. The reasonis that when at that phase the Sun slight is caught slantingly and thereare many shadows on the Moon s surface which greatly reduce the lightreflected.Only about one-sixth of the sunlightthat falls upon its face is reflected bythe. Moon. The light of the full Moonis reckoned as being equal to that of ahundred candle-power lamp seen at adistance of twenty-two yards.When the Moon is in the crescentform if we look carefullv we shall beable to discern the rest of its body asa dull copper-coloured disc. Why isthere this faint light f the rest ofthe Moon were in absolute darkness itcould not be seen at all. There mustbe some source of light shining uponthe Moon to show up the part notilluminated by the Sun.t has been found that thedull copper colour is reallyearthshine. The Sun s raysshine upon the Earth, and ifour planet were viewed from theMoon it would appear very muchas the Moon appears to us. Thissunshine on the Earth is reflectedback to the Moon and lights upits dark side with a dull coppercoloured light.

But the most interestingthing about it is that the earthshine has been used for a verycurious purpose. The planetMars has an atmosphere which,

The Moon is 2 163 miles in diameter and this drawing shows what it would look like i f placed in the middle of Europe95 D*


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HIGH JUMP ND LONG JUMP ON THE MOONThe pull of gravitation onMoon s surface is only about .onesixth that of the Earth s pu 11 andi athletes could be transported tothe Moon and jump with theenergy that they do on the Earth,they would beat all their oldrecords for they would jump sixtimes as high and six times as faras they can on the Earth. Thehigh-jump record would be notas on the Earth six feet eightinches but forty feet and the longjump would b e not twenty-sixfeet two inches but 57 feet.The reason for the lessened pu II ofgraV-itation is that the . Moon smass is so much smaller than thatof the Earth. An object weighingsix pounds on the Earth wouldweigh onlyonepoundontheMoon.It is largelyowing to its l ight gravitational pull that the Moon has lostits atmosphere. The pull was notpowerful enough to hold the gases


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THE STORY O THE MOONPacific Ocean now is. Another theoryis that it was an independent planetwhich the Earth captured and hasever since held as a prisoner. Stillanother theory is that the Moon wasformerly an immense ring encirclingthe Earth as the rings of Saturn encircle that planet and that in course oftime the ring became a globe. No onecan say for certain.Perhaps one of the most remarkablethings about the Moon is that it alwaysturns the same side towards the Earth,so that none of us has ever seen theother side. This is because it rotateson its axis in exactly the same timethat it takes to travel round theJfarth.

There has been mrich speculationas to what the other side of the Moon islike. The Abbe Moreux, the famousFrench astronomer, thinks that thetall mountains on the side we seecorrespond with deep depressions onthe opposite side, just as on Earth thedeep Pacific Ocean is on one side andthe high masses of Europe and Asia onthe other.But perhaps the most interestingthing about the Moon is its scenery.

f we could suddenlv land on the Moonwe should find that its landscapes werevery different from those of the Earth.There are ranges of mountains justas there are on the Earth, but on theMoon they are far fewer in proportionto its area.Alps and Pyrenees on the Moon

The lunar ranges have been namedafter many of those on the Earth- theAlps, the Apennines, the Pyrenees, andso on, and the plains have been calledJakes and seas.The chief feature of the Moon's surface, however, is the extrii.ordinarycraters which mark its surface all over.These are of enormous size, some beingas great as 150 miles in diameter, whilethe largest craters on the Earth are notmore than seven miles in diameter.

f we were to stand in the middle ofone of the giant craters on the Moonwe should not be able to see ;the towercing walls all round.Craters from five to twenty milesacross exist by the hundred. Theyare circular in form, and the wallsreally form a ring of mountain peaks,rising in many places to a height of

2 0 0 0 0 feet above the surroundingcountry.f the Himalayas were as high inproportion to the Earth, Mount Everest

would have to be twenty miles high,instead of rather more than five miles,as it actually is.Sometimes the inside of a crateris 8,ooo feet or more below the levelof the plain outside. Some craters,indeed, are 19 000 feet deep, but othersappear to be filled up to the brim sothat the floor is far above the levelof the outside plain. Here and thereis a crater which is a mere hole, andhas no surrounding ring of mountains.I t is interesting to know that theheight of the mountains on the Moon

can be measured more accurately thanthe giant peaks of the Himalayas.The measuring is done by means ofthe deep shadows cast by the mountains or crater walls when the Sun isshining upon the Moon at an .angle.The measurements are checked whenthe Sun is shining from a differentdirection and casting other shadows.What are these giant craters and howwere they formed? Well, there areseveral theories, and none of themis quite satisfactory.The most likely theory is that thecraters are really extinct volcanoes,and that the surface of the Moonconsists chiefly' of lava and volcanicash poured out from these volcanoes.

ome scientists think the Mpon S craters werecaused by the impact- of meteorites on its crustwhen it was in a plastic state. As shown here,if a snowball be pressed tightly together andthrown down upon a board covered with snow,a formation almost identical with a lunarcrater results. Representations of almost everytype of lunar crater can be formed in this way

On the Earth, the rain and the airacting upon volcanic ash and lava form soil ; but on the Moon there isnothing to change the character of thevolcanic products and so they remainas they were.Other astronomers think that thecraters are really pit-marks made onthe Moon's surface when it was in aplastic state due to meteorites strikingit as the Moon crossed their path.Still another theory is that they wereformed not by volcanic action orby meteorites, but by the bursting ofgreat bubbles of gas which once coveredlarge areas of the Moon's surface.When the interior gases were relieved,it is thought, the thin crust coveringthe bubbles fell back into the cavities.A fourth theory, and a. ~ r y plausibleone, is that the so-called craters arereally blisters on the Moon's face,due to its cooling. As the Mooncooled a crust formed over its surface,but as the interior went on cooling itcontracted, leaving . spaces betweenthe fluid interior and tfut solid crust.The hard crust could not contract,

98

and after a time the pressure of itsweight caused it to fall in, leaving arim.Another curious feature of theMoon's surface is a number of narrowcracks, to which the name of rillshas been given, because they may atone time have been water courses.In addition there are clefts, abouthalf a mile wide and of unknowndepths, which run across the Moon'ssurface for hundreds of miles.Then there are light-coloured streaksradiating from some of the cratersin all directions. . They are hundredsof miles long and from five to tenmiles wide. Like the clefts they gostraight across valley and mountain,and sometimes actually through acrater. Their nature is unknown.The Moon, like the Earth, is beingcontinually bombarded by meteors.On the Earth these mostly get burntup through friction with our atmosphere, but as there is no atmosphereon the Moon they strike the Moon'ssurface with the rapid motion withwhich they move through Space.I t is as though a constant rain ofbullets and shells were pouring.,uponthe Moon, and Sir James Jeans,:Pointsout that the many romanti c writers who.have described human visits to theMoon have quite forgotten that theexplorers would be under this con:..tinuous fire.

A Million Shooting Stars a i ayHe says that at a moderate o m p u ~tation more than a million shootingstars and meteors must strike thesurface of the Moon every day, theirspeeds averaging about thirty miles asecond, or a hundred times the speedof a rifle bullet. -Although there is no atmosphereon 1;he Moon, and no water, andconsequently it has no storms, theremust be some factor at work to breakup tbe rocks and form those jaggedpeaks that we see.It is probably the bombardmentof the. meteors that does this. Theirforce is formidable, for as Sir JamesJeans tells us, ' ' at a speed of thirty milesa seeond a tiny pellet of matter has asmuch energy and also as much capacityfor doing damage as a motor-carmoving at thirty miles an hour, while a,half-pound meteor has the sameenergy as the oyal Scot rushing alongat seventv miles an hour.There would not be much left of ahouse, says Sir James, i f such a meteorfell on it.Some astronomers looking at theMoon through powerful telescopes haveactually seen what they believe to beclouds of dust such as might happenif a fall of rock took place, and it isbelieved that if such clouds were reallyseen they were due to the bombardment of the rock by a meteor. t iscertainly a dramatic thought that menon the earth should be able to see anevent like this a quarter of a millionmiles away.


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P H Y S I C S ~ n account of the Physical Properties of Matter and some of the great NaturalForces harnessed by m nLIGHT SOUND HEAT MAGNETISM and ELECTRICITY

HE T TH T MYSTERIOUSLY V NISHESModem industry and transport, as well as our health and comfort depend so much uponheat that it is important we should know many facts about this subject as described here

ITHOUT fire man could neverhave become civilised. He isdependent upon it for thecooking of his food the warming ofhis home, the driving of his enginesthe manufacture of his bricks and tilesand metal goods. Indeed, i t would takea whole book to give a mere list of theways in which man uses fire to multiplyhis powers and add to his comfort:Now fire cannot be produced withoutheat. Fire is the visible light and heatproduced by the action of a hightemperature on certain bodies but llbodies do not catch fire and flame upat the same temperature.We know how difficult it is sometimesto light the domestic fire in the grate,and we know also how dangerous i t isto bring petrol or celluloid near a hotfire or even a candle light. Why is itthat some things burn so easily whileothers are set alight only with thegreatest difficulty ?Well all combustible substanceshave what is known as an ignitfonpoint, that is there is a certain tempera-ture at which they will burst into flameand until they are heated to that pointthey will not burn. with flame. The

ignition point of some substances isvery low while of others it is very high.For example the.ignition point of coal-gas .is r 198 Fahrenheit, of carbonmonoxide 1,202, .and of hydrogenI 080Explosions in mines occur becausethe gases that have escaped into theworkings are raised. to ignition poihtby the flame of a naked light or of adefective safety lamp, the fusing ofelectric .wires or some similar cause.In the case of marsh gas which iscarburetted hydrogenand is representedby the symbol CH4 with an ignitionpoint of r ,202Fahrenheit, the tempera-ture must be maintained at or aboveignition point for a certain period

generally the fraction of a minute, beforethe gas will ignite.Some substances, like phosphorus, zinc ethyl, and the hydrides of siliconhave such a low ignition point thatthey catch fite spontaneously whenexposed to air. Piles of coal hay-stacks, and greasy or oily rags some-times ignite spontaneously. I t isimportant if we are to understand thereason for this to realise that theamount of energy given out when adefinite amount of a substance suchas carbon combines with oxygen isexactly the same whether the combina-tion takes place rapidly or slowly.When the combination of the sub-stance with oxygen goes on slowly.if the material is ex-posed to the air theheatgeneratedpassesoff and is rapidlydissipated. The tem-perature of the bodyin that case does notrise to ignition point.But if the heatcannot easily getaway, as for examplein the interior of ahaystack, then theh e a t accumulatesuntil at last ignitionpoint is reached andthe material bursts

This drawing shows why it is bad to sleep between damp sheets. Vital heat is taken from the sleeper s body by the sheetsin order to evaporate the damp, and the body is then chilled so dangerously as often to bring on rheumatic fever99


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HEAT THAT VANISHESinto flame. Such substances as hay,oily rags, and so on, are bad conductors of heat, and that is why theheat is retained where the slow om -bination with oxygen is going on.Many very serious fires have had theirorigin in spontaneous combustion.Some time ago the children in aschool after polishing a number ofwooden stools that they had made,threw the rags that had been used.and which were saturated withlinseed oil and turpentine, intoa cupboard. A little laterthe school was dismissed and an hour afterwardssmoke was seen comingfrom the building. Whenthe fire brigade wascalled it was found thatthe oily rags in thecupboard had burstinto flame by spon-taneous combustionand set the school onfire.Linseed oil has a greataffinity for oxygen withwhich it combines toform a hard substarn;;ethat makes it a valuableconstituent of paint. Thehard substance forms aprotective covering for wood.I t is this oxidation of the oilthat makes a brush left withpaint on it grow hard and forms ahard coat on a pot of paint left un

o v e r ~ d and exposed to. the air. Insuch cases the heat generated getsaway, but in the bundle of oily rags inthe school i t could not escape and socaused a fire.Coal stored in the hold of a vessel _without proper ventilation sometimesbecomes overheated by the oxidationwhich is always going on between coaland the oxygen of the air and takesfire spontaneously.An explosion, on the other hand, isthe result of.the very rapid combustionof gaseous mixtures like coal gas andair, or of very finely divided matter,such as the dust ofcoa l or f loursuspended in theatmosphere.The mixture isignited at one pointand the speed of

because its ignition point is low. Wecould light the wood also with a burning taper if we held it to the materiallong enough, but it would take a considerable time with so small a flame as

We light a fire with paper and wood becausethe temperature at which they catch fire is muchlower than that at which coal burns, and s itis easier to et light to paper and wood first.These- ubstances when burning raise the coalto the necPssary temperature to ignite it, andit then catches firthat of a match or taper to raise thewood to its ignition point which ishigher than that of paper.By lighting the paper first we get alarge flame with sufficient heat to lightthe wood and then when the wood is

explained, is the amount of heat requiredto raise one gramme of water onedegree Centigrade, a unit known asthe gramme-calorie. There is anotherunit known as the large calorie usedin estimating food values. I t is theamount of heat needed to raise onekilogramme, or 1 0 0 0 grammes, onedegree Centigrade.The heat of combustion can bemeasured in an apparatus knownas a calorimeter, which means

heat measurer. A givenquantity of the substanceis burned in an inner vesseland the heat developed isimparted to a measured massof water. The amount of heatgiven out by the burning substance is calculated by noting therise in temperature of the water.In measuring the amount of heatwhat is known as the British ThermalUnit, written for short B.T.U., isoften used. I t is the amount of heatrequired to raise one pound of water onedegree Fahrenheit. The B.T.U. isequal to 5 calories.Primitive man had only one way ofgenerating sufficient heat to make afire and that was by friction. It isbelieved that men of the Stone Agemade fire by striking pieces of ironpyrites together and letting. the sparksthat resulted fall on dry tmder madeof grass.L a t e r m a n

. the combustion in creases rapidly tilla maximum speedis reached. Terrificexplosions of thiskind have occurredin flour mills andsimilar factories,and nowadays to

Even coal could be lighted with a taper if the flame of the taper were held to the co ;I longenough to raise the temperature to the coal s ignition point. This, however, would take too long,and so we use paper and wood in lighting a fire, as shown above

learned to makefire by means of thefire drill that isby twirling a bluntpointed stick be-tween the palms ofhis hands so thatthe point generatedenough heat byf r i c t i o n with aslight hollow inanother stick to setlight to the tinder.

avoid such disasters modern flourmills are equipped with costly dustremoving plants.I t is the fact that different substances have different ignition pointsthat explains why we use paper andwood to light a coal fire. I f we placea lighted taper to the paper, thatmaterial bursts into flame at once,

burning it raises the coal to its ignitionpoint and so the fire is lighted. I twould take a very long time with onlythe taper flame to raise even a smallpart of the coal to its ignition point.The heat that is given out whenthe substance burns is called the heatof combustion, and it is expressedin calories. A calorie as alreadyOO

Sometimes insteadof twirling the stick or drill between hispalms he rotated it by means of aleather thong or by a bow. Even todaythe fire drill is used for making fire byprimitive peoples in America, AsiaAfrica Polynesia .and Australia.In some parts the friction is obtainedby a fire plough instead of a fire drillthe stick- being rubbed rapidly to and


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Here we see whath app e n s when westrike a match. Thef r i c t ion generatessufficient heat to setlight . to the phosphorus and then theignition point of thewood is raised highenough for that toburn also

fro instead ofbe ing twirledround. Anothervariation is thefire saw, a pieceof sharp bamboo drawn rapidly backwards and forwards on another pieceof bamboo. n principle, the fire drill,fire plough and fire saw are the same.Even in civilised communities we

A pint of boiling water and a gallon areboth at a temperature of 212 Fahrenheitbut the gallon contains exactly eight timesas much heat as the pi.nt

still use the friction method of generating heat to light a fire or gas-burner.\Ve draw a match sharply across arough surface and the friction generatesenough heat to raise the phosphoruspreparation at the end of the match toignition point. Then the burningphosphorus raises the wax coating of thematch to its ignition point and whenthat is burning it sets light to the wood.Cigarette and pipe-lighters also gen-erate heat on the friction prip.ciple,a steel wheel being rotated against aflint, fragments of which are made

white hot by the friction and set lightto a wick steeped in petrol.That different quantities of heat arecontained in different substances canbe proved by a simple experiment. fwe place a stone weighing, say twopounds in an equal weight of water andthen boil the water for sometime till the stone is at thesame temperature as the waterwe shall find that it containsmuch less heat than the boilingwater.This is shown by pouringthe two pounds of boiling waterinto a bucket of cold waterand placing the stone at thesame temperature 212 Fahrenheit into another similarbucket of cold water.

t will be found on testingthe two buckets of water witha thermometer that the oneA porous bottle keeps water cool in a hotclimate. Water. oozes through to theoutside. of the bottlewhere t evaporates andin doing so absorbs heatfrom the bottle and itcontents thereby m k-ing the w ter insidecooler than it would be

IOI

HEAT THAT VANISHESinto which the boiling water was placedreceived much more heat from thatthan did the other bucket from thehot stone. Though both stone andboiling water were at the same temperature the stone had much less heatin it to give up to the cold water.

f we made a similar experimentwith two pounds of iron we should findthat it contained only about one-ninthas much heat as a similar quantity ofwater at the same temperature whilea piece of lead weighing two poundsat 212 Fahrenheit would contain lessthan one-thirtieth as much heat as asimilar quantity of boiling water.

Now if we take a block of ice with acup-shaped depression at the top andin this place a ball of copper that hasbeen heated to 100 Centigrade or212 Fahrenheit we shall find severalthings result. Some of the ice in thecavity melts and becomes .water andat the same time the temperature ofthe copper ball falls to the temperatureof the ice. We can find the amount ofwater melted by weighing it .

Now if we can take a copper ballweighing twice as much as the formerone, and after heating it to 100 Centigrade in boiling water place thatin a similar block of ice, we shall findthat by the time it has acquired thetemperature of the ice i t will havemelted twice as much water as thesmaller ball.In neither case does thehot copper ball raise thetemperature of the ice. Whyis this ? Where has the heatthat the ball gave up gone?Instead of raising the temperature of the ice it hasbeen absorbed and used forthe melting of some of the

afer as if furns . .nto vapour . a sorbinq , heal . .


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HEAT THAT VANISHESIf water be boiled in a flask and the flask be thenremoved from the gas, the water will cease to boil,but by squeezing cold water on the flask the waterwill again be made to boil. The cold water con aferreducedbelow boilinq point

denses the steam in-side the flask, therebyreducing the pressure,and at less pressurewater boils at a lowertemperaturr

Vapourcondensedreduces thepressurend w ferboils t lessth n 212Fah

ice. The melted portion remains at freezing point. The heatthus absorbed which seems to have disappeared and is incapableof affecting the thermometer is known as latent heat. Theword latent simply means hidden or concealed. The heat isthere in the water, but is concealed, as it were.As scientists say, the latent heat of fusion of a substanceis the quantity of heat required to change a mass of the substance from the solid to the liquid state without rise oftemperature. .The latent heat of the fusion of ice in the experimentdescribed is the number of calories required to change onegramme of ice at o Centigrade into water at the sameternperature.Similarly, latent heat is required to turn a liquid into a gas.

f a kettle of wateris kept boiling forsome time, thewater will eventually boil away.

This picture shows why it is bad to sit in a draught -even a warm one. The current ofair causes the evaporation of perspiration from the skin, thereby takjng heat from thebody and rendering the person liable to catch cold1 2

But although heat is all the time being impartedto the water, its temperature does not rise abovero o 0 Centigrade or 2 1 2 Fahrenheit.Now suppose that we place over a similarflame -in' turn six other kettles of water at thetemperature of the room, say 16 Centigrade,they can each be raised to the boiling point,roo Centigrade, while the water in the firstkettle is being boiled away. In other words, six

equal quantities of water can be raised through84 Centigrade by the application to them of aquantity of heat that causes an equal quantityof water at roo Centigrade to evaporate intosteam. t will be seen that about 500 Centigrade heat units disappear in changing water atthe boiling point into steam at the sametemperature.The heat which has beenabsorbed by the steam iscalled the latent heat ofevaporation, bu t the latentheat of steam varies at different pressures. When anypart of a liquid evaporates there must besome latent heat. I tmay come from outside,as when we dry a damptowel in front of the

A haystack sometimes catches fire by what is known asspontanP ous c o m u s t i o n ~ Heat generated inside the stackis unable to get away, and raises_ the temperature of thehay to fgrtition point when it bursts into flamefire, or it may be taken from surrounding objects,whose temperature then is lowered, as wheneau-de-Cologne is put upon the forehead to cool it.Rapid evaporation always causes cooling.Even when increasingly hot air results in increased evaporation coolness may result, as theheat rendered latent may not be wholly replacedby the hot air.

t is the practice in hot countries to keep thedrinking water in a porous jar. The waterexudes through the pores of the jar andevaporates, but in doing so it absorbs heat whichbecomes latent, and so the jar and its contentsare cooled.Damp clothes on the body or damp sheets onthe bed are always dangerous, because themoisture absorbs heat from the body in order toevaporate it, and so the body is cooled to a pointthat is perilous to health.Wet clothes on a line dry more quickly in awind than in a calm, because as fast as the air incontact with them becomes saturated with the


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NINE DIFFERENT W YS OF M KING FIR

The drawings ori this page show some of the different ways in which manmakes fire that is causes s u s t ~ n c e s to burn with a flame. The earliestmethod is believed to have been the striking together of two pieces of ironpyrites producing sparks to light dry leaves. Later the fire drill was usedin which a piece of wood was twirled rapidly to and fro on another piece.This method is used today by some primitive peoples and the movingstick is made to move more rapidly by using a bow. Another method ofgenerating fire by friction is the fire plough in which a stick is moved rapidlyto and fro in a groove. n the fire saw one piece of bamboo is sawn t andfro on another piece. Till the invention of safety matches the flint andsteel with tinder were used in Europe and the cigarette-lighter of today.is a modern adaptation of the flint and steel which produces a spark to lighta wick soaked in petrol. The safety match is lighted by friction and thefire syringe by compressing air till enough heat is generated to light aquantity_of tinder which lies at the bottom of the syringe103


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MECH NICS How the mighty Forces of a ~ u r e are applied and made 'to work for thebenefit of MankindSTATICS HYDROSTATICS .KINEMATICS and ENGINEERING

THE INCLINED PL NE ND SCREWThe inclined plane, with its developments the wedge and the screw, is,as we read here, of the utmost importance to inankind in all sorts of ways

H N a railway porter at a stationhas a trolly full of bags andboxes and wants to transferthis from one platform across theline to the other, he does not attemptto lift it across. That would be beyondhis powers.Yet he is able to transfer the load,and he does this by wheeling the trollyto the end of the platform down a slope,then across the level line, and up aslope on to the other platform.In carrying out this task he hasused one of the simple machines, ormechanical powers, as they are calledthe. inclined plane. Just as the wheeland axle and pulley are only develop-ments or adaptations of the lever, sothe wedge and the screw, both of verygreat importance, are merely adaptations of the inclined' plane,First of all let us understand theprinciple.. of the inclined plarie. Ahouse may be situated near the edgeof a cliff or precipice, a hundred feetfrom the ground below. .Now .if a load of furniture has tobe taken to the house it would beimpossible for men to lift it up thehundred feet vertically, but they cando .the work quite easily and deliverthe goods by walking up a sloping.paththat leads round the side of the cliff.

The distance they travel is muchmore than a hundred feet, but whenthey get to the house they haveactuall:J carried the furniture up thehundred feet.This illustration will show theenormous advantage which is obtainedin certain circumstances by means ofthe inclined plane. A horse drawing aload on a road with a rise of one footin twenty is really lifting one-twentiethof the load as well as overcoming the.friction and other resistance of thewagon.

The Sloping Mountain RoadBy making a road up a very steephill wind or zigzag all the way, itslength is greatly increased, but theload that has to be raised to the topof the hill .is d ivided as it were intosections, so that it can be convenientlylifted. Staircases are inclined planes, butas they are usually very steep theyare notched into Steps, so that theymay afford a firm footing.If we want to ascend to the top ofthe Monument in London, the columnerected by Sir Christopher Wren tocommemorate the Great Fire of London,

we camfot jump or climb the column,of course,o f l but we canneaction o able keepinq objects n eqw i 'l ium get to the

t op quiteeasily byw l k i n ground andround thestaircase

which winds its way up the inside ofthe column.Mountain roads are excellentexamples of the inc.lined 'plane used for. enabling vehicles to get to the topof a high mountain. .Railways are able to ascend and crossthe Andes and other lofty mountainranges by engineers winding the trackround the mountain-sides.t must be remembered that whena railway line is perfectly level. thelocomotive that draws the train hasonly to overcome the friction of the

wheels on the rails, and the resistanceof the air. If, however; there is arise of one foot in sixty, or one in 120,then the locomotive has, in additfon,to raise vertically 1/6oth or 1/120thof the weight of the whole train. .In the diagram on page I06 we. see a.ball and a box . resting on a planeinclined to the horizontal at an angleof 30. Now .if the plane were quitesmooth the ball and box would slidedown to the bottom, but owing tofriction between the bcrdies and thepiane the former remain stationary,or, as we say in science, in a state ofequilibrium. Friction supplies a forcealong t)le plane which maintains theequilibrium.Before we examine this matter moreclosely let us think of a number ofobjects resting on a horizontal table, asshown on this page. Here, no matterhow smooth the t b ~ e may be, theobjects remain in a state of equilibriumwithout the application of. any forcesuch a.s friction. The ball, jug, bookand box press down upon the tablewith a force due to their mass. They

This diagram shows how objects pressing down on a horizontal surface are kept in equilibrium by an equal force pressingupward in the opposite direction. Every particle of an object presses down towards the E arth s centre, but the united weightof the particles is concentrated, as it were, at the centre of gravity. Similarly, the many small forces of reaction areconcentrated as one force pressing up cfppol ite to the centre of gravity as illustrated in the box ,on the right of the drawing105


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INCLINED PLANE AND SCREWare being pulled towards the centreof the Earth by gravitation.But they remain on the tablebecause its surface exerts a reactionat right angles to the plane and thereaction is equal in magnitude andopposite in direction to the pressureon the plane or table. The two forcesthe pressure of the objects on thetable and the reaction thus counteract .one another and so each bodyremains in equilibrium.

Balancing ForcesBut the case of a body on an in clined plane is very different. Herethe force of gravitation pulls theobject vertically downward. Thereaction being still at right anglesto the plane is not now opposite indirection to the pressure of the objecton the plane caused by its weight.Now as the first diagram on pagero shows the pressure or weight of theo b j e c t and the re action at right anglesto the plane not beingopposite in -directioncannot now counterbalance one anotherand if the plane weresmooth and no otherforce such as frictionheld the body back itwould slide down theplane. I t s directionwould be between thed i r e c tion.s

When bodiesrest on n in -clined p l a n einstead of ah o r i zo n t ls ur f c e t here c t ion isthen not opposite to the pullof gravity but a t right angles tothe slope. The result is that theobject will move down the inclinedpl ne in the direction indic ted bythe arrows unless some forcesuch as friction prevents itof the other two forces the pull ofgravity and the reaction at right anglesto the plane. We get what men ofscience call the triangle of forces.This is explained in the diagramreferred to. In the triangle ABC thesides represent in magnitude anddirection the forces which are appliedto the body on the inclined plane.AB represents the weight CA thepressure or reaction of the plane andBC the friction ; that is the force thatprevents the object from sliding downthe slope-in other words the forcethat keeps it in equilibrium.The dvantage of the Inclined Plane

Let us see the exact advantage whichis gained by the use of an inclined planewhen a load has to be raised a height.In the second diagram on page r rothe man is pushing the cask up theslope AB. When he has brought thecask to B he has raised it througha vertical height equal to CB.

The principle involved is somethinglike that of the lever. f we want toknow what power is needed to push thecask from A to B then the weight ofthe cask multiplied by the verticalheight it has been raised is equal tothe slope AB multiplied by the powerused.Thus if the cask weighs ro5 poundsand the length of the plane- that isA to B- i s twenty-one feet the baseof the plane A to C twenty feet andthe height of the plane C to B fivefeet then the weight ro5 multipliedby the height five must equal thelength twenty-one multiplied by thepower which in thiscase is twenty - fivepounds.

In other words aforce of twenty-fivepound s will push acask weigh ing ro5pounds up the inclined

planetwentyone feet thereby raising the caskfrom its original positionto a vertical height of fivefeet.

These figures hold good wherethe force is parallel with the lengthof the plane. f the force isexerted parallel t the base of theplane or in any other direction a different formula has to be used but weneed not go into that now. Of coursein the calculation given nothing hasbeen allowed for friction but there isalways a certain amount of friction tobe overcome so that a force of rathermore than twenty-five pounds wouldbe necessary.A great drum wound round withlead-covered cable such as we oftensee where electrical lines are beinglaid underground which ten mencould not lift directly may be rolledinto or out of a wagon by one or twomen with the assistance of a beamforming an inclined plane.t is believed that the AncientEgyptians constructed great inclinedroadways to enable them to put inposition the immense blocks of stonewhich astonish us in their giganticarchitecture.The wedge is a very importantadaptation of the inclined plane inwhich the inclined plane itself is

106

moved by force so as to do work inenlarging an opening or lifting aweight.In the third diagram on page r ro thewedge shown has been inserted undera heavy box to raise the latter. Theforce or power has been appliedhorizontally and has moved the wedgeforward a distance of twenty incheslifting the box a vertical height offive inches.In this case the power exertedmultiplied by the distance the wedgeis moved forward horizontally is equalto the weight lifted by the wedgemultiplied by the vertical height itis raised.

f the base of the wedge istwenty inches its sloping facetwenty-one inches and its heightfive inches and the weight ordownward pressure of the box is

erted.wouldpounds.

eighty -fourp o u n d sther i t hewe i gh t oft h e boxmultipliedby.the vertical heightit is raisedis equal tothe distancethe wedge ismoved forward multiplied bythepower exThe power needed in this casetherefore be twenty-one

Wedges are used for splitting logsand in such cases usually have twosloping sides so as to form a doublewedge. The blades or points of cuttingor piercing tools such as kniveschisels choppers planes nails awlspins and needles are wedges and in thecase of knives axes and razors andweapons like spears swords and so onthe blade is a double wedge.The wedge is often used for liftinggreat weights as when a ship in drydock is raised a little by wedgesdriven under the keel. Some time agoa lofty factory chimney owing to adefect in the foundations began toincline like the Leaning Tower of Pisa.By driving wedges under one side anengineer restored it perfectly to thevertical position.

The Value of the ScrewAnother valuable application of thewedge is the screw. In this case thethread of the screw is really theinclined plane wound round a cylinder.The bottom diagram on page uo willexplain the matter. The distancebetween two consecutive turns of thethread is called the pitch of the screw.The force is applied. to the screw atits circumference and if we think ofthe thread or inclined plane beingunwound we shall see that the circumference of the cvlinder is the base ofthe inclined plane one whole turn of


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TWO STRIKING EX MPLES OF THE INCLINED PL NE

This sloping rock on the summit of Tryfan is an example of an inclined plane. f it were smooth l ike ice the schoolboyssitting upon it would slide down but friction acts as a force enabling them to keep their places without difficulty Evena small amount of friction will act in this way as we know when walking up a fair ly smooth road on a steep hill

The wedge is an example of the inclined plane used for doing work. Just as the blades of knives axes and other cuttingimplements are made in the form of a wedge so the prows of boats are wedgelshaped enabling them to cut through the water.The speed of a boat or ship depends to a very large extent upon this wedge-shaped formation of its bows1 7


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~ E X M P L E S OF THE M NY WAYS IN WHICH THEThe inclined plane is one of the simple machines, and on this page we some of its many ingenious adaptations, enablingman to do work that would be difficult or impossible without its aid. We see the inclined plane in its simplest form in thesloping end of a railway platform or in the plarik placed against the tailboard of a van enabling a heavy barrel to be roUed upinto the van. The staircase is another example of the. inclined plane, as is also the mountain road or railway. An ingeniousadaptation of the inclined plane is the wedge, whi.ch is really a double inclined plane. It is used for splitting logs, but weare much more familiar with it as the cutting edge of.such implements as knives, razors, chisels, swords, saws, scissors, planeirons, and the points of pins, needles and nails. Another adaptation is the screw, which is an inclined plane wound round acylinder. The screw is, of course, a .very valuable invention, and it is used in all sorts of ways. We see it in the ordinarywood screw and in the gimlet, in the more elaborate sc.rews u sed for bolts and nuts, and in machines l ike the s c r e w ~ p r e s s thescrew-jack, the micrometer screw, the screw-vice arid worm gear. Without the inclined plane in these various formscivilisation would come to a sudden standstill, for t s found in some form in practically every machine, and is used in every

1 8


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INCLINED PL NE IS USED IN MODERN LIFdepartment of l ife. We could not reach the upper rooms of our houses without the inclined planeIn the form of a staircase the builder could not reach the top of the house he is erecting without hisInclined plane in the form of a ladder and vehicles would not be able to go up hills nor couldmountains be climbed. It is by taking advantage of inclined planes on the face of the mountain-side that men are able to reach the heights. The screw propeller is another form in which theinclined plane is of great service to man. It would be very difficult for the housewife to do hercooking without the inclined plane for i she had no knife with its wedge-shaped edge enabling herto cut and mince up things quickly she would have to break or tear everything and that wouldnot only be hard work but would take a long t ime to perform. Probably when we are eatingsome very nicely cut thin bread and butter we rarely give a thought to the fact that we owe thisproduct.to the inclined plane in the form of the sharp knife that enables the bread to be cut so thin

1 9


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INCLINED PLANE AND SCREWthe thread the length of the planeand the pitch of the screw the heightof the plane.The force exerted in turning thescrew multiplied by the circumferenceor base of the plane is equal to thepitch of the screw or height of the planemultiplied by the work accomplished in driving the screw into thewood.A one-inch bolt :With a Whitworththread has eight threads to everyi n ch I t spitch, therefore is one-

When three forces act upon a body causing it toremain in equilibrium they can be representedby the three sides o f a tr iangle. In this diagramshowing a b ock resting on an inclined plane,AB reprpsents the pul l o f gravity AC the re action of the plane and BC the frict ion that pre-vents the block f rom sHding down

eighth ot an inch and the circumferencethree and one-seventh inches. In sucha screw the mechanical advantagegained by the thread or inclined planeis equal to about twenty-five.The worm and worm-wheel and thescrew propeller of a steam vessel areexamples of the screw in action. Acommon corkscrew is the worm of ascrew detached from the central spindle.The screw is much used in pressesfor exerting a great pressure so asto squeeze oil and juices from seedsapples and so on ; and also for reducing bales of cotton and othermaterial to a denser mass andsmaller compass.Of o ~ r s e it is in the crossing of loftymountam ranges like the Andes ofSouth America and the Himalayas of

~ o r t h e r n Asia that the inclined planeis of such great service. Engineersmake use of it in building their mountain railways and without it therecould be no such railways as thosewhich cross the mighty Andes in ChilePeru and Bolivia. In the Alps there are many mountainrailways worked by cable or by rackand piniOn but these railways of theAndes which in several cases rise toheights exceeding 15,000 feet areworked by ordinary adhesion methods,the track zigzagging and sloping inorder to take advantage of the inclinedplane principle.At Antofagasta in Chile the railsare not much more than a man'sheight above sea-level but 225 milesinland they lie 13,000 feet or approximately 2 miles above the Pacific.In the course of the first eighteenmiles the railway goes up 1,800 feetthe average rise being one in fiftythough at places it is one in thirty. Ata height of 13,000 feet steam is shut

off and the train then travels down bygravity for nearly seven teen miles.The greatest height is at Ptosi on theAntofagasta system, 15,814 feet abovesea level a height greater than that ofMont Blanc.So high is the Antofagasta railway

8

Man pressinq withforce of 25/lJ ;.u. rthis direction

A force o f twenty-f ive poundsexerted by the man up a slope oftwenty-one feet will rais< 105pounds a height o f five feet

8f Sf ofP/f:lne equ l to circumfer,.e(l.C - 0 Fscr ewI IO

that oxygen apparatus is carried onthe trains for the relief of passengerswho suffer from the sudden change tothe rare atmosphere of these highaltitudes.On the central railway of Peru thehighest point reached is 15,806 feet atLa Cima.t will be quite clear that it is only

the enormous advantage which theinclined plane gives that enables passengers thus to travel comfortably bytrain from sea level to such a greatheight.We find the same kind of thing onthe Darjeeling Himalayan Railway ofIndia, which mounts to a height of7 407 feet. The difficulties of ascent

are very great, and the engineers haveonly been able to reach this point bywinding their railway about With loopsand spirals.At one part of the track, about twelvemiles from the start at. Siliguri station,the line returns by various curves toalmost tj:l.e same spot, but at a higherlevel and then it makes a completecurve passing over itself fifty feethigher. This ingenious method of conForce neededto push thewedqe2 nchesforward in ,this direction

quering the mountain is one of theengineering won


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H E M I S T R Y ~ The Science of the Elements o which all the matter in theUniverse is made upThe Wonders of COMBUSTION and CHEMICAL COMBINATION

THE STORY OF LIFE GIVING WATERThe tremendous importance of water can be understood when we know that it is impossible tolive without water for more than a day or two. Here are some facts about this vital substance

As we have. seen water is a chemical temperatures when they have combinedJ \... compound made up of the two chemically their product is a liquid.elements oxygen and hydrogen Water is really an oxide of hydrogenin the proportions of one of the former just as rust is an oxide of iron andto two of the latter by volume and : ..red precipitate an oxide of mercury.sixteen of the former to two of the, ~ T h e weight of a molecule of waterlatter by weight. ' is r times the weight of a moleculeWp.ter is a remarkable example of of hydrogen.what different properties a compound So valuable a substance is water andsubstance may have and indeed so necessary to life that we do notusually does ha ve from the elements of think of it as dangerous nor associatewhich it is composed. a fatal explosion with it. Yet in theHere is a gas hydrogen which burns very production of the water from thereadily with a blue flame but does gases oxygen and h y ~ r o g e n so muchnot support cqpibustion. t combines heat is evolved in the combination thatin a moment .;with another gas oxy g-en the sudden expansion causes a violentwhich will not bum, but is a powerful explosion. Even . when so small asupporter of. combustion and forms quantity as. a s o d a ~ w a t e r bottle fulla substance yvhich neither burns nor of the gases in the proportions of twosupports cmp.bustion. volumes of hydrogen to one of oxygenFurther, at normal temperatures, has a flame . p p l i e d a t the mouth ofboth the oxygen and hydrogen repiain the bottle, the combination of thein the gaseous form whereas at sIJfiilar gases to form water is accompanied by

a detonation like the report of a pistol.There have been many cases of personsbeing killed while experimenting withlarge quantities of the gases for thepurpose of producing water.Water is so abundant in Naturethat it is not necessary to prepareit as we prepare hydrogen and otherchemical substances. Three-fourthsof the Earth s surface is covered withwater. Water vapour is an importantconstituent of the atmosphere, andeven the dry land as we call it haslarge quantities of water in.it.Not only so but all living creaturesanimals and plants alike are largelycomposed of water, and live theirlives in a constant stream of waterwhich is passing through the bodycontinually. This is true of ourselvesand ifthe stream of water is cut off onlyfor a short time we perish. That is whya man can abstain from food for many

This picture-diagram shows how water evaporates, The molecules are in constant motion, and some from the top layers riseand mix with molecules of air, making the air damp. Draughts blow away this damp air, carrying molecules of water with itr J


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HOW GRE T CITY GETS N MPLE SUPPLY OFIn this explanatory drawing, which ~ u i l s across the two pages, we see how a modern city or town gets a supply of purewater for drinking and other uses So(netimes, of course, the water is o . ~ t a i n e d from artesian wells, but here we see ariver as the source of supply, Thel,,Waier is. s u c k e ~ up from the river by powerful centrifugal pumps, driven directly byelectric motors. ,The water is for


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PUR W TER FOR DRINKING ND OTHER PURPOSESsuspension. t then passes through layers of gravel and is collected at .the bottom by means of small pipes which conveyit to the main. This filter bed is also periodically emptied and the impure sand replaced. The main pipe now runs to theozoner, where a centrifugal pump forces it up through a water tower. While the water is in the tower ozonised oxygen ismade to bubble through it, thus further purifying it. The ozone is produced by passing cool dry oxygen between two glasstubes, when an electric current passing between them causes part of the oxygen to become ozone. This is then compressedand forced up through the water. The purified water is next convey ed to a surface reservoir, where it is stored ready foruse. Before being pumped to the street mains as required it is again p s ~ e d through a registering meter. The distributionof water over a vast area like that of Greater London, where the levels vary by as much as 400 feet, is a difficult problem.It would not do to put all the reservoirs on the highest hills, as in that case the pressure at the lower levels would be fartoo great for domestic supply. Water from the Hampstead reservoir, for example, would rise higher than the top of St. Paul sCathedral. The adequate supply of pure water to large and growing cities is one of the problems of modern civilisation

II3


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LIFE,GIVING WATERweeks, but cannot go without water.for more than a day or two.

The leaves of herbaceous plantscontain from 60 to So per cent ofwater, potatoes and most fruits andsucculent plants from S5 to 95 percent, and seaweeds and aquatic plantsgenerally about 9S per cent. Evenwood has 44 to 55 per cent of water,anc;l i t is the explOsions due to theturning of the water into steam in themany little minute compartments inthe wood that cause the crackling ofwood when it burns.

is. not pure,. for in falling throughthe atmosphere i t dissolves certainsubstances in the air.Water is a great solvent, but whilesome substances, like sugar and salt,are readily soluble in water, others, likethe sand on the shore, which is silica,are so slightly soluble in water as to becalled insoluble.Some liquids, like acetic acid aridalcohol, dissolve in water in manyproportions, but other liquids, likeether, will only dissolve in small

Of the r50 p o u n d ~ that make up . Molecules of waterthe weight of an average man . 00 0 . evaporatinqfromIOO pounds consist of water. . o0 0000 0 0 S d Crqsta SIn a fat ox 45 per ;

water in a kettle the mineral is givenup arid deposited on the sides of thevessel which is then said to be furred.Hot-water pipes through which hardwater circulates sometimes get com-pletely filled up with the depositedmineral matter and when owing tothis the water cannot circulate, thehot water in the boiler may be turned

into steam anc1 explode, wrecking thehouse.Hard water is bad for steam boilers,for the coating of mineral matter on thesides of the boiler is a bad conductor ofheat and as a consequence much fuelis wasted.Hard water will

cent is water, in apig 4 r per cent; ina sheep 43 percent, in a chicken74 per cent, in alobster 79 per cent,and in an oysterSi.per cent.Wheri crystals of coninion washing soda are left exposed to the air they gradually ~ ~ o n i e powder,because the water in the crystals evaporates. Washing soda contains about sixty-three per cent of water

not form a properlatlier With soap,but only a curdwhich is really aninso luble subs ta n e resultingfrom the decom-position of thesoluble soap byThe different parts of an animal's

body contain very varied quantitiesof water. Thus in the case of an oxthe brain has So per cerit of water,the heart 63 per cent, the muscles62 per cent, the lungs So per cent,the liver 7 r per cent, and the kidneys76 per cent.Of course, the water in a plant oranimal is not floatingloose like water in ajug or kettle. I t iscombined with theother substances oft h e t i ssues Theamount of water inan animal is found byweighing the b 0 d ywhile fresh ahd then

driving off the waterby drying it at atemperature of 100degrees Centigrade.The drying breaks upthe combinations inthe body and liberatesthe water, which isevaporated, and thenthe difference betweenthe weight of thebody originally andthe same afterdrying gives theamount of watert h a t has Beendriven off.

quantities. Similarly, some gases, likeammonia and hydrogen chloride, arevery soluble in water, while others,like oxygen, hydrogen and carb6ndioxide; are only slightly soluble.There is no known substance that is.entirely insoluble in water. Even pow-dered glass, quartz and gold dissolve invery small quantities.

the calcium salts ih the water.Some water companies now removea great deal of the h a r d i l ~ s s beforesending the water through the pipes,and consumers at once notice thedifference iri their baths and washingbasins when the change over from hardto soft water is first made.Spring water often comes from aconsiderable depth ,where it has been incontact with salts ands imilar substances,and having dissolvedmuch of these it mayhave a decided tasteand smell. . A chaly-beate spring; for ex-ample, yields water

impregnated with ironcompounds.

We take a greatdeal of the water thatwe need iri the animalo r plant food that weeat, but this is notsufficient. t is im-portant that we drinkwater as well. Chemi-cally pure water is

When blue copper suiphate crystals ,are heated in a test tube they give up water, andbecome a white powder, but if wateA.;.be dropped upon this powder it will become blue

In this connectionit is interesting to seehow the mineral con-tent ofvarious bodiesof water varies. Hereis a list given byProfessors LouisKahle"nberg .andEdwin Hart. Thefigures represent thegrammes of mineralmatter in 1,000 litresof water : AtlanticOcean, 35,664 ; WhiteSea, 33,uS; DeadSea, 253,016 ; GreatSalt Lake, U.S.A.,302,r22; Lake Michi-gan, 145 ; artesianwell in London, S34;River Rhine, 231 ;River Nile, 142.Think what this

not good drinking water. No naturalwater is pure, but always has certainminerals or other substances dissolvedn it. Even freshly collected rainwater,which. is the nearest to pure waterthat we know in a natural state,

When the rain has fallen on the landand sunk into the ground it begins todissolve various substances. In alimestone region, for example, thatmineral is dissolved and the water issaid to be hard. When we boil such

means in the case ofthe Great Salt Lake of.Utah. In everypint of water is dissolved .more than aquarter of a pound of solid mineralmatter.There are two ways iri which hardWater can be made soft. t can be


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LIFE-GIVING WATERselves and demand more room. Thatis why water-pipes sometimes burstduring freezing: they are unable toresist the extra pressure.This table, given by Mr. WilliamColes-Finch, will show the extent towhich water becomes denser whilefreezing.

Cen. Fah Density Weightof cub. ft.lb.At o0 or 3200 999884 62417lo 33 s 999941 6242020 3y60 999982 62 423

30 374 1000004 62 42440 ,, 3920 1000013 62425c:> ,, 41 00 I '000003 62 42360 4280 999983 62 42345 l 130 999038 61 823roo 2120 95866 59844-

Distilled water is purer than tap water, but goldfish will die in it, b ~ c u s e there islittle or no oxygen dissolved in it which they can breathe, and so they suffocate

The weight of a cubic foot o waterat a temperature of 60 Fahrenheit is1,000 ounces avoirdupois. This istaken as a standard with which thespecific gravity of all liquids and solidsdistilled, that is, the water can beboiled and turned into steam, when themineral matter is left behind, and thenthe steam is condensed back intoliquid. Such a process, however, istoo expensive for general use owing tothe quantities of fuel that would berequired for the distillation.The other method, which is thatgenerally followed, is to add somesubstance to the water which willcombine with the lime or calcium saltsand form insoluble compounds whkhwill be precipitated as solids. Amongsuch substances are sodium carbonate,generally called soda, borax, andsodium phosphate.Pure water, that is, freshlv distilledwater, is very insipid and fiat to thetaste. Even boiled water that has beenallowed to cool is fiat, partly becauseof the absence of air in it. This maybe largely remedied by agitating thewater violently or pouring it to andfro several times between two vesselsthrough a considerable height. Airthen becomes dissolved in the water.Some people have placed pet goldfishin a globe of distilled water thinkingthat pure water was best for them andhave been surprised when the fish died.They were suffocated because therewas no air dissolved in the water forthem to breathe by means of theirgills.The boiling and freezing points ofwater are very different from those ofthe two gases of which it is composed.While oxygen boils at - 182.5 C., andhydrogen at --252.5 C., water boils at100 C Oxygen freezes at - 219 Cand hydrogen at -258.9 C., butwrtter freezes at o CA peculiar thing about water is thatwhile like other substances it contractsin volume and becomes denser as itcools down from boiling point, at 4 itstops contracting and expands until thefreezing point is reached.As we read in another part of this

book, this strange and unusual propertyof water, which causes ice to float instead of sinking, is of the greatestimportance and benefit to mankind, for it prevents the rivers and lakes inthe temperate and polar regions frombecoming frozen into solid masses ofice which would never melt again.Iron is one of the few other substancesthat behave in the same way. Solidiron will float on molten iron as icefloats on water, because it is less dense.Why these substances behave likethis no one knows. Contraction as the

is compared. t requires more heat to raise thetemperature of a given amount ofwater 1 than it does the samequantity of any other substance excepthydrogen. Because of this, water issaid to have a greater heat capacitythan any other su?stance excepthydrogen, and water is taken as thes t a n d a r d i n d e-termining specificheat capacities.It is because oft h i s g r e a t h ea t

ater is a very bad conductor of heat as this experimentshows. Water can be boiling in the top of a test tube,While ice remains unmelted at the bottom

water gets cooler is due to the molecules approaching each other moreclosely, but at 4 Centigrade or 39.2Fahrenheit, new forces come into playand the molecules then rearrange them-u


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t l F E ~ G I V I N G W TERcapacity of water coupled with the fact millionth of an inch in diameter.that it gives up its heat very slowly The numb_er _of molecules in a singleand reluctantly that islands like Great drop of water, therefore, must amountBritain have a much more equable to a million million million millions.climate than inland countries like - They fly about with a speed exceedingGermany and Russia. The sea having that of a gun, shell, or at the rate ofabsorbed heat when the Sun was twenty miles a minute.shining upon it, gives it up exceedingly Lord Kelvin has told us that i a slowly in winter,and this keeps thetemperature veryeven, and so inlands near the seathere are none ofthose violent extremes found inthe interior of continents.

The amount ofheat required toraise 1 gramme ofwater 1 Centigradeis called a c ~ l o r i eand this is used asa unit of heat. -Water is a poorconductor of heat,as can be provedby a simple experiment. Weight asmall piece of icewith a nail or. othermetal object andplace at the bottomof a test tube. Fillwith water andthen hold theupper part of thetube in a Bunsenflame. Soon the 'water in th_is partwill boil, but theice below will remain unmelted fora long time becausethe heat is conducted through thewater so. slowly.Water is also apoor conductor ofelectricity. Water is vervslightly compressible. Wh e n t h epressure of thea tmosphere isdou hled 2 0 0 0 0volumes of _watera re only c om p re s s e d onevolume. In. all itsforms, snow, iceand liquid, and atall temperatures,wate r gives offvapour - that is,evaporates. Whathappens is tha

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