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 ufntionnl device. 
 
 THE 
 
 HJGH SCHOOL PHYSICS. 
 
 BY 
 
 ALFRKJ) ].. (iAUK, M.A., 
 
 moil HCV ..(,, liOSTON, MASS., 
 
 AND 
 
 C. FESSENDEN, M.A., 
 
 moil BCHOOr,, NAPANKR, OXT. 
 
 A u>homea 6, ^„n-.^. of EaucaH„.f.>r use in t„e Schools of Ontario 
 
 A .nor,.,, ,,, 0...a .fr,.Uic In.ru.io.for „. ,„ .,., LoU , f A.., S..i. 
 
 ' :;: : T"'1 " """' ^— ^->'- -- ^^ a. .e.... ,;.....;!ir- 
 
 ^»./,nn^<.,/ 6, Cou„e,l of P„Uic In.M.tionf„r u.e in the School, of Q^elee 
 
 I'RICP;, 
 
 •Sl.OO. 
 
 ^v. .J. (;agi<: x- company, 
 
 NEW VOKK, U. s. I 
 
 TORONTO, CAN, 
 

 'J 
 
 Kntered according to Act of Parliament o< Canada. In the Office of the Minister of 
 Agriculture, by W. J. Oaob & Co.. in the year one thousand eight hundred and 
 •ighty-seven. 
 
 hrj\- 
 
PREFACE TO THE FOFRTH EDITION 
 
 — OF — 
 
 HIGH SCHOOL PHYSICS. 
 
 rpHE American Association for the Advancement of Science at its 
 -L meetmg in 1879, appointed a committee to consider the subject 
 of Science-Teaching in the Public Schools." The followin.r are 
 extracts from the report of this committee :— 
 
 ' ' T'^"- oug'l books and teachers the pupil is filled up with information with 
 regard to Science Its facts and principles are explained as far™ possUde 
 and then left in the memory with his other school acquisitions! HeTea „: 
 the Sciences much as he learns Geography and Historv Onlv in „ f 
 exceptional schools is he put to any flire^ct^men afwo^upon tt subiort' 
 matter of Science, or taught to think for himself. . . ^ The ? 1, o o^ 
 
 iTeKtse f'^'£?hr • 'i^ ''? ^^'"''^^y ^^ t« incite the%; '' ^"o 
 
 neip himselt. Mechanical school work can give instruction h.,t n- Z„ I. 
 
 develop faculty, because this depends upon seExeTS! £ien^^^^ 
 
 pursued, IS the most valuable school of self -instruction From th^h^if ^ • ^ 
 
 men of Science have been «elf -dependent an S-VeUaTt ^^^^^^^^^^^^ 
 
 ^h;^e^z:ij^'^^-'^- ^'^y '^^ bernt-o^st'idnrer'J; 
 
 That such a state of things should exist is not to be wondered at 
 Any eacher knows how hard it is to dispossess the minds of students of 
 the old inherited ideas of learning from books. They take naturally 
 enough to memorizing a page of text, but seev. >o have no other ideas 
 of education. They are bom blind (to the v u .d of natural objects), 
 and unfortunately have been taught to read before learning to see. 
 
 The system of education pursued for generations has been a study 
 of words rather than a study of things. Pupils have been trained to 
 read and remember what others have written, instead of being trained 
 to ascertain and establish what is true. 
 
 Is it not natural, then, that we should find them not independent 
 seekers after knowledge, but merely receptacles of information? 
 Nevertheless, hard as the task may be through the faults of our 
 ancestors, is it not the duty of the teachers of to-day to train their 
 pupils m that close and accurate observation which develops patiencv 
 
iv 
 
 PREFACE TO THE FOURTH EDITION. 
 
 and cultivates the habit of mental concentration ? Is it not our duty 
 .8 It not our privilege, to change them from passive and imitative to 
 active and creative ? 
 
 hZZ^'^ 7);«»bjects are so suitable for this purpose as physics and 
 biology. But even with these subjects the teacher of elementary 
 science must keep constantly in mind that the primarv object of his 
 work ,s mental training, while the acquisition of info'rmation by his 
 pupils IS a matter of only secondary consideration. Pupils having an 
 ap itude for this work will find such elementary training a helpl" 
 not a hindrance in their advancement as specialists, while those havinc, 
 no particular aptitude for science will find the power and habit o'^f 
 fonmng exact conceptions, which they have gained from a proper study 
 of elementary science, of incalculable benefit, whatever walk of life 
 they may follow. They will leave our schools with hands trained to do 
 a little manual work well, with eyes keen-sighted enough to see thin^^s 
 
 XLIV "^' '^^^"^ ''''''' °^ ''^'"^-^^ "p- *»>- ^'^i-i^ 
 
 About four years ago I was requested to prepare a work on Elemen- 
 tary Physics for use in the High Schools of Ontario. A careful 
 examination of all the works of the kind lately published in England 
 and America showed that, while many contained well-selected experi- 
 ments logically arranged, none could be used as a text-book without 
 taking from the pupil nearly every opportunity of observing and of 
 thinking for himself. I found the experiments described, the attending 
 phenomena carefully described, and the conclusions to be drawn fully 
 stated. Often, indeed, the experiment was given merely to prove to 
 the pupil the truth of a statement previously made. In short I found 
 many capital books with which to cram pupils with information, but 
 none which seemed to me entirely suitable for mental training. The 
 book which approached nearest to my idea of what a text-book on 
 Elementary Physics should be, was the " Elements of Physics " bv 
 Professor Gage, of the English High School, Boston, Mass., and ' with 
 the consent of the author, I have made it the basis of the ''HL/h 
 School Physics." Mr. Gage's experiments, which for the most pal-t 
 are to bo made by the pupil himself, I have retained, making, also a few 
 additions. But m place of descriptions of phenomena and statements 
 of conclusions. I have substituted suggestive. <i.,o.tiong t,> he answered 
 by the pupil after he has made the experiments. By this means the 
 
PREFACE TO THE FuUKTII EDITION. y 
 
 pupils curi.,«.ty i« cjlistod on the aide uf the teacher. Tlie boy .nakes 
 ho e.H,ornnentH, and observes the resultin, phenon.na, becaus t 
 
 Of course I consider the experiments and questions as merely tvnicil 
 and not exhausfve The skilful teacher wUI add to bX I S urn 
 stances sho^. may be desirable. For example, a wrong answer. venW 
 a pupd to one of the questions in the text-book wUl surest .t I 
 
 Very seldom is it necessary for either text-book or teacher to enun 
 cate observed facts. I do not mean to say that the pupil will gene alW 
 enunciate observed facts correctly and fully; far from T Tut the 
 
 rrjirri r'r""'"°" ^"^ completlng'^e enunclti on ,-; n ty 
 the pup.1, should by experiments and questions, point out the e rorl 
 and omissions until the pupil himself constructs a perfect enunciatLn 
 
 in the next process in the study of physics in H,« .^ ■ ■ 
 and c„„,,„iati.„ „, natural W. a, LJ^^'^^^,^ ;"';;: 
 
 and whieK »e are .,.e.,„™ ,»„„,„ /JZ ^rral.^lr"'""''' 
 
 adanrc and jusUy appreciate the fe. «lrM „d°„ tt t H T " 
 
 TT. , „ . C. Fkssendbn, 
 
 High School, Napanee, 
 March, 1889, 
 
COJSI TENTS. 
 
 CriAPTEli I. 
 MATTER AND ITS PROPERTIES. 
 
 Introduction. — Moloculo. 
 
 I" A (in 
 
 Coiistitiitioii of ni.'ittor. — Physical 
 and dicmical clian-es. -Korcc. _ Throe sk of matter.- 
 rhenoniena of attraction, -adhesion, colicsion, etc. .... i 
 
 CHAPTER II. 
 DYNAMICS. 
 Dynamics of rtnids.-I'ressnre in lluids. - Daronieter. -Compres- 
 sibility and expansil.ility of iluids. - Transmittal pressnre. - 
 Siphon. -Apparatus for raisinjr liquids. - Buoyant force of 
 (luids. - Speciilc ,«;ravity. - Motion. - Laws of motion. - 
 Composition and resolution of forces. - Center of -ravity. - 
 Curvilinear motion. - .\ccelerated and retarde.l motioi, _ 
 The pen.hdnm. -Momentum. -Work uud eneri-y - Trms 
 
 forn.ation of em-rj^y. - Machines, -K,,„ilihriun. of forces in 
 one plan(> 
 
 47 
 
 CIIAPTKK III. 
 
 MOLECULAR ENERGY. - HEAT. 
 
 Heat <I»«(lned.-Tem|.en.tnre.-l)ifiusion of Heal 
 hei 
 
 I'at. 
 
 bodit 
 
 Kxp.ansion, 
 
 lOllects of 
 
 Th.-r 
 
 iM.iiiietry. — Law; 
 
 Luivs of rnslon iiiul Itoil 
 
 oC ii'a 
 
 s(;ons 
 
 I US'-. 
 
 potential enerfj:y, and 
 dynamics. — SteMm-eniriue 
 
 I bat 
 
 eonverlible into 
 
 rice versa. — Spedllc heat. — 'i'| 
 
 lermo 
 
 i>< 
 
CONTENTS. 
 
 I-Af: 
 
 CHAPTEn IV. 
 ELECTRICITY AND MAaNETISM. 
 
 f'"nvntdec,,n.i.y._l3atU.i.s.-Km.ts pro.,,,ced hy elocMnc-ilv.' 
 - IMortncal moasurcmonts. - Arn,^notsan,l mni^notism.- r.nvs 
 
 <> -.rronts. _ Magneto-electric and cnrrcnt hulnction.- 
 I l.<'.-n.o.,.lecf,rici.,v. -Frictional electricity. - Electrical ma- 
 chines.— Applications of olcctricily 
 
 • • • ^00 
 
 •r'nAi»Ti;i{ V. 
 
 SOUND. 
 Vibmtion an,l waves. - Sonn.l-waves. - Velocity of sonn.l. 
 flection and refraction of soun.l. -Loudness. _ Piteh. 
 
 bration of strings. —Quality 
 
 lie- 
 Vi- 
 
 20 1) 
 
 CHAPTEK' Vr. 
 
 RADIANT ENBROy- LIGHT. 
 
 Introduction. —Pi,otonietrv.—Ke(le(ii..n i. .• .■ 
 
 ^, , '"•>• ^'*^''<^Hi<>"—l{clraction.— Color. 
 
 -Thermal ertects of radiation. -Optical instrnn,cnts 
 
 ;j2i 
 
 Ai'PKNnix 
 
ily. 
 iws 
 
 na- 
 
 200 
 
 2f)f) 
 
 ELEMENTS OF I'TIYSKJS. 
 
 ;i2i 
 
 37i") 
 
# 
 
B 
 
 s 
 
 ELEMENTS OF PHYSICS. 
 
 CHAPTER J. 
 MATTER AND ITS PROPERTIES. 
 
 INTRODUCTION. 
 Means by which we obtain a knowledge of Nature. — 
 
 W^„..o,„.o,M,lo,l ,viil, .n,.nn«Mfaninin,r.., KnruvlPflaP r.f t,|„> fllilios 
 
 NOTES AND CORRECTIONS. 
 
 and'^'^i.tiot"'" '"'" '^^"-'-I-e^t " permanent " between "in" 
 
 Page 23.— Insert foot-note "Min,, i:,. t i. 
 ehem.cal .lecomposition beS beco.ni.fg lapors » '"°'*^ '"^'''^ ""^'^^'^^ 
 
 Page 102 'I lir f"" ^T""^"^ " ^^^^ " '•^'^'j " CAH." 
 i age 178, 1 / hnes from bottom. -For " 161 " read " 18^ " ' 
 
 part of the stroke." ^ ""^ always" read "during the greater 
 
 Page 109, 2 lines from bottom -For "h .f ^ 
 <h^o. to the heat generated in tTe fm-nace " ^^''"'' "'"^^ ''^^'^ "^''^-'fe'y 
 
 I age 220, at foot of page. -Insert " nl. ■ 
 prove the truth of the fo.Jgoing statem^!;'" ' "''"^ °^ experiments to 
 
 c^iS^^HHoJP'-^^'' "^"^*--' Electricity" read "Electri.i- 
 
1 
 
 We 
 
 ahci 
 Ilea 
 we 
 
 Olltl 
 
 and 
 ieiu 
 thrc 
 edg 
 
 II 
 
 sens 
 
 seer 
 
 heai 
 
 full 
 
 neai 
 
 hold 
 
 twe( 
 
 whe 
 
 man 
 
 whiz 
 
 cxp( 
 
 decc 
 
ELEMENTS OF PHYSICS. 
 
 CHAPTER I. 
 MATTER AND ITS PROPERTIES 
 
 INTRODUCTION. 
 
 Means by which we obtain a knowledge of Nature. - - 
 We are provided wilh means of gaining u knowledge of the things 
 about us. We hear sounds, we see light, we smell odors, we fed 
 heat, we feel the contact of other things, and we taste. Thus 
 we have six gates ihrough which information concerning the 
 outside world may reach us. These gates are called aeuses, 
 and any information received through one of them is called a 
 sensation. Moreover, we Jave the power of reasoning; and 
 through our senses, aided by reasoning, we acquire what knowl- 
 edge of Nature we can. 
 
 OUU SENSKS SOMETIMKS DECEIVE US. 
 
 It must not be forgotten that we are often deceived by our 
 senses, or, it may be, by our reasoning. We believe we have 
 seen what we have not seen, or have heard what we have not 
 heard. Thus to nearly every one, perhaps to every one, the 
 full moon appears larger when near the horizon, tlian when 
 nearly overhead. You may i)rove that this is an illusion by 
 holding a five-cent piece at arm's-Ieng.h, nearly in a line be- 
 tween your eye and the moon in the two positions. Again, 
 when that beautiful meteorite shot across the sky in June, 1884, 
 many who saw it afflrmeU afterwards that they had heard it 
 whiz, while others, having quite as acpte a sense of hearing, 
 experienced no such sensation. Doubtless, the former were 
 deceived through mistaking the meteorite for a rocket. As they 
 
i I 
 
 2 MATTER AXD ITS PHOPKRTrES. 
 
 Imd always heard . .oc-ket win., tl.ov infenvd that the ,neteor- 
 e 1. 11 kew,.e. Jt . pn,vo..|.ial that a stc.-y changes wondc- 
 
 fnll3 as pHHsc-s fron. m.n.th to nu.nlh, an.l U.is has boon set 
 
 dawn..tho ef^et of n^o.al depravlt. in n.nlvind. ,U.t i^^ 
 her the elFect c,f nnxing n,. inleronc-es with sonsa.ions. The 
 ndcntof natnre, ifhe is to snceee.l, nn.st lean., an.on.. oth 
 •ngs, not to allow unconseions inferenee to take the ^he , 
 
 'jbservation. ' 
 
 OUDKU OF NATURK. 
 
 t em. t ,1 c. p... .„ ,u a reg,,!,,,. „,,!,,. ,„„| tl,,,! „,„„„ e,,,,^™ ,„-o 
 
 of 1.0.C „l,o Lave g.,„o heforc l,i,u, c-a„„ot l,„t I Lk-vo h 
 -*'».'/ /-.,v«« ',y c./,„„c.c.. ,t ,s „,e „,„,„.,, „, ,,^, . ,^, 7^ 
 ".v.-»t,ga,. ll„s „r,lc.,- .,f nature. M„d, l,a. b.cu done, , 
 liiucli iiK.ic ifiiiaiiis to l,e doii,.. ' 
 
 A LAW OF NATUHK. 
 
 When we have discovered a fact co„eer„ing ,l,e order of 
 
 We btato a law ot Nat,ue a» fully aud precisely a» wc are 
 acqua,nted w,th the facts of which it is a s.atcnent. ^ rth r 
 
 anothu foi ,t. Ihus, long ago, " Natuie abhors a vacnum " 
 
 was g,vc„ ^ a law of Natnre, and was based on snch , ■",. 
 
 o..» - - been ,n.ade np to that tinte. Hnf further oh . . 
 
 a:;i:'i:"'"" ■™" ''""^' ore hereafter, led „.„ to 
 
 A LA„ OK NATURE IS NOT A CADSE. 
 
 ecm-se. When Newton, from seeing an apple fall in his orchard 
 "s le,l, tron, further observation and powerful reasonh;'' 
 
 1 
 
 
 i 
 
 
 
 
 ■'\. 
 
 to ec 
 
 
 the c 
 
 
 covei 
 
 i 
 
 i 
 
 town 
 will 
 
fiXPEHIi\[ENTATrON. 
 
 3 
 
 to enunciate the sublime law of gravitation, he did not discover 
 the cau3e of the falling. No one since Newton's time has dis- 
 covered the canse of a tendency on the pnrt of bodies to move 
 toward one another, and it may be donl)ted whether any one 
 will ever discover that cause. 
 
 MATTKU AND PIIKNOMENA. 
 
 It nrrs been sai.l that any information of the world abont ns 
 tiiat we receive through our senses is called a sensation. That 
 which, acting on our sensiferous organs, may produce a sen- 
 sation is calle.1 a natural i>henomevon. Again, the source of 
 all natural phenomena is matter. You have a sensation that 
 causes yon to say that you see a rod-hot ball. The licrlit which 
 acting on the sensiferous organs, produces the sonsa°tion, is a 
 natural i.henomenon. The ball itself, which is the source of 
 the light, is a portion of matter. In short, amjthinq you can 
 see, anythfnrj you can smell, anything you can feel, anything you 
 can hear, amjthimj you can ta-tc, is matter, 
 
 MATTER MUST BK PISTINGUISIIKD FROM PHENOMKNA. 
 
 You must be careful not to confound matter with any 
 phenomenon of which it is the source. Sound is not matter • 
 but a bell, (,r anything else that may be the source of sound is' 
 matter. Light is not matter ; but the sun, or anything else that 
 may be the source of light, is matter. An odor is not matter • 
 but a p.ece of musk, or anything else that may be the source' 
 of odor, IS matter. A flavor is not matter ; but a piece of 
 cinnamon, or anything else that m.y be the source of a fl-ivor 
 IS matter. Heat is not matter; but the flame of a candle, o.' 
 imythmg else that may be the source of heat, is matter Pni,, 
 IS not matter; but a sharp thorn, or anything else that may'be 
 llu. Hourrr of i)ain, is matter. ^ 
 
 § 1. Experimentation. ~W, observe phenomena which arise 
 from natural conditions ; that is, from conditions with which 
 man Las nothing to do, and we may learu h from such 
 
■* MATTKU AM) ITS iMJOl'KItTlES. 
 
 Observations if they are carefully attended to. But we may 
 liave some doubt concerning the exact conditions under which a 
 certau, phenomenon arises. We then bring about accurately 
 known artifical conditions, and observe the phenomenon arisiu<r 
 Jn this way our knowledge becomes n.ore accimUe or scientijic" 
 It IS a matter of co.nmon observation that water sometimes 
 freezes, and that it fr.rzes when the te.nperature is low. By 
 tnal we may ascertain the exact conditions .nul.r which water 
 changes mto ice. Whenever we place natural objects under 
 certam accurately known artificial conditions, for the purpose 
 of observu.g the resulting phenomenon, we make an experiment 
 n IS, of course, necessary that we know exactly the conditions 
 present ,n any experiment, or our trial, inste'ad of teachin-., 
 will deceive us. It is not an ensy task to make an experiment 
 and properly to read the lesson that it teaches, because it is 
 very diflicult to exclude all conditions whose presence we do 
 not dcsu-e ; and often conditions are present without our know- 
 ledge. 
 
 An experiment is a question put to Nature. We receive 
 the answer by means of a phenomenon; that is, a chancre 
 which we observe, sometimes by the sight or hearmg, sonre- 
 times by other senses. In every experiment certain facts or 
 conditions are always known; and the inquirv consists in as- 
 certaining the facts or conditions that follow a; a consequence, 
 liie following experiments and discussions will illustrate : _ 
 
 § 2. Things known and things to be ascertained - Wp 
 are certain that we cannot make our right hand occ« sam 
 pace with our left hand at the same time. All Experience 
 teaches us that no t.o portions of matter can occupy ke Zl 
 space at tke sam. tin.. This property which matteV possess" 
 
 tJ^ml: 'T" '"" '^ ""■" ^•'^^^^' >^ -^"-5 -"/>-- 
 
 The of. . '' T"'" ''' ""'*"•' "^^''^'"«- ^'- possesses it. 
 These acts being known, let us p-oceed to put certain inter- 
 rogatories to Nature. Is air matter? Is a vessel full of Ir 
 
1 
 
 THINGS KNOWN AND To JJK ASCKllTAlNEl). 
 
 5 
 
 I 
 
 Fig. 1. 
 
 a vessel full of nothing? Is it - empty "? Qni matter exist in 
 an invisible state'/ 
 
 Experim«.nt 1. Float a cork on a surface of water, cover it witli a 
 
 tunihler, or tall -lass jar, and thrust the glass vessel, nioulh downward 
 
 nito the water. In case a tall jar (Fig. 1) is used, the experiment 
 
 may he made more attractive by placing on the cork 
 
 a lighted candle. Slate hmv the experiment answers 
 
 each of the aljove questions, and wliat evidence it 
 
 furnishes that air is matter; or, at lca.st, that air is 
 
 like matter. 
 
 Experiment 2. ii„ld a test-tube for a minute 
 over the mouth of a bottle containing amuiouia- 
 water. Hold another tube over a bottle containing 
 liydrochloric acid. The tubes become tilled with 
 gases that rise from the bottles, yet nothing can 
 be seen in either tube. Place the mouth of the first 
 tube over the mouth of the second, and invert. 
 Do you see any evidence of the presence of matter? 
 Was this matter in the tubes before they were 
 brought together? If not, from what was it formed? 
 Which one of the above questions does this experi- 
 ment answer? How does the experiment answer it? 
 
 Again, we are quite familiar with the fact that matter exertg a 
 downward pressure on things ui)on which it rests ; and tliat 
 matter, in a liquid state, at least, exerts pressure in other direc- 
 tions than downward, as, for instance, against the sides of the 
 Fie. 2. containing vessel. Dops air exert pressure ? 
 
 Kxperiment 3. Thrust a tnmbh^r, mouth down- 
 ward, into water, and slowly invert. You see bub- 
 bles escape from the mouth. What Is this that 
 displaces the water, and forms the bubbles? When 
 the tmnbler becomes fdleil with water, once more 
 invert, keeping its month under the surface of the 
 water, and raise it nearly out of the water, as in 
 Figure 2. Why does the water not fall out? Wh.at 
 would happen if you were to make a hole in the 
 bottom of the tmnbler? Make the experiment with a 
 glass tunnel, first closing the small . u\ with the linger, and then 
 removing it What conclusion do you draw from this? 
 
(] 
 
 AIATTKIJ AND ITH riU-.PRliTrKS. 
 
 Fig. 3. 
 
 '"'.« I.. iL „ ,01,, .° , '" """"■■ '"■-" "- """-r 
 
 o '-I'-t- lias IE, y/rt.s, ,„^. ^y(./^/^^r, 
 Experiment.';. Exiiaiist tl.<> .,!.. . 
 
 stop-cock to pi-evont th,. r...t. ,. i" '^""" *"''"«' "'« 
 
 The exporimcnts with n>r toach i,s thnt // ,• > .. ^^^ 
 -nee,,,. .na,,r, It can ...lu^Z^ZXt 'n ^^ 
 occvines, it exerts jve.vire mnJ ; "^ "' "^''^ 'P"''' '^ 
 
 AN INVISIBLE STATE. ^ "^ ' "^*^'' ^^^^^ EXIST If, 
 
 § 3. Minuteness Of particles Of matter p. • , . 
 
 J'roatlio, small purtielos of 
 tli.'it siiLstfinco which are 
 floating in the air. The 
 fi'f, for several meters 
 aroun.l, is so.netinies filled 
 "•ith fragrance from a rose. 
 y''^ cannot see anythinir 
 in ^^ ^ -ir. 'uit it is, never^ 
 
 fine dust that floa-., :u , .,oa, t! '. 
 
 J«t sea renders the .'hon '..r'l. "• '""." . '^'"' "'^o'" "^ '-osemarv 
 
 FIff. 4. 
 
 Fig. 5. 
 
 iiil£' 
 
THE MOLErrLE. 
 
 1 
 
 years, by constantly semling forth into the air a dust of musk 
 1 hoi.gli tliu nuaiber of partich's that owcapo ninst Ik; eoiuitless, 
 v».'t th^.v aru so .^auill that the original giaiu doi's not losJ 
 j)i'rc!('p<J>ly ill woigiit. 
 
 Tho init-roscoiH; (.nahlt'.s us to sec, in a sing]., (hoi) of sta-niant 
 water, a world of living creatures, swimming with as much iil.erly 
 as whales in a sea. The larger i.rey ui.un the smaller, and tho 
 smaller find their food in tlie still smaller, and so on, till tho 
 powx.r of the microscope fails ns. The whale and the minnow 
 do not .hirer more in siz., than do some of these animalcules, the 
 largest of whi.h are har.Uy visible to the naked eye. But -is 
 the smallest of these perform very complex oi.erations in col- 
 lectmg an.l assimilating food, we nmst onclude that thoy are 
 eomp..sed no. only of many parti.-les, but of n.any kinds of 
 matter. These minute living forms that p..ople the microscopic 
 w..rl( are exceedingly largv, in comparison with the incon- 
 ceivably nnnute particles called molecuk,, which i.hysicists now 
 " measure without seein<^" 
 
 §4. The molecule. -Experiment 1. Examine carefully a (h-on 
 of water w.tl. t e nakeC c, ., or wiU. a nncroscope. So f^v^^H^ 
 
 mifU with wahT. Fill a test-tul.e with wafer (Fi-r «) 
 Insert a cork stopper, pierced with a jjlass tube ; heat 
 over a lanip-flan.e, au.l note the phenomena produce.l 
 Describe the result. Place it in ice-water. What hap- 
 
 pen.' 
 
 Kep.!at this experiment, usin- otiier li.,ni.l.s, an.l 
 
 (» fill* »'j\o 111 t^ ., _. 
 
 compare the results 
 
 This change of volume 
 
 can 
 
 Expand- 
 ed Btute. 
 
 be explained only on one of 
 
 two suppositions : the space contract 
 
 occupied by the water rnay, as '" """" 
 
 it appears, be full of water, ^^^^__ 
 
 which the heat causes to expand, and occupy a greater space as 
 
 represented giaphically in Figure 7 ; or the body of water may 
 
rig, a 
 
 Kxpancl- 
 
 t'd state. 
 
 Which Contract- 
 ed atate. 
 
 ^ MATTER AND ITS PROPERTIES. 
 
 consist of a dofuute number of distinct particles culled molecules 
 (us represented in Figure 8^ sen-ir-.h rl c. molecules 
 
 „„„„ „ 'fe"i'- o;, scp.iiiited Irom ouy another bv 
 
 spaces so small as not to be perceptible, ^ 
 
 even with the aid of a microscope. Expan- 
 «ioii, in this ease, is accounted for l)y a simple 
 sei)aration of molecules to greater distances. 
 There is no increase in the number of mole- 
 cules, no increase in their size, only an en- 
 I'irgement of sjmce between them. Whiel 
 of these sui,i)ositions is the more probable? 
 
 Kxperlment 2. Place a tinul,lcr full of cohl 
 water in a war... place, and ii. about a.i hour 
 
 l.a.s doscenclocl intL to li^La" ' "' '' "' '''"'"''" ''''' -*«'^« --' 
 
 -;;i;::rh;i-^^^^^^^^^^^ 
 
 «>> the wate,. 1|„H seems to contradict the fn-st of the above 
 
 vatc, s fall of water, leaving no roo.n for other matter. But 
 ccordn.g to the second supposition, the space is not fillea w 
 ^vater; there .s st.U room for particles of other matter in tlo 
 spaces among the molecules of water. Now, as we canno co ! 
 o.ve of two portions of u.atter occupying the same space at 
 tu, same tune («.,., .here air is, water cannot be), we co 
 ude that the glass '^ full of water " is not full of water n a 
 Hlar ma..ner, it n.ay be shown that no visible body con 
 letely fills the space enclosed by its surface, but that there a . 
 spaces m every body that may receive foreigi matter If the o 
 m-e spaces, then the bodies of u.ttor tlu^t our yes a/o te" 
 .n.tted o see are not continuous, as space ;s continuous B.^ 
 every v,sd>le body is an aggregation of a countless numbef o 
 Hei,arate and mdividual bodies called molecule,. 
 
CONSTITUTION OP INFATTRR. 
 
 IVrlbrm, at ^our homes, the two following experiments : 
 
 Kiperinient 4. PiilvcTize oiic-half of a toaspoouful of starch, and 
 l.oil it in two tal)lc.spoonfuls of water, stirrinj,' it nieantinio. What 
 l.hcnomona occnr? What do they teach? What becomes of tlie 
 water ? 
 
 Kxperlmont 5. Fill a bowl half full with peas or I)eans. J.ist cover 
 tlieni with tepid water, and wet away for tlie ni-lit. Examine iu tlie 
 morning. What phcuomena do you observe ? Explain each. 
 
 Strictly speaking, are bodies of mattiT impenetrable? What 
 only is impenetrable ? When you drive a nail into wood, do yon 
 make the two bcxlies oeeiip^' the .same spaee at the same time? 
 Do the wckkI and the iron occupy the same space? How only 
 can you explain this phenomenon, consistently with the i)nnci- 
 l)les of impenetrability of matter? 
 
 § 5. Theory of the constitution of matter. — For reasons 
 whi<'ii ai)pear al)ove, together with many others that will appear 
 as our knowledge of matter is extended, physicists have gener- 
 ally adopted the following theory of the constitution (jf matter. 
 Evenj visible hodij of matter is composed of exeeedimjhj small 
 particles, called molecules; in other ivords, every bodi/ is the sum 
 of its molecules. Xo two molecules of matter in the universe are 
 in contact with each other. Erer>/ molecule of a bod;/ is separated 
 from its neighbors, on all sides, by income ivabhj 'small spaces. 
 Every molecule is in quiverinrf motion in its little space, moving 
 back and forth betiveen its neighbors, and rebounding from them. 
 When we heat a body we simply cause the molecules to move more 
 rapidly through their .spaces; so they strike harder blows on their 
 neighbors, and usually push them away a very little; hence, the 
 size of the body increases. 
 
 This theory seems, at first, little more than an extravagant 
 gues3. But if it shall be found that this theory, and no other 
 theory that 1ms been proposed, will enable U8 to account for 
 most of the known i)henomena of matter, then we shall be con- 
 tent to adopt it till a better can be produced. 
 
 
10 
 
 MATTER AND ITS PROPEHTIES. 
 
 § 6. Porosity. —If the molecules nf n u . 
 ^^l^^olute contact, it follows ^^fc ,' . ""'^^^ ""'' nowhere in 
 
 among, them which may bo occm ic ' '''"I ""^^'^"l^'^'^l «l>nce,s 
 «taiu,es. These spaces are " ' "'"'''''''' ''' '^'■'-''- -"> 
 
 ^•Joth and beans. It is Zalt^ 'T' '''^'''' ^'''^''l'P--« i" 
 -ters the vacant .paco'Lt^r;'"'' """" ' '"'^ '^ --"v 
 -"stances. All matted i '^ ^ 7" ""' ""^'^^'"'^^■^^ «^'"-- 
 t'"-o..oh solid cast-iron .nd IT ' I? ''''^''" '"=^^^- ^'' f^''^'^'^! 
 !--'y,nuch as cha;^w^nV^:r ^^ T"' ^"^^^"' ^'^'' '''^"'^^ 
 •s restrictcl to the invisible ^^uJ '" '"'''' '" ^'''^^i-, 
 
 c-avities that nmv be seen in '''''"'"^" '"<>lc-enlcs. The 
 
 t"^' --0 no .norc^^i; i^'t: IL^!:':^^ '^''^ "'^' 1--' '>"^ ^'oles 
 J'one3-con>b, or the room of 1 o ' ^"^■"' '''^"' ^''^' ^-'"« of a 
 ••~. the pores Of the :^^^^^^^^^^^^^ 
 
 CKsl has s„occ.eclc.cl ha nscer^i '.•''''' ^■'"'•'"'^^'^•'.s, the phys 
 
 -'"1 .. it is thought (hit iLs : .e i' "• ':'i'^"""-' ^^> ti- «i.o Of the 
 "i;P'^"- I" other words, tV olee ' "'''''"' '"'■'''^'^- ^''a.. u,/ 
 
 What an apple is to the Jarth , '".T"' '' ''' '' "''•^l' «'• ^ut 
 
 ^- or„.o,ecu.es i„ . ,,,, La Z:^':^^'''''^^^ ^'> --'t the nu.n! 
 ' ^<^^"'nl. we shonl.1 require S.O.OOo""';;.;: ' ' " *"" '"' """'«" '" 
 
 -^l^^"f5!^;^'':L:i:\;^^ ;vood, apple, p.,tv, 
 
 volumos „f ,|i,n,,,,„t (.,»,, .„,„ ' '■"■''- "" "■'•'fl'l» <>ni„. „,„,„! 
 -a.,o,.s for boli„vi„g Sl I, i''t' """"^ '■'"'•"-' !■- 
 
 liointiiro nr,. t|,s -amo W i' ° 1""«ssimi; ,in<l tl,,. i^,„_ 
 
 matt..,, in „si„.„ :,„„;, .i,;;*;^;';™' «■';' »<>".« MU. |,„vo ,„„,.„ 
 
 ---«H:.M,H,..™.,.c:;,:;:::~;;nt:;- 
 
nowhere in 
 
 ol'itT sub- 
 fippL'ars in 
 It it really 
 L'H of these 
 '>e forced 
 till! liv^uid 
 I' p'O'sics, 
 I»'s. The 
 >iit holes ; 
 cflls of a 
 JO called, 
 e. 
 
 ■i>i"U with 
 Ik' pliysi- 
 It' size of 
 ize of tho 
 ■ ''Jiiiii an 
 of Water 
 tlie iiiim- 
 ullioii iu 
 
 putty, 
 
 I'y have 
 
 di'fer- 
 
 • 4 and 
 B satne 
 St has 
 eenles 
 > tprn> 
 ! more 
 E'ciiles 
 '0 call 
 
 SIM1>LE AND COiMPOUNr) StJBSTANCES. H 
 
 thorn more cleiise. By the «?«<!« nf „ k i 
 
 j'j LUL, mas.9 oj a bony we inidorst-inrl tha 
 
 5 8 Simple and compound substances. - Plaoo a sm^ill 
 .,..-. y ,„• .„,„„. „„ a „„t «tovo. I„ a few n.inaU. cL™: 
 to a black ,„a... Tl,i, l,la,.k «„,«ta„co is f„,„„, ,„ ,,o ,|,a,. 
 o»l, or carbo,,, „„ Ccnist. call it. Evklcntlj- the s,„,a,. ,„,,t 
 ... oonta.no. ca,l,o,„ fo,- U,o o.arl,„„ came fro.n tiro ..J 
 n,c.a,st, aro also abl„ to obtai,. w.aUT f.-om ,„j;.ar. Tl,c I,:" 
 m our «po,.„uo„t, cxpclx tbo water i„ the f„n„ of .,tea,„, ami 
 Ie.ave» tho earbo,,. Carbo,, can be ,..xtraet,.,l from JJZ 
 other ,vav. Prepare a very tl,ick s.vr..p. l-V .li.«olvi„gC" 
 ot wa er, a,„l p„„r „po„ the syrup t,vo or three thue. it 
 
 .'irr:;!';;;::." """■ "-" -'" ''''''^- "'"»'" - "•*^- '.«-« 
 
 By mutable processes, there may be obtained from marble 
 
 dm III "' " "'"'■''°" ■ "'""'""• '^ ■•' "'«" "•tiled cal- 
 
 uum, „lne, looks very ,„„ch like silver , the third Is a gas called 
 
 ox-yge,,, winch, when set free fron. its prison-house i„t .,1 
 
 lib!:;;:.:,. "'""^' """ '"" ^'^^ "^ ">" ■'""■^'" f™" -vi-i' h™ 
 
 If ivc should grind a s.uall piece of marble for many h„n,.s in 
 
 •stni, each little pa.it' ^f :;::.;.: ! •;:':,; rXm.n'.bi;:':.'',r- 
 
 <>n.g".al hnnp. If we .,h„ul,l couli ,„.. ,,„ ' " " « 
 
 :e;r.:"L::r„r;: ;-; .--:.i:;;i;.:;'r:;;;::i;i 
 
 "-AjKit lo nnd iill Iho rnn ecu cs iimt .iliI-M v 
 
 -.allct piece, our u.olecnle. our ni;! l;'' t H.le' ," ' X a",:; 
 
 Bince „,,„.ble is co,np„„.d ,„• tl -ee sni 
 
 * i 
 
 ^•'uni, and oxyjren, we c.nelnde tl „.. ,„„ 
 
 capable of division. No one has been able 
 
 >.sf!ui('o,s, carbon, eal- 
 
 i.'tt our molecule K.self must bo 
 
 to separate any one 
 
12 
 
 MATTER ANn ITS PIIOPERTIES. 
 
 m 
 
 Hi '' 
 
 These ^bstancosS'li" 1^; .T^." """ °-^-~S™- 
 "l> into oU,or substances -nv! n^ °''"' '" '"""k "'em 
 
 ■"-ts. Those sntlwL LT T'*'' ■""■*""•"■' <>'' "'<- 
 
 :'--- a. ea,K, ^Z^T J^Z'^o" ^^ '"^^ "«- 
 l)er of sMl)staiice8 known to in-,,, t.' "'" '■■"S" nnni- 
 
 other snbstaaeos are el:,, i of ' "" "'""™"- ^" 
 clemento. nipounils of two or more of tliese 71 
 
 "Ot lose tla.ir^l,au :.'*:„::: T"">-.""-" •••"•■"■'• ^'^ "o 
 l'""P- Snd, a divi,ion7sc"Ulo,, '"■■;•' •'''''•■''' "'"■'^'»» ""' 
 
 In o</,«. !,„,, ,/;;::"; * ; ••" ««« '■' '» '- ,■<» *„,«, 
 
 llieraselves arc <iivide,I • and ,!■,!„ ' ,'"»•■"■' "'" "'°''^«"l™ 
 
 «.bstanoes. carbon a^ at r nj" ' "?"'""'' '"" *« '>'" 
 sngarno longer ovists • otil, L, , , ■'"■''°"""*'' '» 'lestroje,! • 
 
 Tbemoieenlcof s tist! „ n""?' ""™ ""«■" "» l''"-- 
 
 H '- be,.,, se„a,,:;i tbar::, ' • ' ,t i;;:r;t - 'r ^^w^'" 
 
 pose it. Such a division is ...UnV , ''"'''"" ^'"'^'^ <'«'»- 
 
 these c.han..o.s the substa ; I io ;;'''', '""''"" '^"""- I>"'-i".'^ 
 ha« "oen only a eha,,.., ,n.^^: , "^ '""' "'•^ ""<■ <''"u.i,red. T).,.,-e 
 
 a vc-y intense heat, the ^ ;"•;;, fj ', ^"" ■'^^--' ''^ -hje.tecUo 
 
 •■^ that it Ijecoines eonverted into a 
 
f 
 
 lias taken 
 iictcd from 
 >-\Vgen. 
 break thojn 
 t'<^-'>' or ele- 
 i'lto other 
 lai'ge mini- 
 Diits. All 
 
 these 71 
 
 ^, is that 
 d toithout 
 
 sugar is 
 t, but do 
 'et as th(> 
 eiierally, 
 ide7Uitf/, 
 physical 
 loleciiles 
 <Hvide(l, 
 the two 
 itroyed ; 
 s phico. 
 o which 
 at coni- 
 (Jeiier- 
 'iontitf/, 
 
 RtOilii), 
 
 During 
 
 'i'licre 
 
 rraii^e- 
 
 cted to 
 
 into a 
 
 ANNIHILATION AND CREATION OF MATTER. 13 
 
 mixed gas, consistinj, of two gases, oxygen aud hydrogen. This gas 
 IS not c.ndensable at any ordinary ten.perature. Unlike steam, it 
 burns and even explodes. What kind of separation is this ? Wha 
 has been separated ? 
 
 Blackboard erayons are prepared by subjecting the dust of plaster 
 of Par,s to great pressure, which causes the particles to unite and form 
 the crayon. AM.at kind of change is this? What kind of union? In the 
 experiment (page 5) with the mnmonia and hydrochloiic-acid gases 
 the two gases disappear, and a solid is left in their place. Wliat kind 
 of change is this : chemical or physical? Is it union or separation? 
 
 >; 10. Annihilation and creation of matter impossible. 
 — KxiKTJiiieiit 1. Prepare a saturated solution of calcium chloride' 
 Mix Willi an e(|ual bulk of water and weigh the „(,lution. Prepare a 
 dilute .solution of sulpluiric acid (1 to 4j, ami pcmr an ecpial weiglit of 
 the last solution on the lirst, all at once, and shake gently. Ins^,antly 
 (he mixed li.,uid becomes a solid. The solid formed is connnonly 
 called plaster of Paris. It is an entirely diflcrent substance from 
 either of the two li(|ui(ls used. What kind of change is tliis ? A new 
 substance has been formed. Has matter been created ? Weigh the 
 '.•esulting solid; compare its weight with the sum of the weights^of the 
 two li(|uids. What do you (ind? What conclusion do you draw. 
 
 Solids may be converted into lujiiids or gases ; gases may be 
 converted into li(|uids or solids ; substances nuiy conii»letely lo.se 
 their cluuiuttcristics : but man has not discovered the means hif 
 ivhich a single violccule of maitcr can be created out of nothiny, 
 or by ivhich a simjle molecule of nuttier can be reduced to 
 nothinr/. Midivv cannot be created, cannot be annihilated ; it 
 is a constant (inantity. The discovery of this fact laitl tlie 
 fonndation of the science of Chemistry. 
 
 This statement nuiy not seem to accord with many occurrences of 
 every-day exi)erieuce. Wood, coal, and other substances bin-n ; matter 
 disapi)ears, and very little is left that can ijc seen. Hut does matter 
 pass out of existence when it disappears ir. burning, or does It assume 
 the invisible state known l)y tiie name of jras ? 
 
 Experiment 2. Hold a cold, dry tumbler over a candle-flame. The 
 bright glass instantly becomes dimmed; and, on close examination, you 
 find the glass bedewed with (hie drops of a Ikjukl. This lUjuid is water. 
 
14 
 
 "'""'" '>'•' '" <•."..•«„.„«,. 
 
 Voii may ,/,,, ,^ ,^ HtrflH*,.. n , 
 
 >;'^/'.i-i."i.....x...::: 1^:^ -^- '^^^^^^ «.. «. not n.... 
 
 :. •;•'" "'wuyH .tanw. ov.;S j; :;;''- "^" '"-^'o «a,„o nat.M-o'' 
 '< »a.,..r „f the riv<.r mn« a .M ,. / of k?;'; "" "•" ^'-^--^-t clays 
 
 :f''^^p^^^^ ^"^" " Conn „a.s .ottle- 
 
 'y- '"..I Hhak.. a;«o pour C J!r ; '"*'' *''*-' '"""^'' ^"vor ti. 
 
 '"t all tin, ,„v|„iM., /S,\u 't' r '"^' ""^ ""»y tl.e a.sl e 
 •^ '•""".• .i.at their collec'tl rj :,; ;:;;:""""":'>' /-^"nncl, alu 
 Watordor-H not duhm nni /.#- i 
 
 Nat...... ,. ever array, J tr,:?'^^' ^'" """""""' "-I aU 
 
 - air. IH .■..,.„„.,,.,,.,., 4 » L. ', i*:,;;:''''''!'' ""-■ vapor rise. i„ 
 
 '-'ocea,. whence It can.,. (; "'l f/'*' '''''' ''"'"''"'^ '" 'ivers to 
 
 oco«„H,un,| even the ^' v^urU^^^l^Vr '"* ^''"'' '•"..Mn.nt.s a.m 
 
 well «H whole t,-ll,e« of «,S«.' '''''''' '^ ''"-"' """ ''""ay a s 
 
 ;;• -.nte.| an,o,.« the ,| ^LV"^^^^^^^^^ ^"I ue ^;..;: 
 
 that, havo crnn.bh.l l/.todn.t «. i n '"'"' ""' '""'I'^h will ere 
 
 t'- '/"^«^'V or ..mtter rej^, „*, 1, l^;^.;;^ '"^^'""^ '« ^'-V.npt. Only 
 
Force. 
 
 15 
 
 'lot flamo; 
 ■■ If water 
 not set- i( ? 
 ^•isihle gas 
 •out of the 
 floating in 
 fiatdio. A 
 rt'st days, 
 king a bed 
 Jst, wljioh 
 
 «s bottle; 
 ly does it 
 t was be- 
 vtT tight- 
 ir. What 
 (;nt sJiow 
 ? If so, 
 of sight 
 
 eel vvliile 
 ^ aslies, 
 inewhat 
 I, and it 
 t which 
 
 nor are 
 y^vhcre 
 lysit-'al. 
 ns tlie 
 ises in 
 crs to 
 ts and 
 •ay, as 
 L' may 
 1, ere 
 nposo 
 
 I'll wo 
 Only 
 
 §11. Force. — Experiment 1. From a piece of rardhnnr,! ..,. 
 peud, by a,^, „, .ut tL^-caUs, .,. p,,,,,,,,',, ^ t^Trl;i, 
 
 about 2""' apart. Procure a 
 clean, dry glass tube, about 
 40'^"' long and 3"" in diam- 
 eter. Hub a portion of this 
 tube briskly with a silk hand- 
 kerchief, and hold it about 
 2''"' below the balls. The balls 
 seem to become suddenly pos- 
 sessed of life. They gather 
 about the rod, and strive to 
 reach it. If we cut one of 
 . , , , , . t'lc threads, the ball will flv 
 
 straiglit to the rod, and cling to it for a time. The n,eans by which 
 lie rod pulls the balls is invisible. Vet evidence is positive that the 
 H.d has an mfluence on the balls, -that it pnlls then.. Slip a piece of 
 g^ss between tl;e ro<l an<l the balls; still the in.luence is felt by the 
 
 ^.^^:^''' ""^^^^^^ "'^ ''''''''' '^"-'« "'^^ — ^ the 
 Now slowly bring the rod near the balls, till they touch. They at 
 
 1.-st chng to the rod; but soon the rod, as if displeased with theiJ 
 company, begn.s to push them away. Withdraw the rod; the b^ 
 <lo not hang by parallel threads as before, but appear to be us h 
 one another apart. Gradually bring the pahn of the hand up be a f, 
 the balls, but without touching them. The balls .M-adu'ill . 
 ;'.<• Pnil.of the hand, and con>e together. Hem,; t "L c ' and 
 
 hoy ag^n. fly apart. Matter does not seem to bo the dca 1^ 
 tlMug winch it is often called ; it can push an.l puu. 
 
 Kxperln.ont 2. Haise one of these balls with the fingers and then 
 w. H raw the fingers. Something from below seen.s t<,CclM, an 
 I." 1 the ball <lown again. The san.e happens with <^^ch o, e o^ Tl o 
 
 uu ba N. Cairy the balls ivto another room, the same thin- occurs 
 Ca,-ry hem to any part of the earth, the san.e thing ore " It m si 
 bo that ,t ,s the earth itself that pulls the balls. The .'rt^i p, , '^ 
 fnut and the leaf from the tree to Itself; it pulls all obj" o , .c 
 and more, it holds them there. Attempt to raise any thin' fron t u'. 
 jrround, an.l you feel the earth's pull resistin-r you 
 
 Attempt to break a string, or crush a piece of chalk, and you find 
 • Tables of the Metric MeaBuroB may bo found i„ the Appendix. Section A. 
 
10 
 
 MATTKFl AND ITS lUlOPEnTIES. 
 
 
 t„ «1», ,„„ a„„,„p. ,„ 4:,:;;,",;;^;™"» '° koep tu™, t„g„,„„, and 
 
 vJi^K.TZ^ft"^-'^'' *™'^°°y '° P-'h ™o to 
 
 "or why ."^ »«-■ .s called force. We do not 
 
 another, or to separate from one anZl We do 37 7° 
 nature of force- wo 0..,.,.^^ "'e do not know the 
 
 know that t e „,rt h' '"" ," ""' '^'■•■"'" '"' '"'^ «""l"J' 
 
 The familiar effet, roll "" I- '"'■'"'" ^-^^^ I™""™"- 
 
 eausewe attrihut to foree wZ ' , " " "'"""= ^ '"••" 
 
 mt, we l„o]< for a oa„s ^nd „ T " ^' '" '"°"''" ""'""^ *« 
 
 dflinitio,, that has heen ^ve^is , J^ , f '"■" '■°'"l"'^''^'"'i™ 
 
 i;ni:n, foret\;t,i;r':;:;z.^r"':r:;:,,rjr'';- ^ 
 
 A pushi.,, force is caiied a „,.l,„ f::^:^"'^" 
 
 Placn the roj l„ a »lh " " 7,° r,, ,T '"" "' "'° """• """ 
 Coos the .X|,erl,„.,,t, prme . ' .. T "°'" '° "" "^"''"1 ""J- 
 or to „„t|, „f ,,„„ , fr;,''° '"''"-';"'■;■« "-'""«» to only o„c 
 
 "i«'-r T'" 'r •■■•■"'"' "■'■X":::f,:;;^r'" """■"•"'" « ■■• 
 
 All att) action and reiiulsion betw^^r, .mr. 4 
 mutual. ^ '^'"^^'^ different portions of matter are 
 
 §14. Molar and molecular forces — Tho n-i i , 
 
 «.ev ..ave „,.t to J.':, C" ^tr Z'.-Ttr '^l' ''r" 
 . we pass the hand several times over the part of the rod 
 
MOLAR AND MOLECULAR FORCES. 
 
 17 
 
 that has been rubbed, and over the balls, thej quickly surrender 
 their forces. These forces arc temporary. They are called 
 electric forces, and their cause electricity. The attractive force 
 tiiat draws the balls to the earth existed before the experiment. 
 No manipulation can destroy it or increase it ; it is eternal and 
 unchangeable, and exists between all portions of matter. This 
 force is called the /orce of gravity, and the phenomenon is called 
 ijravitution. 
 
 We ha- 3 seen the effects of attractive and repellent forces, 
 reaching across sensible distances. Have we any evidence 
 that these forces exist among portions of matter, at insensible 
 distances, i.e., at distances too short to be perceived by our 
 senses? Stretch a piece of rubber; you realize that the^^ is a 
 force resisting you. You reason that *f the supposition be true, 
 that the grains or molecules that compose the piece of rubber do 
 not touch each other, then there must be a powerful attractive 
 force reaching across the spaces between tiic molecules, to 
 prevent their separation. After stretching the rubber, let go 
 one end. It springs back to its original form. AVhat is the 
 cause? Compress the rubber ; its volume is diminished. (Does 
 this confirm our supposition respecting the granular structui-e 
 of matter?) Remove the pressure ; the rubber springs back to 
 its original form. What is the cause? 
 
 Every body of matter, with the possible exception of the 
 molecule, whether solid, liquid, or gaseous, may bo forced into 
 a smaller volume by pressure, — in other words, matter is com- 
 pressible. When pressure is removed, the body expands into 
 nearly or quite its original volume. This shows two things : 
 (u-st, that the matter of which a body is formed does not really 
 Jdl all the space ivhich the body appears to occupy; and, second 
 that in the body is a force, tvhich, acting from tvithin outioard, 
 resists outward pressure tending to compress it, and expands the 
 body to its original volume when pressure is removed. This is 
 of course, a repellent force, and is exerted among molecules^ 
 tenduig to push them farther apart. 
 
18 
 
 AIATTKI.' AXI> ITS I'ROI'KUTIEH. 
 
 But it Jias piovioMslv boon shown tl.^f fi • , 
 
 effect, when two fo.-cos aet on .. , , ''.^■'' ^'^'^ ^vJ.jit is ti,(. 
 ^-'t two bovs, at opposite e h 1 of !'/', ;" '''"'"'■^^' <l'>ectio„s> 
 '-^'' P"^'' with ec„ H forc^ . ; H > '' '""'' "" '"'''^- ^'' 
 
 J?'v.'.ter force is n„„lic.(l V , <'"-«"^'t'on in which the 
 
 <^^;-i'ni-i^v,whi;'h:t,:;;::',-^^ 
 
 science, when he first h...,. ""''*' '^ '"'-^^'""f''' in 
 
 -"Other. Jf f,i„, ;J ■; ^-".v ;-ven, ,lo not touc.h one 
 
 ^he thought of m^^u'X^ T'\ "'■ "•'""" ^"'"'"- ^^ 
 tl.ewindsas.so„,„ch,h.l "' '""^" '"''''''' '^^^^'^y kv 
 
 Tiie ancients, perceivino- tint mnff 
 -"-II parts, overcan^e tln^ di^^e 1/' '""•'' '" '"""'^ "'^ ^^ 
 "^"">te particles have hooks o', '' '"''P'^^^*".^ H'-'-t the 
 -"Other. Onr knowc I<r , '' '''''''^' '^''^' ''^'^'l^ «"<' 
 
 We see that the molecnle of ^ ''''"'""^'^'■^^'■^"^^'• 
 
 .-vpart, or fron, scpar. i ^1 •' '"'' '^'^'''^ '•••'^'" ^'"'i"^- 
 
 - also kept f^-on! H^^:^; " .XrH' ^""'^^"''^-^ '^^ = ^"<^ 
 '>.V -> ever-existing repelh>nt^^ b " ' T T""'"'''' ''^'''^''^ 
 
 «ible distances l)ctween n.ol 1 , ' ^'''''' ^'^ -^ inse. 
 
 /-•-. When fbr^ "r e f' ". '• "" "" "^"^^' -"'-"^-• 
 t'-v are called noZ t^'ZoT. V""'''' '"^^"-- 
 ^-ecs; (2)ofn,olecniar fSrees " '""^^'•^^^-"« O of n.obr 
 
 "• '^"'^^"^^ STATES OF MATTER. 
 s 15. Matter i)resont<s JfcL^u- • ^i 
 ''■'/««, .,,,,1 ,wJ„,,,"':i ° . ;; ""7;li'r-.o..t.,tn,o,: .WM, 
 
 -K.io„t i,i,ii„,,,„i.,,, I V ,, air'nT "'"' "' "•■""■ -""■ 
 
 "■'tu..„„i,uiga,osof„i,.. 0„ Uus account 
 
THltEE STATES OF MATTER. 
 
 19 
 
 they called thorn, to,.:c.lh..r with fi.., ck.nents or pri.narv matter. 
 Ihcy rannot now he .so regariled IVoin a eheniical point of view 
 bec.au.se each of then, ha.s heen sei,an.te,l into still n.ore simple 
 substances; „or fron. a physical stan,|j,oint, because, as will 
 soon be shown, most substances amy exist in auy one of these 
 states. 
 
 i;' 16. Characteristics of each of these states 
 O.MHTi.n.nt 1. ITovide two vessels, a cubical dish ami a jjoblet 
 e d. luun , a capacity of about 200-., Also provi.Ie 200™n. ,,( ,, ' ' 
 in.)-, of water, a.ul a cubical block of wood containing.- 200-' 
 G msp the block, and place it in the cubical vessel. Atten.pt to do the 
 san... hn.^^ w.lh the water. Why ca.. you not ..n-asp the water ? Pour 
 a port-on of the water n.to the cubical vessel. When you move a por 
 tH,u of the block, the whole l>,ock uk,vcs. When you pour a^ort^ . 
 
 \\ 1 y s this ^ Why .s .t that we can dip a cup.ul of water out of a 
 P -If ul, without raisin,, the whole? Pou,- all the water into .he ^o let 
 he water adapts itself to the shape of the goblet, and the vesse lis 
 tilled. Attempt to place the block of wood in the ,.,blet. What dif- 
 
 ^Z .'"f '""";' '" '"" "'•"'"" • '"''^y ""•^' '""•--»- ? Vo r 
 
 e w ""v "T"' '" ''""•'• " '""^'^*'^ '''''' t« the shape of each 
 
 ossel. Why? Drop the bh,ck of wood on a table. Pour water on the 
 tul^e. How does a IKpud behave when there is no vessc o cou in" 
 
 Kxperl.„ont 2. Throw small particles of sawdust into the «ol et of 
 ^vate. ; you can thus reu.ler perceptil,le any n.otiou of the water in the 
 joblet. Just as, by thn.wiu, blocks of wood on the sn.ootl .tfaco ' 
 a ver you can discover the n.otion of the river. Noti.e the ease 
 "•-th winch the particles „,ove about, rise, and sink. As they become 
 <l".et, shghtly jar the vessel, or tap it with tin- end of a HM,cil am 
 oUce the ease with which disturbance is pnKluced throu^l.: a 
 •l"..l. Now rap the si.le of the block with a hannner, and observe 
 how numovable are the particles of wood. ouscnc 
 
 Our experiments teach ns that the molecules of solkh are not 
 msily moved out of their places; conseqnentlv, solid masses 
 form such a frmly connected tohole that their shape is vat .asih. 
 changed, and a movement of one part causes a movement of the 
 ^rhole On the other hand, the molecules of liquids have scarcely 
 any fxcdness of position, but easily slip between and around on^ 
 
20 
 
 MATTKU AND ITS PUOPMtTrKS. 
 
 sclto vossol, a,Kl arc easily separated into parts. 
 
 ,K,tatoos are ,k>,„c.,1 r,-o„, haskot t„ basket. Wh , im i.llt 
 II :« lloiiml, ,n,.kc«k-« glide past o„e another ' '"' 
 
 It ,s „ot so easy to stml.v the eharaeteristics of gases, becaasc 
 we cannot us„„l ,■ see then,. ]!„t we nnu- l,e ai,le,I I, „ > 
 Similar to that en„„o,e., to n.ake the „,o veUlt ^f ll^rWs^ 
 
 or h^ra=;-!frt ';:,■.::■ ":,:,""""■ r-r^' - ™»" --" 
 h. the path or the .,«„.;.,:"„„ n . r;:,':"'*!.,?! """' "y"^ 
 
 *^^, and freedom oj motion amony themselves is almost perfect 
 The, appear to he in a continual state of repulsion, aJlconZ 
 <:nently have a tendenc;, to expand to greater and grecUer volumes 
 li.cy cxi,ancl UKleflnitoiy, unless confined by pressure while 
 l",n.ds and solids tend to preserve a unifonnlty of ^1^ 
 I-.qu.ds do not rise al)ove wl.at is called thei. surface, and we 
 
 mte s^crface an. there is no such thing as a vessel half full of 
 MS. On the other hand, if gases are subjected to pressvre their 
 
 .^ ^"'^"^ ^^ssel maybe compressed into a pint vessel 
 
 Pr even luto less space, if sufficient forge ia u^cd. m ,om- 
 
THREE STATES OF MATTER. 
 
 21 
 
 pression of liquids is barely perceptible, even when the pressure 
 is very great. 
 
 § 17. Philosophy of the three states of matter. — We 
 conclude from the difficulty which we experience in separating 
 the parts of a solid body, that the molecular attractive force in 
 solids is very great. From the ease with which we usually 
 separate the parts of a body of liquid, we might conclude that 
 this force in liquids is very weak. But before arriving at any 
 conclusion, it is necessary to consider how the difficulty of sepa- 
 ration of the parts of a liquid is to be measured. It is very 
 easy to tear off a portion of a sheet of tinfoil, but we should not 
 surely regard this as an evidence that the molecules of tin have 
 but little attraction for each other, for in tearing such a body we 
 only apply the force to a comparatively few molecules at a time. 
 We can form a just estimate of the strength of molecular attrac- 
 tion only by attempting to separate the foil into two portions by 
 such means as that the separation may take place no sooner at 
 one point than at another. So, too, it is very easy to separate 
 a drop of water into two portions, but this is no measure of the 
 attractive forces unless we take precautions that we do not apply 
 the separating force successively to different molecules. If we 
 succeed in preventing such a successive action, and there are 
 certain methods of doing this more or less perfectly, we should 
 find the process much more difficult, — more so, indeed, than to 
 produce a similar change in many solids.^ 
 
 There is, however, a difference in the molecular action in 
 solids and liquids ; such that, in the latter state, the molecular 
 forces offer no resistance to a shaping force, while in the former 
 state, change of shape can only be brought about by the appli^ 
 cation of considerable force. 
 
 In a gas, on- the contrary, there is little attranfion between the 
 molecules ; but as they are constantly hitting one another, and 
 thereby tendmg to drive one another apart, it requires an external 
 force to keep them together. 
 
 » Tb* wbwjv* /or«? pf yrm )» dt least 132 iba. per nt^me Jncb, - Wajjwij*, 
 
22 
 
 MATTER AND ITS PROPERTIES. 
 
 notlo,, of each mole;uirh TkeZt" f 1 * " '"""" ' '" ""'"I"' "■<= 
 It is almost or quite Impoii 1 , o ? "'"" '" " """^ "">«'> wLc-ro 
 urouncl, aucl ha^e so^'ir,™ Cjlde 't'o" «t"""" ^" '' "-"^ "■"' 
 
 Practically, the condition of any nortion «f m... j 
 "PC" ite tempeart„,-e and pressu/c/ Se" § Ls >, r"''" 
 ordinary pressures water is a solid a liodd if •■" "' 
 
 to i^ ''■"Perature.so anysnj; „:'l'';;„r't:"""''""^ 
 
 :r:ra[ t:;r " --- ■^ *- oriXeriretr:: 
 
 been ohtainod in a lin nd rff/ , " ™1»">^<"'. I'as never 
 
 "«. ha, neve 1^3c„ fd tl^T^' '"'""" ""«*"" '>«- 
 "Ot .,e n,el,ed unlLs e pre rr l^^^^^^^^^^^^^^^ " ™'""-' '"" ™"- 
 • entimeter. For a similar ™!! , ^™"" '""■ *<1"»''° 
 
 ;- -not .eit. •a^^'rz.Te i! rrt:;!:*, r'-T 
 
 l>cen ji )le to mwlimo . ♦ "-"^ >^<^'"s iiuve physicists 
 
 a»^,LT*iflt:„rr:rT »'."'-'^" <«*™" ->«'«"- 
 
 and nitr„„ "'"'^'•cnt states, there is great diversity. Ovv.ren 
 
 errs sfs <"•,""■•-""'"" '^ " ""•^""■'' <"•"■» '"'- 
 ti/o:,rwr i::l: ,:r:^:r^^ 
 
 nressnre On the other hand, certairsl:^, 'L r™';:,: 
 
 »»% „„ „» lemp„-„,ure ami p,;,,,,„;. : „„ ,,,,„, „„„,, !!'J .*^™* 
 .egurded as sin„.,v n„.t,.r in a IV„«.n state", eve'; i,: i ^riT 
 tcr ,„ a n,elte.d state, and eveiy gas as n.atter in ■I.ZttT^^, 
 
t>iiENOMENA OF ATT H ACTION". 
 
 Kvery liquitl has been solidided and volatilized, and every gas 
 has been liquefiptl and soliditied. Air was one of the last of tlie 
 gases to surrender its reputation of being a " perujanent gas." 
 Not till the year 1878 was it reduced to hunps. We may predict 
 the future of our globe. If its heat increases sufficiently, the 
 whole world will become a thin gas. If its heat diminishes indefi- 
 nitely, all earth and air will become a solid mass. 
 
 III. PHENOMENA OP ATTRACTION. 
 According to the circumstances under which attraction acts, 
 we have the various phenomena called gravitation, cohesion, ad- 
 hesion, capillarity, chemism, and magnetism. Sometimes these 
 terms are used as names of the unknown forces that cause the 
 phenomena. 
 
 § la Gravitation. — Thnt attraction which is exerted on all 
 matter, at all distances, is called gravitation. Gravitation is 
 universal, that is, every molecule of matter attracts every other 
 molecule of matter in the universe. The whole force with 
 which two bodies attract one another is the sum of the attrac- 
 tions of their molecules, and depends upon the number of mole- 
 cules the two bodies collectively contain, and the mass of each 
 molecule. Tiie whole attraction between an apple and the eartli 
 is equal to tlie sum of the attractions between every molecule 
 in the api)le and every molecule in the ear*h. 
 
 § 19. Weight. — It is scarcely necessary to state, that what 
 U understood by the weight of a body is the mutual attraction 
 l)etween it and the earth. The term mass is e(iuivalent to the 
 expression quantity of matter. It follows, then, that weight ia 
 proportional to mass. Why do we weigh articles of trade, such 
 as sugar and tea? 
 
 § 20. Does the apple attract the earth with as much 
 lorce t*i= TJno caiou auoritcis tne apple v — i.i-L us »-xiiiiiiiie this 
 question. First assume M.at tlic nioleeulos of tlie apple and tlie eartlj 
 have equal masses, i.e., are horaogeueous; tlien tlie attraction of any 
 
 %\ 
 
24 
 
 MATTER AND ITS PROPERTIES. 
 
 i 
 
 'r 
 
 M I 
 
 molecule in the apple for any molecule in the earth is Pn„ni i .u 
 attraction of any molecule in the earth for any moCle inTh }" 
 
 That J ,,,, ,,,,, ^„, ^,,^ ^^^^^ ^^^^^^^^^ eLh o a Igle i^ke r: 
 cule, their attract on for each other wnniH ).„ i ^ * °'®" 
 
 .he .pp,eco„..i„,.„„.„^7i'e:i';:;;ri:rrc„,!:"'v7rr''"' 
 
 w,.h Which one molecule ...ract, a„„.|,er be rpreseMed t ' r° 
 each molecule of the anule atlracf, tl,. «. "'',*'"■"' "^ »■ Now 
 with a force of 5 „ . tTi^ , " , '" """''"l"' 1" the earth 
 
 the eartK with a f:;ce ofTo „ Ou™ ^l "" , "■»'"'>■ '-vouid «.tr«,t 
 of the earth attract, the moLu of t faUriira T'""'^ 
 
 soniu« Will i^;i;j^z tZio'tt.^ r;:f 7- 
 
 whose masses diBTor, „„d consequently between rjr, T u"!' 
 
 rs-r,: ;t^r:f ,r-rl" --t ""'•"' -" 
 
 the former? ^ '' "' '*'""^'^ **^ ".e latter attracts 
 
 If the apple attracts the earth as stronjriv no +i.„ 
 
 «» . Ship. His purr<-™r tiT.;'.: r:? huVthrrr,'"" 
 
 not appear to move. But if flve hunrlrerl m„n ^'^ ''''*'^ 
 
 together, the ship would be ."oln to mfvo Dill Z^' ''''^' '''''''' 
 motion? If so, then would the fl^e hundred men nrodn:; '^ '" '" 
 since five hundred times nothing is nothing? '^ ''"'' "^ '"°"""' 
 
 You will learn, in the next chapter tliaf u.n =..„ ., 
 a given force moves a hody in a 'J ^t 1 "'^'^^^.'^ ""-o-'fe-h which 
 
 the mass of the body. Does IL f^ctlT" T"' '"'"'''^y *« 
 nomena? ^ "' ^"'' '^P'*''" "'e foregoing phe- . 
 
 from'; J^n^rOb^^^^^ .-^*^ *^« <^i«tance 
 
 ^"- the force of ,ravit, ^: t":;:::?:::^;:''^ 
 
 the greater is the force o^rlvt tI e ^"''^ '' "" ^"^^" 
 earth IS about 26 miles less thnn ifc I .". " '^' "' '^'*^ 
 
PHENOMENA OF ATTIIACTION. 
 
 25 
 
 poles is 13 miles less than to the surface at the eqnator. This 
 considerable difference in distance from the center oc(!asions an 
 appreciable difference between the weight of a body (having any 
 considerable mass) at the equator and at the poles; and, "since 
 the distance of the surface from the center constantly increases 
 as we go from the poles toward the equator, the weight of all 
 objects transported from the poles toward the equator constantly 
 diminishes. 
 
 . It is obvious that any object raised above the eartli's surface, 
 as in a balloon, must weigh less than at the surface of the earth.' 
 But the bights with which we commonly have to deal in our ex- 
 periments are so small in comparison with tlie earth's ividius, that 
 the differences in weight due to diircrencos in bight at a given 
 place can scarcely be detected by most delicate t(>sts. 
 
 The statement that " weight is proportional to mass" (§19) 
 must, therefore, be restricted to a comparison of masses at the 
 same place and at the same aUitiide only. The pi-opriety of 
 making a distinction between the terms' mass and weif/ht is 
 now apparent, as the former impHes tliat which does not ciiange 
 when a body is translerred IVom place to place, while the 
 latter ma3- change. 
 
 If the earth were of uniform density, bodies carried below its 
 siuface would lose in weight as the distance below the surface 
 increases. At one-fourth the distance to the center there would 
 bo a loss of one-fourtii the weigiit. At one-half the distance 
 the weight would be one-iialf; and at the center nothing. Is 
 weight an essential property of matter ? State certain condi- 
 tions on which a body would have no weight. 
 
 The terms up and down are derived from the attraction be- 
 tween the earth and terrestrial objects. J)own is toward the 
 center of the earth, or it is the direction in which a I)odv <alls or 
 tends to move in consequence of gravitation. Up is the opi)o- 
 site direction. It is apparent that the up and down of one 
 place cannot correspond with the up and down of any other 
 place. 
 
^J6 
 
 MATTER AND ITS PROPERTIES. 
 
 QUESTIONS. 
 
 must y„„ tun, yo„r face In order to look "p ? '"'"°" 
 
 the, „„tl, look i/tho Ze ZS;, " "'■ °' "" "'"■"'• """"^ 
 
 e. What is the ori^Mu of " water-power" ? 
 <• vv hat IS the cause of tides? 
 
 wefu:.";:"/!";™:::;";'"' "'° "™ "'>"'- «-"'«» 3- wo,,,, 
 haiate.'irt e : aL°''or«:r;:r'''' °'-"'"" "■"""" """ -p*s- 
 
 ' 'I 
 
 t i 
 
 § 22. Cohesion. -That attraction which holds the molecules 
 of the same substance together, so as to form larger Tc i^ is 
 ealled cohesion. It is the fornn fi,of . ooaits, 13 
 
 all bodies from f. 11 prevents our bodies, and 
 
 . au oouies, Horn falhng down into a mass of dust T^ Ja fi 4. 
 force »^,ie„,.o,i3t, u fo,.ce te„di„« to b,.e„k or d, 'a , .^ " 
 
CRYSTALLINE CONDITIONSOF MATTER. 27 
 
 fluid condition, then, bv nrp«j<5iiro tu^ i 
 
 tie, VISCOUS, malleable, ductile, and tenacious ' 
 
 §23. Crystalline and amorphous conditions of matter 
 -If our v.s,on conkl be rendered keen enc.^b to enib^. T* 
 
 u..ox„lo,„, „„,,<,, tl,o,.o „„l„„ nndouh „ ; tS ; ' 
 wonders .and be.intios of wi.;, i. i untoided to us 
 
 tlie same substaiiue), we ijet, i)os8i,,lv U- ,,:«■ * ' 
 
 and b,il,iant diamond iu l,,„ „„e cjctb^ft ' '''"""^'■""- 
 
 >ooM„g graphic i„ .„„„.„., a^iT-u^t f,;„r;';',:r'''"; 
 
 shapeless ehareoal poious, Dlaek, and 
 
 jou get rough and "^d smtcl T °"'" *"'=''°"' ""'' 
 
 
 I 
 
28 
 
 MATTKU ASt) JTH flU,l'FAmm. 
 
 
 m 
 
 of elium,„|. ' «''"' «'""" "'"' '-"'l-'X i" tlic luim 
 
 wal..r; HUsp.nU . u, I n^t^:^ '"'';' ''''' '"-"^" "> ^O^-" of hot 
 
 """""'« '"•• -' - ^' ' ., '^'z:xz:iZ;:r '''''"' '"■^' 
 
 "I- »»ll|"lr.'. All,,,! Il„. n„„,V, '""■.»■'••)"' »"ril»,T ,lrhl,„.i,|„ 
 
 »-ii.i„., wi.,a,,,,,,;:';,,,i^r/;, ::;■;;*,-■' /• '■'■ "• «■« 
 
 y<)ijr..yi;„;, " "^^ '« '^ "'«>■>« «(,ii,g ou before 
 
 ""I .v"" "viii I ,,,„. „„rr,.,... m„C „,..?, 'r' " "'"""■■'«"■• 
 
 >', wuuoii. „„ui„, „„. .„„, „^:.,„„.,i ,t:t;,r"K::' ';;:: 
 
 of 
 iiK 
 
 est 
 
 Riippi 
 from 
 incre 
 we fii 
 by cc 
 
OHAXGE OF VULUMIO 15V CUVSTALLIZATION. 29 
 
 ofico fs a mass of crystals, so closely packed together that the 
 individuals are not distinguishal)lo. 
 
 § 24 Change of voltime by crystallization. - This tend- 
 ency of matter to structural arranooment is not only very inter- 
 esting, but very important in the arts. It is very natural to 
 
 Fig. 10. 
 
 snppose that the new arrangement of molecules, when nassina 
 Iron, tlie ii(,ui<l to the .solul state, should <,cc..sion ni'her ,1 
 
 increase or diminution in vol We are not surprised when' 
 
 we find that water, in freeziug, disregards the law of c.-ntraction 
 by cold, and that the molecules are not found so closely packed 
 
 * '• 
 
 *t| 
 
so 
 
 Matter and its WiopEHTifia. 
 
 
 m 
 
 "li 1 
 
 ii ' 
 
 togethor, in the now and strac-tural state, as when under th. 
 inHnenoc of cohesion alone. ™ 
 
 The force exerte.l by tlie molecnies in clmnein. nosition, :, 
 so e„om,ons as to hnrst the stronges't vessels. Hence ote 
 
 ee.p„K.s a,-e h.n-st when water is .allowed to free.r 1"^ , ' 
 Huge rocks arc dislo,lgod from their restin<..nlaces rthr,, , 
 
 s;" "■» ■"™'"'^'"-*'^ "3- "'er getti^^'ii : .;: 7 
 
 ee^, g, expanding year after year, and pushing the ^1,3 
 Horn then- support. Cast-iron and many allovs „fcbT. . 
 met.,,, expand o„ .soli.liiying. ,s,„„ Jeu'^^'t a f ^ 
 
 t e mold. Most uietals oontmct on solidifving. Hence ..old 
 
 that the ™„,eeu,cs of iron, whe,. sl,:i.«:^p": i tl/e jZ™ a ! 
 free to arrange themselves in their peculiar Lth«^^ 'Z t St , 
 this new arrangement, the cohesive force is weakened. 
 
 §25. What 18 the cause of this almost universal t.„ 
 
 denoy of matter to orystaUize? w„ 1 ""™™»' "n- 
 
 knowledge of the doinmTr^ . , ™ "° "^""''"^ 
 
 B^ lue noings m tlie molecular world Rut „.« i.„ 
 
 »«spe.,U each needle bv .br;..^., hT t ]° H T"*'""' "'"' 
 burlzontal position. Brine the „„ ,r ° b«l»iice<l In a 
 
 other. Brin. the „«,e^ r„:r„:4 z :,r i/t: r: 
 
^1 
 
 HARDNESS. 
 
 3] 
 
 Bring the point of one near the point of the other. Brintj the eve of 
 one near the eye of the other. What is the result in each case? Til 
 
 ITX. ''' ''''''' "'' '""' °' "'' "'''"' '^^'''^' ""'' ''''' '^^ 
 
 Now. break one of the needles into two pieces, and experiment as 
 before. Break U.em into still smaller pieces, and the smu J Lee 
 that you can obtain possesses polarity, as certainly as the ori i ,al 
 needle nnvgine the work of division to be continued till t mole' 1 
 
 poSyT ""'' '° "^""^ ""* "" ^"«^-"^« --^y P--e- 
 
 Throw a dozen of these magnetized needles on a sheet of paper and 
 
 r Xhr tv"" tT "' ?%'"""^ "^'""^^^^^^^ ^''^ -^ appLra" e ;' 
 legulanty? If two arc to forma rigid group, how must they be ar 
 ranged? How is it with a group of three? 
 
 Experiment 2. Next place a magnet beneath a sheet of paper and 
 s.ft ,ron l^hngs over it. Gently tap the paper. This renunds us o'f the 
 effect of jarr,ng on the car-axle and cannon, where molecules, once set 
 in .not.on, tend to arrange themselves according to some guiding prh, 
 
 th'nrai;" it' ' "" ""'""' "' ' ''' "' """ """^'^'^ ^-^ § ^««). -d 
 We pass readily from these facts to conclusions respecting the mo- 
 ecular arrangement in the crystal. Only grant the supposition that 
 the molecule .s endowed witii something similar to polarity, and we • 
 can picture to ourselves the molecules, like the iron fllings. wheelin' 
 into Ime in obedience to their polar forces. Crystals are more easilj 
 cleft m some dn-ecfons than in others; n.ay not this be accounted for 
 by supposmg that, like the magnet, the attraction on some sides of the 
 molecule is greater than on others? 
 
 § 21. Hardness. -Name some metal that you can scratch 
 with a finger-nail. See if you can scratch a piece of copper 
 with a piece of lead, and vice versa. Get as many specimens as 
 possible of the following substances : talc, chalk, glass, quartz 
 iron, silver, lead, copper, rock-salt, and marble. Ascertain 
 whu.b of them will scnrtch glass, and which are scratched by 
 glass. What term do we employ in speaking of those substances 
 that are easily scratched ? To those that are scratched with 
 difficulty.? Which is the softest metal that you have tried' 
 
 .) d 
 
hi 
 
 MATTRIl AND ITS riiOPERTIES. 
 
 Ihoh^dst: Whi.hs the softer .etal,lr.n or load? Which 
 W^ ": r '" ■ '''^^ "^'•'''"- ^M.end upon don^^ 
 
 When vv.ll ,,1,0 sMbstu.K-o sctitch a.iothor? 
 
 io onahlo MS to express ,h.o,,>c.s of. hardness, the followhicr 
 *uble ol reference is generally- adoi^ted : _ "^ 
 
 MOIIIfS SCALE OF HARDNESS. 
 
 1. Talc. 
 
 2. Gypsum (or Rock-Salt). 
 ^. Calcite. 
 4. Fluor-Spar. 
 5- Apatite. 
 
 ■< 
 
 fi. Ortlioclase (Feldspar). 
 
 7. Quartz. 
 
 8. Topaz. 
 
 9- Conindiitn. 
 
 10. Duunoud. 
 
 o„e or «,„ n^..i:: r;rt:: ;L:-^-';r«:' ,::;r";? ^ 
 
 Fiff. 11. 
 
 §27. Flexibility. -Such substances as may be bent or admit 
 of a huige-hlve movement among their 
 molecules, are calhdjlexiljle. AVhat 
 difrerence have you noticed in differ- 
 ent jack-knife blades ? How can ^on 
 tell a soft blade from a hard ])lade^ 
 
 molc^ulcr^ntlof'' "-J" '''^"" ^^' '^ ^^ ^^^P^-"^ ^hat the 
 r tHe f?r ..^''''' "^' "i"«t be separated from each other 
 a httle farther than usual, and that they must Inve su2Z 
 
ELASTICITY. 
 
 88 
 
 iron, zmc, and lead. Stretch the piece of rubber. What change 
 la Its molecular condition must occur when it is stretched? 
 What molecular force causes it to contract when the stretching 
 force 18 removed? Compress the rubber. What chancre of 
 molecular condition takes place in compression? What^foroe 
 causes it to expand when the pressure is removed? Bend eacli 
 one of the above strips. Note which completely unbends when 
 the force is removed. Arrange the names of these substances 
 •n the order of the rapidity and completeness with which they 
 unbend. *' 
 
 What change takes place among the molecules on the concave 
 side of the bent strips? What, among the molecules on the 
 convex side? What two forces are concerned in the unbendin<.? 
 Twist the cord of a window-tassel. What causes it to untwist' 
 Ihe property which matter possesses of recovering its former 
 shape and volume, after having yielded to some force, is called 
 elasticity. To what forces is elasticity due? Does all mat- 
 ter possess this property in the same degi-ec? Does the rub- 
 ber possess the same ability to unbend, as to contract after 
 being stretched? In what four ways have you 
 tested the elasticity of substances ? Does a sub- 
 stance possess equal power of recovering its form 
 aft«r yielding to each of these four methods of 
 applying force? Why are pens made of steel? 
 What moves the machinery of a watch? What 
 is the cause of the softness of a hair mattress or 
 feather-bed ? 
 
 A common spriug-balance used for weighing con- 
 sists of a steel spring wound into a coil. The weight 
 of the body to be weighed straightens or draws out 
 
 AX A A. . , *^'''"^" ^ 1'°'"*^'' moving over a plate which is 
 
 divided into equal parts shows how much the sprint ha- h-n ^-a-n 
 out But the entire virtue of this apparatus consists in the elastlcliy 
 of the spring, or its power to recover its original form after bein- 
 drawn out. Give other illustrations of the application of elasticity 
 to practical purposes. 
 
 Fig. 12. 
 
 •31 
 
 I 
 
84 
 
 MATTER AND ITS PROPERTIES. 
 
 ; "■ 
 
 n 
 
 Any alteration m the form of a body due to the application of 
 
 .lodl h'-' n ;\'''''"'"' ""^^ *''^ ''''''' "^y ^'"^''^ ^'«« strain i.s 
 produced ,s called the stress. A body which, having experienced 
 a strani due to a certain stress, completely recovers its original 
 conchtion when the stress is removed, is said to be perfectly 
 elastic. Liquids and gases are perfectly clastic (see § 48) . Solids 
 arc perfectly elastic up to a certain limit, which varies greatly in 
 
 tl7^T 1":T' '' "" ^'"^^ '""'''^^ ^ certain nmii,\he 
 foim of the solid becomes permanently altered, and the state of 
 the body when the permanent alteration is about to take place, 
 8 called the hnut of perfect elasticity. In sof^ or plastic Llies 
 tins unit ,s soon reached. , What is the result of overloading 
 carnage springs ? * 
 
 .n^i^^H"**^^^^^-"^'''''^' '^'^'^' ''^""'^ ^'th a hammer to 
 each of the substances whose hardness you have tested (§ 26) 
 and ascertain which are the most easily broken or pulverized.' 
 Obse.ve that some substances suffer a permanent change in form 
 when subjected to a stress which exceeds their limit of elasticity, 
 while others break before there is any pcrmaiuut alteration in 
 lorm. Ihe latter are said to be brittle. 
 
 J 30. Viscosity. -Support in a horizontal position, ar one 
 of Its extremities, a stick of sealing-wax, au.l suspend i'romits 
 nee extremity a small weight, and let it remain in this condition 
 several days, or perhaps weeks. At the end of the time the 
 stick will be found i)ermanently bent. Had an attempt l,een 
 n.u e to bend the stick quickly, it would have been found qi.ite 
 little. A body which, subjected to a stress for a considerable 
 ^me suffers a permanent change in form, is said to be viscous. 
 Hardness ,s not opposed to viscosity. A lump of pitch may be 
 quite hard, and yet in the course of time it wiU flatten itself out 
 
 Oy Its own weiffht, jind fl«w /1„..,., k;i! i:'— 
 
 ij ., ,., ': ' '■"'' '»" *"iy a scitaiii of syrup. 
 
 I;.qu.d8 like molasses and honey are said to be viscous, in djs. 
 tinction from limpid liquids like water and alcohol. 
 
 
TILNACITV. 
 
 S5 
 
 §31. Malleability and ductility. - Some stibstanoos pos- 
 sess, in tlie solid state, a <rrtain a.noni.t oi' flauUt,! ; that is, 
 tlieir molecules may be displaced without overcominjr" their oohe- 
 sion. Place a piece of lead on an anvil, and hammer it Jt 
 spreads ont nnder the hammer into sheets, witiiont being broken 
 though .t is evident that the molecules have moved about amon^^ 
 oiie another, and assumed entirely different relative positions" 
 Heat a piece of sort glass tube in a gas-flame, and, although the 
 glass does not become a liquid, it behaves very much like a 
 liquid, and can be drawn out into very fine threads. When a 
 solid possesses sunicient flui<lity to admit of being drawn out 
 into threads, it is said to be dmtik.^ When it will admit of 
 being hammered or rolled into sheets, it is said to be malleable. 
 
 iJ^ ""n"! 'I? '^»''^*"^'' '^'"' substances thai are dnctile are also mat- 
 leable. But the same suljstance does not usually possess tlie two 
 properties in an equal degree. Platinum is the u,ost ductile metal. It 
 can be drawn into wire finer than a spider's tln.ad. It is the seventh 
 metal ,u the rank of malleal.ility. Goi , the most malleable metal. 
 It can be hammered into leaves so tuin, that it would require 300 000 
 o make a book one inch thiek. It nu.ks next to platinum in ductufy 
 I.ou at a red heat, is very malleable and ductile. What metals can 
 
 sheetTr' '""''^ ^^''' "'''''^' '*° ^"^ •■""'^' ""' hammered lute 
 
 ti^l 
 
 . .§ 32. Tenacity. — Tn order that a substance may be ductile, 
 
 Jtis evident that it nmst possess a strong cohesive force, so as 
 
 ft^p^event rupture. The power that matter possesses of resisting 
 
 ^^ra|)tiifre,,l)y;a4)uiling ftM-ce, is called tenacity.'' A body may C 
 
 gemeious^tkout being- Aiktile, but it cannot be dnctile ivithom 
 
 mmg mhcions. It is remarkable that the tenacity of most 
 
 metals is increased by being drawn out into wires. It would 
 
 seem, that, in the new an-angement which the molecules assume, 
 
 the^cohesive force is stronger than in the old. Hence cables 
 
 made of iron wire twisted together, so as to form an iron 
 
 ' Ductile, draw-able. , Malleable, as 1{ were malltt-abU. 
 
 » Tenacity, properly of holding. 
 
 I ' 
 
Is"! 
 t.'l 
 
 |! a 
 
 86 
 
 MATTER AND ITS PROPERTIES. 
 
 rope, are stronger than iron chains of equal weiglit and length, 
 and are much used instead of chains, where great strength is 
 required. ^ 
 
 §33. Adhesion.— Grasp with your finger a piece of gold- 
 leaf, and, honest as you may be, it will stick to your fingers ; it 
 will not droi) off, it cannot be shaken oflT, and to attempt to pull 
 it off IS to increase the difficulty. Dust and dirt stick to clothing. 
 Thrust your hand into water, and it comes out wet. You can 
 climb a pole, because your hands stick to tlie pole ; but if the 
 I)ole is greased, climbing is not so easy. We could not pick 
 anythmg up, or liold anything in our hands, were it not that 
 these things stick to tiic hands. 
 
 Every minute's experience teaches us tliat not only is there an 
 attractive force between molecules of the same kind of matter 
 but there is also an attractive force between molecules of unlike 
 matter. That force which causes unlike substances to clin^r 
 together, is called adhesion. Is adhesion a molar or a moleculaT- 
 force? How does it differ from cohesion? Wliy do not gold 
 watches, and other articles of gold jewelry, appear to stick to 
 tlie fingei-s? Wiiat keeps nails, driven into wood, in their places? 
 What would happen if all adhesion between tlie different parts 
 of the building you are in should 
 be suddenly destroyed? When a *''^- '^• 
 
 liquid sticks to a solid, what term 
 do we usually employ in describ- 
 ing the phenomenon? 
 
 Experiment 1. Shake a small 
 quantity of olive-oil In water, and ob- 
 serve the form assumed by the frag- 
 ments of oil as they rise throiigli the 
 water. Does this experiment indicate that the adhesion between the oil 
 and water or the cohesion in the oil is the stronger ? 
 
 s- -- >-._.j!.i!!. „ p.atc ot giuss, about «»="> scmare. from 
 
 one arm of a scale-beam, attaching the threads to the^plate with seallZ 
 wax. Balance it, and place a dish of water under the glass, so thati?« 
 
CAPILLARITY. 
 
 a7 
 
 tinder surfac. will jnst touch the surface of the water Add small 
 we ghtuntilthe glass leaves the water. Kxan.ine the unc.-^ of t " 
 glass. Have you separated the j^lass from the water, or have you torn 
 the water apart? Do you inf,.r from your experiment tMt^hlnT 
 h^^r. ^een the ,lass and the water, ^r the c^rLr ^^Lm!; 
 
 Is there 
 
 Glass is wee >y water, but is not we )y mereurv. 
 no adhesion bacween morctiry and glass 
 
 E.peri.„e«t 2 Substitute .nercury for water in the last expori- 
 ment Do yor. Hnd any indication of adhesion? Is it j^reater or less 
 than that between glass and water? ^ 
 
 It is probable that there, is some adhesion hetioeen all cuhstanees 
 when brought in contact. If a liquid adheres to a solid more 
 firmly than the molecules cf the liquid cohere, then loill the solid 
 be wee by the liquid. If a .solid is not wet bv a liquid, it is not 
 because adhesion is wanting, but because ooliesion in tlie liquid 
 IS stronger. That gases adhere to solids is proved by the 
 phenomena of absorption desci-ibed in § 37. 
 
 QUESTIONS. 
 
 1. Why will not water wet articles that have been greased ? 
 
 2. Why is it difficult to lift a Ijo.ird out of wat.T ? 
 
 8 Why does water run down the side of a t.nnhlor when it is 
 
 venti'g'lt " "' '•"""' """""^ ' '"-'"^^ •'^'""•' -'"">'' "^ P- 
 4. In what does the value of cement, glue, an.l mnrilair. consist ? 
 6. What enables you to leave a mark with a pencil or crayon ? 
 
 § 34. Capillarity. — Examine the surface of water in a goblet 
 ^ou Hnd the surface level, as in \ (Fig. 14), except aroun.l the 
 edge next the glass, wiiere the water is curved upward so as 
 to resemble the interior surface of a watch crystal. Mercury 
 placed in a goblet (B) has its edge turned downward. rcso,nb!in.r 
 the exterior surface of a watch e.,..aK This seems to indicate 
 
r. 1 
 
 m 
 
 Matter and its pRoPEUTiEd. 
 
 
 a repulsion between mercury and glass. But a previous e^peri- 
 nent (i^ge 37) has shown that, instead of repulsion, thereTs . 
 slight adhesion between 
 
 Fig. 14. 
 
 these substances 
 
 Pour any liquid on a 
 
 level surface wiiich it 
 
 does not wet, — e. q., 
 
 water on parafline or 
 
 wax, or mercury on 
 
 glass. It s[)reads itself 
 
 over the surfiice, but the 
 
 edges are everywhere ^ 
 
 rounded or turned down ^ 
 
 like the edges of mercury in a goblet. Surely those romulcl 
 
 edges are not caused by the repulsion of the sides of a v 
 
 The edges of all liquids will be turned down unless the adhes on 
 
 between the.n and the sides of the vessels exceeds the cole io 
 
 I" the hquul The glass does not cause the turning down o 
 
 prevelitT '""''"'^' "' "'" ^''^'''' ~ ''' '""^""'^' ^^ '■^^''"- ^« 
 Thrust vertically two plates of glass into water, and gradu- 
 all3 brmg the surfaces near each other. Soon the water rises 
 between ^o plates, and rises higher as the plates are brought 
 nearer. Thrust a glass tui,e of very fine bore into water; the 
 a traction w,t lun it, on all sides, will raise the water to twice the 
 lugh ,t would reach when between two plates whose distance 
 apart ,s equal to the diameter of the bore of the tube. Thrust 
 a tube of the same bore into alcohol ; this liquid rises in the 
 t..be, but not so high as water. The surfaces of both the 
 Water nn.l the alcohol are concave. If the tube is placed 
 in mercury, the opposite phenomena occm- : the mercurv is 
 depressed, and its surface is convex.i Both ascension nnd 
 
 'The SCODC Oi' Uil« Jmnk "'M! r\r.t a-l-n!' -f 1 
 
 o.pl]I„I„. The .■ud.M ,™ „„V. 1,°,: J ; ° LlTt "'""":""» ,"' ■I"' Pta.o,.™„ „, 
 
CAPILLARITY. 
 
 39 
 
 depression dimiuish as the temperature increases, being greatest 
 at tlie freezing point of the given liquid, and least at its boiiin^ 
 point. (Regarding heat as a repellent force, can you give any 
 reason wliy the ascension should be less at high than at low 
 tcn,perat.n-cs?) Inasmuch as the phenomena are best shown 
 in tubes having bores of the size of hairs, they are in such 
 cases called capillary' phenomena, and the tubes are called 
 capillary tubes. ^ 
 
 The phenomena of capillary action are well shown by placing 
 FJ815. various liquids in U-shaped glass 
 
 tubes, having one arm reduced to a 
 capillary size, as A and B in Figure 
 15. Mercury poured into A assumes 
 convex surfaces in both arms, but 
 does not rise so high i' the small 
 arm as it stands in the large arm. 
 Pour water into B, and all the i)hc- 
 nomona are reversed. C is a glass 
 tube containing water and mercury, 
 and showing the shapes that the surfaces of the two liquids take. 
 Generalizing the above facts, we have the /owr laws of capil- 
 lary action : — 
 
 Liqnifh rise in tubes when they ivet them, and are depressed 
 when they do not. 
 
 The ascension or depression varies inversely as^ the diameter 
 
 of the bore. • 
 The ascension and depression vary ivith ^ the nature of the 
 
 substances employed,. 
 
 The ascension or depression varies inversely with the tern- 
 perattire. 
 
 lUuHtrations of capillary action are abundant. It feeds the 
 lamp-llame with oil. It wets the whole towel, if one end is lef. 
 for a time in a bu«in of waiter. Ft draws water into wood, and 
 causes it to swell with a force sumcient to split rocks, and to 
 raise large weights. I low does a little water in a wooden tub 
 prevent its falling to pieces? 
 
 ' ^"1 ""y- *«'>-""^<'- ' ObBorvo that thro,.Rl,o.,t tl.U troatlHc U,o word „8 exnrewe. 
 
 «u .x«ot proponluu. Wla.» U,.r. 1« n.t an exact proj.o.llon, the word Jk ia 3 
 
 I. 
 II. 
 III. 
 
 IV. 
 
40 
 
 MATTER AND ITS PROPERTIES. 
 
 noL!!; °^°l«<'^lar Phenomena. -Besides the phe- 
 
 nomena we nave just st.ulied, there are a great many others 
 ependmg .n part on n.oleeular attraetion, but much a.o e on the 
 •no ecuhu- n.ot.ons, of which we learned in § 5, page 6. Many 
 
 evenXrir"^^ ^"•"' "^' •'"'^^^'^"^^ ^"^ tl/explanation 
 nete r •'"• '^ ^'''"' ^' "-'^"3' complicated and ineom- 
 
 Wnr ^'T:^f "'""'' ^^''" ^'^^'^" phenomena are.o;«^/o„, 
 oOsorption, and diffusion. 
 
 su face of water. Soon water is drawn up into the pores of the 
 nnp by cap.llary action, and the whole lump, including the 
 
 ll umn r '""'^"'' ;r™^^ "'"•^^- ^-^ you discover that 
 the 1 mnp becomes smaller, and slowly disappears in the water. 
 ^Vhen a sohd becomes diffused through a liquid, it is said to 
 
 1.0 dissolved. The dissolving liquid is called a solvent, and tl^ 
 result, , , J, ^^j, ., ^ ^^^^^^.^^^^ ^ ^_^ ^^.^,^ ^j.^^^^^^ ^ 
 
 solid, 07ihj When the adhesion hetioeen them is greater than the 
 ^^oAc^jon zn the solid. A liquid always dissolves a solid more 
 i.'p.dly at first, less rapidly as the adhesion becomes more nearly 
 satisfied ; and when it is completely satisfied, or is balanced by 
 the cohesion in the solid, the liquid will dissolve no more of the 
 sohd and th > solution is said to be saturated. When a solution 
 wul take nwch more of a solid, it is said to be dilute: and 
 concentrated, when it will take little or no more. 
 
 If the solid be first pulverized, the liquid has more surface on 
 winch to act, and the solid is dissolved much more rapidlv 
 irmt generalh, weakens cohesion more than it tveakens adhesion: 
 hence, with few exceptions, hot liquids dissolve solids more 
 rapidly and in greater quantities than cold liquids. Boilin^ 
 water dissolves three times as much alum as cold water; conse! 
 quently, when a hot .aturate<l solution of alum is allowed to 
 eool at least two-thirds of the alum must be restored to the 
 «ol.d state (see Ex'i,. 1, §23), while one-third, or the amount 
 
ABSORPTION OF GASES BY SOLffiS. 41 
 
 that the cold liquid is capable of dissolving, remains in solution, 
 llie ie„,a„ung solut.oii ,s called the mother.lup.or. Lum and 
 u few other substances, are dissolved better in cold w;"! 
 t rystals of such substances are only obtained bj gradual evap- 
 oration of the solvent. ^ ^ 
 
 Water is the great solvent. When we speak of the soh.bility of a 
 substance, water is always lUKlcrstood to be the solvent . 
 
 other liquid is speeiiied. AVhy is it fortunate ,t J^ ""' 
 
 «o,ventP Nan. substances that waSl t^^ ^ :^ ^^^U^ 
 many substances insoluble in water, sou.e, as phosit^r^un^ ^d 
 .esni, flud a solveut in alcohol ; sulphur, in bi-s, iphile of c^rClea 
 >» mercury; and tats, iu ether or bcu^iue. Would von .?,' ' 
 
 « t^rniture with alcohol. How are ^JIZJZ:::^^. 
 
 \i 
 
 m„luMhu altmclmc, and is ,i,.„„nll,i »„j„,yi,,-„(. c,.,.tai„ soli,!, 
 pc.,s »o strong ,u, uttracti,,,, fo,- j,,.., timt thov not onl • d Iw 
 U.0 ga.cs ,nto tl,o s.nall oavitic, or l.olcs within then,, l,nt groZ 
 condense ti,e,„ tlK.,-o. It should be earefnlly „„,^,, |f4 'ho 
 attraction i„ this case is generally between L . Js tri the 
 »,/««, or ».«,■„», and is hence called ..,«;:, in dU 
 tmetron fron, „U,>n,M,c,aar a,t.«,ion, which is tife „a„e given 
 to the phenomenon when gases arc taken tato the porJotl 
 
 r.-oshly.bumed charcoal placed in dry air, may, i„ a few dave 
 have ,t, wcght increased one-tiftieth in conse,, , nee „f The a^ 
 that ,t absorbs. (Has air weight?) The attra tion of d arcoj 
 for „„x-,ous gases is especially great, n.aking it very e fflcto^ 
 .0 cleaning the air i„ hospitals, au,l in .^noving ,,o^i„ ! 
 odors Irom p„tr,d animal and vegetable matter by abs^l Z the 
 foul gases that are generated. It docs not check d ca^' but 
 rather hastens it. K r.,t, which had been baried in cha eol 
 
 teft but he ha,r and bones, yet no bad odor was perceptible. 
 v» liy do farmers mix muck with manures ? 
 
42 
 
 I l.;i 
 
 i 
 
 
 MATTBIt ANf, m VHOl^mTtEH. 
 
 §38. Absorption of irai,e» by linuidM ,/. , 
 teinp<.rut.iro of 0° CV„ Ja ,<«,.„, , '^"'"^""Mf' Water, at a 
 
 cim.,.,.i with ti.iH ,.1 :::,'' :; ""^-"^* ^-^ wat.- thus 
 
 of gUH that a n,uid Hi , IC IH """" r*"''-'' '^^''« """-'"' 
 water" i« «iu pir^aflr '^^ " «o^J- 
 
 '.".-or..e.t,ji.;:t;s::t,::^ 
 
 u»e l« removed a Iur«t' „art „f ^^»'«" .the prcss- 
 
 efferveseence. "^^ ^^'^ m-apt-M, cnushig 
 
 i^ 39. Free diffii«ion ^ liauid« ,/ 
 
 Hii« cttwc-'/ ' ni\%\u^ or (liffiLsioii in 
 
 I ''X|><^rim<fn( », Take rIm ,^ * , . 
 
 "'• i''<n u, «Tm Jl , I*" f '^ """•«" Particle 
 
 HnU any l,„ll,.ai ion« „t mn.\m^ "''•' ^'"' ** week. Do yo« 
 
 ttihc nearly /ill 
 
 It In poHHlhl,., and even /•r<*l«iM«' flmt i,. ,i,i 
 there Is u ./..«,>«/ ,.„,!„ \^^ZuZl "'";":"""'"'''' "^ "'-^ter, but 
 
 of heat d«.eo„,pom.. ihe hy<lr«i trn^i. T^ '^"* '"*' ''''"' "f'l'Ii'atiou 
 which lutt..r then .Heup.^ ' S;/Tr *" T'' """ «"'"""'"' «'»«> 
 dioxide (.„„„»». -alld.a^lj^, '" :7'*"";' ->'«^'-" -r ca;bo,; 
 .•.xt..nt there Ih a ehe.nleal unlonXeln H ^ *""'"' '""""'''^ "' -""« 
 
blFPUSION OF LIQUIDS. 
 
 4A 
 
 Fig. 17 
 
 If, (lm-;.;jr the operation of diffusion in 
 
 he last tlu-oc experiments, yon exanane 
 he l.quKlw,th a microscope, yon ^ill not 
 be able to trace any cnrrents ; hence the 
 raot.o„ of liquids in diffusion is not in 
 mass, but in molecules, - a kind of inter- 
 mole, ular motion. We learn (hat s„me 
 J'Q'nds, even when stirred together, will 
 "ot remain mixed; while others, who.e 
 densities are very different, when tnerely 
 placed m contact with edch other, slowly 
 mix of themselves. ^ 
 
 porous partitions. -Osmose 
 
 bottle ? r k""°'" "' '' ^'>"i<^->l"«>'H„c.,I 
 
 .lass t«.e pas^i u^^h u(;"- :^;;^ "t^^^:, - ^-^^ '^-'-^ ^ 
 
 a piece of gold-bouter's-skia or ^J,l\' " '^ •'^■'''' ^''" ''"«"'" 
 
 With a concentrated sohu';;; :; .^^r ""' ^"" """"• 
 
 corknUothebottlesothattlu,liq„Ki ' K,,n rl'T"' """ "'•'^•^^ ""• 
 •say at a. Now snspen.l the an a ' "' "■"^' "" ^"« t"'^^'' 
 
 the >tton, n.ay be covere.l. I ave f,- '" ?'^"' "' ''■'''"'' -' »"«' 
 carefully examine. What Is fho .'<!.,; i ' ', "T '"' """"' ''""' ^'"■" 
 Pl-OVC? '*'^''"^ ^^ '"'t (Iocs this,. xp,.,.i,„ei.L 
 
 When liquids or ^ases fnr.u. <i • 
 «epta,^ and mix wi^ . ;,, '''''V^'V'''"'^'' "^*'•^- 
 osmose:^ To distinon Ll ' '" •''*^'"""" '^ '••■'"^"<S 
 
 Of the liquid ^gns«::Tf" '*''''^ 
 
 ^ s towauls that which nicreases in volume 
 
 See Appendix, Section B 2 ^,.„, 
 
 N i'ff 
 
 :.i 
 
 h 
 
 t. 
 
 HI 
 
 I 
 
 m 
 
44 
 
 MATTEU AND ITS iMtOI-KIlTIEH. 
 
 Fig. 18. 
 
 l::f ' -^^.....,. .uul the opposite oun.nt is cal.e. .... 
 are called o;;r/orfre:r% ^'"-^^"""'^ -'^^'^ 
 
 The principle ,f un;,.::^;;^:;^:^ Lll^'^^'T^-^ 
 
 finds important a,)plieation in chenncV "^ '"''^'^ 
 
 and pharmaceutical laboratories. For 
 example, from a rod (Fig. ]8) is sus- 
 pended a glass vessel having a bottom 
 of parchment paper. «„,(, ., ,,,,,,., j^ 
 
 called a .Z.«/y.,,. j^ y^^ ^ 
 P aced, for instance, the liquid contend 
 ot the stomach or intestines of a detid 
 annual, suspected of containing some 
 IXiison, and the vessel is floated in a 
 vessel of water. If either arsenic or 
 
 SZnir" '' ^''''''"* '^^•■" ^^P-'^^'-'^te from 
 
 the albummous matter in the food, and 
 
 pass through the septum into the water Th. 
 
 -atmg ,„i.ed liquids b, osn.ose iltu;., ^^^"^ ^^ ^^P" 
 
 a"fl tl.rust i„ a li-^hted siXor '«st-tul,o with oxj-cn ^as, 
 
 than i„ the air. l' a c r tnhl" ^'; '"^'V'"™'^ '"'"" '"-^' ™P^W 
 tube hivortcHl Mor t is 1 • l'''^'' '"ydroj^en gas, and keo, the 
 there will ho 1; ^.! ^^:;'S.f «";; «'^-' *!-- M.htcr thai. ^ " 
 the gas takes Are, am ,^ n ,^^t ' ;r n ^''''''' '" '^ "^'"^^"*' «''""^^''- 
 Next ,11, one tube wit "J.; 'an t T '' "" '""'*" "^ "'« ^'"^- 
 Place the ,„outh of the .at ; ;Tho 0^;^^^^^^^ ^'"^^^'^"' ''^' ^'^^ 
 
 : En<inRmose, h„aard impulse. 
 fixosmoae, outimrd impulse. 
 
 ' Crystiilloid, like crystal. 
 * CoUoM, like ffum. 
 
DIFFUSION OF GASES. 
 
 45 
 
 ii 
 
 o.v,„„ ana ,,,v.,,.„«.n,r;;;',;:;::;;:: :':,;;;;; "'"- ■'-" -' '^- 
 
 Many pairs of liquids do ii,>t ilimi 
 
 FifT. m. 
 
 /,„«,7,rt- ., ' ■" "'■'*'^ '"to <;ac]i otlicr, hut fcprw 
 
 .7- .^ir«... ..o e..-, o,.r,a.^ .„, ^ is i.npo.sihle t<Mn-evo.rt^ 
 
 gases Ironi mixing wl.on phicc-d in contact. 
 (It IS tlionght best to introdnco the snlyect of 
 <''<i"s.on of li.inids an,l gases in ti.is place, 
 <l'<"'yli It lias little or no connection with tlie 
 «"l.)eet of adl.esion. 'J'he explanation of 
 (I'M.is.on n.nst he deferrcl to its proper place 
 in the chapter on Heat, § 128. 
 
 In c„nsoqnon,.c of this universal tonclency to 
 '" l-Mo,,, ^...s,-s will „„t remain se,)arated, -e> 
 
 u S " n f "'^ "T'! ' '"^"^^' "^ '^'^ '--^^ "i-n 
 
 ".lU.. Jlus ,s of nnn.ensc importance in the 
 -•.>non.y of nature. The largest portion of our 
 atmosphere consists of a mixture of oxygen and 
 "Urogen gases. There are ahvays present lo 
 -.all .iuantitles of other gases, such as carbomc- 
 acul gas, anunonia gas, and various other gases 
 ^vluch are generated by the decon.position of 
 orgame n.atter. These gases, obe.Uent to gravity 
 alone, would arrange themselves aeconhng to 
 
 then- weight, -earbonie-acid gas at the botC 
 .-. nitrogen, annnonS, d o ^r^!^: ^ "Sr' ''"^T"'' '' ^'^^- 
 
 P.oporUo^..l^th;^r;— '^^^^^^^^^^ 
 
 S 42. Diffusion of gaaea through porous n«rt-iH™. 
 
 motion; very complex, m^eQufcw 
 
 Ml 
 
 1 II 
 
46 
 
 MATTER AND ITS PROPERTIES. 
 
 Experiment. Take a thin 
 
 Riinsen's battery (§ ifiS), a.id pi 
 
 unglazed earthen enp, such as is used in 
 
 end witli a cork tl 
 
 g up the open 
 
 ^ V, , irough wJiJch extends a "-l-iss 
 
 tube. Place the exposed end of the tube in a^ cup 
 
 ^dro^-en or coal-gas, over the porous cup, as in 
 
 ■gure 20. Observe carefully what takes pLe 
 
 Ion do you explain the result? Ren.ove the glass 
 
 e d to .' ''"''"' ''"^^ *"« '-^^-^'t •" "-case 
 lead to the same conclusion as that in the lirst^ 
 
 of a t.ghtly-stopped vessel, and having another 
 
 «hiss tube pass through 
 an(,tlier perforation in 
 'he same coik, water is 
 i'>iced out in a jet sev- 
 eral feet in hight, when 
 tlie liydrogen jar is held 
 over tlie porous cup. 
 
 Cliildreu well understand that toy-balloons 
 ^vluch are made of collodion and filled with 
 e^ml-gas, collapse in a few hours after they are 
 "•fiatcd. What is the cause? Nature furnishes 
 an dlustration of osmose of gases in respira- 
 tion. In the lungs the blood is separated from 
 
 vpi„« P 1 • . **"■ '^^ "'*^ *'^'"' "lembranous walls of the 
 
 veins. Carbon.c-acid gas escapes from the blood throu-.h these sentr 
 and oxygen gas enters the blood through the same sepia ' ' 
 
ised ia 
 
 CHAPTER n. 
 
 DYNAMICS. 
 
 )ons, 
 with 
 fare 
 shes 
 ai ra- 
 re m 
 file 
 pta, 
 
 IV. DYNAMICS OF FLUms. 
 § 43. Equilibrium, pressure, and tension. _ That branch 
 of scjeneo vvh.ch treats of forcv an.l the ^notions it pro h 0'" s 
 caned dynamics. It has boon shown that force may a "n 
 bodytop..duce „.otionorrest; also that two or Joi'fo 0* 
 may so act on a body as to neutralize each other's effect T 
 the la ter case, the body continues in the same condit on either 
 Of mot.on or rest as if it were independent of the action o e 
 forces, and .s saul to be in e,uiU,nuW and the forces Tctbl 
 on It are also said to bo in cqnilibrinm. Inasmuch as no boy 
 IS ver free from the action of force, it must be that a loTyZ 
 rest IS in a state of equilUmum. ^ 
 
 If any portion of a force is r.ot effective in producing motion 
 - t.e., If par or all of it is exerted against other forces - 1 ^^ e 
 may result w at is called a ,.....•. on the body; T^hen wo 
 push on a wall or on a heavy sled n.oving over the ic or ! 
 book presses the table. The same force which cans a i;" to 
 fall when unsupported, causes it to press on any obsU^cl "^i,,^ 
 ♦preven s ,t fron, falling. Or, if the force is e4rted o VZlv 
 m whK.h the n.olecular attraction is strong, -_ " T^^' 
 we may have a pull or tension, as when Z. Inn.. ^ .u 
 ^a^s„ma.bberband. If t,.; bod^:;: Z^-^ 
 
 convenient. ThecaseofunifLvi:^;;^;:^:::-^^ 
 
 » Equllibriuin, equal balance. 
 
48 
 
 I>VNAMICU 
 
 liv f !;♦?>.. • ''^i^'"^'^'''y a»tl those occasbnofl 
 
 »)3 difference m compressibility an-l exDausibilitv li. 
 
 gasesaregcvernedbv the same laws Ts. u^^^ T '"^^ 
 
 then together, in so far as they uralike ^ n Ir h ""' '""'^* 
 
 of jitud. ' ^"^ common term 
 
 Of"™;: 'Tllr'"" """ "•" "- i"-«' ™- «- '.orders 
 
 air reqnn-es spec a e\'i)erimoiif« ir i ["^s^uit or 
 
 w.litni.s.ilh..ter;lj:e^ 
 
 lH•a^ ,er thereby ; the downward pressu.e is not felt, liut wlut 
 v.. ra.se a pailful ont of the water, it suddenly become LJ^^ 
 
 If we could raise a pailful of air out ^' 
 
 of the ocean of air, might not the 
 
 weight of the air become perceptible ? 
 
 If we dive to the bottom of a pond 
 of water, we do not feel the weight 
 of the i)ond resting upon P'^. We'do 
 not feel the weight of the atmospheric 
 ocean resting upon us ; but we should 
 remember that our situation with ref- 
 orence to the air is like that of a 
 diver with reference to water. 
 
 Experiment 1. Fill two glass jars rPlcr 99^ „ •.,, . 
 glass bottom, B a i.otto.n provi,lc7hv ^ ^^ '''''*"*' ^ ^''^'"^ * 
 
 tightly over the rim. inve t b n T"!' " "'""' "' •^heot-rnhhor 
 
 iiiNtii uotli i„ u lap^rer vessel of water, C. 
 
 Fig. 22. 
 
 
PnESStTRE m PLTTIDS. 
 
 49 
 
 What forre sustains the water in A ? WT.nf „ i .^ 
 
 ;;.;ta... ..h ......u. W . L^hC h;tr:r b^^ 
 
 j:::s^tr«;S^;-x^^^^^^^^ 
 
 feel? Why? Remove the finger, and he ZZ the t h f '"" 
 sinks to the level of the waterin vessel C ^v?"' " • r^"' 
 
 we find that the downward pressureof air ^es ri ^an "nf '''": 
 pressure in the liqnid. In this respect fluids S!Z wil-U i ^ ^ 
 whose molecules are so rtrndy hehl to-ether tl.af u .„ '"" ^"''''•'• 
 pushed in any direction, that pirtdra^stlTerJstwit'h^i^^^^^ '^"^ '''' '' 
 ^n7n vT'"'' ^''^""*''^ f^'- «'at«^ '>ci"ff sustained in the vessels A B 
 
 the a?; T VI '^''"''""^' '^'•^'•"'^^ '^^ "- downward pTetu'e of 
 the air. Does this downward pressure cr-afe in ,.,»,..,.. i ^ ^^^ ^^ 
 
 ...0 ..r use,,, »„ .ha. ,r .„„ vLe,, L^nrLr^Tl^Sr^ho' 
 Finr. 23 water will not fall out? 
 
 Experiment 2. Keeping the finger pressed on the 
 
 vlr T^'"' ;* "'r'' ""' ^'"-"^^"^ «"' «f the 
 water. The water does not fall out. Why? Slio 
 
 a tlun glass plate, or a piece of thick pasteboard, 
 
 under the mouth of A, and. pressing it against the 
 
 mouth, raise the vessel carefully out of the water 
 
 and remove the hand from the plate. The water 
 
 does not fall out, nor does the plate fall. Why? 
 
 Experiment 3. Force a tin pail (Fig. 23), having 
 
 Does ilowuwaid pressure cause a lateral pressure? 
 
 Eiperlment 4. Make holes, at dlUferent depths In the sM„ „f . 
 «el ,F,g. 24) co„u,„„« water. Water Issue, '»"„„:," ,:'o™' 
 erable force, from tjie orifices. Why? consid- 
 
 ExperimentS. Bind a piece of thin sheet-rubber tightly over a 
 
60 
 
 BVKAMIOs. 
 
 *'ig. 24. 
 
 Is the result no matter in 
 what position the bottle is 
 placed? What lesson does 
 this teach? 
 
 Experiment 6. The 
 Majrdelnirg hemispheres 
 (Fig. 25) are two hemis- 
 pherical cups, having 
 their edges made smooth 
 so as to be "air-tiglit" 
 when placed in contact. 
 
 tl'o^Z'^sLT:',"'"''^ '""''^- ^"^ "^ ^"« '--"- -»--ts of 
 
 The stJinM K " '■'"^'' "•" '"'" •'^'•^'^ '*^''"" ^-onnected by a screw 
 I he stem has a bore passing througl, it, and a stop-cock 
 
 screw t?!f' . "r '"'^^ *" '""''^^'' ^^'»°^« the ring, 
 tl.e ail f om the sphere; then close the stop-cock and 
 
 J'o ding tlie sphere in any position they choose, can onlv 
 
 with great dilHculty pull them apart. Why?' ^ 
 
 Boys amuse themselves by lifting l,ricks (Fig. 2f5) with 
 
 Fig. 26. a circular piece of leather, ii.oisteued and 
 
 pressed against the surface of f.ic brick, so as 
 
 to exclude the air. The pressure of air against 
 
 tlie leather l)iuds it to the brick in whatever 
 
 position placed. 
 
 We (oiulude that gravity causes pressure in a body 
 ofjtidd in all directions. 
 
 § 45. Pressure increases with the depth. - In the ex 
 IKMument with the vensel with apertures in L side (iT^ 
 we find that the deeper >.e orlflee, the greater the velooify of 
 t^.e stream. And in the experiu.ent with the wi,Ie-,„ou. .e, 
 br^ le covered w.th rubber, we (h.d that, at the same depth, the 
 1^.. I pr.,-.,<o i.,„,a.d ecjually in aii directions, but, as it ia 
 carried to greater depths, the piossure is increased. 
 
 I 
 
PRESSURE EQUAL. 
 
 51 
 
 I 
 
 Fig. 28. 
 
 Fif 27 1? ^'" '"''' ''""* '" *'"^ ^°'''" represented by a, 
 
 Fig. 27, place mercury ,n the lower part of the tube, so as to fill the 
 short arm, and ir,-adually lovver the tube into a 
 deep vessel of water. Sink tlie tube to diflerent 
 depths, and carefully watch the column of mer- 
 ci»ry. What Is the result? What does it teach? 
 
 M 46. Pressure at any point in a fluid 
 jgual in all directions. - Experiment i. 
 Introduce anc.iher tube, containinjj mercury, of 
 the form represented by 6, Fig. 27; lower both 
 tubes so that the orillces in the water shall be at 
 the same level. Observe the column of mercury 
 lu each tube. What does the experin)ent prove? 
 
 :- Experiment 2. Gem- one end of a lamp-chimney (Fi... 28) with a 
 orcnlar piece of leather, and suspend" fron. the hand by 
 ."'.•u.sofa string attached to the center of the leather 
 and passmg through the chimney. Hold the leather 
 flrndy aga.nst the botton, of the chimney, and lower tl^ 
 covered end a little way into a vessel of wat^ 1^^ 
 m.y now drop the string, and the upward pressure of 
 the water will keep the leather in place. Pour witer 
 -slow y.nto the chinn.ey until the leather falls. Wha 
 i«ht does the water reach in the lan.p-chimney befor 
 
 1 nJiif r'V :' •''"^' '"""'^ '^ ^"^■" '■'^"'^ ^^'>y ''-« "o 
 a i)iUlful of water in a well sociii licavj? 
 
 ■ '*''!'' T"!'! "f «''l'«"">"nl« thus far show that, at even ,»,■„( 
 .,.«(,.,;, o/JI.„;i, ;„nvU,j «.„« pressure ,o l,e «,,.,( J„ ,? 
 . « *r«,o,„, anatlua ,n ll„a„s tte pressure increasj. ZZ 
 <hph rncrea.e.. It »ho„l,l also l,o observed that, th, .UreJZ 
 
 uglU avglen to the surface. 
 
 
 i>. ■ ; 
 
 i' ': 
 
 '1l 
 
 -i) 
 
 ,'iif 
 
 I 
 
52 
 
 DYN Allies. 
 
 Flir. 29. 
 
 cnry In the closorl arm will sink abont 20- to A, and will rise 2cm („ the 
 open arn, to C; l,nt the surface A is TO- higher than the surface C. 
 i ins can l)e accounted for only by the atmos- 
 pli^.'c i)ressurc. The colunni of mercury BA 
 containing TO'-", is an exact counterpoise for a 
 t-ohnnn of air of the same ,liameter extending 
 from C to the upper limit of the atmospheric 
 ocean, —un unknown liight. 
 
 Tho weight of tho 7G™"' of mercury in the 
 column IJA is l();?,'5.3s exactly, but, for 
 convenience, nmy be said to be about 1^. 
 Hence the wei-ht of u colunni of air of 
 1'""' section, ('xtentling from the surface of 
 the sea to the ui)i)er lin)itof the atmosphere, 
 F'g- 30. isat)outl''. Rnt 
 
 gravity causes 
 equal pressure 
 in all directions. 
 Hence, at the 
 tcrd (if the .sea, 
 
 all hijtUes arc pressed vpon in all 
 diredm,s by the afmosjyhere, with a 
 force of about P per srpmre centi^ 
 meter, about 15 pounds {exactly 14.7 
 lbs.) per square inch, or about one 
 ton per square foot. Fluid pressm-e 
 is <X("iierally expressed in atnios^ 
 l)heres. An atmosphere (when the 
 term is used to denote i)ressin-e) is 
 the pressure of P per square centi- 
 meter. 
 
 . ..,.„, , , '^ ""^" "f average size sustains an ex- 
 
 ternal pressure of about lifteen tons. If the area of the bntto.n of .an 
 •• empiy pad is one sc,uare foot, the downward pressure on Its bottom 
 is a little more than one ton; how can any person carrv such a paU? 
 ftoa whj' l8 Its bottom ijo( forceU vut? 
 
BAEOMETER. 
 
 53 
 
 Fig. 31. 
 
 § 47. Barometer. — Figure 30 represents another form of ap- 
 paratus, which is moreeommouly used for ascertaining atmosplienc 
 pressure. It consists of a straight tube about 8o"" long, closed at 
 one end, and filled witli mercury. When this tube is inverted, tlie 
 
 open end having 
 
 been covered with a 
 finger and plunged 
 into an open cup of 
 mercury, and the 
 linger withdrawn, 
 the mercur_y in the 
 tul)e will sink till 
 it balances the at- 
 mospheric press- 
 ure. This experi- 
 ment was devised 
 by Torricelli, an 
 Italian. The ap- 
 paratus is called a 
 barometer. ^ The 
 empty space above 
 the mercury in the 
 tubeis called a Tbr- 
 dceUian vacuum. 
 The history of this 
 experiment is very 
 interesting and im- 
 l)ortant, inasmuch 
 as it was the rirst 
 demonstration of 
 the pressure of the 
 atmoaphore, CSee 
 Whewell's History of Inductive Sciences, Vol. I., page 345.) 
 The hight of the barometric colinnu is subject to fluctuutions ; 
 * Barometer, weight meatunr, 
 
54 
 
 DYNAMICS. 
 
 this shows that the atmospheric pressurp ,'« «„w . * 
 
 frpm vaMous causes. The baron^^T IXa.^^^ 
 
 tor of all changes -'n atmospheric presst^^ 'it if """"" 
 
 able as a weather indicator Not fiT '*" '''^^"'- 
 
 which mercury may stand Zl f"' ^"^' ^'^^'^^^^^^^ point at 
 
 mercury constantly falls as the ascent increas s T^.f ' '' 
 that the pressure is greater near the bottom of fh'o , ' 
 
 than near its top. It is found tZ, T """^"^^ ^'^^" 
 
 rapidly near tl o hnfV ' ^"^ ^'""''"'^ ^"^^'^^ses very 
 
 Fig re 31 T ,^" *"'"' '^^^ ^"^-^3' be understood by studying 
 air Th Vr '^'"^' '^^^' '^' ^'-^"^"^^ i" density of hf 
 
 Xre^;;^—::::^^^^^ r "^ '^^^^^^ ^ ^^'^^ 
 
 the mercury, i^ inches tL f ^orresponding hight of 
 
 cobjnma,«;,,^-:,^-age^ 
 
 It will bo see,, that the deusitv at a hi.rl,t „f a , ■ ■ 
 mtio „„.o tha„ , the density at tJ-ole:.,' f "a't ;:: S 7 .'•^ 
 
 ""'y Ttmni' so that the greatest part of the at,inn«„i,....<. [ , 
 within that distauoo of the surfa e of he elrtr ar,:™', 
 hand if ™ opening conld be n.ade i nthe ea.'th 3V ile 
 depth below the sea-levcl, it is cale„lato.I tmt th' d.!,' itvt^^,,;" 
 mr at the bottom wonld be 1,000 tin,es greater a, a Ve n 
 level, so that water would float in it Ai,. l,„ i, " 
 
 to this density. '^" *"" '''<''' """P-essed 
 
 To what hight the atmosphere extends is unknown u ■ 
 variously estimated at (h>m 50 to 200 .nill IfHl , " 
 were of unifo™ density, and of the ^mt de , " [' i ir: 
 the sea-level, its depth wonld be a little short oflve Ze 
 Certa,,, peaks of the Hin.alayas would rise abo " it "iT t 
 
 ' bjt the aid of a barometer. ^ 
 
 > 
 
 reauii 
 
 ma 
 
COMPRESSIBILITY AXD fiXPANSIBILITY. 
 QUESTIONS. 
 
 
 the Dottom ? ^ " '^'"^'° «"»■". "»' Hf would at 
 
 life ? °^ "" '» '• B"™ tunc, in c.-dcr to sustato 
 
 pa'o?trz.,:rrof„,fr: n,r°°" '»-' -"- ■'-'>''«>°-> 
 ati:,r;;:':;:;*;;;;r /"'■'- -"^» -»^™; >v„„t ,s ..e 
 
 §4a Compressibility and ejtpansibility of saa«, t., 
 ."c-aase of p,v..,„-c attending the inc-oase L floitr i,rh!t,' 
 .qinds aud gases, is readily explained byti.e faet tl th lo 
 layers of fluids sustain tl,e weight of all |,e la™, te 7°" 
 seaueutly, if the body of fluid is of uniform d.,!,' • 
 
 nearly the case in liquids, the pressnr wi ^ ' Z'Z7 
 
 the same ratio as the depth increases lint Z ''*' 
 
 fa,, f,™ bohtg of nnifo,™ doi.sity, i , L "! , ! 7;f °7" " 
 compressibility of gaseous mutte^ The e rast Zt "" 
 
 and air, in this respeet, m.ay be seen i, theZt i f ' "'" 
 Jeeted to a pressure, of onl atinos,;::: ' Jr, f S;'™"; 
 
 The pressure at difforonf rIor>tJic :., v -i 
 by piling several biiX To ot anotl ' !!1 T' '"'""■'"'" 
 difforem bricks snstai,, I^ d^c ; ;Tth "th " TT".' '""' 
 the upper surface of the pile. On ,e ot ' ' 1 ' '""' 
 gases at different d,.,.,h, T-,'^,"Jr f ' "'''"""™ "' 
 
 wool one on anothe;: ^ SI, ce h vl l,™: !? "'"7 ""'""'" 
 «eeoc varies with the weight it bl! t";rel::l| .'rr 
 eat fleeces snston, are not proportional to their respective dept; 
 
 
 :: 
 
 pi I 
 
t)6 
 
 PVN'AMHH. 
 
 
 below the upper mirfiieo. of fj,,. njlp At t^t ,u ^ , 
 
 to.-ce exerted i,. .v,!,.,., , tl 1 Jrr'' '"T'^ 
 will «!«,. c„m„I...,v 'r.^Z ZtrSlTT"''"'' " 
 
 i« removed, i,i,,„i„, .J ,»,.,,„1"" ,^"j^ r^J;" "-; p'-^-' 
 
 St ted fli.if maur,. ' < ia»ticity. It }m» jilready been 
 
 Seated that matter in a ga«eoi,i, »tate expand* fndefinifelv 
 ui)le,ssri.Htmine(Jbvcxf<.nj'tlforr.« 'ij ** , imietimtely, 
 to the . nth hv f 1. f ; - ^^'"'^I'^'ere is confined 
 
 tu LUC > uMi oy t)](! fojw! of gmvjtv. 
 
 oompletoly InfJuti,,^ tJi,> |,ttiu,„v ,,,^.,. j^ 
 ...uler the gluHH rocdv... of «„ «),.,„„, 
 (lM<.r. 31) tt.ui .xhau^t tl.« «Jr, What i. 
 
 tlie receiver, the balloon 
 HiiHtaiijK a prewurc of ir, 
 f'oiindH OM every w/uare 
 i'H'li. Wlmt prevent* a col- 
 li I>HemKi,.r thfx prexfiire? 
 I'mwrnuch m the balloon 
 
 hIi'»VV« no «ltff,« o/ dlMt*;n- 
 
 'i'"'. or colhip^e. until ^^ 
 
 plficed nnder the ree«»Jvpi- u ^. ,. 
 
 tension of tiLrtvH:. *" "*'^'"^ ''«'«"'^«^« ^>y ti. 
 
 for the purpose of m^al!^Z'i^£::n "l'" ^'"^j/'''^'^^ (Fig. 33) 
 orUlnar, Ue.lt,. the, are -ci Jf^r^:;:, ,j;^--« - o. 
 
 f1|t. 32, 
 
 Fig, 33, 
 
THE AIE-PUTMP. 
 
 5T 
 
 ,4 
 
 1^' 
 
 ments of ^^lass in all directions ""'"'' *'™^'"« ^^'^S- 
 
 to ulax rtse 1, wlien opporttnuty is given. Since this elastic 
 ^rco at the bottcn of the cohnnn exactly balances the fo e of 
 g av.ty act.ng on the whole column, i.e., equals the weight of the 
 whole column, it follows that, at the sea-level, the elastic fhZ 
 of mr IS ordinarily P per square centimeter. ^ 
 
 § 49. Air-pump. - The air-pump, as its name imnlies is 
 used to w.th<h.aw air fron, a dosed vessel. Figure 34 wk serve 
 
 to illustrate its oper- 
 ation. R is a glass 
 receiver from which 
 air is to be exhausted. 
 B is a hollow cylin- 
 der of brass, called 
 the pump-barrel. A 
 plug P, called a pis- 
 ton, is fitted to the 
 interior of the barrel, 
 and can be moved 
 up and down by the 
 handle H; s and t 
 are valves. A valve 
 ^ acts on the principle 
 
 of a door mtended to open or close a passage. If you walk 
 .igamst a door on one side, it opens and allows 301. to pass ; but 
 If you walk against it on the other side, it closes the passaoe, 
 and stops your progress. Suppose the piston to be iu the let 
 of descendmg. The compression of the air in B closes the valve 
 , and opens the valve ., and the enclosed air escapes. After 
 the piston reaches the bottom of the barrel, it begins its ascent ; 
 when the air above the piston, in attempting to rush down 
 
 f. 
 
 m 
 
58 
 
 DYKAMICS. 
 
 ■/■I 
 
 ! 
 
 to fil the vacuum that is formed between the bottom of the 
 barrel and the piston, closes the valve .. But as soon as a 
 vacuum is formed above t, and the downward pressure on the 
 valve removed, the air in R expands, opens the valve t, and fills 
 the space u. B that would otherwise be a vacuum. Btlt, as the 
 au- m R expands, it becomes rarefied ; and, as there is less air, 
 o there .s less tension. The external pressure of the air on R 
 emg no longer balanced by the tension of the air within, pressed 
 the ece.ver firmly upon the plate L. Each repetition of a 
 doube stroke of the piston removes a portion of the'air reml! 
 ing m R. The air is removed from R by its own expansion 
 owever far the process of exhaustion may be carded he 
 lec iver will always be filled with air, although it may be exceed! 
 .ngly rarefied The operation of exhaustion is practically ended 
 
 valve t "' '''" "' '" ^ ^''"'"'' ^" ''''''' ^ "ft '^« 
 
 D IS another receiver, opening into the tube T, that connects 
 the receiver with the barrel. Inside the receiver is placed a 
 bammetei-. It is apparent that air is exhausted from D as well 
 as trom R; and, as the pressure is removed from the surface of 
 the mercury in the cup, the barometric column falls ; so that 
 the barometer serves as a gauge to indicate the approximation 
 to a vacuum. P^or instance, when the mercury has fallen 380°^ 
 (15 inches), one-half of the air has been removed. 
 
 QUESTIONS. 
 
 1. Why is it difflcnlt for a person to lift the receiver from the nnmn 
 after the air is exhausted from it ? ^ ^ 
 
 2. Why is it easily raised before the air is exhausted ? 
 
 3. Suppose tliat the air in the pump-barrel, when the piston is raised 
 is one-eighth of all the air in the pump, including the ^r in the rece v- 
 
 4 w. r'"'" "' '^'' "''' '' •'•^'"^^'^^ "^y "»« «rst double stroke? 
 
 4. What portion of the original amount of air is removed at the 
 second double stroke? ^cmuvea ai tne 
 
 5. Which double stroke removes the most air ? 
 
 6. If there were no force required to lift the valve t, why could not 
 a pertect vacuum be obtained ? ^ °' 
 
 7 
 colu 
 
 crse 
 
 A 
 may 
 
 i 
 
 stitute 
 in Figi 
 faucet 
 lated. 
 will adj 
 To the 
 means ( 
 which J 
 
THE AlE-PtiMp. 50 
 
 7. It Is a very good pump that reduces the M^m ^f .,, 
 
 column to 3-™. ^hat portion of the afr ha. h^ ' mercurial 
 
 ■SA 9 *' "u ui me air has been removed in that 
 
 1?^^ 
 
 crse ? 
 
 Fig. 35. 
 
 An absolute vacuum has nev.r beon flHn,-r,«i ^, ,. 
 
 may be readily understood. IcccMr. to t T '""'"^'^ 
 
 Accoiaing to the most recent cjii- 
 
 ^"•'^t'ons, the number of molecules 
 contained in a cubic centimeter 
 of air of ordinary density is some- 
 thmg like 21,000,000,000,000,. 
 000,000 (twenty-one million tril- 
 lion) ; consequently, when it is 
 reduced to one-millionth its us- 
 ual density, 21,000,000,000,000 
 (twenty-one trillion) molecules 
 are still left. The exhaustion 
 may be carried much farther than 
 '>y purely mechanical means, by 
 lieating a piece of charcoal in 
 tlie receiver while the pumping is 
 going on. Heat expels the air 
 in its pores. After the pump- 
 ing bus ceased, the charcoal 
 IS allowed to cool, when it 
 condenses a large portion of 
 the remaining air in its pores. 
 (See § 37.) 
 
 stitnfo fx,. • ^ ^^^'^' ^^^^P ^"^^ efficient sub- 
 
 in figure 86, m wh.ch a is a,; . i^vated tank of water iiavin.. a 
 fau^t * by which the rapid,., .; the flow of water may be ":„! 
 lated. The tube o should be as long ,« the hight of'the oom 
 
 lo the end of the branch-pipe e there mav be connected bv 
 m a„s of rnbber tubing k, a f iass tnbe leading to a vesse 7^0™ 
 rtich air is to be exha«s..u. Water fallhrg freel/Ihroi^ha 
 
 * hIHiI 
 
 t 'jS 
 
60 
 
 DYNAMICS. 
 
 f 
 
 Fig. 36. 
 
 vertical tube ei:ert8 no lateral pressure ; consequently there is 
 no tendency to enter the branch e. As the v.iiej ui xalling 
 increases in velocity, it tends to separate, leaving between the 
 cylinders of water vacuous spaces. The lower end of the pipe c 
 being immersed in vater, air cannot enter there ; 
 but the air in the recaiver g expands and rushes 
 through the tube e, to fill these vacua, and thu^ 
 exhaustion is eft jOuCd. In Sprengcl's air-pump 
 mercury is substitutt d for water, and air is reduced 
 by it to less than one-millionth its usual density. 
 
 < 
 
 Experiment 1. Take a glass tube (Fig. 36), having 
 a bulb blown at one end. Nearly fill it with watei , so 
 that when inverted there will be only a bubble of air in 
 the bulb. Insert the open eiul in a glass of water, plr';e under a 
 receiver and eximust. What happens? Why? What will happen 
 when the air is admitted to the receiver? 
 
 Experiment 2. Through a cork of a tightly-stopped bottle pass one 
 arm of a U-shaped glass tube C (Fig. ;57). 
 
 Introduce the other arm into the empty Fig. 37. 
 
 vessel B. Place the whole under a glass 
 receiver, and exhaust the air. What ; ' ,> 
 nomena will occur? What will hapi. i 
 when air is admitted to the receiver? 
 
 § 50. Marlotte's Law. — The 
 experiment illustrated by Figure 32 
 showed that the volume of a given 
 body of gas depends upon tlie pros- 
 sure to w)iieii it is subjected. To 
 
 find more exactly the relation between tliese quantities, proceed 
 as follows : — 
 
 Experiment 1. Take a bent glass tube CFig. 381. the short arm 
 being closed, and the long arm, which should be at least SSe™ long, 
 being open at the top. Pour mercury into the tube till the surfaces in 
 the two arms stand at zero. Now the surface in the long arm supports 
 the weight of an atmosphere, i'herefore the tension of the air en- 
 
 do 
 poi 
 sur 
 red 
 
 striiif 
 cury 
 6. A 
 thefli 
 oury : 
 atmos 
 inche! 
 the 16 
 
mariotte's law. 
 
 61 
 
 }re IS 
 .Uino: 
 
 der a 
 
 ippen 
 
 s one 
 
 3eed 
 
 arm 
 
 oiig, 
 
 s in 
 
 orts 
 
 en- 
 
 Fig. 38. 
 
 Indstn h arm whieh exactly balances It, must be about 15 
 
 pounds to the square mch. Next pour ni.rcury into the long arm till the 
 surface m the short arm reaches 5, or till the volume of air enc lo ed is 
 reduced one-half, when it will be found that the hlght of the columnl C 
 is just equal to the hight of the barometric column 
 at the time tlie experiment is performed. It now 
 appears that the tension of the air in A B balances 
 tlic atmospheric i)ressure, , s a column of ujercury 
 A C, which is equal to anotlar atmosphere; .-. the 
 tension of the air in A B =- two atmospheres. But 
 the air has been compressed into half the space it 
 formerly occupied, and is, consequently, twice as 
 dense. If the length and strengtli of the tube 
 would admit of a column of mercury above the 
 surface in the short arm equal to twice A C, the 
 air would be compressed into 
 one-third its original bulk ; and, 
 inasmuch as it would balance a 
 pressure of iliree atmospheres, it- 
 ten'^ ion would be increased throt- 
 
 1 erlment 2. Next take a 
 glass tube (Fig. 39) open at l)oth 
 ends, and about 24 inches long. 
 Tie three strings around the tube, 
 — one 3 inches from the top, 
 another 6 inches, and the third 21 
 inches.. Nearly fill a glass jar, B, 
 25 inches high with mercury. 
 Lower the tube into the mercury 
 till it reaches the string at 3. 
 Press a finger flmly over the up- 
 per end, and raise the tube till the 
 string at 21 is on a level with the surface of the mer- 
 cury in tl ' jar. The mercury in the tube will stand at 
 6. At first the air enclosed in the tube between 8 and 
 the finger withstands an upward pressure of the mer- 
 cnrj' suffic =:t to sustain a column of i,,. rcury SO inches high, or one 
 atmosphere. When the tube is raised and the mercurv stands at 6 15 
 inches high, one-half of that upward pressure is exerted in sustaining 
 the 16 Inches of mercar5^ and the other half is exerted on the enclosed 
 
 
^:l' 
 
 (52 
 
 DYNAMICS. 
 
 lated as f„lto„»'- '"" ""' °' "^P"l>nents may be tabu- 
 
 Pressure .... i j i « „ . 
 
 Volume . . • • I't'^'^'^^- 
 
 Density . ' ' ' i ,'' ^' *' ^' '^^■ 
 
 Elastic force' :;■••'' ' ?' .^' ,*' ^<^- 
 3. i. 1, 2, 3, 4, &c. 
 
 From tlK..„. results we learu that, at twice the pressure th„„ 
 
 . s a"'"Ar;::rtr''"' '"^ ^-^''^ -" »'»* ~ 
 
 uistovuul It at about tlic same time. This law i, tn.n f^- 
 
 »n gases w.thiu certa mits, hut under e.treuV or ,s , e tt 
 
 - -^on ,„ volume is greater than iudieatecl ,,;," ^V "tt 
 ;|- ..>t™ from ,t occurs with those gases tha't are mos't Ltty 
 
 QUESTIONS. 
 
 1. Into the neck of a bottle partly fllle,! with water (Fig. 40) in- 
 
 ^'^- 40- t^h/ '°? "''^ "^""^' ""•'^"^'" ^^''"'^l' P'»«««« « glass 
 
 tube nearly to the bottom of the bottle. Blow forci- 
 bly into the bottle. On removing the mouth, water 
 will flow through the tube in a stream. Why? 
 2. How can an ounce of air, 
 
 in a closed fragile vessel, sus- FiJ?. 41. 
 
 tain the outside pressure of 
 the atmosphere, amounting to 
 liH^^H ^'^evcral tons? 
 
 3. What drives the peljots 
 
 from a pop-gun ? 
 
 Ilg^_ 4. I'lgure 41 represents a 
 
 dropping-bottle, much used in 
 
 chemical laboratories. Why 
 
 do bubbles of air force their 
 
 way down into the liquid? - ' " ' ■ »iia ^ 
 
 ^^ stop the upper ormce, .„a .he U,„.a ^ ,„,„y, ^^ ^ ^^^ 
 
 9. S 
 
 contracti 
 44), int< 
 condense: 
 
>p. 
 
 Flgr. 42. 
 
 CONDENSER. go 
 
 IS just below the surface of the boiling liquid As 
 the water evaporates, and its surface falls below the 
 mouth of the bottle, an air-bubble enters the bottle 
 expands, and pushes out enough water to cover once 
 more the mouth of the bottle. Why does not the air 
 push out all the water from the bottle? 
 
 7. Fi^-'ure 43 represents a weight-lifter. Into a 
 hollow cylinder s is fitted air-tight a piston t. The 
 cylinder is connected with an air-pump byarubi)er 
 tube n. When air is exhausted the piston rises lift- 
 lug th<' heavy weight attached to it. Why? 
 
 onnt V ^''l'"''"' "^ ^^"^ ''''''''■ '"'■'■^^^ «f the piston is 
 2010™, how heavy a weight ought to be lifted when 
 the air is one-lialf exhausted ? 
 9. Suppose you tightly stopper a bottle at the top of Mont Bianc, car- 
 
 ry it to the sea-leveJ. 
 insert the mouth of 
 tlie bottle in water, 
 iiiid withdraw the 
 stopper; what would 
 happen ? 
 
 10. Show that the 
 labor of working the 
 kind of air-i)nmp de- 
 scribed (§ 4l>) increas- 
 es as the exhaustion 
 progresses. 
 
 Fig. 43. 
 
 § 51. Condenser. 
 — Ill the experi- 
 ment with the bottle 
 (Fig. 40), air was 
 condensed in the 
 mouth by muscular 
 contraction and forced intx> the bottle. An apparatus A (Fie 
 44) uitended to condense air in a closed vessel, is called "a 
 condenser. Its constructi, a is like that of the barrel of the 
 
 ff 
 
64 
 
 DYNAMICS. 
 
 air-pump, except that the position of the valves is reversed. 
 (Compare with Fig. 34.) What differences do you notice in 
 respect to the valves? What happens to 
 
 the valves when the piston in the condenser ^^^'*^' 
 
 is forced down? If the condenser is con- 
 nected with a closed vessel B, how much 
 iiir would be forced into it at one down 
 stroke? VVHiat prevents the air from o.s 
 eaping during an up stroke? If, after air is 
 condensed in B, the cylinder C is connected 
 witli it by a screw, and the stop-cock t is 
 suddenly turned, what would happen to the 
 bullet .s ? Wliat name would you give to such 
 "n apparatus? 
 
 The Western Union Tclosraph Company, in 
 V'w York City, employs atmospheric pressure in 
 forwardinj? messages to its central olHce from 
 
 frnr^lT? ''^"^''^'^' '*^"^"' "' '^"'' '''^'- '^"^'^^ «f ""'form siso, free 
 from sudden em-vatures, and laid under ground, connect the bmuch 
 
 on ces with headquarters. Rolls of paper, or lettc -s to be des- 
 patched, arc deposited m a cylindrical box c (Fig 45) whieli 
 fits the interior of Mie tube. The box being dropped into the 
 
 6^ end of the tube 
 
 at a, and the air 
 being exhausted 
 from tlie tube at 
 the end h, by 
 ^^^i^X^^'^^M ^^^^^'^^^P means of an air- 
 
 Steam, air rushes 
 
 , , , ™ - -.^-- -- .--V . «!^*>^5^^-s^!s^w5^ss»ss5^5%.^x^^ in at a and pushes 
 
 I" l>ox tiirough the tube with a foree of several pounds for every square 
 i.c . of the end of the box. The operation is still further facilitated 
 riy tJie aid of a condensing-punip worked by steam at the end a. 
 
 § 52. Pressure transmitted undiminished in all direc- 
 tions. - Fill the globe G (Fig. 4.1) , and about one-fifth t lu- ..ylin- 
 dor C, with water. The water in the tubes a, b, c, and d, will rise 
 
 Fig. 45. 
 
! 
 
 PRESSFHE Tl; ANS.M ITTED. 
 
 65 
 
 to the same level wit!, the water in the cvliuck-r C. N<nv Ibree 
 the pisto.i !• into the cylinder, and the downward pres.sure will 
 ^'UMse ,,et8 of water to issue from eacl, of the tubes. But the 
 s rounds from the tubes «, 6, and c, rise t« exactly the same 
 hight that the stream fro.^. the tube d does, although the li(,uid 
 Fig. 46. ia the latter tube re- 
 
 ceives the direct ac- 
 tion of the downward 
 force. It thus ai)pears 
 tliat the pressure is not 
 felt alone by that por- 
 tion of the liquid that 
 lies in the path of the 
 force, but is folt equally 
 iu all parts and in all 
 directions. 
 
 If the globe is filled 
 with air, and subjected 
 to pressure as aK.uv'e, 
 eurrents of air will is- 
 sue from the several 
 tubes with equal force. 
 This i)roperty of trans- 
 mitting pressure equal- 
 ly in all directions, 
 which IS peculiar to fluids, is due to their viobility and i»'rfeci ' 
 elasticity. 
 
 Figure 47 represents a number of elastic hooi)s enclosed iu 
 the vessel AIU'D. A weight, plaee.l on a, communicates to 
 It a downward pressure. It is evident, that not only is the 
 pressure communicated to the hoops below it in succession, and 
 finally to tl.e h^MUm of the box, but there is also a lateral pres- 
 sure due to th(' elastic property of the hoops. The hoop (;, 
 receiving pressure fio.ii b, above, reacts, exerting an upward 
 pressure ; it also prosses laterally upou the side A, and the hoop 
 
 ' HrJNil 
 
 <'m 
 
m 
 
 66 
 
 DYNAMICS. 
 
 >V- 
 
 nl „i„" 7'"' "'" """r'"'^ """" "'''"'" -1 "■«"''- 
 
 iiius, upward, downward, and 
 
 laterally, a force equal to the "^ ''' 
 
 dovvuward pressure of the weight 
 
 W. Hence that portion of the 
 
 bottom Ininiediatcly under the 
 
 weight receives no greater pres- 
 
 sure from W tlian an equal area 
 
 of any otiier part of the bottom, 
 or than an ecpial area of either 
 of the sides, A and B, or the top 
 C. This operation illustrates, 
 somewhat imperfectly, the meth- 
 od by which elastic fhiids traus- 
 nnt pressure uuthminished in all 
 directions, 
 
 if we take a quantity of water 
 !" ^ ''«^««I A i^\- 48), shut ^^^^m 
 
 1^- a^d^^rtd .:; "' ^k ^'^^ ^^-^^^^ ^^- -«P-tiveIy 
 
 ;'-atte;isn:rirt:^;rr\^^^ 
 
 former, but that it will require a -l,)-gram 
 ;7;P'''^^-^ ---to preserve equilihHm" 
 
 nt te area of the piston i is 4..... ,vhile 
 
 Ik. piston a contains four such areas- 
 hence It follows that a pressure of 10« is 
 transmitted to each of the 4- of a, and lust 
 supports the 40-gram weight. Had the am! 
 of the piston b been 1-.-, then the 10-gram 
 
 weight ph.ced on it would require a 1 GO gram ^— 
 
 weight placed on a to balance it • t Lf ^ 
 
 would be exerted on eve^ .quare^: ^^^ ;, f^^^ ^' ''' 
 
 Obviou.1, UU« for. or apparatus cannot .. ....Uo to woric well on 
 
 Fie. 48. 
 
'e to 
 aus- 
 
 at 
 
 HYDROSTATIC BELLOWS AXD PRESS. 67 
 
 ' account of the friction of the pistons- hnf 
 
 the pistons and weights colum.fs of m' . ^^ ""'"^ «nbstitute for 
 -nncctiug tube and^the lon-Tplt of ?h h' ^^ '"'''"''' ''' ^"« 
 cury; the two free surfaces wiU t tt sT ?" f""' "'"> '"^'- 
 of any liquui, e.rj. water is no.nwi . i ^'^ ^''^''^- ^'''^ ^f 108 
 
 § 53. Hydrostatic bellows — Ti.;. . • • , . 
 
 tal;.!,''""'''''^'''^^'^^^^thcr at- 
 tached to then- edges, as to form an air- 
 tiglit vessel called tlie bellows. A <.lass 
 t'lbe a, having a bore of l-i-. sertion' 
 communicates with the interior of the bel' 
 
 ows Let water be ponred into the tube 
 
 ^ til the boards, is raised a few centime- 
 
 tei. The water wilUtand at the same 
 
 .m the tube and bellows. Now, if 
 
 oO« of ,,,tcr be poured into the tube, it 
 H.Il requn-e a weight of 20,000« to be 
 
 placed upon 6 to prevent its rising. Any 
 
 the oO« of water ,f ^n^fl T' """ ''''' "'" '^ ^'^'^-^ '>y 
 i>eIlows, a Teton t.. / ""TZ ''^'"^ '"^'•'^''--^ '"^> the 
 
 eaaii,raise\.m:^t^ni:^^:;i^t::ir '-' ^"^^' '^^ -" 
 
 na.e Of the inventor. You see t::;i;i::nXFS:: JIT 
 
 Ml 
 
 £ 1 
 
 Ik 
 
 Mi' 
 
i 
 
 m 
 
 DYXAMrcS. 
 
 m^. 50, 
 
 The area of the lower surface of t is (say) one hundred times 
 
 that of the lower surface of s. As the piston s is raised and 
 
 depressed, water is pumped up from the cistern A, forced into 
 
 the cylinder x, and exerts 
 
 an upward pressure 
 
 against tlie piston t one 
 
 luindred times greater 
 
 than the downward pres- 
 sure exerted upon s. 
 
 Thus, if a pressure of one 
 
 hundred pounds is applied 
 
 at s, the cotton bales will 
 
 be subjected to a pressure 
 of five tons. 
 
 The pressure that may 
 
 be exerted by these press- 
 es is enormous. The hand 
 of a child can break a 
 strong iron bar. But observe that, although the pressure ex- 
 erted is very great, the upward movement of the piston t is 
 very slow. In order that the piston t may rise P-", the piston s 
 must descend 100-'. The disadvantage arising from sic .mess of 
 operation is little thought of, however, when we consider the 
 great advantage accruing from the fact that one man can pro- 
 duce as great a pressure with the press as a hundred men can 
 exert without it. 
 
 Tlie press is used for compressing cotton, hay, etc., into bales, 
 and for extracting oil from seeds. The modern engineer finds 
 it a most efficient ma(!hine. whenever great weights are to be 
 moved through short distances, as in launching the Great East- 
 ern steamship. 
 
 § 55. Pressure in fluids diiA +.n o-ratri+w ._ tr 
 
 rtViH? con - 
 
 sidored the transmission to the walls of the containing vessel, of 
 external pressure applied to any portion of a surface of a liquid, 
 
i>KESSUKE IN FLUIDS DUE TO GRAVITY. 
 
 GO 
 
 Fig. 51. 
 
 we MM examine the effects of pressure due to the weight of the 
 liquids themselves. Suppose that we have three vessels filled 
 
 with water, A, B, 
 andC (Fig. 51), of 
 equal depth, and 
 liaving bottoms of 
 equal areas. It is 
 plain that the bot- 
 tom of vessel A sus- 
 tains a pressure equal to the weight of the column of water 
 abed, or just the weight of the water in the vessel. The pres- 
 sure on /y, a portion of the bottom of vessel B, is equal to the 
 weight of a column of water ghji. But this pressure is trans- 
 mitted undiminished to the surface fh; consequently, the 
 pressure on//t is equal to the weight of a column of water of 
 the size of ef/,g, and the pressure on ^7 is equal to the weight of 
 a column ijlk. Hence the pressure on the whole bottom fl is 
 equal to the pressure of a column of water eflk, or the same 
 as the pressure on the bottom of vessel A. But the weight of 
 the water in B is less than the weight of the water in A. Hence, 
 (1) the pressure on the bottom of a vessel may be greater than the 
 weight of the water in the vessel. 
 
 In vessel C, the side mq sustains the downward pressure of 
 the body of water mqn; and the side pr sustains the pressure of 
 the body orp; while the bottom r/r sustains only the pressure 
 of the column nqro, which is equal to the pressure on the bot- 
 toms of each of the vessels, A and B. Hence, (2) the pressure 
 on the bottom of a vessel may be less than the weight of the water 
 in the vessel. 
 
 We conclude, therefore, that (.3) the pressure on the bottom 
 of a vessel depends on the depth and area of the bottom and 
 the density of the liquid, and is independent of the shape of the 
 vessel and ike quantity of liquid. -The important fact that the 
 pressure on the bottom does not depend on the shape of the 
 vessel is often called the hydrostatic paradox, beeauae, though 
 true, it aeems at first absuYd. 
 
 i 
 
 
70 
 
 tnrsAMum. 
 
 rrj;ir ^'"^' - "--^ i."urr r^ir;; 
 
 ««ch. Etuih vernal, m^^^^ "' " 
 
 when Id UPC 1h sup- 
 ported by the tripod 
 "• The dl«k is sup 
 ported and presst-d 
 up «tron«;ly against 
 the bottom of the 
 vessel by means of 
 a string passin/if up 
 through the vessel, 
 and attached to a 
 sprlng-bahince. Let 
 water be poured In- 
 to vessel C, and reg. 
 ulate at pleasure the 
 amount of down- 
 ward pressure nec- 
 essary to push the 
 bottom off and al- 
 low the water to 
 «8cape. Note the 
 depth of water 
 when the bottom IS ,^,^^^^^^^^^^^^^^ 
 
 ^^^'"^^'^^^^ -Ubth. pointer/. Also 
 
 vessel A for vessel O. PourL l!!i . *"' ''*'«'^«' Substitute 
 
 bottom will be forced off at U^^ ^J.'Zf , ""^"**''' •^' ^'"^" ".e 
 
 pointer, and by the same pn!..^! ' ^ 'f ^^^ ^ *""^^" ''^ the 
 But much less wat«r is re.u^Xn Tj '7 '^ *'"' •»l'r»"«-balanoe. 
 experiment, repeated wfU^eCl ? wTu.' *1.'''"' ""' *'^-"''*" '^ ''"« 
 ase of a stlU Urn qaantHy of waL!r * ••**^' ''^*«'^ ^1"' ".e 
 
 -v?^ ,. j^ ^^-^ ^ _ ^^ northern , f ,t. 1 ' '-' ^VW^S srnose 
 
QUESTIUNS AND PKOBLEMS. 7^ 
 
 suppose the area of hj, of the bottom of vessel R ^'Fi*, k^\ • 
 
 JOO . And, since the weight of one cubic centimeter of water 
 IS one gram, the weight of the column is 900«, which il the 
 pressure on the surface /,•; and the pressure on eac o i'e 
 equal surfaces //. and jl being the same as on /y, the pressure 
 on the entire bottom is 2700«. pressuie 
 
 Evidently the lateral pressure at any point of the side of a 
 vesse, depends upon the depth of that point; and, as depth 
 at different points of a side varies, hencc^S) o find t~ 
 sure upon an, portion of a side of a vessel, IJftncUke Jig t of 
 a column of r.ater rohose base is the area of that portion of Z 
 ^,ana .hose hight is the average depth Jtha, pZ^^^ tC 
 we compute the pressure on the side ah of vessel A (Fig 51) 
 n- multiplying the area of the side 90- (dimensions, 9 x 10-' 
 
 of I ot water, which gives 405« for the pressure on the sUie 
 QUESTIONS AND PROBLEMS 
 
 bottom. How rapidly should its thickness 
 increase ? 
 
 2. At high tide, suppose the flood-gate of 
 a dock to be closed, leaving the surface of 
 water on the inside and outside of the gate 
 at the samo lovel. From which does the gate 
 siistaiu the {rreater pressure, the water in 
 I lie dock, or the ocean of water outside? 
 Why ' 
 
 „. , , ,^ ®- "^^^ interior dimensions of the rectan- 
 
 depth. Tiie vessel is full of water. Hnmnu*.. fKo *of«> -r^«„^ 
 each of the six sides. """" " ''°*^' ^'^'^"^ °° 
 
 i« L^"?r ""'^ '^' P'"^ " ^^'^- ^*>' *^« ^f«* «f whose end is 4aem. 
 
 what ;!TrH T '""" '''' ''"''''' "' "'« ^'**«'- ^'^»^ the force of 100. 
 what additional pressure will each side of the vessel sustain ? 
 
 Fig. 53. 
 
 
 H ^ 
 
72 
 
 DYNAMICS. 
 
 6. How great will be the whole pressure that each side sustains, 
 due to the weight of the liquid and the external pressure? 
 
 6. Suppose mercury, which is 13.6 times heavier 
 
 than water, to be employed instead of water, what '^^ ''*• 
 
 would be the answers to the three preceding ques- 
 tions? 
 
 7. Into the top of a keg filled with water, a brass 
 tube lom long is inserted, a transverse section of 
 whose bore is l-icm. Tlie depth of the water in the 
 cask is 30'=™, and the area of the bottom of the cask 
 is 40iem. (a) Compute the pressure on the bottom of the kes 
 (6) Compute the pressure on the bottom of the cask if the tube is filled 
 with water, (c) What is the weight of the water in the tube that 
 causes this extra pressure? 
 
 8. What crushing-force on each side would an empty cubical box 
 tiie area of one of whose sides is li™, sustain, if lowered l^m into the 
 
 9. What crushing-force on each side would this box sustain from 
 the atmospheric pressure at the sea-level, if the air were completely 
 exhausted therefrom? i'lcieiy 
 
 10. Suppose the top of the vessel (Fig. 54) to be the weak part of 
 the vessel, not able to sustain more than 508 pressure on 10«<=™ what 
 pressure applied to the plug will burst the vessel? 
 
 § 56. The surface of a liquid at rest is level. —By jolt- 
 ing a vessel the surface of a liquid in it may be made to assume 
 the form seen in Figure 55. Can it retain this form ? Take 
 two molecules of the liquid at the points a 
 and 6, on the same horizontal level. The 
 downward pressure upon a is the weight of 
 a column of molecules ac, and the downward 
 pressure upon b is the weight of the column 
 bd. Now, since the pressure at a given depth 
 is equal in all directions, bd and ac represent 
 the lateral pressures at the points b and a respectively. But bd 
 IS greater than ac ; hence, the molecules a and 6, and those lying 
 m a straight line between them, are &ot^A nnnn u.r fw« ..^l„^^ 
 forces m opposite directions. There will, therefore, be a move- 
 ment of molecules in the direction of the greater force toward 
 
 Fig. 55. 
 
I 
 
 ARTESIAN WELLS, ETC. 73 
 
 a, tUl there is equilibrium of forces, which will only occur when 
 the points a and b are equally distant from the surface ; or in 
 other words, there will be no rest till all points in the surface 'are 
 on the same horizontal level. 
 
 This fact is commouly expressed thus: "Water always seeks its 
 owest level." In accordance with this principle, water flows down an 
 
 a^i"Ition"f'H ""' "? "™"" ^"^P^^ "P- ""- "^-*-tion oTtho 
 a, phcat on of this pruic.ple, on a large scale, is found in the method 
 .supplyu.g cities with water. Figure 5G represents a modern a que- 
 duct, through which water is conveyed from an elevated pond or river 
 
 Tn a'c'lt'v ;/ 'T- t' "'" " ^"" '' "^"-^"^^ ^ -"^y ^' '- ' '--vo^" 
 m a city, from which water is distributed by service-pipes to the dweu' 
 
 Fig. 56. 
 
 lugs. The pipe is tapped at different points, and fountains rise theo- 
 retically to the level of the water in the pond, but practically not so 
 high, on account of the resistance of the air and the check which the 
 ascending stream receives from the falling drops. Where should the 
 pipes be made stronger, on a hill or in a valley? Where will water 
 issue from faucets with greater force, in a chamber or in a basement? 
 How high may water be drawn from the pipe in the house/? 
 
 §57. Artesian wells, etc. -In most places, the crust of the 
 earth is composed of distinct layers of earth and rock of various kinds 
 These layers frequently assume concave shapes, so as to resemble oups 
 placed one within another. Figure 57 represents a vertical section 
 exposing a few of the surface-layers of the earth's crust : a is a stratum 
 of loose sand or gravel; b, a clay-bed; c, a stratum of slate; d a 
 stratum of limestone ; the whole resting on a bed nf jrranH» . if ,,«., 
 hoUow out a lump of clay, and pour water into the cavitv, you will 
 find that the water will percolate through tlie clay very sl'>wi Water 
 tliat falls in rain passes readily through the gravel a, till U • l.-hes thf 
 clay-bed 6, where it coUects. Hence a well, sunk to the cl --bed, wlU 
 
 ^J: 
 
 m 
 
74 
 
 DYNAMICS. 
 
 
 Fig. 57. 
 
 Pig. 59. 
 
 a p.1.0 tiuougl, a .like by tl,. seashore, and c,n-ve it „ wari ho 
 
 r:,r;r :;';;f '■ '^"" '"^ --'-■• - --i^ '^e pi: 
 
 Notwithstanding that every-day 
 experience teaches that " liquids seek 
 a level," it may soem strange tliat 
 the If.rge quantity of water in a 
 teapot is balanced by the small 
 quantity in the nozzle. Why, for 
 inst.ance, should the liquid in the 
 small arm B balance the liquid in 
 the large arm A, of the vessel in 
 . 1 ,. ^''^' ^^'- Imagine the liouid in A 
 
 columa e. It is dear that the downward pressure ot any 
 
 one 
 ward 
 is nc 
 othei 
 
 r^ 
 
 §59. 
 
 f erring 
 of atmo 
 
SIPHONS. 
 
 76 
 
 one of the columns a, b, o, d, or e, will balance the down- 
 ward pressun. of anyone of the other COJU..US, and that there 
 .8 no reason wh^. e should rise above any one, or all, of the 
 
 . o9. Siphon. - A siphou is a an instrnment used for trans- 
 fernncr a hqmd from one vessel to anothei- through the acreucv 
 Of atmospheric pressure. It consists of a tube of any material 
 
^f^f- 
 
 IMAGE EVALUATION 
 TEST TARGET (MT-S) 
 
 4- 
 
 m/^ 
 
 4- 
 
 
 Cj?/ 
 
 A 
 
 f/. 
 
 
 
 1.0 
 
 
 
 =^ !? ■** l-^.^ 
 
 
 
 e: i^ ^ 
 
 
 I.I 
 
 Hut. 
 
 lllll^^ 
 
 m 
 
 1.25 
 
 U. 1.6 
 
 ^ . 
 
 6" • 
 
 Til 4 U- •_ 
 
 rlluuj^-dpiuu 
 
 Sciences 
 Corporation 
 
 23 WEST MAIN STREET 
 
 WEBSTER, N.Y. 14580 
 
 (716) 872-4503 
 
 <^ 
 
 4^^ 
 
 5V 
 
 c\ 
 
 \ 
 
 <^ 
 
 
? 
 
 ,.<!' 
 
 .V 
 
 .«■ 
 
 .% 
 
 4^ 4 
 
 C??/- 
 
 
76 
 
 DYNAl^IJCS. 
 
 mbber IS often most convenient) , bent into a shape somewhat 
 
 .qud, stop each end with a finger or cork, insert one end in 
 the hqu,d to be transferred, bring the other end below the lev.I 
 of he surface of the liquid, remove the stoppers, and the liquid 
 ;v.l .mmeduately flow. What are the forces acting on the wat r 
 •n the siphon A (Fig. 59) F What forces tond'to move th" 
 water ft-om the tall jar to the short one? What forces tend to 
 move the water the other way ? By how much does the one set 
 of forces exceed the other? When will the water cease to flow? 
 
 tall jars or bottles, experiment with it as in B and C. State and 
 explaira the results of your experiments. 
 
 The remaining diagrams in this cut represent some of the 
 great variety of uses to which the siphon may be put D F 
 and F are different forms of siphon fountains, 'in D the 
 s.phon lube is filled b^ blowing in the tube /. Explai^ the 
 remamder of the operation. A siphon of tie form G 
 always ready for use. It is only necessary to dip one end into 
 the iq,„cl to be transferred. Why does the liquid not fl^w out 
 of tins ube in its present condition? H illustrates the method 
 by which a heavy liquid may be removed from beneath a lighter 
 vpll • ^ r''"' ""^ ' "^P^^" " ^•'l"'^ ™^^ ^« removed from a 
 
 t^rth. r ' ^'^T l""^' A liquid will not flow from this cup 
 till the top of the bend of the tube is covered. It will th^n 
 contuiue to flow as long as the end of the tube is in the liquid. 
 The siphon J may be filled with a liquid that is not safe or 
 p^ asant to handle, by placing the end J in the liquid, stopping 
 the end A; and sucking the air out at the end I till the lower end 
 13 filled with the liquid. 
 
 Gases heavier than air may be siphoned like liquids. Vessel o 
 contains carbonic-acid gas. As the gas is siphoned into ll 
 vessel p, It extinguishes a candle-flame. Gases lighter than air 
 are siphoned by inverting both the vessels and the siphon. 
 
..^ 
 
 Apparatus fok raising liqujlDS. 
 
 77 
 
 QUESTIONS. 
 
 1- "What is the greatest hight to which the bend r (in A, Fig. 59) 
 can be carried, and aliow water to flow? 
 
 2. What would be the greatest hight if mercury were used? 
 3 Suppose the bend r is IS" above the liquid; what theoretically 
 ought to happen when tlie end h is unstopped? 
 
 4- What would happen if the long arm were tut ofl" at e ? 
 What would happen if it were cut off between e and a? 
 What would happen if the siphon were lifted out of the liquid? 
 W' at would be the effect of lengthening the long arm? 
 Must the two arms of a siphon be of unequal length? 
 How far can a liquid be carried by a siphon? 
 Will a siphon work in a vacuum? 
 
 Imagine that some such condition of things as is represented by 
 the apparatus K (Fig. 59) exists in the earth, and that the siphon a 
 has a smaller bore than the siphon c; 'can you account for intermittent 
 springs which flow and cease to flow at nearly equal intervals of time? 
 12. If the siphon A were carried to the top of a mountain, would 
 the water run through it more rapidly or' less rapidly, or at the same 
 rate? Your reason for your answer. 
 
 6. 
 
 6. 
 
 7. 
 
 8. 
 
 0. 
 10, 
 11. 
 
 Fig. 60. 
 
 § 60. Apparatus for raising Liquids. — The siphon can 
 only be used for transferring liquids over hights to a lower 
 level. Liquids cannot be transferred to a 
 higher level by atmosoheric pressure alone. In 
 fact, atmospheric pressure is only a conven- 
 ience, and never does work. If the piston a 
 ot& syringe (Fig. 60) is raised, the air is rare- 
 fled below it, and the atmospheric pressure will 
 force water up into the syringe ; but to raise 
 the piston, against the atmospheric pressure 
 tending to force itdovenward, requires as much 
 muscular energy as Would be required to raise 
 the same quantity of water to the same hight as that to which 
 il is raised in the syringe. 
 
 The common lifling-pump is constructed like the barrel of an 
 air-pump. Figure 61 represents the piston in the act of 
 
 m 
 
T8 
 
 t 
 
 .■si 
 
 -. i 
 
 \ * 
 
 \i 
 
 i: 
 
 II 
 
 DYNAMICS. 
 
 Wg. 61. 
 
 rising. As the air is rarefied below it, water rises hv ^fmr.. 
 Phene pressure, and opens .. lowe. valVl Ve TeigttT^^^^^ 
 water above the piston closes the upper valve, and 
 the water .s discharged from the spout. When the 
 piston IS pressed down, the lower v.ive closes, 
 
 botto'Tf 2u "'T' '"' '''' "^^^ ^^-«^- the 
 bottom of the barrel and the piston passes through 
 
 the upper valve above the piston. How hifh 
 can the bottom of the barrel be above tlie surface 
 
 How high If it is mercury? 
 - ^g- 62- '^h® liquid is sometimes said to be 
 
 laised in a lifting-pump by the " force 
 of suction." Is there such a force ? 
 
 Experiment. Bend a glass tube iuto 
 a U shape, with unequal arras, as in Figure G2 Ffli rhn 
 tube with a liquid to the level c6 Clo^.h II 
 a finger, and try to suck the ' '''' '°^ ' ^^'^ 
 
 liquid out of the tube. You . g. 
 
 fhP fl. * ^'"^ '^ impossible. Remove 
 
 t^ finger from b, and you can suck the 
 
 liquid out with ease. Wliy? 
 
 The piston of a fone-pump (Fig. 
 63) has no valve, but a branch pipe 
 leads from the lower part of the bar- 
 rel to an air-condensing chamber a, 
 ftt the bottom of which is a valve c, 
 opening upward. As the piston is 
 raised, water is forced up through the 
 valve c?, while water in a is prevented ,■ 
 
 ^roed into tUe cha.t:: e!';.. :^:^;C'j'^\ " 
 
 The eia,tlcity „, t„o condensed ai,- fo^s ^ ^Z ' ! "t/' 
 hose 6 m a continuous stream. * "' ""' 
 
BUOYANT FORCE OF FLUIDS. 
 
 79 
 
 atmos- 
 b of the 
 
 a 
 
 ill the 
 b wftU 
 
 V. BUOYANT FORCE OF FLUIDS 
 
 less tm it is .o^^C^C^^TXxTr'^'"' .^"'"^"^ '^^«"- 
 and note the change il ZxZtti .^Z"" T"" '^ ^"* "' *^^ ^^'^^^^' 
 the stone from a sL„° baS. '*.^"'^'^^«« f^^o™ the water. Suspend 
 
 ascertain itsToss o^S^^^^^^^ '' '\^'^ ^"^^ *^- '" --'-, and 
 
 pieces of iron, wood Tud other ,1 . ' ''f '^' ^'^^ experiment with 
 it beneath a s^rTace of water ^TT^^' ^"^'^'^ * ^^^^'^^'•' «°d ^ «rce 
 and it wi:I ris^t tVe toTof tlae ;^^^^^^^^ "^'^^ '^"«- -"'^ -^^--. 
 
 unLnLThTheTttr"*' l! "'"^ ^^ '' ^^"^^^-^ ^ ^^e fluid, 
 
 lose an, portion of their weight ZZX:^^ ^^ idf •^' 
 
 bef^atr^^iU': wtiVa'lt IT^^ ^ ^ ^^^^P- ^^ a balance- 
 ascertain the^pparent'loss ofwdA;" '' '''' ''^'^ '" water, and 
 
 (Fig. 64), and weigh the beaker of water 
 
 with the stone immersed. How much more 
 
 do the beaker and its contents now weigh 
 
 than before? Compare this gain with the 
 
 apparent loss of weight of the stone in 
 
 water. Is any weight really lost? Repeat 
 
 the expadment with a block of wood 
 
 Experin.ent 3. Make a saturated solu- 
 tion of salt in water. Weigh the same 
 stone ,n air, fresh water, and salt water 
 Compare its apparent loss of weight in 
 salt water with that in fresh wnt^r n 
 
 ence? Throw a piece of tor In n ^''''' ''''''''"' ^"'' t''^' <li«Vr 
 
 like cork on watef Fill a ve sel witrr^ •" "-^'^ "'*'"' "^^-"•>' 
 bubble, and drop it into the ve 1 It w 11^^ t T '' ''"^ ' ^"'^"• 
 rolls over the side and taultTf] « ^' ""'' '" *''" ^'««''^el, but 
 
 have greater ly^^^tVrfX^^^^^ .'* '''""■'''' '''' ^^^ «>-'« 
 
 in Palestine, is so salt thTA ^ ' ^'''' "'^*''" «^ "'« ^^^ad Sea, 
 the reason? ""' ' '''''''' ^°«« "°' ^'"k in it. Can you se^ 
 
 Fig. 64. 
 
 
 fti'il 
 
 ! II 
 
80 
 
 DY-SAmCfi. 
 
 V\g.65. 
 
 65) to .e . eubieal Moo. of^3f i^L^^^^^, ^^ ^^t 
 obvious that the downward pressure upon 
 the surface (Za is equal to the weight of the 
 column of liquid edao. The upward pres- 
 sure on the surface cb is equal to the 
 weight of a column of liquid echo. The dif- 
 ference between the upward pressure against 
 cb and the downward pressure on da, is the 
 weight of a column of liquid ecbo less the 
 weight of a column of liquid edao, which is 
 a column of liquid dcba (ecbo -edao = dcba) 
 But a column of liquid dcba has precisely 
 
 ltr!:Zv' "" ^^"' ^"""^'•«^^^- '^^--^-^' ^ -''-'^ '-'^ buoyed 
 up by afluv^ rnnsequence of the unequal pressures upon its ton 
 
 that fluid equal in volume 
 to the solid immersed. The 
 last proposition is gener- 
 ally stated as follows : A 
 solid loses in iveight as 
 much as the tveight of the 
 fluid it disj)laces. 
 
 Flpr. 66. 
 
 Experiment 4. The last 
 statement may be verified 
 with apFiaratus like that 
 shown in FiKiireCG. Fill the 
 vessel A till the liquid over- 
 flows at E. After the over- 
 flow ceases, place a vessel c 
 
 under the nozzle. Snspend m 
 
 a stone from the balance-b.-an, R ...h r,^^.^. -, ■ ■ 
 
 carefully lower it into the lion i '. " '^ '" "''"' ""^ tl'^n 
 
 into the vessel c. Tl.e ve e c h! "" T" "' "'' ''''"'*• ^"' ^^^ 
 
 vessel c having been weighed when empty, 
 
'g- 
 
 BUOYANT FORCE OF FLUIDS. 
 
 81 
 
 weigh it again with its liquid contents, and it will be found that its 
 increase in weight is just equal to the loss of weight of the stone. 
 
 Experiment 5. Next suspend a block of wood that will float in the 
 liquid, and weigh it in air. Then float it upon the liquid, and weigh 
 the liquid displaced as before, and it will be found that the weight of 
 the hquid displaced is just equal to the weight of the block in air 
 
 Fiir. 67. 
 
 Hence, a floating mass displaces ?is own weight of liquid ; in 
 other words, a floating mass will 
 sink till it displaces an equal 
 weight of the liquid, or till it 
 •reaches a depth where the buoyant 
 force is equal to its own weight. 
 
 Experiment 6. Next, partially 
 fill with water a glass (Fig. 67) 
 graduated in cubic centimeters and 
 fractions of the same. Note the level 
 of the water. Drop one of the solids into the water, and note again 
 the level of the water. The difi-erence between the two levels is the 
 number of cubic centimeters of water that the solid displaces. But 
 one cubic centimeter of water weighs one gram. How does the num- 
 ber of cubic centimeters of water displaced compare with the number 
 of grams the solid apparently loses in weight? 
 
 vil 
 
 There is au adage that " a pound of feathers weighs more 
 than a pound of lead." Is there truth in the statement? 
 
 Experiment 7. 
 
 Fig. 68. 
 
 Instead of feathers, we will employ a hollow globe 
 a (Fig. 08) ; in place of the " pound of lead," we 
 will use a counterpoise 6, of any metal whose 
 weight is just equal to the weight of tlie globe. 
 Then when the globe and counterpoise are sus- 
 pended from the opposite arms of the balance-beam 
 c, the beam will be horizontal. Now place the whole 
 on the plate of an air-pump, cover with a receiver, 
 and exliaust the air. What happens when the air is 
 partially exhausted? How do you explain the 
 phenomenon? 
 
 
 • '-i'i 
 
 il 
 
82 
 
 DYNAMICS. 
 
 A pound of feathers displaces more air than a pound of lead, 
 
 Dold'ofr^°''L-^".Tf "P"^°''^^'^^ ^^^' consequently the 
 pound of lead which balances a pound of feathers in the air, 
 
 ZZT^fT" '^'"^ ^° " ""^""'"- ^« ^^^r» ^'^^ the experil 
 menttl,at^od^es^eigh less in air than in a vacuum, and that 
 
 rjiL '"" ""^'^ '-^^ ''''^' ^^^^^ "^- --^^^^ - 
 
 i!^ 
 
 the sir ace of tt ^h'* ?t ^^^^^^^^ <>f '^^ atmosphere is greatest at 
 whilP Td? , '*'^*'- ^ •'^^y ^'•"^ *« '"^^'^ cannot remain at rest 
 
 lo^n M .^ *''f "^'^ '^"" ''' °^^° ^«'Sht of a fluid; therefore a bal 
 
 ban aTrt th' ' 'T 1'' '"^1 "'*^ ^^'^^ ^^-* fourt;en tSighter" 
 than air at the sea-level. will rl^e till the balloon, plus the wei^^ht of the 
 
 car and cargo, equals the weight of the air displaced. tL a o^^^^^^^^^ 
 
 Wishing to ascend still higher, throws out a portion of his cargo Th 
 
 Soo^n h?°'' '' f '"^ ^""^ ""^ ''^ ^'^ '« --P« at th t'op orthe" 
 balloon by means of a valve, which he controls by means of a cord 
 passing through the balloon to the car. ^ 
 
 QUESTIONS. 
 
 1. Why is it difficult to stand in water reaching the neck? 
 lift whrnoTo";^" '•'^^^^^ «*«- -^— '-. Which he cannot 
 
 Pll wl^nraungr "^'^'^ '""' "^^* "^'^^' ^' ^'^ ^^ ^^ dis- 
 place? ^'"'' ''''^^' ""' ""''""'^ '''" * P'"^^ '^^ ^'•°" weighing 600« dis- 
 
 VI. DENSITY AND SPECIFIC GRAVITY. 
 
 §62 Density. We speak of a piece of cork as beinc 
 heavier than a nail, at the same time that we speak of cork al 
 l.ght and iron as heavy. This seeming contradiction is ac- 
 counted for by the different meanings which we attanh to the 
 terms hgnt and heavy. In both cases, light and heavy are used 
 as terms of comparison. In the former instance, we compare 
 
iH 
 
 SPECIFIC GBAVITr. g3 
 
 the weights of the two particular bodies, without reference to 
 vohime ; m the latter, we call cork light and iron heavif, having 
 no particular bodies in view, but because we know by experience 
 tha cork is not so dense as iron, i.e., a given volume of cork 
 contains less matter than an equal volume of iron. The term 
 we^ht refers simply to the number of grams, kilograms, etc., 
 that a particular body weighs, without reference to the materia! 
 or the volume. The density of a body can be stated only by 
 expressing (or understanding) two quantities, viz., mass and 
 IJTn' Z^" '"T;"^'^'' '"PP""" ^^^' ^ '^^^^'^ «f ^«°^^ "^e^^ures 
 
 then ""« « '°1 s'o'o' "^"""if ^'''•' ""^^^'^ ^^^' ' '^' ^^^^^^y is 
 "2^10 X 2u — jD?y = 0.75 gram per cubic centimeter. When 
 we speak of cork as lighter than iron, it is evident that we are 
 comparing the densities of these two substances. 
 
 § 63 Specific gravity. TJ^e specijlc gravity of a sitbstance 
 IS theratio of the density of that substance to the density ofanother 
 substance assumed as a standard; in other words, it is "the ratio 
 of the weight of a certain volume of that substance to the 
 weight of the same volume of another substance which is taken 
 as a standard. 
 
 To facilitate comparison of densities, uniform standards are 
 adopted. Distilled water at its maximum densitv, at 4° C is 
 the standard of specific gravity for all solids and'liquids In- 
 asmuch as one cubic centimeter of water weighs one gram 
 when the weight of one cubic centimeter of any substance is given 
 m grams, i.e., when its density is given in its usual metric 
 units, the same number also expresses its specific gravity. 
 Ihus, one cubic centimeter of water weighs one gram, and 1 
 IS the specific gravity of water. The density of silver is 10 53« 
 per cubic centimeter ; hence the specific gravity of silver is 
 10.53. The standard for gases is air' at the average sea- 
 level density, and at a temperature of 0° C. The weigJit of one 
 cubic centimeter of air, under these conditions, is O.^Oo"l2932« 
 or about ^1^ of the weight of one cubic centimeter of water. 
 Let G = the specific gravity of a substance ; D — its densitv 
 
 » qomu physiclste adopt l.ydrogon jfas «« a standard for gases. 
 
 !i<i;i 
 
 'i' 
 
84 
 
 DYNA3HICS. 
 
 in grams per cubic centimeter ; V = the volume of a given body 
 of it in cubic centimeters ; W = the weight of the given body 
 in grams ; W = the weight in grams of an equal volume of the 
 standard. Then, as shown above, D = ^, and, by definition, 
 tr = — , • G is numerically equal to D, and W to V. 
 
 Since the loss of weight of a solid immersed in a liquid is 
 just the weight of an equal volume of that liquid, it is evident 
 that, if we divide the weight of a solid in air hy its loss in weight 
 when immersed in water, the quotient will be its specific gravity. 
 
 Experiment 1. Obtain small lumps of glass, iron, lead, marble, 
 granite, etc., and weigh each in air. Partly fill with water a measur- 
 ing-beaker graduated in cubic centimeters, and note the level of the 
 water. Drop a lump into the water, and note the level again. The 
 rise of water, as indicated by the graduated scale, gives the volume 
 (V) of the specimen. With these data find the density (D) , employing 
 the formula D = — . Next weigh each of these lumps submerged in 
 water, and find its loss in weight; and, from the data obtained, ascer- 
 tain G from the formula G = ^^. Prepare blanks, and tabulate your 
 results thus : — "' 
 
 Name of 
 Substance. 
 
 W 
 
 e 
 
 ii'lint glass. 
 
 435 
 
 V 
 
 ccm 
 
 D 
 
 orG 
 
 134 
 
 3.24 
 
 09— 
 
 W 
 
 water, 
 
 e 8 
 
 W 
 
 435 
 
 305 
 
 130 
 
 G 
 orD 
 
 3.34 
 
 .01+ 
 
 Av. 
 
 3.29 
 
 .04- 
 
SPECIFIC GRAVITY. §5 
 
 When the result obtained differs from that given in the table of spe- 
 c flc gravities (see Appendix, page 402), the difference is recorded in 
 
 "'inn TVk '"■""■,' ^'' • '^*^'" '^' '^"^^^ '' 8'"«^*«r 'b*° the latter, it 
 is Indicated by a plus sign affixed to the number; when less, by the 
 mmus sign. The results recorded In the column of errors are not nee 
 essar ly real errors ; they may indicate the degree of impurity, or some 
 peculiar physical condition, of tlie specimen tested. 
 
 Experiment 2. Obtain good specimens of cork, oak, elm, and poplar 
 woods, all of which float on water. Tie to a specimen a piece of lead 
 heavy enough to sink it; immerse the two, thus attached, in a measur- 
 ng-giass, and find the number of cubic centimeters of water displaced by 
 them. In the same way find the amount displaced by the lead alone. 
 Subtract the amount displaced by the lead from the amount displaced by 
 tlie two, and the remainder will be the amount displaced by the specimen. 
 Then, regarding the number of centimeters of water displaced as so 
 many grams, apply the formula G = ~ . 
 
 W 
 Example. Find the specific gravity of a piece of elm wood. At- 
 tach to It a piece of lead weighing (say) 408. 
 
 The combined solids displace 28.6« of water 
 
 The lead displaces 3 gg „ 
 
 The elm displaces ^^ „ 
 
 The elm weighs in air [ 20.08 " 
 
 The specific gravity of elm wood is . . ! 20.0-^ 25=. 8. 
 
 Experiment 3. Find the specific gravity of alcohol, a saturated so- 
 lution of common salt, sea-water, naphtha, olive-oil, pure milk, and 
 mercury in the follow :, manner: ascertain the loss in weight of a 
 sinker in each^one of these liquids, also in water, and then apply the 
 formula G = — . Here W «nd W represent the loss of weight of the 
 sinker in the liquid and water respectively. 
 
 Example. Compute the specific gravity of alcohol from the foUow- 
 mgdata: — 
 
 A piece of marble weighs in air .... 66.808 
 
 The same weighs in water 36.808 
 
 Loss in water 2o!oob 
 
 _, 56.808 
 
 The marble weighs in alcohol 40.968 
 
 Loss in alcohol j^ 
 
 il! 
 
 ^1 
 
86 
 
 DYNAMICS. 
 
 Since 20K and 15.84* are the weights respectively of equal volumes of 
 water and alcohol, and since G=^,,then -^^=Jd2. the specific 
 gravity of alcohol. 
 
 FIk. (;<!. 
 
 § 64. Hydrometers. — Experiment. Take a unif f)rin rod of 
 light wood about a foot long, and mark off on it a scale of eciual i)arts. 
 A convenient size is. » inch square, and a suitable scale is 
 Inches and half inches. Coat the rod with parattine to pre- 
 rent its absorbing water and swelling. Bore into the end 
 marked zero a hole about 2 inches deep, and drive in bullets 
 till the rod will sink in water (Fig. 69) just to some inch- 
 mark, and stop the end with parafflne. If it sinks too deep, 
 cut off the upper end of the rod. 
 
 Suppose the rod sinks 8 inches in water; then, if it is | inch 
 square, it displaces 2 cu. in. of water. The weight of the 
 water displaced must just equal the weight of the rod (see 
 §61J. Now immerse it in alcoliol; it sinks deeper, say to 
 the 10-inch mark; that is, Y cu. iu. of alcohol weigh the same 
 
 V 8 
 
 as f cu. in. of water; therefore, G = -^=— = .800. If in 
 brine it smks only C| in., G = — = 1.20. 
 
 10 
 
 "J 
 
 Apparatus like that described is called a hydrometer. In- 
 stead of a rod of wood, a glass tube is generally used, ter- 
 minating in a bulb containing shot or mercury. Tlie tube _ _^ 
 contains a scale with numbers corresponding, which express the specific 
 gravity, so that no computation is necessary. Make solutions of 
 various substances, and test their specific gravity with your hydrome- 
 ter, and test the accuracy of the results so obtained by other processes. 
 
 The most direct way of finding the specific gravity of liquids 
 and gases is by employing vessels that hold definitJ weights of 
 the two standards, water or air, and then weighing these vessels 
 when filled with other liquids or gases ; .ind, after deducting the 
 
 weight of the vessel, applying the formula, G = — • 
 
 W 
 The specific gravity of a solid that is dissolved by water may 
 be found by weighing it in a liquid that will not dissolve it 
 (e.g., rock-salt in naphtha); and, having found its specific 
 
lIYnUoMHTKUa. 
 
 ft7 
 
 gravity as compared witli the liquid used, multiply this result by 
 tile specific gravity of tlie liquid. 
 
 From the formula D = :| , we have V = | ; hence, the volume 
 
 of an irregnlar-shaped body may he found in cubic centimeters by 
 diouhng Us neight in grams by its density. 
 
 Again, from the formula D = ^, we have W=Vx D. Hence. 
 
 tvhen the volume and density of a body are knotvn, its weight in 
 grams may be found by multiplying its volume in cubic centime- 
 ters by its density. 
 
 ! 
 
 QUESTIONS AND PROBLEMS.' 
 
 1. How high can sulphuric acid be raised by a lifting-pump? 
 
 2. VVhat IS tlie weiglit of 508 of water in water? 
 
 8. Find the specific gravity of wax from the following data : weight 
 of a given mass of wax in air is 80^; wax and sinker displace 102 88ccm 
 ot water; sniker alone displaces 14'^='". 
 
 4 Why does a light liquid Ce.g., oil), introduced under a heavier 
 liquid (e.g., water), rise? • "«*^'*-'^ 
 
 6. Glass is about three times heavie- . water- how then can « 
 glass tumbler float in water? ' ' *° " 
 
 6. How can iron vessels float in water? 
 
 7. A block of ice containing oOOecm is floathig on water; how many 
 cubic centimeters are out of water? "vv luauy 
 
 8. Will ice float or sink in alcohol? 
 
 9. How much more matter is there in 500<=em of sea-water than in 
 the same volume of fresh water? 
 
 10. In 50k of gold how many cubic centimeters? 
 
 11. What is the density of gold? 
 
 12. What is the density of cork? 
 
 ture'of To'cV' '''" '"'"'"'^ "' "'' ^* "^''""'^ P''^^^^'-^' *"^ ^' * t«™P«ra- 
 
 14. An Irregular piece of marble loses 53« when weighed in water 
 How many cubic centimeters does it contain ? 
 16. When will a body sink, and when float? 
 
 iAj «.,!^fi „,, muuu as !•"" of water? 
 
 > Consult the Tables of Specific Gravities. In the Appendix. Section O. 
 
 ii:u 
 
 M 
 
 ill 
 
88 
 
 DYNAMICS. 
 
 17. 
 
 is. 
 
 19. 
 SO. 
 21. 
 
 How much will Ik of copper weigh in water? 
 
 What does a piece of load 20 x 10 x 5cm weigh? 
 
 What will it weigh in water? 
 
 What will it weigh in mercury? 
 
 What becomes of the woiglit that is lost? 
 ..1 " i^lu^ f " ^"^ dissolved in Ii of water, without increasiuff the 
 ^^olume of the liquid, what wi,l be the specific gravity of Ihe^som! 
 
 vacu'umi """'' '' ''"" """^^^ '" '° ^''- ^^'^^ ^"' '^ ^^'^S^ in a 
 
 24. A mass whoseVvelght in air is 308, weighs in water 26« anrt <n 
 
 another liquid m What is the specific 'gravit? of «r"othlr 
 
 26. A silver spoon, weighing 1508, is supported by a string in water 
 
 s:^^:rb;tt ::;s; '^ --^-^ ^^- «--- -L;rtt 
 
 S" tr Ta^'fT'"" ''^"" ^' '''''''■ "''^ "'"^'^ ^-«« it weigh? 
 It displace? '"""" ^''''^ *° *'^ ""'"'^ '''^ ^^'^ watef would 
 
 28. If the boat is capable of displacing lOO'bm of water whnf w^i^hf 
 must be placed in it to sini< it? *^ ° " oi water, what weight 
 
 29. An empty /:;lass globe weighs lOOR; full of air it weighs 102 4k- 
 
 S'ktrgiv""' '' ''''''' '''■''''■ ""'^^ *« ^^« speciflri^v:; 
 
 ^^ ?0. What weight of alcohol can be put iuto a vessel whoso capacity 
 
 31. You wish to measure out 50? of sulphuric acid. To what num 
 hereon a beaker graduated in cubic centimeters wUl ttt coJr" 
 
 Ing'beater ''°'' ^°" """"'^ ""'''"'"' """^ ^^' "^ "*''''^ "^'"^ '=" ^ '"^««"'- 
 
 weight VtretptS'" "°*^'"^ ''''^ ^^ ""'^""•^- ^~ »« *»- 
 
 vn.?" wi^'"!'''^^ '' ''''"^''^ ^^"^ ''"^''^ *h« ^"''f'ice of water in a reser- 
 sulZlT '""*'"--^°^'^« P-«^-- ->''-ter must it be capable of 
 
 85. A cubical vessel, each of whose sides contains 2500nem ig fl,|„j 
 
 6 T: „7r f """" '^^^^ ''' ^'""'^"^ «-*'""^ 0"->^ it« nicies' 
 »>,,!;, ? ^^ ^ ''* * '''^''**'" '"'■P"' '" '^ "yuid when the vessel 
 which contains it is in the air; If the vessel is placed in . v«.nn J 1^ 
 tne solia siuk, riae, or remain stationary? """ ' " 
 
MOTION. 
 
 89 
 
 ;!i 
 
 VII. MOTION. 
 
 ;n *e o«. ever, j„,:;;ir t:? ::~: r:;^^^^ 
 
 Ins attent on to obiects hv tha «,„ -^ ' ' ^^*^ '"'" f^»ect 
 
 "-t »„ i„ t,.e ea7a« i'' S' t'aU ' "" °T "' '"'«°-- 
 i-eferecee to certain object^ a Ji„ T" ""^ ""^ "' «'^' "'«' 
 
 object eonsidered apart ' ^arotC "1^ """"""""^ "^ "■' 
 object except with refcre,. ^Z[ .! . "•■""""" '<"="«<= «n 
 ceive o, eJnge or' ^Z:^^^^^':,' ""v"" ^" «"'• 
 i» relation to some other object £ "'''"•'' "«"?' 
 
 rate of sixty miles an ho„t knows^M tlLTT ' "°""^ ■•" '"« 
 till he looks away from hi, baZn ] " """'"« "' "". 
 
 passing in panotla 10"^ ""^ ""'^ "■"• '"""^ 
 
 §66. All matter is in motion 77,. . 
 absolute rest in the universe. There i«Tn! " T "^'^ ''"'^^^ «^ 
 cept t. indicate, with -fereLe ^l":,':; ^^ t^^^r ' - 
 objects that are movin- in tho «„ , ' ^''^ ^'O^dition of 
 same velocity. For e ":«! . j'"""'""" «"^ ^'"' ^he 
 
 riage, at the L of te" mTel au hoT """ '""'"^ ^ -- 
 to each other and the ^ ria " T,' T "''' ^^"^ ^^^^''^^^^ 
 only be .sed in an ex r mefv ii J , ' "''^" " ^' ^^«' " «-« 
 language refers only to t e ec^^^j ^^^^ ^^"r' '"' '' ^^'"'"^^ 
 to that on ^vhich it stand as . J T ^^■"'* "'"•''' ••^^f^'-«"^« 
 of the earth. It isZtCLT '' "' "^ ^'''''' ^'^ -"'f'-- 
 
 'notions of the eanhtULCiror^' 7' ^^ '^'"^' ^'^^ 
 as being at rest. ' ^ ""^ ""^^ terrestrial object 
 
 chf^;i:L:x;:::r^^^ - « who,e, or visile „.. 
 
 then.ss,-an^n;ir:je:r:„r\rr-^ 
 -not «ee the movements of the mole^Ie^ ^^"irbut Zl 
 
 m 
 
 m 
 
 «h: 
 
 III 
 
 m 
 
 tl 
 
90 
 
 DYNAMICS. 
 
 know that they exist by their great power, manifested in moving 
 machinery. 
 
 § 67. Velocity. — Uniform and varied motion. — All 
 motion taltes time ; hence the term velocity, which refers to the 
 space traversed in a unit of time. Motion may be uniform or 
 varied : uniform, when an object traverses successively equal 
 si)aces in aJl equal intervals of time; varied, when unequal 
 spaces are traversed in any equal intervals of time. Varied 
 motion may be accelerated or retarded : accelerated, when the 
 paces traversed increase at each successive interval of time ; 
 retarded, when they diminish. The motion of a train of cars, in 
 Btarting from a station is at first accelerated, afterwards tolerably 
 uniform, and when the braked are applied, it becomes retarded. 
 Strictly speaking, all motions are varied ; there is no illustration of 
 absolutely uniform motion in Nature nor in art, though we may 
 conceive of its possibility and have very closely approximated to it. 
 
 The velocity of a body having accelerated or retarded motion can 
 l)e given only at some definite point l)y an estimate of the distance it 
 would traverse in a unit of time, were it to continue in uniform motion 
 at the speed it lias at that point. For instance, a railway train passes 
 us, and we estimate that its velocity is 30 miles an hour, although in a 
 few minutes its speed may be reduced to 10 miles an hour, and a little 
 later it may come to rest. When we assign a velocity of 30 miles an 
 liour, we have no thought of whether it will run 30 miles during the 
 next hour, or whether it will riui an hour; we mean that, should it 
 retain its present speed, it wiU be 30 miles away from us at the end of au 
 hour. 
 
 VIII. FIRST LAW OF MOTION. — INERTIA. 
 Now, what is it that sets in motion that which was previously 
 at rest? We may call it force; but what idea does this terra 
 convey? Let us question our own experience. We leave an 
 apple lying upon a table ; have we not entire confidence that it 
 will continue to lie there, unless disturbed by some otlier body? 
 If on returning we find it gone, are we not sure tliat it has been 
 removed by the action of some body other than itself? An 
 
riEST LAW OF MOTION. 91 
 
 apple falls to the ground, and although the action is one of the 
 most mysterious m all nature, yet do we not almost instincUvl 
 
 1 he ball at rest ,s put m motion by a bat ; but must not the b.t 
 
 Again, the bat, having received motion, is capable of immrt 
 ■ng motion ,„ the ball , but, having sot in motion one ba™ s 
 equally capable of putting in motion another ball ? C« " mis 
 impart motiou and retain all its motion? Is it not lit. „ 
 meroial transaction, a trade, to which thcr ai^ tw parties"™' 
 a buyer and the other a seller? that is, arc not all tCsaeU^ns 
 
 tine of a transfer, which should ho entered on the debitside of 
 one s account, and the credit side of the other's? Tj^l 
 (2) that «„(,o,. ,„ „„e bo,hj is cause) only by another boWs 
 imrundwill, so^ of,tspo,oer of producing LL " 
 
 it a sled, on which a child is sitting, is siiddenlv ni.t i„ „,„ 
 .on the child is left in the place from wliie, "sled srt"d 
 If theoiUd and sled are both in motion, and the edtsid' 
 deny stop,«l, the child lands some distance ahead ft 
 sled IS started slow^, the child partakes of the niotbu o 
 sed, and is earned along with it; and if the sled „rad ,.,llv 
 stops, the child's motion is gradually checked, and i it "^ 
 place on the sled. This shows (3) that .^ass^s of mXrZZ 
 rmtmi graduuUy and surrender it gnuluall,,. 
 
 p„,!ti.,„ ton , t„h ? , i|"art2.,a,i,l is placccl l„ a„ olova.,.,l 
 melteawax. When ceo,, u.„Ues^„ IsV^'cllT^irrsl^J-X;' 
 
 
 I. )i 
 
 m 
 
92 
 
 DYNAMICS. 
 
 l"rf ZeS*' The nlT; T"^*'' ''"'' ^"^^^^^ «°'^ -^^^^e the lines 
 are traced. The plate is then placed beneath the orifice of the tnho 
 
 and exposed to a shower of sand. The velocity of the sand-grains s 
 not at Its maximum at the start, but is constantly accelerated tiUthl 
 reach the plate, where their velocity in turn is^radua't given up"^ 
 The wax, on account of its yielding nature, gradually bXftheVtn 
 Tuii at it " t"' -*-^«-*-^-g its harLss, cLnrip t^^ 
 quite at its surface; and, therefore, it suffers a chipping act on from 
 he sand. Thus the soft wax affords a protection from the acHono' 
 the alhng sand of all parts except those intended to be cut As tm 
 Ttul Fo:^?""'"^^r" *^ *^« ^^'^^ "^y «*-™ blown thtough 
 
 metals like 1!^' ''''"" ''' ^^P"'"'"^ '' ^*"«^ * «««'^-*'«*^- Hard 
 metals like steel are engraved in the same manner. Yet the hanrt 
 
 may be held in the blast several seconds without injury. (What is the 
 difference in the effects of catching a base-ball with hands held rgidlv 
 trballPX ' ''' ''''' '" '"'' ^'^"^"'^^ '^ the moS Of 
 
 m^^. 'I.y^^'.^'' I ^^^P^*' - '* «o«° «tops ; roll it on a smooth 
 maible floor _, rolls much farther. On a perfectly smooth sur- 
 face it might roll for hours. If we could provide such a surface 
 and dispense with the resistance of the air, how long would il 
 roll These conditions are impracticable? True. But have 
 not the heavenly bodies rolled for millions of years through fric- 
 tionless space, unchecked because unimpeded ? 
 strST- ""°^^;r^*^^ *« I'^'-Petual. Motion undisturbed is in a 
 strSt , "^'""^.^^"^^ ^»l ^ ^-^-ble roll more nearly in a 
 
 straight line, a smooth or a rough floor? What if the floor were 
 perfectly smooth? • 
 
 The relations between matter and force arc admirably and 
 oTZZr^'''''''^ '" '"^'''^ ''' ^"""^^ '"' ^''''''''' Three Laa,^ 
 rel ^ndat^^^ ""^ Motion.-^ body at rest remains at 
 
 That nnrtnf tho l""' ivh!/-v. *-• 
 
 t\.^ r„ .r. " " P^'Kiins to motion is brietlv 8nmmari7PH In 
 
 the r.«ar e.pr.,*., •• pe.pu„„, „„„„,,. .. ^ p^p^fj^^ po'^. 
 
re the lines 
 f the tube, 
 icl-grains is 
 ed till they 
 
 given up, 
 gs them to 
 stop them 
 ction from 
 } action of 
 It. A still 
 'n through 
 ast. Hard 
 
 the hand 
 /^hat is the 
 2ld rigidly 
 motion of 
 
 a smooth 
 ooth 811 r- 
 . surface, 
 would it 
 Jut have 
 ugh frie- 
 
 hI is in a 
 irly in a 
 3or were 
 
 biy and 
 
 ee Laws 
 
 \ains at 
 
 ty in a 
 
 change 
 
 irized in 
 ion pos- 
 
 SECOND LAW Ot MOTIOK 
 
 QQ 
 
 pos^bie „««^, ,;,„;*;^ .^znrir'r^--^-' »<'■- «•- 
 
 to do who mm establish oem.,,..! ".'"'""•■>■ What has a ncrson 
 
 "'« it is in with rXnce to 1, "' *" '^'»"'» '" ""= ««e 
 called ».«„. Eviden iXC'::. hT '"" '"'^ """""'^ '^ 
 
 «o'- ia o,to. app.,.^toCu:drL ^"LS ^"^ - 
 
 ^t^ek by a bat he wolt^Kr V" "■" """o" "' - "all 
 . ( ; m wlut direction It moved, and (3) Ac,™ 
 
 at I »d D%r'"o'; rr^'i' 
 
 hu hof J ** ^ "® struck 
 tively to B and E in one second V. h""" *'^ """^ ''^P"^"" 
 their starting-points ; tlXs AB?nd nP "^''^ ^ ""O » "■•'' 
 tion of their motions, and he lifts „f« ,"""*"' ""' "''■"c. 
 the distances traversU and th! ? , ' ''"'=" '"P^''""' both 
 
 "pplied. In readtagthe dirtct r "*"^'*' <>' «» 'oroes 
 order of the Iet,^:t A B t^d d E ' '" '""'"'"«' "^ "■« 
 
 -=4TtrBtrsrd^--p- 
 
 let a force of tb„ i,.t„„."" 7'?°"<' ' "'"'- "t the end of the seeo„d 
 
 tion Of the former 'Cattlir^trd""^'- " T *'•- 
 
 second, — It would move 
 
 -- *•> ""never, wn 
 
 forces should be applied at the 
 
 however, when the ball 
 
 I'- vi 
 
 m 
 I 
 
 .Ni 
 
 »sat A,io^/<ofthe8e 
 
 uame time, then at the end of 
 
 a 
 
94 
 
 t)YNAMICS. 
 
 %r 
 
 
 Its path will not be A B nor 
 
 Fig. 71. 
 
 second the ball will be found at C. 
 AD, but an intermediate one, 
 AC. Still, each force produces in 
 effect its own separate result, 
 for neither alone would carry it to 
 C, but both are required. Hence, 
 the 
 
 § 69. Second law of motion. 
 
 — A given force has the same effect 
 
 in producing motion, lohether the 
 
 body on which it acts is in motion or at rest; ivhether it is acted 
 
 upon by that force alone, or by others at the same time. 
 
 § 70. Composition of forces. — It is evident that a sino-le 
 force, applied in the direction AC (Fig. 71), might produce tlie 
 same result that is produced by the two forces AB and AD. 
 Such a force is called a resultant. A resultant is a single force, 
 that may be substituted for two or more forces, and produce the 
 same result that the combined forces produce. The several forces 
 that contribute to produce the resultant are called its components 
 When the components are given, and the resultant required, the 
 problem is called composition of forces. The resultant of tivo 
 forces acting at an angle to each other is always a diagonal of a 
 parallelogram, of ichich the components form two adjacent sides. 
 Thus, the lines AD and A B represent respectively the direction 
 and relative intensity of each component, and AC represents 
 tlie direction and intensity of the resultant. 
 
 Tlie numerical value of the resultant may be found by com- 
 paring the length of the line AC with the length of either AB 
 or AD, whose numerical values are known. Thus, AC is 2.23 
 times AD; hence, the numerical value of the resultant AC is 
 4x2.23 = 8.92. 
 
 When the components art at right angles to each other, as in 
 Figure 71, the resultant divides the parallelogram into two equal 
 right-angled triangles ; and the intensity of the resultant may be 
 
 c 
 ti 
 
 ce 
 on 
 inl 
 
be AB nor 
 
 it is acted 
 
 it a single 
 reduce the 
 
 and AD. 
 igle force, 
 rocluce the 
 sral forces 
 mponents. 
 [uired, the 
 nt of tioo 
 onal of a 
 zent sides. 
 
 direction 
 epresents 
 
 by coni- 
 ither A B 
 C is 2.23 
 nt A C is 
 
 ler, as in 
 wo equal 
 t may be 
 
 BESOLUTION OF FORCES. 95 
 
 i«und by calculating the hyi.otiienuse, havin-^ two s^des of .M^ 
 triangle given. Thus, V l^X^^- « n, '"« '^« ^.d^s of eitlier 
 the result-xut A C \\T J ." "^'^''^'"""'-''■'^'''^'^^^"eof 
 cicsultaut A C. A\hc-n the two components of a force F 
 
 lin« J . ^ ^""^ '" '' d^-ection at riglitan-Ies to its 
 
 hne of action, it is evident that the resolced part of -t for^^ 
 any irection has tiie total oifect of that forced tl^t ' i L ' 
 
 A B ' "7V"'^""' "^' '"' *"^ ^-^'^"^^-^^ «f the coinpo^n s 
 A B and A C, ,M each of the four dingnun. in Fig. 72 Also 
 
 Fig. 72. 
 
 if, 
 
 assign appropriate luunericul values to each cou.ponent and find 
 the corresponding numerical value of each resultant! ' 
 
 yVhen more than two components are aioen ii»r1 ih. 
 
 . ™.v t.o of,,.,., ,„ of,,,. m,,«„:/;:r:^:t ::r: : 
 
 tM evenj oo,npoue,U ,ms l,,en used. Thus, in Fig 7^/0°" 
 Fig- 73. tlie resultant of A B and A D, 
 
 and A F is tlie resultant of A C 
 and A E, i.e., of the three forces 
 A B, A D, and A E. (Invent 
 several problems similar to this, 
 in whicli three, four, or moie 
 forces are to he combined, and 
 work out the results.) 
 S71. Resolution of Forces 
 certain distance in a eertai,. .,i,.~tio,::TcT;;; rT)" ^^^ ! 
 oueoftho foree, a.at produce, ..us 'n,„tio ir.J, i;™"! il 
 .uten»,tj, and direction, by the lino A B; what mus b ', 
 
 lif ! 
 
 
 
96 
 
 DYNAMICS. 
 
 Ma' 
 
 intensity and direction of the other force ? Since A C represents 
 in intensity and direction the resultant of two forces acting at an 
 angle to each other (§ 70), 
 it is the diagonal of a paral- 
 lelogram of which A B is one 
 of the sides. From C, draw 
 C D parallel and equal to 
 . B A, and complete the par- 
 allelogram by connecting 
 
 the points B and C, and A and D. Then, according to the prin- 
 ciple of composition of forces, A D represents the intensity and 
 direction of the force which, combined with the force A B, would 
 move the ball from A to C. ' The component A B being given 
 no other single force than A D will satisfy the question 
 
 Had the question been. What forces can produce the motion 
 AC- an infinite number of answers might be given. In a like 
 manner, if the question were. What numbers added together 
 will produce 50? the answer might be 20-f-30, 40-f 10 20-1- 
 20-f.lO, and so on, ad infinitum; but if the question' were 
 What number added to 30 will produce 50? only one answer 
 could be given. 
 
 Experiment. Verify the preceding propositions in the following 
 manner: From pegs A and B ^ujio^ing 
 
 (Fig. 75), in the frame of a ^''^- '•'• 
 
 blaclcboard, suspend a Itnown 
 weight W, (say) 10 pounds, by 
 means of two strings con- 
 nected at C. In each of tliese 
 strings insert dynamometers' 
 X and y. Trace upon the blaclc- 
 board sliort lines along the 
 strings from the point C, to 
 iniUcate the direction of the 
 
 two component forces; also _^^^^^^^^ 
 
 trace the line CD, in continuation of the line WC t • ^- , 
 
given, 
 
 RESOLVED PARTS OF FORCES. 
 
 97 
 
 be found that 10 .- „un,ber of pomK, ''T" *' * ^'«^"'^«'- ^^ ^i" 
 - : C D : Ca ; also that 10 nlbe of "l T' r" "" ^y-mometer 
 
 I'ounds must act i„ the direction C D to n \" ^^' ^ ''"^'^^ ^«»'^« «f ^0 
 l>'-"<'»ced hy the two compone" s IW '"T' ''' ■'^'^™^ ^*^«"^* ^^at is 
 ^ra.n represent tke intJuyTn^ dU^UoZ/: '"""''" ''•^«^-«^'^"'- 
 '^'"^"""^ re^r..e«^. ^/.a resultant Y'vyZ { m '^"'"^'"'"'^ Z"^^". ^A. 
 strings fron. different points, as E and p A aSl!'! "^ '"'^'"'^"^' "''^ 
 
 § 716. Finding the Resolved Parts of Porno. r, 
 SHke of bievitv we will ..^l! . f Forces. -For the 
 
 the length of i C Ind "■"''^""'•^'"^ ^" '"^^"-'y by 
 
 FiG.A. 
 
 in direction by the 
 
 direction of A C, the 
 
 force A C. Let D B 
 
 ^e a rectangle (Fig. 
 
 A). From what has 
 
 been said (page 95), 
 
 it will appear that the 
 
 forces A B and A D 
 
 are the resolved D-ii-fQ r^t *u .. 
 
 « an. A D, re^^/ely^^^ 'Z\^ l^ ''' ''^^^ «^ A 
 
 -"gle C A B is given, A B and A D t ^'^'"' ""^ '^^ 
 
 ever, as we wish to d^al onlv I'tt '"'^ ^' ^^""^- ««^- 
 
 nmtics, we shall consider onlv thll ^^'^ .^^e^^entary mathe- 
 
 C A B is 30% 4.5°, or 60°. ^ '^''' '" ^^'^^ the angle 
 
 If the angle CAB is "in" fi c • 
 
 A C, and. flw.i.-.f....„ * T. \/> 2 
 
 A C, and, thei-efore A 1} = ^l 
 angle C A B is 45°, A B 
 
 AC (Euclid I., 47). If the 
 
 therefore A B 
 
 '8 60°, the angle A C B 
 
 -J A B = BC (Euclid I., 32 and 6), 
 
 2-AC(EuclidI.,47). If 
 
 and 
 
 the angle CAB 
 
 IS 
 
 md therefore - B is ^ A C. 
 
 
 1 
 I 
 
 
 m 
 
 im 
 
 fi 
 
98 
 
 DYNAMICS. 
 
 From the above, of the truth of which the student shoukl 
 thoroughly satisfy himself, it will be seen that, the resolved pa^t 
 of a force Fin a direction makiny an an>jh of 6'(f ivith its line 
 of action is F x-^; if the angle is 45° the resolved part is 
 
 ^ X - "2 ' -^ ^''^ ""^^^^ '^ <^^°' '''« resolved part isF x i; and if 
 the angle is .90°, the resolved part is zero. These fo.ir fuels 
 should be carefully remembered as they will be found exceed- 
 ingly useful. 
 
 QUESTIONS. 
 l^n^■h ^'""i the vertical and horizontal resolved parts of a force of 
 100 lbs whose Ime of action makes an angle of .JQO with the vertical 
 
 2 A picture weighs 20 lbs., and the two parts of the suspM.sion- 
 cord make with each otiier at the nail an angle of GO"- whitisX 
 tension of the chord? ' *''* '** *''*' 
 
 3. If a body is in contact with a smooth surface, why is the 
 pressure between the body and surface nt ri.kt an, l. to thesnrfa ? 
 
 4 Why ,s the pressure between a fluid and a surface in contac 
 with It at right angles to the surface? 
 
 5. Why can a sailing yacht make progress in a direction at ri-hf 
 angles to the direction in which the wind is blowing? m v^hrpos mon 
 must its sail be set? position 
 
 6. What is tiie force which raises a kite? 
 
 FiG.B. 
 
 if! 
 
 resolved 
 
 Composition of Forces 
 by First Finding Resolved 
 Parts. - Let two foices of 
 H lbs. and \0 lbs, act along 
 the lines A B and A C (Fig° 
 B), and let the angle BAG 
 = 60°. Find the resultant 
 of these two forces. 
 
 For the force of 10 lbs., 
 
 substitute its resolved parts 
 
 along A B and A H. 
 
 angle C_A H =30°, these 
 
 ind 5 VS lbs. along A II. 
 
COMPOSITION OF FORCES. 
 
 ent should 
 ■.solved part 
 t'th Us line 
 
 Jed part is 
 
 : h ; ami if 
 
 four facts 
 id exceod- 
 
 a force of 
 J vortical. 
 siKsp.'ii.sion- 
 vvliiit is tlie 
 
 liy is the 
 
 lie surface? 
 
 in contact 
 
 on at right 
 at position 
 
 if Forces 
 Jesolved 
 
 forces of 
 act along 
 V C (Fig. 
 :Ie BAG 
 resultant 
 
 ■ 10 lbs., 
 
 'ed parts 
 
 [. 
 
 0°, these 
 
 ig A II. 
 
 Draw A H, 
 making the angle 
 Ji A II = 30°, 
 and draw N A K 
 at right angles to 
 AH. 
 
 Now it is easi- 
 ly seen that — 
 tlie anglos 
 
 B A N = (J0° ; 
 
 C A N = 4r>° ; 
 
 C A K = 4r>°. 
 
 For the force 
 of 6 lbs., acting 
 along A B, sub° 
 s.it,,.e^i. „so,voc, part«, 3 ft,., „,„„g A N a.d 3^8 lbs. 
 
 Now, 
 
 since 
 
 these forces = 
 
 Let three forces 
 
 tant of 
 
 along A B, 
 
 m 
 
100 
 
 DYNAMICS. 
 
 A C, and A D (Fig. D). Let the angle BAG- 60°, and let 
 
 Draw H A K at right angles to A C. 
 Now it is easily seen that — 
 
 The angle R A 11 = 30° ; 
 " " D A K = 30°. 
 
 Fig. D. 
 
 For the force of 8 lbs. acting along A B, substitute its re- 
 solved parts, 4 lbs., along A C, and 4^/3 lb.. . .long A H. 
 
 For the force of 6 lbs. acting along ., . .ub«titutP ifo 
 resolved parts, 3 Ibs.,along A C%nd 3 V 3 lbs alol I K 
 
 ~ Ja i^ilon^ A u: ^^^"^ ^= ^' -^ ( W3 - 3 V 3)lbs. 
 
 . -W- .-^co A C and A H are at right angles, the resultant of 
 o^ ':. >p* forces =-■ V ( 12 )^+ ( v'S ;'^ ibs. 
 »»VT47 1b8. 
 
COMPOSITION OF FORCES. 
 
 101 
 
 syli::iT\Zo:r'' "^^^'^ ^^"^^ ^'^ -^^^-^ ^^ oth. 
 
 !• Two forces each of 
 flml the resultant. 
 
 QUESTIONS. 
 
 50 lbs. act at a point at an angle of 60°; 
 L ^r?' "^ ^^ ""^' 2« ^^'- respectively act at a 
 
 point at an angle 
 
 "fGO°; And the resultiuit. 
 
 ataiiaugleof fr,^; anii 
 
 4. T»o forces each of e lbs. act at a point 
 the resultant. 
 
 6- forces ot 10 ami 12^3 Iba rcsDeellvelv .„, .. 
 angle of icoo; And the resnltant "*""'""''' '« " « !«"»• « an 
 
 at Tn an'lJf of'r""' "' '"" ""•'=" ""'""' '" '"--""Sat. -Int 
 
 flnd'-thr^esfS. """ """'^ '"■'■ '"' " » "»'"' " - -«•= of "' 
 lorce oi 5 V 218 ; find the angle between the fcs. 
 
 lb"; «l7;;ri;;f °"^^"'"* ^"'^"^•^ «^ ^^^-^ "-ir resuuunt ,« lo 
 
 , .^^' ^ ^^ "i^ i" '^ ^'•°'"''"« having thr angle at A ^ coo- A D C R 
 intersect in E; fcs. act along A B, A D A E and R v ^ a ' "^ "' ^ ^' 
 
 .lojea.othe.,„nan,.g„,t„ae„nhe,;'re!'„irt,^'^^^^^^^ 
 
 >1 
 i 
 
 M 
 
 m 
 
 
 
102 
 
 DYNAMICS. 
 
 
 ^*' The resultant of two eoml fn= „ *• 
 1^«-; find the components ''*'"^ ^* *" ^°S'« o^ 135° is 20 
 
 15. The resultant of two fc^ in fi, 
 -«Ieof l50Oi.sO4„,,, find the Ws '''" '''••'' ^^'^'"^^ ^^ - 
 
 10. The resultant of two fcs artincr .* 
 one of the components is 9 Ibs.rflud I'otLr" "'''^ ''''° ■'^ '' "^•^- ' 
 
 § 72. Composition of parallel forces Tf tr. . • 
 and CB (Fig. 75) are brought near to oTot Tk '*""^' ^^ 
 
 ponded from B and E, so Ihat t^e ^no^ Vo V" "'^"^ ^"«- 
 dimiuished, the component force nt^r ™'^ ^^ *^^'" ^« 
 mometers, will decrease, 1 11 the Vo T '"''' '^' "^^ ^^^"- 
 when the sum of the com 1 . ""'"'' ^^"^"^^ Parallel, 
 
 Hence, (l),..,,J,t;3-2/;^^^^ ;^r "" ^^^^'^^^ ^^' 
 ac^m^^a^e ../.en they are 7arau! a , /'' "'' '' ^^^e greatest 
 ^^^ich case tkeir reZtZ ^;:;^,::lZ ''' ""^ '''''''^-^ ^^ 
 
 oZ '^::t::::t^:::T': '-' -'-^'-^ ^-^ -h 
 
 necessa,, to supp^i:: ^g^tc^^ n.!;^ 'ITV'' '-"- 
 actly opposite each other, when the two ^"^' ^^'"^ '•^- 
 
 other, and none is exerted T., „ ^^J " "'"*''"''"^ ''^'^' 
 weight. If the two ^:i^:Z^''''''^ ^"^^^^^'-^ ^'- 
 weight (the weight bein? sZ^oted^^^^ 
 
 pulled with equal force ttSf? ^ ' "'"'^ ''^^"^)' '^^"^ 
 is pulled witl'a force of 15 ZTl \T """ ^"^ '' -« 
 -• 10 pounds, the weigh^m^^ tlhTdi e\^^^^^^^ ^ '^^ 
 force; and if ., tliirf dynamotnptm. i, !» , '° «''"'"«'■ 
 
 the .id. of tl,e weaker LT" ol^Tl "^ "° "'='«"'• <"• 
 of 5 pounds must bo anXd ,1 ""'' "" '"'<"'''""'' foree 
 
 ««o&r; ldtk/ZTo}laJllf "''" ™'"'^<« /™™ »'« 
 to tteiV di^cmice. '' ^"""^''■'iira.iJTO/iort/oMate 
 
COUPLE. 
 
 103 
 
 Fig. 76. 
 
 When i)aralk'l fon-es are not applied at the same point, the 
 qtiostiou arises, Wiiat will be the point of application of their 
 resultant? To the oi)positc extremities of a l)ar AB apply two 
 sets of weights, whicli 
 shall be to each other vm 
 3:1. The resultant is a 
 single force, applied at 
 some point between A 
 and B. To find this 
 point it is only necessary 
 to fnid a point where a single force, applied in an opposite 
 du-ection, will prevent motion resulting from the parallel forces • 
 m other words, to find a point where a support may be applied 
 so that the whole will be balanced. That point is found by trial 
 to be at the point C, which divides tlie bar into two parts so 
 that AC : CB : : I : 3. Hence, (3) ivhen two parallel forces art 
 upon a hotly in the same direction, the distances of their points of 
 application from the point of application of their resultant are 
 inversely as their intensities. 
 
 The dynamometer E indicates that a force equal to the sum 
 of the two sets of weights is necessary to balance the two forces 
 A force whose efTect is to balance the effects of several ccmpo- 
 ucnts IS called an equilibrant. The resultant of the two com- 
 ponents is a single force, equal 
 to their sum, applied at C in the 
 direction CD. 
 
 Fig. 77. 
 
 § 73. Couple. —If two equal, 
 parallel, and opposite forces ar*> 
 ai)plied to oi)posite extremities of 
 a stick AB (Fig. 77), no single 
 force can be applied so as to keep tlie stick from moving ; there 
 will be no motion of translation, but simply a .o.../on around 
 Its middle pomt C. Such a pair of forces, equal, parallel, ad 
 opposite, but not in ti.e same line, is called a couple. 
 
 % 
 
 (i 
 »' ' ■ I 
 
 III 
 
 i fil 
 
104 
 
 15TNAMICS. 
 
 1 A PROBLEMS, ETC 
 
 2- If the vv! 2^ ""^y ^""PP^" 20^ ? ^"'^ '^'^^"Id "'e weight 
 each s/ppttr'^"'* ^- P^-e. 4o™ ,ro. «,e .an how „ . 
 
 3- Suppose thaf«h ., ""*"' ^^^^ much would 
 
 Wide, ancHs ro ved wlfh '^''''"' ^'"--^"y across a H . 
 shore in half an hn ? '^ ''"'""'^y »hat w„„ici lalufl "" ^^^^ « '"'Je 
 the boat down th T' '' '''''' ''''' "« ^"rrtt '"' ' "P^" '^^ °PPo«ito 
 ".e boat land? ''' ^'"^"^ ^' *^« -te of one nJiie^ t^^'Z ^^"^^"^ 
 
 *-Howfarwiin.traveIP 
 "• How long will ith^i 
 
 •• A ship l/sail IL^ °''°'''°S ""> --iver? 
 ""r" ;',:*; rr;*^ ™'«""''"' "' "^ "'' "' "• ""- 0" "our, 
 
 Venfy the rcsulta With ,„„.„„„,^^^ 
 
 X. OTHER APPLICATIONS OP THP =. 
 Let I • '""'• - ^™^°« ™ OHA V^ '^'^ °- MO. 
 
 -«^«^ measured bv fhn ,„ • ,/ "'^ ^^'''ce is 
 
 "ally downward ;"e '7? "*'"« ™«i- 
 -<1"«U their sum ;,d, /"!,"'*""' "f "hi-'h 
 
 position the My XtZTJ^'^''' 
 
 ^/^Ae rm^^a?^if .,^ a// fhZ / '"^^^Me, the point of appH.^f.-^ 
 ^hole weight of the hodu mLZ """ "''^"^ Purposes the 
 
 its center of nravitu W ^ ''"^'■^"'^^ '« ^'« coL^L/; ! 
 
 '-U that point Where the 0.,^;^::^^^^^^^^^^^^ 
 
CENTEB OF GRAVITY. 
 
 
 105 
 
 ' Jong, sup- 
 tlje weight 
 
 luch would 
 
 »alf a mile 
 3 opposite 
 nt carries 
 ''■Jiere will 
 
 >erhour; 
 
 y of two 
 
 i weight 
 
 F MO- 
 
 nee, a 
 force 
 J'ce is 
 eciile. 
 denies 
 verti- 
 vhich 
 ction 
 las a 
 ;ever 
 •oint 
 •titer 
 tiuri 
 the 
 ' at 
 fa 
 
 Let G in the figure represent this point. For many practical 
 purposes then we may consider that gravity acts onl/upon Z 
 pom , and m the direction GF. If the stone falls freely th s 
 pomt cannot in obedience to the fir.t law of motion de'v^^ 
 
 LTall "J'na P'^^^ j— ' ^^^ the body may rotat'e durh^ 
 Its fa L Inasmuch, then, as the e.g. of a falling body always 
 descnbes a definite path, a line GF that represents this ".t .' 
 o. the path m which a body supported tends\o move, is cd^' 
 the hne of direction. The centre of gravity is often called 
 the centre of inerUa, and this latter name is very api^pri! ' 
 when we are speaking of this point with reference to the ma s 
 of the body instead of with reference to its weight. (§§ 7 19 
 
 onnl > '• '^.'°' *^'* '^ ^ ^^'"'^ ^'l"^^ *« '''' «^« weight and 
 opposite n. direction is applied to a body anywhere in L line 
 
 of du-ect^on (or its continuation), this force will be the equi- 
 
 hbrant of the forces of gravity ; in other words, the body sub- 
 
 jee ed to such a force is in equilibrium, and is said to be sup- 
 
 ported, and the eqailibrant is called its mpportiru, force To 
 
 support any body, then, it is only necessary to provL a support 
 
 for us center of gravity. The supporting force must be apS 
 
 somewhere m the line of direction, otherwise the body will fall 
 
 hemisphere of lead. The toy will not *''^' '"■ 
 
 lie in the position shown in the figure 
 
 on a horizontal surface ab, because 
 
 ^j— .., ..^ ,„,.... apphuu immeaiateiy * v 
 
 under its e.g. at G; but, when placed horizontally, it immedi- 
 ately assumes a vertical position. It appears to^^he o W 
 to rise ; but, regai-ded in a mechanical sense, it really falls, be- 
 
 lli 
 
 i 
 
 M 
 •''iii 
 
 Urn 
 
 
 i' 
 
 U 
 
106 
 
 DYNAMICS. 
 
 cause its c. g., where all the weight is sunnnse.l fn h 
 
 trated, takes a lower position ^"PPOsed to be concen- 
 
 bounded „v the string is the ba '7 lie tb k ^fvt t T 
 base of a u.an whe,. standing on one foot? ™ two fiotP) "' 
 
 F!^. 80. .ere will be an equilibrium of forces, and 
 
 the e.g. must be iu the same line with the 
 equilibraut of gravity ; hence, if a knitting- 
 needle is thrust vertically through the no- 
 tato from a, so as to represent a continua- 
 tion of the vertical line oa. the e.g. must 
 he somewhere In the path an made by the 
 needle. Suspend the potato from some 
 ^other pomt, as b, and a needle thrust verti- 
 cally through the potato from 6 will also 
 pass through the e.g. Since the e.g. lies 
 
 their point of Intersection. U win Velurtl^ \T' '' ^* '' 
 
 §75. Three states of equilibrium. - The weight of n. 
 body .s a force tending downward; hence, a 6oc^/w^ JJ 
 sume aposUion suck that its e.g. null be as low as pisWle, 
 
 5 ^Tllrf. '• '''y *.« «"PP°''t « ring on the end of a stiolc. a« at 
 c >miau. oupport It at o. Have you any difficulty hi sup- 
 
STABILITY OF BODIES. 
 
 107 
 
 be concen- 
 
 ^ther or not 
 f a body is 
 3 of a body 
 ng around 
 ular figure 
 ^hat is the 
 t?) 
 
 body. — 
 
 tack, as in 
 nes to rest 
 brces, and 
 e with tJio 
 !i kuittiug- 
 jli the po- 
 coutiuua- 
 c.g. must 
 ^de by the 
 'om some 
 rust verti- 
 will also 
 3 e.g. lies 
 it be at c, 
 ver point 
 mder the 
 is found, 
 le of the 
 
 it of a 
 
 s to as- 
 
 ki as at 
 r. of the 
 imain at 
 ^troyed, 
 iu sup- 
 
 Fig. 81. 
 
 porting it in this position? Disturb the ring and remove the disturbing 
 force. Wliat happens? Why? 
 
 A body is said to be in stable equilibrium, if its position 's 
 such that a disturbance would raise ite 
 e.g., since in that event it would tend to 
 return to its original position. On the 
 other hand, a body is said to be in nn- 
 Htable equilibrium when a disturbance 
 would lower its e.g., since it would not 
 return to its original position. 
 
 A body is said to be in neutral or in- 
 different equilibrium when it rests equally 
 well in any position in which it may be 
 placed. A sphere of uniform density, 
 
 resting on a horizontal plane, is in neutral equilibrium, because 
 its e.g. is neither raised nor lowered by a change of base. Like- 
 wise, when the support is applied at the e.g., as when a wheel 
 is supported by an axle, the body is in neutral equilibrium. 
 
 It is evident that, if the e.g. is bclotv the sxqjport, as in the last 
 experiment with the ring, the equilibrium must be stable; but, as 
 in Figure 79, a body may be in stable equilibrium, though its 
 e.g. is above the point of support. (When is this possible?) 
 
 It is difficult to balance a lead-pencil on the end of a flno-er ; 
 but by attaching two knives to it, as in Figure 82, 
 the e.g. may be brought below the support, and it 
 may then be rocked to and fro without falling. 
 
 Fig. 83. 
 
 § 76. Stability of bodies. — The ease or diffl- 
 culty with which bodies supported at their bases are 
 overturned depends upon the hight to which their 
 c. g. must be raised in overturning them, and 
 upon the weight of the bodies. The letter c 
 marks the position of the c. g. of each of the 
 ies A, B, C, and D. To turn any one of these bodies over, 
 its c. g. must pass through the arc ci, and be raised through the 
 hight ot*. By comparing A with B, and supposing them to be 
 
 rFiff. 8.S) 
 
 \ c - - / 
 
 four bod- 
 
 um 
 
 'n\ 
 
 (i ' 
 
 ,'■:' 
 
 111 
 
 
 i H^l 
 
 n 
 
 H 
 
 
 HI 
 
108 
 
 ni 
 
 DYNAMICS. 
 
 rmsed high„, and &, thereflTlJ^"^ J "''''"' ''« """' '^ 
 A comparison of A and C. ,7 """"' *™'^'- *»;«*» 
 
 leases and Wghts, but D ia made .1 ^ ? "'"' ^ '"""> ^l"*! 
 '-« -•'» 0.,. „, ,,„,, , ^ -^^ ^-y at the bottom, and Ibis 
 
 Fiff. 83. 
 
 1. 
 2. 
 
 3. 
 4. 
 
 QUESTIONS. 
 Where is the e.g. of a box? 
 
 •'0..0 Of e,„., „eig„."r '° "" '"""•""-<'. « '»«0 of Hy or a ,„„ „, 
 
CUEVnjNBAB MOTION. 
 
 109 
 
 ^ifght and 
 ' must he 
 difficulty. 
 of equal 
 ' weights^ 
 ve equal 
 and this 
 
 ontal 
 iieij- 
 
 sed? 
 
 of 
 
 in 
 
 XI. OTHER APPLICATIONS OF THE SECOND LAW OF 
 MOTION. -CUliVILINEAli MOTION. 
 According to the first Ia^v of motion, every moving body pro- 
 ceeds ui a straight line, unless compelled to depart from it by 
 some external force. If the external force is continuous, Le, 
 acts at every pomt, the direction is changed at every point, and 
 the result is a curvilinear motion; and if the force is constant 
 and acts at right angles to the path, the curve becomes a circle. 
 
 Thus suppose a ball at A (Fig. 84), suspended by a string from a 
 po „t . to be struck by a bat, in a manner that would cause ir to move 
 m the du^cfon Ao. At the same time it is restrained from taking that 
 path by the tension of the string, which 
 operates like a force drawing it toward 
 d. It therefore takes, in obedience to 
 the two forces, an intermediate course 
 toward c. Atcits motion is in the di- 
 rection CM, in which path it would move, 
 but for the string, in accordance with 
 the first law of motion. Here, again, it 
 is compelled to take an intermediate 
 path toward e. Thus, at every point, 
 the tendency of the moving body is to 
 preserve the direction it has at tliat 
 point, and consequently to move in a 
 straight line. The only . ason it does 
 not so move, is that It is at everv nnint fv... i , 
 by the pull of the string. Bull when tt. u *" "' ""*"''"' P"^" 
 
 -.. i" the direction ., which i^ ii:^;; I i:zz IX S!'. 
 
 This tendency of a body moving i„ a curvilinear path to fly 
 off ma straight hne has been erroneously attributed to a sup- 
 l)osed 'centrifugal force," which is constantly urging it away 
 from the center, it. escape being prevented only b^ a Tree 
 pnlhng it toward the center. ^ ^ 
 
 Centrifugal force has in reality no existence ; the resulte that 
 
 pi 
 
 « 
 

 k4J 
 
 !1; 
 
 I' 
 
 110 
 
 DYNAMICS. 
 
 line. Th s Tv t r r ' *""" ''''''^■■'"™ '-» " ''-i^ht 
 .evolutions are Z! "' t'"? *''' '""■''• «• "h™ 30 
 
 '".00 wiu JCi7 o^ie'';:;:: x^ '^r'-t^- "■' 
 
 -d tho velocity regain, o/Za thi ftTCf 'J/°:r/ 
 
 f«e square of the velocity increases and n^ fh^ 
 increases. «■<" etweif, ana as the mass 
 
 velocity HantLLr?. co„sa,„ently the greater its 
 
 the g-eatast tender 0% ffC 2'"°%'"' "r"'"^"''^ 
 whichistonentraliJin ""'^°'*' ""« "f*"* <>« 
 
 is oai^uiated :,:: i::,;"„:!rart\? 'T.r *'"^''^- " 
 
 Hfc either nnl^ ,',, ^ ^*^ ^^^^ ^* *h« equator than 
 
 earth's velocity wefe in . ''""" °^ 17; hence, if the 
 
 equator .oul^^eiirLrr ^^^^"^-^°^^' <^^>«^« ^^ ^he 
 
 nnZ^- '"''" ^^'"^ ^'"'^"^ ^§2^> *^^* ^ body weighs more at th« 
 poles in consequence of the oblateness of the eartk TM . 
 
 .«ated to make a difference of about J S! \ "" ""'"' 
 
 weigh at the equator about ,^,71 - ,^7^\^, ^^^^^ -» 
 poles. " ?8?J + 2*Tr - Tk less than at the 
 
 ^Mis, . ,.a,i„ar, straight n„e pa«.„, through a bod, about which H roiat^a. 
 
 !:fi 
 
QUESTIONS. 
 
 Ill 
 
 Experiment. Arrange some kind of rotating apparatus, e.g., A 
 I'ig.Si. Suspend a skein of thread a by a string, and rotate. Sus! 
 pend a glass fish aquarium e, about one-teuth fuU of colored water and 
 
 rotate. Pass a string through the longest diameter of an onion c, and 
 rotate. State and explain the result In each case. 
 
 QUESTIJNS. 
 
 rofatedT''^ '^°'' "''^ '^' '^''"■' ^ ^^'^^ ''^ '''"""^ ''' P°«^""" wl>«» 
 
 2. State the various facts illustrated in the act of slinging a stone. 
 
 3. («) When will water and mud fly off from the surface of a re- 
 volving wheel?, (i) Why do they fly off? (.) lu what direction do 
 
 theh" orWtsJ '' *""' '"'■'' ''''* ''""''" *^' '"''^ ""^' '''' ''''''' l'l^"«t« in 
 5. How do you account for their curvilinear motion ? 
 
 ti± 
 
 mi 
 
 I 
 
 
 nm 
 
 11 
 
 ' 1 
 
 
 
 11 
 
 ''11 
 
 ^11 
 
 ill 
 
 
 n 
 
 H 
 
 'tH 
 
 1 1 WH 
 
 1:1' 
 
 M 
 
 '1*1 
 
112 
 
 ! '^ ! 
 
 I>YNAMlCa. 
 
 5 77. Accelerated „„«o„ „, ""'™''- 
 
 ™se of motion „„,er ZI^LZt^T^-^ ""' "«' "->y 
 ''7 «t"diccl is .h«t of curv llear l^"""""^ '""c that „e 
 "«ta at an angle .„ the <lircc to"r„.r "' '" ""''=•' "■" '"■« 
 and so tl,» ,li,.ection of «,. 1?™ "'°"°" "' '■'">• Point 
 
 *e motion Ukcs piaoe ^1:^^: ZS^v' '"^''^''^' ""' ^' 
 •he forec acts, we simll have one of , J ' "^ ""' '" ""'"I' 
 "^fcned to in § 67. °'^ ""^ '^''^^ <>( varied motion 
 
 aWo^rr!,!:;;;:™' j;;';j^';'^';"«' « l-«vy car we „V be u„. 
 
 ti.ey continue to JenlZJl'T,,"' """^ "=»»<'«! bnt, ^f 
 greater and greater veloeitrnnt^L t T '' "'" "'°^« ""'■ 
 creases with the velocity) hec^^e, ! , °® '*'™ (""ch i„- 
 
 ,7°- This continually incrrir.!", T """ "'"Jied by the 
 ated velocUy. " ""'"^'•'S velocity is tenned aaeler 
 
 fr^t-stovy window. Inasmuch as T . ^"'^ ^ J""^P ^''^'^ » 
 
 « «o great tl.at there is not 1 1 ?« ' ''^'f ^' "' '^"'"^ ^«^'es 
 
 "'-r fall, we must resort to si„eT?1 '''''''''''''^ ^'"-'"^ 
 
 vcIoaty,witlK>utotherwisectn:n:the r '' ^'^^''"»" ^''-^ 
 E-Per,„,e„t Tak. '""^"^^ "'« -'^-•'-eter of the fall. 
 
 higher than the other. Sus- 
 pend within easy view 
 a string (about lmio„„-) 
 
 anddr"''*'"-^^"'*"'"'"' 
 
 Vibration, and, at the instant th« h„n ^®'' ^^^ ball. Set iMn 
 
 to roll down the inclined Xe ' r ^t ^ "T'' ''^ ''"'' ^^* « ™arb5 b ' 1 
 hoard thatthfi ba!i rearh !" , ^*"°*^er person mnru .}.„". ® "^^'^ 
 
 ^ijf. 8a. 
 
ACCELERATED MOTION OR VELOCITY. 
 
 113 
 
 at the end of the second and third swings; also verify the precedlniF 
 points by several trials; If there Is a difference, take the mean Z 
 tanee between the points obtained at the end of a given swi, gTor In 
 Uproxlmate result. If the experiment Is conducted with car^ lty.ni 
 e found that during the first swing, which we call a unit of time (T) 
 the marble moves through a certain space, which we represent by the 
 xi>re s>on J A-; during the second uult of time it moves throu-^h ^k 
 luoo t.mes the space that It did in the first unit of time, and durin.: 
 tho tlurd unit of time it moves throu-h ;] k. " 
 
 ^^^Arrange the results of your obsermions in a tabulated fom as fol. 
 
 No. of units of 
 tiiue. 
 
 Total distance 
 passed over. 
 
 Distance passed Increase of ve- 
 over In each locity in each 
 
 1 
 2 
 3 
 
 4 
 etc. 
 
 unit; also av- 
 erai/e velocity, 
 
 1 (U) 
 4 " 
 9 " 
 IG " 
 etc. 
 
 3 «' 
 
 5 '< 
 
 7 " 
 
 etc. 
 
 unit, i.e., «c- 
 celeration. 
 
 Velocity at the 
 end of each 
 unit. 
 
 2 " 
 2 " 
 2 " 
 etc. 
 
 4 •' 
 
 G " 
 
 8 " 
 
 etc. 
 
 The marble, under the influence of gravity, starts from a 
 state of rest, and moves through one space in a unit of time. 
 Gravity, contmuing to act, accomplishes no more nor less dur- 
 ing any subsequent unit of time. But the marble moves throu<.h 
 tiiree spaces during the second unit; hence, two of the spaces 
 must be due to the motion it had acquired during the first unit. 
 In other words, if the action of gravity were suspended at the 
 end of the first unit, the marble would still move on, and would 
 pass through two spaces during the second unit. It therefore 
 has at the end of the first unit a velocity (V) of two spaces (k\ 
 per unit of time. But it started from a state of rest ; hence the 
 constant action of gravity causes, during the first unit, an 
 acceleration of velocity equal to two spaces (fc) per unit of time • 
 and It causes the same acceleration during every subsequent 
 unit of time. The distance k [. called the acceleraiion per unit 
 of time in a unit of time, due to the constant force. A body im- 
 pelled by a single constant force, and encountering no resistances 
 always has a uniformly accelerated motion. ' 
 
 'I- ■ 
 
 !lff| 
 1 
 
 : :.!t m 
 
114 
 
 DYNAMICS. 
 
 f "i4r n::^r r;r::: rr " '^r • - 
 
 Let M = measure of initial velocity. 
 
 V = 
 .s == 
 
 ii 
 
 n 
 
 k( 
 
 U -)- V 
 
 final 
 
 aeeelcmtion. 
 
 time of motion. 
 
 (listjinee traversed in time t. 
 
 X t 
 
 But 
 
 
 X t 
 
 mi?anIk°n°dyL«ef' Tf '°'^ independent of its 
 
 """I. soem as tkougl, a I.eavv bo,,,, fal r, ,st „ 
 a lio- it hnrlv n„Ti -^ raster tljan 
 
 '^iit bo(h . Gal.leo was tlie first to si.ow tl.o falsitv 
 of this assumption. He let rlmn f..^ ^ ^ 
 
 iron balls of ^iee /'''."-^ '""P ri'oin an eminence 
 "uij uaus ot difrerent we (I- Its . fi,,„, „ii , , . 
 
 ground at the same instan ir! ^ """"^''"^ ^^' 
 
 that the velooityoTa Z ) ' ''" concluded, 
 
 mass. (Thi7cXb rf e^ '' ' '' "'^^^^^^^^^-^^ «/ ^'^^ 
 ueated h ^^^^''"'-^ted experiment should be re- 
 
 ideated by every student.) 
 
 He also dropped '.alls of wax with the iron balls 
 
 Uie iron balls reached the ground first A 
 
 .nds Of matters affected mort stro J t . ' .^ 
 
 than others? If a coin o,ul o f .1 ' fe'-i^Xation 
 
 longgUss tube (Fi.. 87 and ,' T "''' ''^'''''' '" ^ 
 
 turned end for end I -u u ^'^''^^^"«ted, and the tube 
 
 -. end, ,t W.11 be found that the coin and the 
 
RETARDED MOTION. 
 
 115 
 
 feather will fall with equal velocities. Hence, gravity attracts 
 all matter alike; but, iuasmuch as a wax ball presents, accord- 
 i»g to the amount of matter in each, more surface for resist- 
 ance of the air tiian an iron ball, it falls more slouly We 
 conclude, therefore, that all bodies fall with equal xelocities in 
 a vacimm. 
 
 When the body falls freely, and the unit of time is one 
 second, we use the letter g instead of Jc to represent the 
 acceleration. Experiments show that in the latitude of 
 Ontario the value of g is y.8-, or about 32^ ft. per second ; 
 that IS, tne velocity gained, if the force of gravity acts one 
 second, is 9.8-" per second, and the body would fall'ln the first 
 second 4.9"', or 16-iJj ft. 
 
 § 80. Retarded Motion. — If we reverse the order of the 
 hgur.'s in Fig. ««, tlie same diMgram will represent the motion 
 of a body rolhng upward, or tiie motion of a body under the in- 
 fluence of a retarding force. The formulas given (§ 78) for 
 finding velocities, etc., of bodies having uniformly accelerated 
 moti.;n, nn.y be used for linding velocities, etc., of bodies havin.^ 
 undormly retarded motion ; but in the case of uniformly retarded 
 motion Jc is negative, and 
 
 V = u ~k t 
 .' . .s = a t—h k t'\ 
 
 Of course, if the acceleration begins when the body is at rest, 
 
 n = o. 
 
 u 
 
 
 the 
 
 PROBLEMS. 
 
 (Solve these problems from 1 to 12 i„ both the metric and the English measures.) 
 
 1. Disregarding the resistance of the air what distance will a body fall 
 from a state of rest in five seconds ? 
 
 2. Wiiat distance will it fall during the fifth second ? 
 
 3. What is its velocity at the end of the fifth second ? 
 
 4. A stone, dropped from a balloon, strikes the ground in seven seconds 
 xlow higri is tiie balloon ? 
 
 6. Under tiie influence of a constant force a body moves from rest 
 500'" m a minute. How far will it go in an hour ? 
 
116 
 
 DYNAMICS. 
 
 8- A bodv f..liv f.. '*- """"«^ t'le flfty-niiith niinnte? 
 
 '-« -teJt^u to :;;;:;:r;;sr '"^"^^^^""^^ 
 «. What is it. cf!:, x;;^ j^r'^^f r'"'- "'^ ^--"^^ 
 
 ^0. What is its vortical vol, tv af tho T'r . "" '""'•^" ^^^^'^'^ 
 "• With what vertic-il vol, i^ ""' ""^ "'^^ ^""'•^'' «^'C">h1? 
 
 tl.ree seconds? "'"'^"^ ^^'^'^^'*>' '""-sta body start that it may ascend 
 
 ^er second ; when and wlie e w ' "' '''"' ' ''^''""'^^ "^ -'^"'-^ ^■^- 
 
 14- In Que f 1' . "'*^ stones meet? 
 
 when and where wouk. theThave"met-r'' "''" ''"''''''' ^«>vn wards, 
 
 a velocity of 200 ft. per second Twt."^.''"''*"^^""^"''^^ 
 after the first stone slarted ' "'<^"' ^ 'stance apart, 4 seconds 
 
 ou^- a^;r if dr;:;;d'xrt'"r '?^^ *^^ ^^ ^ ^-- ^ «- 
 
 as to overtake the othe' in ; Tecond!? ''' ™"'' ^' '^ *^^«^° «" 
 
 17. How high will a body rise wbiol, i= f. 
 
 seconds from starting. ^^locity, fl„d the space described i« g 
 
 -o';.Avr.taf rr,:,rwuM;:''vrr'r " ^»» '-' -- 
 
 ond? »'" ^^'" "s velocity be 80 feet per see- 
 
 20- A stone fallinL' from rest fn,. f 
 
 »-'e Of glass, thereby^losinTone- h :i of Tts"T"'r '""" ""'""^'" ''^ 
 ^'.-ound three seconds afterwards find fh f .^f^'^'*^' «"^ reaches the 
 
 21- Gravity at the surf- 0^0^ .? , ''"'''"'* "^ ^'^'^ ^'"««- 
 timesasgreatasatthesurftceof tt ' 71 "■"^'^'- ^^"'^' ^'-"^ ^^« 
 a.K, velocity acquired by! 4"TaC' /"'«'" ''^^^'"^ ^••---'^' 
 Jupiter from a point near its surface "^ "" '""""^^ *°^^'"'^'« 
 
 pe:ict.n::,L^:r;:::;;:;;^:- -^ ^^nu. veio^ty of «« .et 
 
 second, how high will ft rile? '""^ ^' ^''' ?«'' ««^«"d per 
 
hour? 
 
 i ; meantime 
 ■ ill a liori- 
 lie gromul? 
 I'th socorul? 
 secoiul ? 
 way ascend 
 
 ft- per sec- 
 Jf 22r>A ft. 
 
 'vvnvvards, 
 
 • per sec- 
 ards witii 
 I seconds 
 
 er 3 sec- 
 lirovvn so 
 
 upwards, 
 
 lass, and 
 iJed in 6 
 
 feet per 
 per sec- 
 
 roiijLfli a 
 !lies tlio 
 
 out 2.6 
 iversed 
 awards 
 
 CO feet 
 Jd per 
 
 PROJECTILES. 
 
 117 
 
 >.a«„„« t,„.„„,.„ a Stopper at I, a,,., i„. r. JZZ f SZ''"^ 
 Keep tho can /i]|e,l with water, lien.l tl,e lower mttlriZrll , ?' 
 
 int'l'^'of 'J'""'"!""' J"",""™ " "■'"'""™ representation of the 
 fr2„? , .'"■''J™"''^^'' »"«■> »» ea„„o„.baIls, stones thrown 
 ftom the hand etc. The horizouUtl distance that the proJectUc 
 atoms ,s called its n«^. „r ra,ulon.. Theoretically, the gteat! 
 
 o3'« *"?" "' "" ""°"" of «°^ "•" l»otiealf on 
 account ol the res.stanee of the air, it is at a litUc less than 40". 
 
 Fig. 88. 
 
 M^v. ty, and (2) tho resistance of the air. It also has a eertiin 
 velocity and direction at the instant of projection. If this 
 velooty and cJirection are kno.vn, and the resisfinoo of th' .ir L 
 cl.sregarded, the path of a j,rojectiIe can he determined." ^TI^, 
 suppose that a projectile is thrown from A (Fig. 89) at an 
 
 ♦Projectile, a body thrown, 
 
 III 
 
 M 
 
 u 
 
 11'' 
 
 I' 
 
 *!■■ 
 
 I 
 11 
 
 #1 
 
118 
 
 bs.t 
 
 DYNAMICS. 
 
 eively, are B, C and n V, , ""' ""'^'= "»''« ^ucces- 
 
 unite of tim« TheTr J' "'' "' ^°"='' »' ""^ ««' 'hi-ee 
 
 curved iJaB'^"D" Tne'^tr'T?'''?"' ""'^^ ''""^' ''-= 
 "e equal, it must reaeh i*f . "' ""^ ""«'"' "'«' <'<^'«»»' 
 the tWrd ulit wll„ -M *'■''"'" ""■''■="' ""Sl't at tl:e end of 
 
 D'm"t round If " •', """• '"■" '"'"■ »' <•-'=«"• 
 acribed is known as a 1 .'"; ""' '"''""'"■- '^'' P="" ">"« "e- 
 praetioaily „r„::';rthe 'T""' "•"' '"»""«" as this is 
 
 describes a Xtr Ih . r^T "' "" ""■' " "> '-'i'^ 
 ' """ P""" """sd a bmstic curve. Tlie curve 
 
 rtf. 89. 
 
 -Ut „ou.d cause it to .aerJ^a^tCd^^rttl.^S 
 
 in XlTllT^T" "'■ '"" ''""" '^^ »' "■<"'<»> - 'o„„d 
 ground rprue,;rirr''''',''°"^°"'»''^' ""' '■-«'' «■« 
 
 mot nn o 1^.J,. u . '"o'Jt" inat IS, nUV previnna 
 
 o7gravity:^u"u: 'L^;! *'■"'"" '""^ -'" ^^'^^ ■"'^ -ti"" 
 
ind that the 
 nits succes' 
 inimpeded, 
 that direc- 
 Combining 
 )", reached 
 ! first three 
 to change 
 units, the 
 id descent 
 ;he end of 
 of descent 
 I thus de- 
 as this is 
 in reality 
 'he curve 
 
 wn from 
 velocity 
 I unit of 
 
 8 found 
 ach the 
 3d from 
 Tevious 
 action 
 
 H^VESTIGATION OF PROJECTILES. 119 
 
 thf f "^^'^r* '^' ^"PP^'* *'^^ ""^^ ^^''' « ^"f' ^ (Fig- 90). bent into 
 Zl doTn H ' ""'T,^' •^'^«»* ^"■" "P-*. a-' - ^ituatei that a ball n. oil 
 
 So conl r* "^ ' ^ V'"''"'"^^' '^""^ ^'^^"^ '" * horizontal direction. 
 So connect the wires of an electric battery . with these bars, that while 
 the non ball n rests upon them the circuit is closed, and the iron ball 
 m .s supported by the attraction of the electro-magnet e. Now allow 
 
 broken T ".T T"""' ^^^'^ '^^"" '' ^^^^ '^^ ^^ ^^e c7rcu?'^ 
 b.oken, e instantly loses its power to hold m, and m drops. But both 
 
 balls reach the floor at the same instant. If the horizontal velocity of 
 
 Fig. 90. 
 
 n is varied, by allowing it to start at different points on the bars so as 
 to cause it to describe different paths, the two Lis wm. n eve y' ase 
 ac.qu>re exactly equal vertical velocities. This experiment marbe varied 
 
 Shan contain the Imc of direction of the ball m. If, in this case the 
 balls are far enough above the floor at the start. what'si^ouM h„;p;„$ 
 
 §816. Investigation of Projectiles. Continued -In 
 mvest.gat,ng the paths of projectiles we shall find it oontniPn" 
 to consider the horizontal and vertical motions separately 
 
 If we neglect the resistance of the atmosphere, which, for the 
 sake of simplicity, we shall do, the horizontal motion is a uni- 
 
 
 [ 
 
 li; 
 
 ■J .tin 
 
120 
 
 m 
 
 ! ' ! 
 
 DYNAMICS. 
 
 veloeu/ :::; :::n j;;;;-f - the inula, absolute 
 
 the direction of 1 , ot Uo 7 'l- "" ''" ^' Projectiou), 
 
 point of path, the al h ' ' ""' "' ^*«'^*' *^« '''S^^ ' 
 
 -moment after ^ro ee ior;„ t,"'''''^''/^'''^'*^ ^' -»3^ g'>o„ 
 
 any given mor^nt::^;^^^ ^^^'^^ ^^ *'- P-J-tile at 
 
 TJ.e horizontal veloci v - -- "f .' "' '"" "^«^'*^ «'« ^'orizon. 
 
 " initial vertical ' ^ Z ll' ^' '^^^ ^'' «««o'k'- 
 
 The acceleration due to LM-avitv'- •v>i f" * " " ^§ ^'*)- 
 ond(§79). feiaMty = .]2i f^et per second per sec- 
 
 The ball will continue to rise until fl>„ o f 
 vertical velocity to zero, an, n ""''T "' ^"""^'''^ ''^'^"^^^ '^s 
 
 strikes the plane. Sinc^ its o lei , '/""'' '''.'^' ''' '"'^'^'^^ ""til it 
 dnring the fall as well asduH . , e j t.r;" '"i"'""""' '^>' ^'^^'^y 
 as the time of rising. (§§ 78, 80.) ' "" ^''"'"*' '' '"^ '^"^^ 
 
 Hence, the time of flight = I!? >, o ,,, 
 
 32^ -^ -^ == *'^ seconds. 
 
 ^ To tl„d the highest ^oint < f paH S:' T l'^' ^ ''^^ ^-^• 
 
 1-ving an initial upward vertior v i^v ^ 'l:^ V" -'-" '^ -ly 
 rise in 24 seconds. ^eiocitj of n 2 feet per second will 
 
 Therefore, highest point of path = 772 x 24 
 
 = 92(J4 feet. 
 
 32,i 
 2" X (24)' 
 
 Tc And the position of the ball nt th^ , . 
 ;i^ht at .his moment by cc^. i ,t it^ T^^'' ^^^.""^•''' «»^ '^s 
 
 itshonzontalpo.itionatthe.samemo u.nfL M """'""' '"'"^^ ""^^ 
 
 motion. Having its hight and W V ^ ^'""'^''''"■i"?,' its horizontal 
 
 tionisofcour.se%xed.^;;^:i'thi:lu't. ""'''"'' "'^ '^'^'^"'"'^ P-' 
 
SECOXD LAAV OF MOTION. 
 
 121 
 
 To find the absolute velocity of fho haii „* ♦!, 
 
 square the ,neas„re« of its veftic U td ! f"'^ *'' '' ^^^°"^«' 
 
 moment, and extract the squa "oot of th?" "' T""'"'''' ^* *^'« 
 Why? ' ^ '°°' °^ the sum of these squares. 
 
 The horizontal velocity is uniformly 772 /'^ fn»f 
 
 QUESTIONS. 
 
 1- A particle is projected at an an-'le of 450 wifJ, f ho 1, ■ 
 
 a point on a hon/..,ntal plane witi. T 7-7 . ^'th the horizon from 
 Find its range, an.l ftnc ts dist'Zl V ?'''^ ^''^^ ^''' ^'' ««««nd. 
 e.Kl of 5 seconds. "'' ^'■^'" "'« P°'"' "^ projection at the 
 
 2- A particle is projected at an an^le of fino f..«.., 
 horizontal piano, and its total range is 5 000 feet JuT- If"'"' ''" " 
 of projection ? ' ^^^*- ^^^^^^ »s the velocity 
 
 3 A particle is projected at an angle of Sfto f^nm n ■ . 
 horizontal plane, and tho l.i.ri.„.f • . ! """^ * Po'"<^ on a 
 
 j,i 
 
 ' ''■! 
 
 Ill 
 m 
 
 Xiri. OTHER APmCATIONS OF THE SECOND LAW OF 
 MOTION. -THE PENDULUM. 
 
 in F^7er"*D;aw'ira";ul o"'*"''', f"^^""' '^^ '''"''' '-'^-^ h«"«. as 
 B xna; swing tl^Z^H ^^^^ :^; iT .f ''''"^"' '''^'^'^' ^ ".at 
 C moves n.nch faste tharB ' id ^1 . . '?" "' ''"' '^""'^ '"«*'">t- 
 
 swing, hnt hoth ^o..j^;;::,:T:^z:i^^jr''''''' ^* -^'^ 
 
 •iio, ui viuration, In the same time. 
 Hence, (1) «« „•„;« „„„pi„„ , „,^ ««**,» 
 
122 
 
 DYNAMICS. 
 
 ■- "^ (I 
 " ■■ i! 
 
 '■ V. 
 
 Bay this law be regarded as practically trae. The pendulum 
 or a short one, but the difference becomes perceptible only when 
 
 .trrrortrt ir„,r :.r-- ^ -^ = - « sw,„, to. 
 
 Fig. 91. 
 
 ^'eiher; the shorter the pendulum, the 
 
 faster it swings. Make B im long, and 
 
 F ,m long. Watch in hand, count the 
 
 vibrations made by B. It completes 
 
 just 60 vibrations in a minute ; in other 
 
 words, it " beats seconds." A pendu- 
 lum, therefore, to beat seconcls must 
 
 be im long (more accurately, .993™, or 
 
 3J.09 m.). Count the vibrations of 
 
 i ; It makes 120 vibrations in the same 
 
 time that B makes 60 vibrations. Make 
 
 G one-ninth the length of B; the for- 
 mer makes three vibrations while the 
 
 atter makes one, consequently the 
 time of vibration of the former is one- 
 third that of the latter. 
 
 Hence, (2) the time of one vibra- 
 tion of a pendulmn varies as the 
 SQmre root of its length. 
 
 1 W^ . QUESTIONS AND PROBLEMS, 
 
 two vIlTOtlons per ml„„te? '""""»" '° '"■» »ecoud,? Tliat makes 
 
 § 82, Center of oscillaf inn 
 hollo „...-, , »Jo«-uiaT;ion. — ExnAritnpn* i «-,^ ^ , 
 
 "••!i.^, at intervals or IS*;"' bv Dassintr o ,.,; • 7, "*""*• *■ ^^untiect six 
 
 mannerof pendulum A. TWs forms^a / ' "™"^^ "'^'"' «'^«'- ^^e 
 
 forms a compound pendulum composed 
 
I pendulum 
 
 ration than 
 
 only when 
 
 after many 
 
 C swing to- 
 
 CENTER OF OSCILLATION. 
 
 ry as C? 
 
 ? Quar- 
 t makes 
 
 ons per 
 
 ect six 
 ter the 
 upoaed 
 
 193 
 
 actuated on v bv tho hnii r u .„^ 1^ -u ^ "^^^ " ^ "^ere 
 
 ball a were free wo^ d U 'J'/"*' '" ""'^°» ^'''^ ^^ ^^ the 
 constrained to ml'toLher tlT? 7'"' *'"" "^^ ''"^' ^^ ^^«>^ -e 
 
 motion of/, and the tendencvof^' ^' '-"'1 ""' "^ '' *^ '"''^'" "^^ 
 is quickened less than ^ and J ll ..' '''' *^' "^'^"^^ ^^ «• B»* « 
 Checked by / less than a, and c e sTtha:";^ lUs^'^ ^^'^^ '^"'' ^ '^ 
 must be some noint hot JZ. f ! '^ apparent that there 
 
 ened nor checked by thoonmV Z ^',^'"'' ^"'"^'^^ '« "^'^l^^"- 'l"'^^- 
 it. and where ? ^s w,eC "'"^ °' *^' '""^ '^^^^^ '^"^ "elow 
 number o/':^^t:^::;i^^ ir'' '"^^^ ''^ -™« 
 does. Shorten nendnlnm n Ta ? l ^"^ compound pendulum 
 
 i« called *r«r„7Sir ""^ *' "'"''°'' ■>"'■"• ™» "<"»' 
 
 whose lengk ., .j„a( to rte &<*<.nce. 6e,„«, „« cenlerofoZl' 
 
 center of oscillation detennmes the rate ot vibration, wlienever 
 toe e.pressio,W.^« o/„™«„» i, „,«,, it ™„st l,o nlrlo, 
 to m.a„ tlHs distance. Strictly speaking, a simpU peM^, 
 
 couise sna a pendn „m cannot actually exist ; Init the leaden 
 JV,«, Ml, suspended by a thread, is a near approximatLa 
 
 ffp:e™t-„??j;.sro^t%r^^Lr£! 
 
 flud the center of oscillation ot the latu to be? At, 
 
 r::pr:rTr„;e'tii'ir£ 
 ^n.tirtirc\u;:ros=io-.f.-r 
 
 lowered by the addition of the weisrht Mnvl T 7 . 
 up the lath; the vibrations are quickened ?What Tthe offl f 
 pendulum bob?) ^ ^^ '^ ^'^^^ o^^^e of a 
 
 Experiment 3. Remove the weight, bore a hole through the lath at 
 
 lii 
 
 
 ii»\ 
 
i24 
 
 DYNAIMICS. 
 
 vibrate about the needle CoZ rn ^ "'"."f^"''- ^ause the lath to 
 
 its period Of vibraru When suspended T ^^ ;'''"'^'"" ""^^ -'"' 
 Can you explain it? «"«Peiided from A. What is the result? 
 
 You have virtually two pendulums, B C and C A n. ,, 
 in conjunction or in opposition? ^ and C A. Do they vibrate 
 
 changeable, m other words, there are always two points in a eo^n 
 pounapenaulum about which it will oscillL in tZZZr' 
 
 t J "^T^ " P"''""^ "'-^y ^^ fi'^^^'^g the center of o c I'la- 
 tion and the equivalent length of a compound pendulum Fn^ 
 
 same number of vibra- ^ ^' 
 
 tions, in a given time, 
 as from its usual point 
 of suspension : that 
 point is its center of 
 oscillation ; and the 
 distance between it and 
 the usual point of sus- 
 pension is, technically 
 speaking, the length of 
 the pendulum. It will w mi^i^^^^m 
 
 strike It horizoiifilly near it, „„,,„... ""'""""os, and wfih a cU,l> 
 
 ■nove, ,„ t.e ^^rJZVTZ::TZ OsTatX °' "■" '"" 
 causing a sudden ierk on thp «*..,•«„ , • . . ^" '' ^^ ^^^^ *'»™e time 
 the lath in the san directti n ar it^'f '' '''' '^ "'^ ^'^"^- Strike 
 of the lath now mov^sfn „ d ^ V """' «^tremity; the upper end 
 
 jerk of the string. Next strike 
 
 the lath 
 
through the 
 the Jath to 
 
 1 uow with 
 the result? 
 
 liey vibrate 
 
 are inter- 
 in a com- 
 le time. 
 ►f oscilla- 
 im. For 
 vliich the 
 
 om the 
 
 md Uie 
 1 a chii) 
 ihe latJi 
 le time 
 
 Strike 
 aer end 
 
 at the 
 lie lath 
 
 APPLICATIONS OF THE PENDULUM. 12o 
 
 rfrnttMtn '"'■"? ''^'''' ^''^ '^'^'^•^^ *^«^« "« lower extremity It 
 
 ;:tLrrrh?.!^:i^r;r^^ 
 
 of motion. The base-ball player soon learns at what point on 
 ns bat he can deal the most effective blow U> the ball and Z 
 the same time feel the least tingle in his hands. 
 
 TllTnJT^.r^^ applications of the pendulum. - 
 The orce that keeps a pendulum vibrating is gravity Were 
 .t not for frietion and resistance of the aW, a pendufum,lc 
 set n. motion, would never cease vibrating. Since the fore 
 
 of vibration of a given pendulum must be determined bv the 
 intensity of this force. Hence it is apparent, that i the'ra 
 of vibra ion IS known, the intensity of the force of gravity mav 
 be calculated. It is found by experiment that the time ^1 
 brauon vanes inversely as the square root of the force of gravity 
 8o the pendulum becomes a most serviceable instrument ffr 
 measuring te intensity of gravity at various altitudes and a 
 differen latitudes on the earth's surface. (Compare § 21) j 
 •s also the most accurate instrument for measuring time thai hns 
 been invented. Its value, as a time-measurer, 'depend p^ 
 he absolute uniformity of the rate of vibration as long as it 
 length IS constant, and the length of its arc very small. But a 
 heat IS ever modifying the dimensions of all visible bodes 
 various devices have been called into existence by whidrheat 
 may be made to correet automatically its own mischief. Clol 
 that do not have self-regulating pendulums are fast in wint 
 and slow m summer. (How would you regulate thea 2) 
 
 Is >\ .9 
 
 nm 
 
 ! ■ fm 
 
 ' : '"If! 
 
 U f 
 
 ;■ !!■ 
 
 m 
 
 I 
 
12G 
 
 DYNAnrrcs. 
 
 1 w, QUESTIONS. 
 
 that swings it) should a b owbS "" ;;;?."; '^»;' ^^'^"'^^ «-^ «-"'.„ 
 When held in the hand at one- extremit^; '' ""''"'""^ dimensions 
 
 4- Which end of a bat th. h "'"" "'" '^^'^^^ 
 
 hands? Why? '' "'" ^^"'''«'' ^' ''^hter, should be held in the 
 
 § 85. Momentum Tf i, i 
 
 ttat al; bodies, under tl'.e acLn f '^ ^ '"""' *'"""'<' <§ ^9) 
 Tbu., if a ouelpound i ouMI f '""'^' ""' "'"esarae rate 
 "■'owed to fall V„ ;e"' d^H " '""'■''°™'' ™- •""' ^ 
 end Of t.,e second ^a. aoiS v o il^ 4?;'"; '"«" "" «'■' 
 Now, o„ the „„e.po„„d tali a force Those ll!- "^^ "^'^■ 
 lias acted conlinuouslv daring 1! '"'eMity ,s one pound 
 
 pound ball a force whose ™f ,1 """""^' """■' °" "«= '"<- 
 continuously during one seco d r '" ""> P"-""' las acted 
 force has, during tl,e second tl T^" ""' "f*'^'^- Each 
 locity, namely,'from r "to's". feeT ' '"°'° """"^^ "' ^- 
 produced the same chanl of It f ?^<"'»<' ^ but has eacl-. 
 each produced the saj effectT'T' .'" "'"'" """f^. has 
 pounds u> be divided int tw" eoual S"' ,""' '""^ "' ""> 
 you imagine these two eqna fori, '°™^ »' one pound ; can 
 effeet other than twice ,1?, 1 T?""' '° "'" '""""^ ™ 
 -me time? Evidently not X:^""^" "y one of them in the 
 quantity of motion inVtwo-pI'dbTd"?":"""'''^ """ '"e 
 
 32*fcetperseco„discxa:«/d „b,e2i:r* " "'""'''^- <" 
 I'avmg the same velocitv L^in e one-pound body 
 
 ball to fall from rest freel'v dtril, ^'"' "^ ""^ one-pound 
 the two seconds, is fo L H "T"'' "' »""c end of 
 
 second. Now, a one-po „d flt"° ," """"'^ °' «^i ''^^t per 
 two seconds must produrinH 'T.e'r':""^^ ""^'"^ 
 force acting continuously during one econdo °' '"' '™^ 
 
 conclude that a body of one Doundl-r. "™™ "^ """«' 
 •^ ""^ P°"™ ""li a velocity of 64| feet 
 
MOMEMTUM. 
 
 127 
 
 ixe? Why? 
 t of the arm 
 1 dimensions 
 
 ssion is pro- 
 5 held In the 
 
 ^ed (§ 79) 
 same rate. 
 »n ball be 
 ich at the 
 Br second. 
 )ne pound 
 
 I the two- 
 has acted 
 ;s. Each 
 ?e of ve- 
 lias eaci: 
 rds, has 
 
 of two 
 ad ; can 
 seond an 
 B in the 
 hat the 
 ocitj of 
 id body 
 3-pound 
 
 end of 
 
 'eet per 
 
 during 
 
 s same 
 
 3 must 
 
 II feet 
 
 per second has exactly double the quantity of motion possessed 
 bya one-pound body with a velocity of 32^ feet per second. 
 To that which we have called quantity of motion tlie name 
 momentum has been given. • 
 
 It will be seen from the foregoing that the momentum of a 
 
 Sti J"'n f^ " '''' """ "' '^^ ^«^'^' ^-> -•«- varies 
 d r ct,,.as the velocity of the body. Hence a proper measure 
 
 of the momentum of a body is the product of the measure of 
 
 Its mass mto the measure of its velocity. We may also say that 
 
 a uniform force actin, on a lody produces a ckanye of .Leu- 
 
 tu.x imts own direction proportional to the intensity of the 
 
 themselves, the heavier weight will, of course, fall, and the light r^le 
 
 DuLv Tf H !. "''''''^ '' ^-^ ^*'^' b'-'^i'l^^ the cord and the 
 
 pulley. If hese latter are very light their motion may be ne-^lLed 
 Now you already know that a mass of 32^ lbs. if initially at r^ t Ina 
 left to the ac ion of its own weight, that is. if left to the action of a 
 force whose intensity is 32^ lbs., for one second, wi in t a t^>^e 
 
 T:L^zt:r\ '' '''- 'r ^°' ^^^-^^^^ ^^ th^endonirer : 
 
 a velocity of 32^ feet per second. Through what distance, therefore 
 do you expect the weights in the experiment to move in ;ne second 
 and what velocity do you expect them to have at the end of the sec- 
 ond ? Compare the result of your reasoning with that of the expe ime t 
 Allow, he same weights to move during two seconds, three secZs 
 
 :;s.'r:;;treSt -*^*^--^- ^^^-^^ -e expenmentr;;; 
 
 The second law of motion (§ 69) may now be extended so as 
 
 !rr \j i^'"''''-^"''' '*«^ ^^« ^«^« effect in producing motion, 
 whether the body on which it acts is in motion or at rest ; whether 
 
 !J' T^Z^'' ''y '^^'f^''^ ^'ione, or by others at the same time, 
 and that effect is to produce in its own direction a change of 
 rnomentum proportional to the intensity of the force andpropor- 
 tional to the time during which it continuously acts. 
 
 t 
 
 If 
 
 •'■I 
 
 m 
 
 M 
 
 I 
 
 
 II 
 
128 
 
 DYNAAUCS. 
 
 ill' 
 
 Kl .: 
 
 ii 
 
 iml 
 
 QUESTIONS AND PROBLEMS. 
 
 1. Compare the momenta of a car \vpi.r},i„„ m * 
 
 per minute, and a lump of ice wrigl nj^ewt at t^""' ^7'"^ '' '^ 
 second of its fall. "'^•fe'»»fe 5 cwt., at the end of the third 
 
 2. Why are pile-drivers made heavv? VVl.v .„!„ w 
 
 4. A body has a certain momentum after fallinfr ti,r»,. i 
 
 XIV. THIRD LAW OF MOTION. 
 
 § 86. Third law of motion t* i,„ i 
 fhnf «,^*- . "^°"o°- — It lias been shown f S 67\ 
 
 that motion cannot originate in a sindo borlv hnf • V^ 
 
 ever „„o body .-eoeivos ,„„tio„, anothe,- body always pa Jwth 
 mofon, or ,3 set in motion in an opposite direction tlTat J 
 
 bodies oppositely affected. 
 
 Experiment. Float two bloclcs of wr.r..i ^f 
 water, connecting thorn bva shoflL Z \ ""*'''"^' ™*«««« «« 
 end the band wuf et ^oth in l^ h T'""' ^^"^^ ^^«* ^'« '''- blocks, 
 the greater ve7ocitr ' '"' '^' ^™^"^^ ^^«^^ ^-" ^^^ve 
 
 othtrd\'f\\Ssr£t/anrh*^^ ^""^ ^* °"^ ^•^^ "^ ^ -P«' ^'^^ 
 as the first man L a IL ^r. T""' ''^^ ^^'"^'^^ ''''^' «« '""'^" 
 towards each otL ; L' in ^nlfsH^ .T'" ''^*' "'^^^^ ^'" '"-« 
 
 .ov t ,, ^; t Lrtt sre^r ;r^r ' -^^ 
 wi^::rct::nr^--;^:-- 
 
 f erent velocities, yet with equal momenta. ' ' ^" '''''' '^'^- 
 
 usu.11^ distxagmshed by the tenn« action and region. We 
 
. -^v 
 
 THIRD LAW OB' MOTION. 
 
 129 
 
 return to 
 
 ^'i"ff 10 ft 
 tho tliinl 
 
 It hi^'hts? 
 
 liavc tho 
 
 ic rate of 
 
 a certain 
 3 its nio- 
 
 a (§ 67) 
 les from 
 
 can lift 
 ect, bnt 
 
 When- 
 rts with 
 -t is, in 
 '■ast two 
 
 Lsses on 
 
 blocks, 
 
 ill have 
 
 )pe, the 
 IS much 
 11 move 
 Jat will 
 
 s boats 
 th dif. 
 
 Bd are 
 We 
 
 measure these by their momenta. As every force is either a 
 l)iish or Ji pull (§ 12), and produces equal momenta in two 
 bodies in opposite directions, hence, the 
 
 TiiiUD Law of Motion : To every action there is an equal 
 and opposite reaction. 
 
 The application of this law is not always obvious. Thus, the 
 apple falls to the ground in consequence of the mutual attrac- 
 tion between the api)le and the earth. The earth does not 
 appear to fall toward the apple. But, allowing that their mo- 
 menta are equal, we are not surprised that the motion of the 
 earth is imperceptible, when we reflect that the velocity of the 
 earth must be the same fraction of the velocity of the apple as 
 tlie mass of the apple is of the mass of the earth. (§ 85.) 
 
 QUESTIONS. 
 
 1. The velocity nf the rebound or "kick" of a gun is slight when 
 compared with tlio velocity of the ball. Why? 
 
 2. Ill rowing a boat, what are the opposite results of the stress 
 between the oar and the water? 
 
 3. Point out the results of the action and reaction that occur when 
 a person leaps from the ground. 
 
 Fig.M. 
 
 4. If there were no ground or other object beneath him, and he 
 were motionless in space, couid he put himself in motion? Why? 
 
 6. A boy, running, strikes his head against another boy's head. 
 Which is hurt? Why? 
 
 6. Suspend two balls of soft putty of equal weight, A and B (Fig. 
 94). Draw A one side, and let it fall so as to strike B. Both balls 
 
 !l 
 
 ii^^ 
 
 
 ll- 
 
 1 
 
130 
 
 DYNAMICS. 
 
 I 
 
 Show .,««,,,,,„, ,;-rjrrrsr:/r^^^^^^^ 
 
 veloclt;-, While A Is brought to rest Show .STr """" •*'' °*°'" 
 tent with the third law of moMon *" '''"" '' '°"»'»- 
 
 t« E, E to'^F. .ri'lins .otr.",""" v".' l"""""^ comnunleate, It 
 With C's original veloeUv Trf^I fh , ^"^ '° <'°"""™'<:«te It. moves 
 
 «. whaf woai. Soh Sx r„rTrr z:^""^ 
 ri'rss:::rt,tr^--^^^^^^^^ 
 
 o^e the ha. e.erts7s tr.tVtrS:ih: L,„^roJTf 
 JRg^M, would be necessary to project C toe WTien 
 
 an das«c body strikes another fixed elastic 
 oody, It rebounds with its original force. 
 Experiment 2. Lay a marble slab A fFic 
 
 tlTn * "^^^^^ '"'^ '°" "" *^«''J' ball in the 
 line DC, perpendicular to the surface of the 
 slab; the ball rebounds in the same line to D. 
 f. n ^^^" *° "'' "°*^^C; it rebounds in 
 
 path ...e, with the s.™e perpenaiclar:',: fZl^^rofTZl' 
 
 It .s found by raeosuremcnt that Uiese aneles arc ^„,„1 wl,.n 
 
 tt. two ood,„. are perfectly clastic. This equality is 'expressed 
 
 by the Law o. Replkction : When the strik^g My „„dTS 
 
Work. 
 
 131 
 
 ;d with A's 
 
 es B, com- 
 
 r when it 
 
 nd let fall 
 ■ collision, 
 ion. 
 
 ills, which 
 's original 
 is consis- 
 
 ke D. D 
 
 inicates it 
 It, moves 
 ■orghout. 
 
 :Flg. 94) 
 '^able, C's 
 covering 
 t in this 
 of D as 
 y. When 
 d elastic 
 e. 
 
 ' A (Fig. 
 11 In the 
 3 of the 
 ne to D. 
 unds in 
 its Ibr- 
 ruck, is 
 creating 
 flection. 
 
 I when 
 >res8ed 
 lebody 
 
 '• to the 
 
 XV. WORK AND ENERGY. 
 
 § 88. Work.- We have learned (§ 43) that a force may pro- 
 duce either motion or pressure (or tension), or it may produce 
 both eflfcLs at the same time and in the same body. But a force does 
 work, in the sense i' vhich this term is used in science, only when it 
 produces motion. A irson may support a weight for a time and 
 become weary from txic continuous application of force to prevent the 
 weight from falling, or, in other words, to prevent the force of gravity 
 from doing work, but he accomplishes no work, because he effects no 
 change, i.e., causes no motion. The body that is moved is said to have 
 work done upon it; and the body that moves another body is said to do 
 work upon the latter. When the heavy weight of a pile-driver is 
 raised, work is done upon it; when it descends and drives the pile 
 into the earth, work is done upon the pile, and the pile in turn does 
 work upon the matter in its path. 
 
 Whenever a force causes motion, it does work. A force may act for an 
 indefinite time without doing any work; but whejiever a force acts 
 through space, work is done. Force and space (or distance) are essen- 
 tial elements of work, and are naturally the quantities employed in 
 estmiating work. A given force acting through a space of one meter 
 will do a certain amount of work ; it is evident that the same force 
 acting through a space of two meters will do twice as much work. 
 Hence th*. general formula, 
 
 W = FS, (1) 
 
 in which W represents the work done, F the force employed, and S 
 the space through, which the for^^e acts. 
 
 In case a force encounters resistance, the magnitude of the force 
 necessary to produce motion depends upon the amount of resistance. 
 Indeed, in cases in which the body having been moved through a 
 given space comes to rest in consequence of resistance, the entire 
 work done upon the body Is often more conveniently determined by 
 mtdtiplying the resistance by the space through which it is overcome, and 
 our formula becomes by substitution of resistance, R, for the force 
 which overcomes It, 
 
 W=RS. (2) 
 
 Tor examrV, a ball is shot vertically upward from a rifle in a vacuum; 
 the work done upon the ball iiuiy be estimated by multiplying the 
 average force (difficult to ascertain) exerted upon it by the space 
 through which the force acts (a little greater than the length of the 
 barrel), or by multiplying the resistance offered by gravity, i.e,, its 
 weight (easily ascertained) by the distance the ball ascends. Also, la 
 
 I 
 
 I J 
 
 
132 
 
 t)YNAMICSl. 
 
 w 
 
 M 1 
 
 It! 
 
 li^ 
 
 gravlw b, the first foZ„ „»„ 1. Is"™ T <'" ""^ "»» "J- 
 that part of the work don/l„ „ ? ! ^""■'"' '° estimate onir 
 
 «lven .n SHwniTfot: I'T^flTr"""' '"= '°™""^' 
 
 p4e?wh«tsta,:';^tta'::„t'o/tT ?""- "■' """ ™- 
 
 § 96 ivlll be defined the nnf, ^f the elements of work. (I„ 
 
 employed asafSrof ,™'krT f .rT'T" """° ""= '»-'"» 
 French Is the work done In raSnl nV" f "">* """P'"' "y the 
 It Is eaued a mo,ra:V::,!^^SZ:^;i%"^r 'l'^' <>' '"• 
 work Is that done In raisino- oL „„ T '' " >^"gh»h unit of 
 
 ;>™nd. ThckilogZm eflsahont °rw°° "'°'' "" " «'" "/»»'■ 
 the foot.po„„d. Now Shu e tl^e w ? 'r'' "'=™"'«'J. '-233) times 
 l'", the work Of r.,,„;J" r, ImZH, ' ZJ" ?:1"« " ■■" '"«" '» 
 the wo. done ,„ ralslnj ,. |.„ hSl Tn^r 're'rl^ahrrSr 
 
 ButX":,t7rermrrtrertr^v;'"°'™*-"'^'>- 
 
 any other direction is just the sal «! T,? P''^^"^'""^ motion in 
 
 easy, in all cases in whLuhetre,ellV""f ''''''''-' '^ '« 
 and spa.e) are known, to find t eo h a ' in J"[\^^'"' '^^'^^^°«« 
 weight vevtically. By thus securinrn '^''''' ^^"^ '"^ ™'«ing a 
 
 nlentofwork^v^LreaSe trc^nnare r""'" '''"''''' ^^ "'^'^^"re. 
 other. For instance, lot s compare h 7 ?T' '' ^^"^'^ ^^'"^ -»y 
 through a stick of vv'ood. w,rr: .11''^ '' T "^" '" ^'^^^'"^ 
 resistance of 12^ with that done by a Tui lun V ."'""'* "" "^''^•'^S^ 
 depth of 2cm against an average ro,i«f f"''''*''"^'*^^'*"'^ to a 
 
 10'" against 12^ resistance" eo.l I '^ ' ''''• ''"^■'"»" ' ««- 
 doing 120^.™ Of work; a hul,et:r „t L^aT n^'ir ?" ''''' "^ 
 as much work as Is required to raise 2004:",.,''^ T^'*^^ 
 of vvork. 120 -^ 4 - qn fim^. , '*'»''' o"" 200 x .02 - 4 Vgn. 
 
 the bullet. '' "'"''^ '' '""^'' ^-••'^ ^o»o l,y the sawyer as by 
 
 § 90. Rate of doing work — Tm „ .• .. 
 
 „ .., ^oijv dune, the tune consumed is unt f..i' • ~. " 
 sideratiou. The work dn,.P 1», i , '''^" '"*« con- 
 
"le force are 
 there is no 
 I body falls 
 his case by 
 timate only 
 le formula." 
 titiited for 
 
 tmit em- 
 vork. (In 
 
 force is 
 ed by the 
 a:ht of im, 
 ih unit of 
 ed a /oot- 
 ids) times 
 '" high is 
 
 > same as 
 us raisiiis"; 
 
 : weights, 
 notion in 
 ion, it is 
 esistance 
 raising a 
 measure- 
 vith any 
 
 1 sawing 
 average 
 ank to a 
 g a saw 
 I'gh, or 
 ice does 
 
 2 = 4 '"■gni 
 
 r as by 
 
 totn! 
 
 o con- 
 
 1 ,000 
 
 s it ID 
 
 POTENTIAL AND lONETIC ENEEGY. I33 
 
 a day or a week. But in estimating the power of any agent to 
 CO work, as of a man, a horse, or a steam-engine, in other wo.^f 
 t^te at which it is capable of doing worf, it' is evidm:; 
 time IS an important element. The work done by a horse, in 
 raismg a barrel of flour 20 feet high, is about 4000 ft.-lbs 
 but even a mouse could do the same amount of work in time' 
 Ihe unit m which rate of doing work is usually expressed is a 
 horse-power. Early tests showed tliat a very strong horse ma^ 
 perform 33,000 ft.-lbs. of work in one minute. lo 1 TJ'. 
 power = 33,000 ft.-lbs. per minute = 550 ft.-lbs. per second 1 
 nbout 4570^- per minute = about 76^- p,, ,,^ J^^ ^'^^^"^ ~ 
 
 §91. Energy. -The energy of a body is its capacity of 
 doing work, and is measured by the work it can do. Doma 
 work usually consists in a U^asjer of motion, or energy, from 
 the body doing work to the body on which loork is done. Wher- 
 ever we find matter in motion, whether in the solid, liquid, or 
 gaseous state, we have a certain amount of energy which may 
 often be made to do useful work. ^ 
 
 § 92. Potential and kinetic energy. _ Place a stone, 
 weighing (say) l()^ on the floor before 30U ; it is devoid of 
 energy, powerless to do work. Now raise it, and place it on a 
 shelf (say) 2- high ; in so doing you perform 20''«"> of work on 
 It. As you look at it, lying motionless on the shelf, it appears 
 as devoid of energy as when lying on the floor. Attach one end 
 of a cord 3"' long to it, and, passing it over a pulley, wind 2'» 
 of the stnng around the shaft connected with a sewing-macliine, 
 ooflee-m.ll, lathe, or other convenient machine. Suddenly with- 
 draw the shelf from beneath the stone. It moves, it sets in 
 motion the machine, and you may sew, gi-ind coffee, turn wood, 
 
 etc., with the power triven to iho rpoohino 1— fi» -^-.,-- 
 ' •• - — .!...! Nine i/j tiU; atOiiu. 
 
 Surely, the work done on the stone in raising it wa.. not lost; 
 the stone pays it back while descending. There is a very im- 
 portant diflference between the stone lying ou the floor, and the 
 
 II 
 
 iii 
 m 
 
 I 
 
m 
 
 hrNAmcQ, 
 
 see no differen* «*» / ^' motionless, and you can 
 
 at rest ,s not necessarily devoid of e„e»r Ttht «^ ° , "^ 
 passively on the shelf there exists a n^^.\ !, . "" '*"'8 
 
 th.t possessed by the stone whth El ^r^, T' ■""" "^ 
 great velocity. '""°''' """"g 'reely, has acquired 
 
 diii::;at:'aX:tTri;:fin '" ""-^^ -' '"° '^"^■^ 
 
 K my exist as oo J. L^^, e7«L vSb.r" " '° ""r"""- 
 c">tion, or invisible a, in th. ^T , ' "^ '" mechanical 
 
 it may exist in Lm ^ molecular motions called heat; or 
 
 *;' mrrCcZ r*'t'/^ •" *^^ ^'°"' '^-s on .:: 
 
 ene.^; in the kZ it t Tw "" *"'*'' ("""^'"S) °' «<*■«" 
 
 -TmvTioiTtirit™' *" '*"" "'• "^-'"^^ <•- '"'- - 
 
 gradually in the movLento S tht I ' *" "" *'"^ »»' 
 when we bend the to " raTse .hi K """"""»'y- ^e store it 
 rai«. any body above^^; e^L'sur^a^ ""' ""'*'''" ""' ""<> 
 
 do work, as when mills a.t driven bv 17 " '■""i"^'" '" 
 water; but the water i, fl~,t j ■ J P""" of falling 
 
 one^of thesunfhe ri^JSlf 2 ''' •'""^"'' ''^ "■° 
 a motive power • but el„«t,v , • ^ ^ """"^ " ^mp'oyed as 
 
 Which the'mrc'ul 'o 'CS Lv^t "" 1™""^' "' ''°''«''» 
 force appBed to tliem. ^""^'' '" "o^^iue-ee of 
 
 We conclude, then, that a 6oA, «r..^.^. , 
 
 «Ae», m w«ue 0/ ,oor* <fo„e „«„„ ft'',-: ™. '"*""•«' e««w 
 
FORMULA FOB ENERGY. 
 
 135 
 
 tUmergy expended can be at any time recovered by the return 
 oj the body to Hs original position, or by the return of its mole- 
 cules to their original positions. 
 
 A^l Energry contrasted with momentum. - Problem. 
 
 A bullet weighing 30« is shot with a velocity of 98n. per second from a 
 gun weighing 4- requir. d the momentum anc^ the fnergy of both^ 
 
 kilogram, the meter, and the second as units, the momentum of tl,P 
 ball IS .03X98 = 2.94 units. If the ball were 'shot verrairupl^ 
 Its velocity would diminish 9.8« per second; so it would iise^= lo 
 seconds and therefore, before its energy is expended, to a"hight 
 
 iion ,f! J ""'^''^ ^^^ "'^ S"°' ^y the third law of mo- 
 
 units ir*"? 7 "'"f '" ^"'^ '''' «^™« *« *h^* of the baU, 2 94 
 735 '' '^°'' ^"^^ ^ '^ = •'^''" P^^ «««°"^- Then 
 
 '^= 9X= -^^^ '•'^°°^' the hight (supposing the gun to be raised verti- 
 
 T:^'^^t:':n:,i:r'''^ = ^-^ >< -O^^- = •02766»; and its energy 
 
 While, therefore, the momenta generated in the two bodies by the 
 
 burning of the powder are equal, the energy of the bullet is ilil = 133 j 
 
 times that of the gun. (Why are the effects produced by\^h! bullet 
 more disastrous than those produced by th» recoil of the gun?) 
 
 § 94. Formula for energy. _ We can find, as in the above 
 
 example, Uy what vertical hight a body having a given velocity 
 
 would nse, and thus in all cases determine its energy ; but a 
 
 ormula may be obtained which will give the same result with 
 
 less trouble ; thus : , ""mm 
 
 Let E = measure of energy, in foot pounds. 
 
 '' velocity, in feet per second. 
 
 " acceleration due to gravity, = 324. 
 
 " hight to which body would rise, in feet. 
 
 " weight of body, in pounds. 
 
 ♦* time of rising, in seconds. 
 
 " V = 
 
 9 = 
 
 i( 
 
 (( 
 
 S = 
 
 " w = 
 
 " T = 
 
 (( 
 
 » i'ii 
 
 !lM 
 
136 
 
 DYNAMICS. 
 
 T = 
 
 i"* 
 
 or T''= 
 
 
 if 
 
 9 
 
 But K = ^v s 
 
 _ 1^ ^'' 
 
 Itis Pvident that, when the wehjht ( W) of n hnrh. ,. • , 
 
 tls momentum, as wc h-ivo lo-.m,.! "^ ''''^^""'^' ''^^"■'^' 
 
 rn other words thHii . 7 ' '•^" ^"•'''"^'•^'■"««^ «<^ *V. .e/oaVy. 
 
 ^.>iciiy than its .o^ear^'rihi:;::^^^^^^ 
 
 to four tunes the depth that it did in'the fonne'; "" " "''^'^^'•^^^^ 
 moving with a ve'ocitv of ion e. , "^^'<^*"i etc. A bullet 
 
 "ot twice, b„t fortii ."L!"!;': z"": *'"' "r-"'-""' 
 
MEASURE OF A FORCE. 
 
 137 
 
 Which a hor.se draws a wagon is SO-^ ; that is, a spring interposed 
 be.ween the horse and tlie wagon is stretclied just as much as it 
 would be by the force of gravity acting on a mass of 50" hung 
 from the spring. But often it is impossible to measure the 
 force except by the motion it produces. Experience has shown 
 that a useful ana accurate measure of a force is the momentum it 
 produces or destroys in a second; if the body is already in mo- 
 tion, we must say the change of momentum produced in a second 
 iov example, gravity we know will impart in three seconds, 
 f % oL^"^'"^ ^ '"''' ^^ ^'^^^) ^'' ^"^^ ^'^^ ^ f^"' a velocity 
 
 & X 6 X 980. Then, by definition above, the measure of the 
 force of gravity on the body is 62<3^9^ == 5 ^ 980. When the 
 centimeter, gram, and second are taken as the units of len-th 
 mass and time respectively, the system of units of measurem'eni 
 based on them is called the C.G.S. system, and in it the unit 
 of force IS called a dyne. 
 
 A dyne is that force lohich, acting for a second, wiU give to a 
 gram of matter a velocity of one centimeter per second. In the 
 example above we have a force of 5 x 980 = 4900 dynes 
 
 The gravity unU of force is the weight of any unit of mass 
 e.j7., a gram, kilogiam, pound, or ton. In distinction from 
 gravity units, the C.G.S. units are called absolute units. Gravity 
 muts are easily changed to absolute units; thus in Ontario the 
 foice of gravity acting upon P of matter .free to fall will 
 give It an acceleration of velocity of 980- per second per sec- 
 
 :bi^ J;:r [ti: ^'^^ ^^''''^^' ''- ^^^^^^^ -^^ ^« ^^-^ ^^ ^^^ 
 
 Let M = measure of mass of body, in grams. 
 
 (( 
 
 (( 
 
 W 
 F 
 
 
 9 = 
 
 u 
 
 weight of body, in dynes, 
 attraction between earth and bodv, in 
 dynes. 
 
 acceleration due to gravity, in absolute 
 
 units = 980. 
 W == F = M ^. 
 
 
 
 
 ' Ml 
 
138 
 
 DYNAMICS. 
 
 11 
 
 The equation is a general one ; tliat is, wlienever any two of the 
 three quantities specified are known, the third mav be coi^put^ 
 
 con i^pl r' "^^'J'''^ ^^^^' ^"^^'^y' ^"* ^^'"^t resistances 
 considered as constant, such as the forces shown in cohesion 
 
 elasticity, etc, the equation will still be true, only a should b^ 
 replaced by some other letter, as a. J y "uia oe 
 
 Now let us learn what is the 
 
 J.^ , ^T'^^f *^« ^fi'^^* <>^ a force. - One measure we 
 know already, -the product of the force into the distance 
 through which It acts; that is, the work done, or the enerau 
 imparted to the body moved, is a wmmre of the effect of afoZ 
 If the fon^e is measured in dynes, and the distance in cenfa- 
 meters, the work done will be expressed in a C.G.S. unit called 
 an erg An erg is the work done or energy imparted by a force 
 of one dyne working through a distance of one centimeter. Be- 
 sides the erg we have the common gravitation units, the kilo- 
 grammeter, and foot-pound ; that is, we have another measure lust 
 
 Tu^tT/ ''7 '"""^ '^^^ '' "»^— ^- commor;Li,gr 
 
 just as, for mstance, we may express lengtJis in inches, meters, 
 01 miles ; masses, m grains or pounds, ct«. 
 
 Experiment 1. Suspend by a long cord a heaw bodv ink«,«.^ 
 
 SL' ;ro^-i ^- -- -^^^o^^^i^ic z 
 
 (B^)T7tZ^\ ^r'l"^ ^^ * '*'^°« ^"^ *°°8 * «*°°« ^hose mass is 
 (say) 5k. Attach to the stone a No. 36 cotton thread- thU win «.,.. 
 
 port about 1.^. Pun the baU slowly to one sideTwhenT^s boen 
 
 drawn about 20em from its place of rest, the thrsad wlU brel^L , " 
 
 ban w,U swing back to the other side like a pendulum^ an^t> when 
 
 passes through Its lowest point It has a definite momentum 
 
 in/th^/i ""Z ^'^T ""^ '^'^^^' *°^ P"" ™°^« a°d more quickly, bmk- 
 
 Dg the thread each time; the motion produced Is less and !;« As 
 
 the strinc Is strftjffhfon^H ♦!,« i^-ii! — «- • - ^^ 
 
 ty,^ „ " \^' — ° '" P'^" "" " increases I'rom zero to 1^- «r» 
 
 nir^Z^ZTV"^' V^r *'^ ''"^^'^ *" gravitation units! 
 nearly or exactly ^k. Here, as before, with the same force the manu,^ 
 
 tumproduced varies as the time during u,hich the/orve ^ ^'^ 
 
MEASURE OF THE EFFECT OF A FORCE. 139 
 
 But If we use stronger and stronger threads, we may pull more and 
 more quickly than at first, ani yet give to the ball init the TZ 2 
 
 7oZV" ^\'"'V''^' *«• '^' ^ff'<^^ of a greater force acting for a 
 shorter time is to produce the same momentum. 
 
 So far then as our experiments go, they teach that the prodnct 
 of a force into the time it acts, or the momentum produced, is a 
 ir^asure of the effect of a force. We may draw the same conclu- 
 sK>n from our last equation, F = M^ : multiply both sides by T, 
 the time during which the force acts, and we have FT = M«T 
 = M V = Momentum. If T equals one second, we see that 
 the momentum of a moving body is the measure of the force 
 that would in one second give it this motion. It is evident 
 that if motion is to be produced by a force acting for a very 
 short time, the force must be enoi-mous. 
 
 We have, then, two measures of the effect of a force, —mo- 
 mentum^nd energy. The first is found by multiplying the force 
 by the time it acts ; the second, by multiplying the force by the 
 spa^e through which it acts. The latter can also be found by 
 multiplying the momentum by one-half the velocity. One is 
 M V ; the other is ^ M V^ Wliich is the con-ect measure ? Both 
 are correct; so the question now is, Which is the more useftil? 
 Experience shows that momentum is a useful measure only in 
 cases where the force acts all the time in the line of motion, aa 
 m falling bodies, or where it acts for so short a time that the 
 body does not sensibly change its position during the action, as 
 m the cases of a blow, a jerk, coUision between balls, etc. 
 Kxperience further shows that energy in all cases gives a useftil 
 nieasure. 
 
 § 97. Summary of mechanical units, and formulas for 
 their determination.'-Thc following tables show the quanti- 
 ties measured, the unit of each in the C.^S s-«t-m -nd *he 
 formulas for the determination of the derived quantities "1 " 
 
 •dT.n«KJ.tudent cannot ftll to be greatly pwflted by iu„«to?^^ *"' """ 
 
 S ! 
 
 ll V- 
 i i 
 
 i, 
 
 ll' 
 w 
 
 % ■ M 
 ll 
 
" DYNAMICS. 
 
 FUNDAMKNTAL QUANTITIES AND UNITa 
 
 Lengtii (L or S) icm. 
 
 Mass (M) 28 
 
 T*™«cT) ;:;:;: 1,,^ 
 
 PERIVED QUANTITIES. UMTS, AND FORMULAS 
 Velocity (V^) - rate of motion ; unit, ic™ per sec. ; in ...iform n^otlon. 
 
 ' T' (1) 
 
 Acceleration (A) = rate of change in velocity; unit, an increase of 
 velocity in 1 sec. of ic. per sec. ; body starting from rest under 
 constant force, A =r — . 
 
 ^'" pj'il '''''^' °^ "^^^'^ ^°^^'^ (I')' ""^it. 1 erg per sec.; 
 
 Momentum; unit, U moving with a velocity of i™ per sec or^that 
 
 produced hy 1 dyne in 1 sec. ; Momentum = M V 
 From (2) amU3) we have the very useful equations, F = MJ and 
 
 ~ M~' (6) and (7) 
 
 A body, mass M, acted upon by the force F. starting fVom rest will 
 acquire in time^T a velocity V = O^ . The acceleration, which- 
 from (3) IS =- -, Is a constant quantity, and the whole space 
 
 veTcl"'Thrl'fr'"''*\*^' "'"'^ '^ '"•"^'^''^^ ^y'^<^ '"^"'^ 
 ^eloc^ The latter^ ^s^one-half the final velocity; hence, mean 
 
 - 2 jj. and S = ~ (an equation of great importance). (8) 
 To find an expression for the energy of a moving body combine (4) and 
 C3):W = y?,butFT.My,...E = & ' [.^ 
 
 1 « - 98,000,000 ergs; 1 foot-pound == 13,650,000 ergs 
 1 horse-power = 447,000,000,000 ergs per min. 
 
 § 98. Transformation of energy. _ In ti.e operation of 
 raismg the stone (§ 92),kiuctic energy is UaasformedVnto poten- 
 
PHYSICS DEFINED. 
 
 141 
 
 tial energy. During its descent it is re-transformed into kinetic 
 energy. If, instead of boing attaeiied to maciiinery, and tl.ereby 
 made to do work, tlie stone is allowed to fall freely-, it acquires 
 great velocity. On striking the ground, its motion as a body 
 suddenly ceases, but its molecules liave their quivering motions 
 accelerated. Mechanical motion is, thereby, transfonned into 
 heat. We shall often have occasion to examine the transforma- 
 tions of energy, as into electric energy, heat, etc., but never 
 of momentum. We shall study Joule's equivalent (§ 14G), 
 expressing the relation between the unit of energy, or work, 
 and the unit of heat ; but it is certain that there is uo relation 
 between the latter and the unit of momentum. 
 
 § 99. Physics defined. — All physical phenomena consist , 
 either alone in transferencis of energy from one portion of 
 matter to another, or in both transferences and transformations 
 of energy. Transformations may be from one condition of 
 energy to another, as from kinetic to potential ; or from one 
 phase of kinetic energy to another, as from mechanical motion 
 to heat ; or both may occur, as when the falling stone does 
 work, a part of its energy being expended in producing mechan- 
 ical motion, and a part being transfoimed into heat, occasioned 
 by friction of the moving parts. 
 
 Physics is that branch of natural science which treats of trans- 
 ferences and transformations of energy. It does not, however, 
 in its usual limitation, include a group of phenomena which occur 
 outside the earth, and also a group whose essential character- 
 istic is an alteration in the nature of the material considered. 
 Tlie study of the former group is the object of Astronomy; of 
 the latter, that of Chemistry. 
 
 QUESTIONS AND PROBLEMS. 
 
 1. Does the energy expeuded in raising the stones to their places in 
 the Egyptian pyramids still survive? 
 
 2. What kind of energy is that contained in gunpowder? 
 
 8. What transformation of energy takes place in b ling coal? 
 
 4. When steam works by expansion, its temperature i^ioduced. Why? 
 
 f iH 
 
 \4 
 
 h} 
 
142 
 
 DYNAMICS. 
 
 
 .' I: I 
 
 A. li I 
 
 •■ How much work fs done uer Imnr ip ook . 
 
 9- «.) What enorsy m„,l l,e h marM ,„ I ^"""' *" P" °"»"««' 
 m»y rise 4 sew,,,*? (ft, iWriZvll^, "^^ "''''«'''°« '"«•'« " 
 l.=parted to the same bout t mu T/ /" '""'"' '='""■» """« bo 
 
 «t .he i,„w„„ u,e body I" th'rowl " «'"° "" ""' '^' 1°«»"<">. 
 
 H,.i»mr„;t::r:h:rra'e°ep' ^^ "*- -■* ^»'." a 
 
 Of L";:™!™""' "°""' "■" -""= ^-"^ -«- " •' had . velocity 
 
 H "err::: r^fe t:tnr,;.;r' — --» ™-.y „„„« 
 
 the'Ugyr"" ■■'" "■""«" «'-"■. """would become or. p.^t o, 
 
 .nufa, 'el„'ct:*"2;":.tl"feclV' t" "" T'"°"^ """''"' """ « 
 J3. What become., of-itVrS du"" .^ ^ trtT "" '" 
 
 a velocf.;^ofTC llZT^TX^ """' """""'"^ '»'■ »°1 "avios 
 
 -, ..v,„, a iity o^r^rirrTft^^- ra 
 ..itpi;t;zr.rar:-;-'-^^^^^^^^ 
 
 "• Expla „„ a e|,iy „j ' iin ,oi ? 
 
 welshing jsol. " ""' "" ^^ <^«» draw a carriage 
 
 ■stand, a. ^J» '" af rrt' h" """" ""' '""■- '» «^-h'" 
 
 the rate l„ h,„.„„-p,,„,rr (See 5 » " ''°"'"'^' » E^f"''' 
 
 aci.:,;tr;;r™^*:;^:a:'::;„;?r »'.'■"-» p-" > p.oww,th 
 
 they travel at the mte of 3" per hZ' ""'""' '° '"''"' '"" P'"" " 
 h„„", '"'" '""-Pow" '■>.■■. engine will raise 1,350,000. 6. ,„ an 
 ^^^»0. How lon, will It .ate a 3 horse-power engine .0 raise 10 tons « 
 
 S».. How lona would I, "a?e , ~r? TT*'°° "" '° ''° "o"' 
 orworj a man can do In", day bernraTo^ ^toXr"' "^ "■"°"°' 
 
USE OF MACHINES. 
 
 143 
 
 Fig. 96. 
 
 A 
 
 f 
 
 §100, Usesof machines. — Experiment 1. Obtain from a 
 hardware store two or three pulleys, and arran-e apparatus as In Fi- 
 96. The dynamometers a and h read 4 lbs. each, showing that the 
 povver (P) employed to support each weight (W) of 8 lbs. is just one- 
 half of the weight.' If the power applied in each instance is slightly 
 increased, the weights - , .- ise. liaise each of the weights and meas- 
 ure the distances trav;iv«»ed I'c 
 si)ectively byW and ; in ,;;i,oh. 
 Through whaidista -ce Must 
 P move that W may be raided 
 one foot. What amoiv of 
 work is done in raising 8 lbs. 
 one foot? How does this worlj 
 compare witli the work done 
 at P? Is any work saved by 
 raising the weight by means 
 of the pulley instead of lifting 
 it directly ? Is a force of less 
 intensity required when the 
 pulley is used? Since the 
 string is light, and passes 
 round freely revolving pul- 
 leys, its tension may be sup- 
 posed to be the same through- 
 out its length. What is Its 
 tension? Since the 8 lbs. is 
 supported by two parallel 
 portions of the string, Miiat 
 must the tension be? What 
 iloes the dynamometer P show the tension to he? 
 
 r.„ cvciy 1 11. tliat W moves Xnw •> (fi ^ v m /n 1 
 
 32 foot-pounds of work done by P. Lx 4 m ' i 8 1^ I 'V 7 
 ponufls nf ..roH' -"-.j.- ,. «- ■--- '"'"*'-"'>' X «> (lus.) =i{-' loot. 
 
 o. app,.n. the ^^-u:^^::^::!^'^:, r zr 
 
 « A email allowance muHt be n.a I. fo,- ,.,o weight of .ho movable pulley,. 
 
 '1 A 
 
 \ 
 
 iP 
 
 ^ — , 
 
 .. 
 
 
 
 sib, W 
 
 p; 
 
144 
 
 LAW OF MACHINES. 
 
 i 
 
 •'y the use of apparatus? We found tbnt w 
 
 and, consequently, with twice the y7olytL7v '"'''I '''''' ^' ^'' 
 
 It thus appears that, if it should be SI ,! ""'"'■ 
 greater velocity than it Is poss ble or ' ™°^' * '''''^''' ^'"' 
 
 n>oveJtn.aybeacco„,plisLlt?roth h T''"* '"' *^^ P^^^'^-" *« 
 applying to it a power pro on o,,nf "'''''*"''" °' * ™^^1"»«. by 
 apparatus is one of ZvcoT ^ ^'''''''' "'^" '^^ ^veight. This 
 
 advantages derived fro. th:te IV^Ichn r/s^:. -^'"^ ^' *''^ ™^"^ 
 ««^/^ ^'"^ """-^ '""''* '" ^^ -"•'^"'-^ « /«ra. resist 
 
 the forces of wZ 'f' "'^ strength of animals, 
 
 t-iastt.:or;;L;:s.:^^'- ^---« 
 
 The ratio of the weight to the power In any syste.n 
 of pulleys may be easily calcuLed , " 
 
 makinguseofthefact that the tension 
 of the same string is the same through- 
 out ^ts length so long as it passes romul 
 T1.0 weight is. Of course, the r s S'ofto'f ■"•'"'"'""^"P""^^'' 
 '■'"" <'-^ power is the tension or the Isl-u't if h'T"'? ^"PP°r*'»g 't, 
 's supported. What Is the ratio of , e lei'^J ^' " ^"■^*-^•^^ ^vhich it 
 
 wcignr, to the power In Fi;;. !>;? 
 
 §101. Lawof Machines. — Let Pl»ti,„„„ 
 
 timo, W the weight moved or extern! eL'e7„ '" " '''™" 
 "' M.e .„.canee through which it i lov , T 11^™°""''. ""'' 
 the. the ,„eeh.,.|«„ w„rk .ap„,ie„ to the : L , vZ T" '' 
 k..ogran,„,eters or f„„t-p„,„„„) , .,„„ „,„ ,„„,, i .^^^ r,;;" 
 
LAW OF MACHINES. 
 
 145 
 
 tTj:J;Z'''^! -ad.ines without exception, the follow- 
 
 No machine, therefore, creates or increases energy. No ma 
 elnne gives back more energy than is spent upon ft P can be 
 made as small as we please by taking, c.,eat e oth i^ fh 
 ease we see that -P^'o^omo/a.^o«;:/.;'Itr Z 'rfl 
 or velocity is lost. On the other hanrl tr^^'"'"^: '.'^'' ^*^^«'^^«' 
 
 ^^(ti. distance traversed; W-.^;L^;rZ 
 
 may be mcreased indefinitely by taldng P larg^ enough tS 
 
 case, asvelocity, time, or space is gaiZa, poS / Zol " A It 
 
 leceives. A bank will give you in exchange for a fiftv-dolkr 
 note fifty one-dollar notes; or, for fifty one-dol,ar nofes "/ 
 posited successively, it will return toyou a fifty-dollar note Tn 
 a similar manner, if you aonlv tn n Ln i • ^^ 
 
 to move 50 lbs 1 n ^ '""^ "" P^^^'' sufficient 
 
 In our v^liscussion hitherto we have io-nm-«^ tu^ • x , . 
 
 --.,.1-,.,.. „ „...., „5';«; i'Ji'S'; "vt 
 
 wo* to be done by the ,„.,cl,i„e is concerned. Let I re ,« 
 
 .acne, as .o;,.ed in i^; ^ tfXS:,' b^j: '°' 
 
 (2) P2)=Wt«-fr; 
 that is, the work applieU to a machine i. e.ual to the effective 
 
 m 
 
 1 ■ 
 
146 
 
 t)YNAMICS. 
 
 1:^ ).: 
 
 |:v 
 
 hi 
 f'2 
 
 » 
 
 work plus the internal work done by the machine. So, that su 
 far fl-om any machine being a source of energy, as is sometimes 
 enoneously supposed, no machine practically returns as much 
 energy as is applied to it. 
 
 By division. Formula (1) Pp = Wtv becomes ^ 
 
 (3) ^=-^ 
 
 i.e., weight : power : : the distance through which tfie power 
 moves : the distance through which the weight is moved in the 
 
 same time. Prob- 
 lems pertaining to 
 machines may gen- 
 erally be solved by 
 Formula (3), and 
 afterwards suitable 
 allowances may be 
 made for the in- 
 ternal work done. 
 Thus, suppose that 
 P (Fig. 99) is 10 
 lbs., and it is re- 
 quired to find what 
 weight (W) it will 
 raise. By experi- 
 ment, and also by 
 geometry, we find 
 that P travels 8 ft. 
 ft rp. ,,^, ,^ while W travels 4 
 
 The 20 lbs. in W ,8 just sufficient to balance the 10 lbs. in P- 
 anything less than 20 lbs. will be raised. 
 
 It 18 to be observed that, as we saw, § 89, work is not always 
 or even usually, expended in raising a weight, but in overcom^ 
 
 tlm«. .7"; '' ' "■'' ™''''^' »»t«'pret Formula (3) 
 
 thus . resistance : power : : the distance through which the powei 
 moves : the distance through which the resistance is overcome 
 
QUESTIONS AND PllOBLBMS. 
 
 147 
 
 QUESTIONS AND PROBLEMS. 
 
 1. If the power applied to any machiue is 2^ and It r,n,r»o «,ui. 
 
 velocity Of 10- per second, wi^ what velocity InTZTrZjZ: 
 
 n,«t, u ' """""'"S ^'''■''"Sh a space of lOQm, i^. paoable of 
 
 moving how many kilograms through a space of 2".? What adva^t^e 
 would be gained by the use of the machine ? advantage 
 
 3. Watch the movements of the foot in working the treadle of a 
 sewing-machine, also the movements of the needfe Tn sewing and 
 determine what mechanical advantage is gained by the machine ^' 
 J^ n^r^" three levers, as in Figure 98; and, calling the distance 
 (ab) of the po>.er from the prop the power-arm of the lever and he 
 
 eCimenullout^ "^-'* ^^^ *^^ P-P the ..-.. J^^e^fy by' 
 experiment the foUowmg special formula for levers : — 
 
 .^=.£ _ power -arm 
 P w weight-arm" 
 K.B. — EqulHbrium must first be established bctwopn th^ ♦„„ 
 lever, by placing weights on the 8hort arm. * *^'* """^ "' *« «"* 
 
 6. Ascertain the advantage that may be gained by each lever 
 
 a power oT2V:r:?"^" ' ""'''' ^""«* *^« P-P ^ P'aced in order that 
 
 end may move 4k at -^— ^— — — Slg.89. 
 
 the other end? What 
 
 will be the pressure 
 
 on the prep? 
 
 7. Show that the 
 results obtained in 
 the last problem are 
 consistent witii the 
 third law of parallel 
 forces (§72). 
 
 8. What advantage 
 Is gained by a lever, 
 
 when Its power-arm ^ ^^^^^^^^^^^^^^ 
 
 Is longer than Its weight-arm? What wsIT^^^^^^^^^^^^ 
 
 9. Two weights, of 6^ and 20^7*: .' ^'^ht-arm is longer? 
 
 lever 70- long Where mu^Mh!;."'f'"^'^ ^•"^^ '^^ ^^<^^ of »- 
 ,ft „., . ^ , "^'^^"'"^t the prop be Placed thnf.th»v!Ti-»vh«i=,-, 
 
 th. Otter side of JfulirClu W IT U '"'''''''^ ' """'" "^"' 
 
 !i 
 
 i'' if 
 
 fi i 
 fli 
 
 I it 
 
148 
 
 DYNAMICS. 
 
 II' uZ7T ''? ""'^' ""* ' ''''■ "' *^^ ^'*^ *h« «''^™o steelyard ? 
 
 14. How many meters must the power travel (Vi^r u^n^ t • , 
 bucket from a cavity 10". deej.? ^ ^ ^ ^" ''''"^*= ^^'« 
 
 16. (a) lu the train of wheel. (Fi... loi), if the circun.ference of 
 ^^«- 100- the wheel a is 30 in, and that 
 
 of the pinion 6 is 4 in, a power 
 of 1 lb. at P will exert what 
 force on the circumference of 
 the wheel d ? (b) if the cir- 
 cumference of the wheel d be 
 30 in., and that of the pinion c 
 6 in., the power of i lb. at P 
 will exert ,vhat force on the 
 circumference of tlie wheel f? 
 Wheel/be 40 in, and that of the axle . rV^^'' -•'•^""'ference of the 
 wm be nece^ary to prevent^t^lSl :;t;:'t::: '^^.Z:ZZl 
 
 ^^•^'*^- weighs llu.? (rf)IfWhasa 
 
 velocity of 5 ft. per second, 
 what will be P's \elocity? 
 
 16. Prepare a special for- 
 nmla for the solution of prob- 
 lems pertaining to the wheel 
 and axle. 
 
 17. The weight W (V\>r. 
 102), In traversing the in- 
 clined plane AB, only rises 
 thron-h the vertical hight CB, 
 while P must move throiigli a 
 distance equal to \B. J.ct 
 L represent t; . .t], of an 
 inchned plan.^ ii.,i i ? s hight 
 
 i«». A Skid 12 ft. long rests one end on a car t f f i i , 
 o«,cr c,„, „„ t„o ,ro„.d. w„„. force ,„„ ."a^ , l^ ^ ^J^l 
 
 ». During one revolution a screw aUvanco. » distance e,ual to the 
 
QUESTIONS AND PKOBLEMS. 
 
 149 
 
 distance between two turns of the thread, measured in the direction of 
 the axis of the screw. Suppose the screw in the letter-press, Figure 
 103 to advance ^ ,n. at each revolution, and a power of 25 lbs. t^ be 
 applied to the circumference of the wheel 6, whose diameter is 14 in 
 What pressure would be ex- 
 erted on articles placed be- ^- 103. 
 
 iieath the screw. [The cir- 
 cumference of a circle is 
 S.U16 times its diameter.] 
 20. The toggle-joint (Fig. 
 104) is a machine employed 
 where gi-eat pressure has to 
 be exerted through a small 
 space, ci ia punching and 
 
 shearing iron, and in print- 
 
 iBg-presses, in pressing the types forcibly against the paper. An 
 
 illustration may be found in the 
 
 Fig. 103. 
 
 joints used to raise carriage-tops. 
 Force applied to the p,g, j^^ 
 ioint c will cause 
 two links ac and 
 be to be straight- 
 ened, or carried for- 
 ward to d, while the 
 guides move through 
 a distance equal to 
 (ac + be) - ab. It 
 dc = lO™, ab = 98""^ 
 
 ,, ^ and ac + bc= lOOcm, i i 
 
 then a force of 80. applied at c would exert what average pressure 
 oi. obstacles in the path of the guides? pressure 
 
 21. Show that the hydrostatic press conforms in its operations to 
 the general law of machines. I'citt.iuiis lo 
 
 § 100 b. Moments and Equilibrium. -We often find it 
 convenient to consider the tendency of a force to produce rot.- 
 .on round an axis wliicli is at right angles to the plane in which 
 the force acts. Take, for example, any one of the levers in Fi^ 
 98, the force P has a tendency to produce rotation about the 
 point b m one direction, while W has a tendency to produce 
 rotation about b m tiie opposite directiou. If the lever is just 
 
 
 If 1 
 
 II 
 
 (i is j 
 
 n'''l 
 
 1^ 
 
150 
 
 DYNAMICS. 
 
 ii ;« 
 
 MM. 
 
 balanced, these t„o teudeueios a™ cvideoU... oc,„a. and op- 
 
 This tendency of a, force ic ,>;oduce rotation about a noinf 
 ■s called the moment o( that foroe with i-esnect t„ TT ' ' 
 -« b,.ie«,, the .no,„e„t of that J^ l^ 1° 2""'' '" 
 
 F,„„. c::„.„„en,s a!,„ad.v umde >,i.h levc-s i-' „i, 'he seen 
 
 p«..w;..«f™„o..,t;::h.":-^r:rti^^^^^^^^^^^^^ 
 
 we take a. tm measure of the moment of F about O iul ^' 
 
 and opposite to the momeut of a force Q about o",." "T^ 
 When, in a system of forces, the moments about n point 
 
 at:t::::o-t-:!:rcr:t:rrt:ar^^^^ 
 
 n.on,o„ts about O, of those fo,ce, of the'^jslTu h tl" 
 K the moments of a system vanish about a r>oi„t (l i . ■ 
 
 iX:r ^^"» '^^ "■» - --' o' '^e'riant' :m:: 
 
 If the moments of the system vanish al)on< r> n i , 
 about P, What do you .no. about th^ret, l^'cT hit Z^ 
 If the moments vanish about each of three po^ O ' tt, n 
 what do you know about the resultant ? If o .l, o ^' 
 
 in the same s,....;ght line, what do you k I " . , ,,',.? "I'f ""i 
 Can the line, . .tion of the resultlnt of ,",;"", 
 
 other than a s„.„ht line? If a svsten, . .brcL L in 
 
 libriura, that is, if the forces are' such 
 
 ountoract ouo 
 
op- 
 
 MOMENTS AND EQUILIDRIUM. 15l 
 
 ar^othcr, and, taken together, produce no eflfect, is there any 
 point about which their moments do not vanish? 
 
 If the resolved parts (§ 70) of a system of forces alon- a 
 line m one direction are together equal to the resolved parts^of 
 the same system along the same line in the opposite direction 
 the resolved parts of that system are said to vanish along that 
 
 If the resolved parts of a system of forces vanishlilong the 
 hne A B, can that system, as a whole, produce any motion 
 along A B in either direction. In the above case, what do you 
 know about the resultant of the system? 
 
 If the resolved parts of the system vanish along A B, and also 
 along C D, what do you know about the resultant? If A B and 
 C D are not parallel, what do you know al)out the resultant? 
 Can the hue of action of the resultant be at right angles t« 
 each of two lines which are not parallel to each other? If a 
 system of forces is in equilibrium, is there any line along which 
 their resolved parts do not vanish? If the resolved part^ of a 
 system vanish along each of two lines not parallel to each other 
 IS the system necessarily in equilibrium? Is a couple (§ 73) in 
 equ.hbrium? Do the resolved parts of a couple vanish along 
 each of two lines not parallel ? 
 
 From a careful consideration of the foregoing questions the 
 pup.l will see the truth of the foUowiug propositions, which are 
 very important : — 
 
 EQUILIBRIUM OF FORCES ACTING IN THE SAME PLANE. 
 
 1 . If a system of forces is in equilibrium the resolved parts of 
 the system vanish along any line tvhatever, and the moments of 
 the system vanish about any point whatever. 
 
 2. Jfth^ moments of a system of forces, all in the same plane, 
 vanish about each of three points not in the same straight line, the 
 system must be in equilibrium. 
 
 3. If the moments of a system of forces, all in the same plane 
 vanish about one point, and their resolved parts vanish alo^ig eoc/I 
 
 »%;• 
 
 If III 
 
 i 
 
152 
 
 .^Dynamics. 
 
 If it is known that a system of forces is in equilibrium the 
 
 fact that the system is iu equilibrium. Why? ^ 
 
 Example 1. 
 
 A uniform beam, 
 A B, 20 feet long, 
 weighing 300 
 lbs., rests with 
 one end against 
 a smooth, verti- 
 cal wall, C D, 
 and the other 
 «f')donasmooth, 
 f.orizoutal 
 plane, C B, this 
 end being tied 
 by a cord, C B, 
 16 ft. long. In- 
 vestigate the 
 forces acting 
 The beam Is evidently acted uoon hv fn„.. f "^"" *''^ ''''^'"• 
 
 own weight. 300 lbs., which since th.h •''' ""'"'^^ •" ^''' ^'' 
 posed to act at K, the Jd 1 ;: U :f 1"b "2i";lT' ''''' '^ •^"'^■ 
 string C B, . lbs., acting at B in^he diritn b C sf t^"'" °' ''" 
 the wall, X lbs. acting at A at rWU an' os to tb. n ^''''"''^ "^ 
 
 smooth; 4th, the pressure of thl n '''''"' ''"^« "'« «'«» '« 
 
 angles to the' floor^S^ L^; if rio^h'^' '^"'"^ '^^ ^ ^ ^'^"^ 
 
 ::;u:s:;:' !!^^^"^ ''- «-' ^-^->"- -^^ov^ we-ha;;';.;:^;;:::!;: 
 
MOMENTS AND EQPILIBEIOM. I53 
 
 _^__B.c.„se.her„„,ve<,p„.,„, .,, „,^„ ,„„, ,,„__^ _ ^^^_^^^^_ 
 
 Because the resolved parts vanish along a vertical line. 
 
 y = 300 (2). 
 
 Because the moments vanish about any point (say) H 
 2BH=:300OH ^3^ ' 
 
 Iherefore. substituting in equation (3) we have ^ 
 
 12 z= ;J00 X 8 
 
 300 X 8 
 ^= j^ — ^ = 200 
 
 But x = z 
 
 .-.2 = 200 
 
 Example 2. Let A 
 
 B (Fig. F) be a smooth 
 inclined plane, tlie angle 
 A being 30". Let a 
 heavy particle placed at 
 T> be kept at rest by a 
 string, D B. Investigate 
 the forces acting on this A 
 particle. The particle 
 is evidently acted on 
 by three forces, namely : 
 1st, its own weight, lOO 
 
 ltri'nr?ihf ' f "'''^^'^^ ^''•"^''"y ^"^"^^'•^ ' 2c1 the tension of the 
 n!' ^t D ' "h •' '" ""' "" ° ' •''' '''' P'-^««"^« «f t'^e plane y lbs 
 the pfa:e '' "'' ""^'^ *'^ P'^"« '« --*^' -*'"«" »* right a^^es i 
 
 A^r?,J^ '""'"'' ''^'''' '"""'"^ "'''''^ ""'' '''^^ "^^ ^<i the angle 
 
 X = 100 X i = 50. 
 ^Because the resolved parts vanish along D H. and the angle E D H 
 
 y = 100X^-2 = 50^3 
 
154 
 
 DYNAMICS. 
 
 To ai)ply the above piopositioo correctly, the pupil must be 
 esreful to take note of all the forces of the system in equi- 
 I.brmm, and to make no m--- h.' . « .pressing the resolved parts 
 and momenta. Resolved parts are discussed in §§ 70 and 716. 
 
 QUESTIONS AND PROBLEMS. 
 
 a Lio^rlir. ""''' '" *"^'' *"' '^"^"°" °" "»« ^«*=* '^^' tl^« moments of 
 wiTh ho .'' •" ^^"'"''••'"'» v«"-h about auy point whatever. and 
 
 wish the equation not to involve a particular force of the sys em 
 Where should you choose the point? If you wish the equation notTo 
 
 u^rts of th """"^ ^^ *" '''"**'°"' *''^''*'^ °" ♦^'^ f»^* '^^' the resolved 
 paits of the system vanish along any line whatever, not to involve a 
 particular force of the system. . V* line .borld you choo« 
 
 2. A uniform beam 37 feet V .^ -^ ..[ang 400 lbs., rests with one 
 end against a smooth wall, and the other end on a smooth Jo th s 
 
 iZxT, Tr^ '^ " '''''' '' ''''' ^°"^ ^° ^ ^'^ -t the bottom ofUe 
 wall ; find the tension of the cord, 
 
 3 In Example 2, if the weight of the beam acted at a point i of its 
 length from i he lower end ; And the reactions of the wall and iir 
 
 4. A beam A B, ^eighuig 120 lbs., a -ting at its middle point is 
 made to rest against a smooth v rtical wall and on a smooth floo by a 
 force apphec .rizom.ily to ti, -bot; find the force if the inclination 
 of the beam is ^n) 30^, (6) 45°, (c) 60°. '"cunation 
 
 6 In Example 4, if the weigh . .1 the beam acted at a point ? of its 
 
 SioV 7 ."» ':)»""" ■■' '°"°' '" '"="'-"°" "' «"""»- -- 
 
 ''• Taking the incllnatl, oft, beam a*, in p^o„.^i„ ^ a , , 
 a weightof 120 lbs. must be ^^aspended IXt I^-^ ttll rce^^: 
 ing m IbT " ""' '" ^''^"^'^ '' "^^' '^'^"^ '^^•"^ -^^-"' -d weS- 
 
 8. A uniform beam, weighing 60 lbs., rests wifh np. ....^ „^-._-. 
 peg in a smooth horizontal plane, and the other end" on a "waif" The 
 point Of contact with the wall divides the beam into parts as 3 I 
 
QUESTIONS AND PROBLEMS. 
 
 156 
 
 tlon of the beam being (a) 30°, (i) 45°, (c) 60°. 
 
 9 What relation must tl.e moment of the whole weight of a body 
 weights of Its several parts about the same line? 
 
 nnint' n ,.^^*'»^^''*1« ^""^ «f the moments of several forces about a 
 pomt or hne Ks understood, the excess of the sum of the positive mo 
 ments over the sum of t. negative n.on..nts. It is customary to con- 
 sider a moment positive wi.en the tendency to rotation is in the direction 
 opposi e that m which the hands of a watch n.ove when you are Tol 
 ^g at Its face, and negative, of course, when the tendency to rotation 
 Is In the same direction as the hands of the watrh. 
 
 10. How may you apply the answer to Question 9 to find the distance 
 of the centre of gravity of a body from a given line, when ,he weights 
 of the several parts of the body are given, and the distance of the 
 centre of gravity of each part from that lin. is also given? 
 
 gravity" (§'?!;). ''''^*'' ""^ ^ ^""^^ ""^^ ^' '"^P°^^^ '"^ ^'^ *' "« ««°t'-« «f 
 
 11. How may you apply the answer to Question 10 to tind the posi- 
 ti . of the centre of gravity of the body in Question 10? 
 
 Three uniform rods are placed so as to form a right-ancled 
 \so8celes triangle, the longest being 8^2 feet; find their C. G 
 
 13 A heavy tapering rod, weighing 160 lbs., balances about a point 
 4 f.-et from the heavy end, when a weight of 30 lbs. is attached to the 
 he other end; find the point about which it will h.,lanc. if he^ iS 
 Is removed. "^" 
 
 14. Four weights of 5, 7, 9, and 11 lbs. are placed a- the comers of 
 a square plate weighing 12 lbs., whoso side is 10 inches- find the 
 distance of the C. G. from the centre of the plate. 
 
 1«. Weights of 4, 6, 8, 7, 3, 2, 13, and 1 lbs. are placed at the cor 
 
 C. (j., the side of the square being 16 inches. 
 
 18. Aumfonnsquare plate whosesidcisl0inches,andweightl21bs 
 has a weight of 18 lbs. attached to one co ,.er ; where must it be sus' 
 pended by a cord so as . o rest horizontally? 
 
 17. A square table, weighing 40 lbs., rests on four legs, one at 
 each^ corner. Can you determine the pressure on each leg? If so! 
 
 '• .11 
 
 E '-I 
 
 m 
 ill 
 
 i « 
 
166 
 
 DYNAMICS. 
 
 18. From a circular plate whose radius is 8 inches, a circular plate 
 whose radius is 4 inches is cut away, the distance lietween tl^e two 
 centres is two inches; find the centre of gravity of the remainder. 
 
 „ ?K ^?™.* "'''^''"" '^'''''"^^'" ^^'^^' ^'^o^^ diameter Is 10 inches, 
 another disc, having for its diameter the radius of the first circle, is cut 
 away; find centre of gra\ity of the remainder. 
 
 20. From a circular disc whose diameter is D, a circular disc whose 
 diameter is d is cut away; find tlie centre of gravity of the remainder, 
 tne distance between their centres being a. 
 
CHAPTER III. 
 MOLECULAR ENERGY. - HEAT. 
 
 XVII. WHAT HEAT IS. -SOME SOURCES OP HEAT. 
 In the preceding pages the theory of heat has been several 
 
 § 102. Mechanical motion convertible into heat -F, 
 
 SLt- Zi Tr ""■"" ■"°«' '»»' ""O" » -P°"^ re'vln, a'; 
 
 .r .t:: r.:,:3r.'r ,::,■=:; zz 
 
 checm Ucomes heat. When tl» brakes are applied T he 
 wheel, of a rapidly „,ovtog railroad train, its motion is a^e^" 
 verted ,„,<, heat, ™„el, of which may be found in the wheTls 
 brake-bloeks, and rails. The meteorites, or " shooting.rr^ " 
 wh,ch .are seen at night passing through the upper afr some 
 ™es strike the earth, and are found I be stene' heawTt 
 l.ght-g,vmg state. Thev beeome heated when they reaeh our 
 a mosphere, m conscquenee of their motion being cl>eck<^ by 
 the resistance of the air. ' 
 
 § 103. Heat convertible int/^ «,«„>,»_,•— ■ _.», 
 Expenment. Take . .1„„ g,a., «.";k-A,'Fig7re"7;n,,rhT?uT. 
 with water, ftt a cork air-.,,,,.. ,„ ,., neck. Perto™^ X c"r" 
 ■ A lto<Kl w., B „.k. . crt „|r.„,M 1. B „„t „ ,„ „„,^ ^„„„ 
 
 ii 
 
 !l 
 
 
 • l.'r'l 
 
 Iff? I 
 
 ; '"pl 
 
 
'■4 
 
 H • 
 
 158 
 
 MOLECITLAR ENERGY. — HEAT. 
 
 
 IL 
 
 ^ Here heat produces mechanical motion, and does work in rai^ 
 -g a weight in opposition to gravity. Ever, steam ^gineTa" 
 Pi. 10. hm-eyu,ine. AH the power of steam consists 
 
 in its heat. The steam which leaves the cylin- 
 der of an engine (see § 148), after it has set 
 the piston ;n motion, is cooler than when it 
 entered, and cooler in proportion to the work 
 done. Furthermore, it will be shown (§ 145) 
 that heat and work are so related to eadi other 
 that a definite quantity of the one is always equal 
 to a definite quantity of the other. 
 
 Now, when the appearance of one thin<r is so 
 connected with the disappearance of another, 
 that the quantity of the thing produced can be 
 calculated from the quantity of that which dis- 
 appears, we conclude that the one has been 
 formed at the expense of the othor, and that 
 they are only di'^erent forms of the same thing. 
 We have, therefore, reason to believe that heat is of the same 
 nature as mechanical energy, i.e., it is only another form of 
 kinetic energy. j ">- uj 
 
 r "^ ,.^*** defined. -A body loses motio,. i,> comrauni- 
 ca ing ,t (5 67, The Lammer descends and strike, the I- 
 V.1; lis motion ceases, hut the anvil is not sensibly moved; 
 the only observable effect produced is heat. Instead of the pro' 
 
 f„'rr.h""""°;; "' "'" "'""■"" ■" ^ ""*• "■««= - "«". "oc^rd. 
 
 ■ng to the modern view, an increased vibratory n.otion of the 
 
 ma,..„fe, that compose the hammer, -a^ere cLn,e of ItZ 
 
 n W andlocalU,. Of course, this latter motion's invisibTe! 
 
 . "~ ~: — ' '" ^^'"^ ''^"' '^ molecular molion. A bodv is heated 
 
 by avmg the motion of its molecules quickened, and-cooledty 
 parting witli some of its molecular motion. 
 
HEAT DEFINED. 
 
 159 
 
 n,i.,\, generated by chemical action. -Experi- 
 
 ^ono f T^*"-^ "" ^'^^'' '''^-^''^'' "^'^ ^"" «f ^«ld water, and pour Into 
 can th Tn r'"'"' "' '"'^^'""'^ '^^'^- W'^^^t '« the effect? Whence 
 
 thT Ti, "r""' ™"'""'" ""^'"" ^"^^^ ««- •^o- the volume : 
 
 oped The invisible oxygen of the air combines with the vari- 
 ous fuels, such as wood, coal, oils, and illuminating gas, and 
 gives nse to what we call burning or combustion, by which a 
 large amount of heat is generated. In all such case; the heat 
 18 generated hy the combination or clashing together of mole- 
 cules o substances that have an affinity {i.e., an attraction) 
 for each other. Before their union they are in the condition 
 of a weight drawn up; while approaching each other they are 
 like he falling weight; and when they collide, their motion, 
 like hat of the weight when it strikes the earth, is converted 
 into heat The chemical potential energy of the molecules is 
 converted, in the act of combination, _to molecular kinetic 
 energy, — into molecular motion. 
 
 § 106. Origrin of animal heat and muscular motion — 
 The plant finds its food in the air (principally the carbon dioxide 
 m the air) and in the earth in the condition of a fallen weicrht • 
 but, by the agency of the sun's radiation, work is performed 
 upon this matter during the growth of the plant; potential energy 
 18 stored in the plant, -the weight is drawn up. The animTl 
 now finds its food in the plant, appropriates the energy stored 
 in the j)lant, and converts it into energy of motion in the form 
 of heat and muscular motion. The plant, then, may be re- 
 garded as a machine for converting energy of motion 'received 
 from the sun into potential enngy ; the animal, as a machine 
 lor transforming it again into the energy of motion 
 
 1« 
 
 R- 
 
 i li : 
 
 
 
 r£.<i 
 
 > 
 
160 
 
 MOLECULAR P^NERGY. — HEAT. 
 
 
 I 
 U 
 
 II i 
 
 I'i I 
 
 § 107. The sun as a source of energy. - Not only is the 
 
 un the source of the energy exhibited iu the growth of plants, 
 
 as well as of tlje ..uscular and heat energy of the anirnul'but 
 
 IS the source, d.rectly or indirectly, of very nearly all the ner^y 
 
 en^i^oyed by „.an in doing work. Our coal-bedsf the results;' 
 
 the deposit of vegetable matter, are vast storehouses of the sun's 
 
 energy, rendered potential during the growth of the plants n.any 
 
 ages ago. Every drop of water that falls to the earth, and rolls 
 
 nowT7of"t ":: -"f'buting it. n.ite to the unbounded water- 
 
 powe of the earth, and every wind tiiat blows, derives its power 
 
 directly from the sun. ^ ^ u^ power 
 
 If a man were to make use of the ocean tides for drivin.. 
 
 TZ7 r ""' ' 1" "^'"^^-"-g^ •^--ived from what soured 
 B3 the friction ol the tides against the coast the water and 
 and are warmed. What is the source of this energy? t 
 tins source inexhaustible? U it increased itself fSn any 
 other source? If not, what must be the effect of this co ' 
 
 t":? "^"" '''• '' '''' '''^'"^' ^«-^ ^- ^« ^ound outsMe 
 
 XVIII. TEMPERATURE. 
 §108. Temperature defined. —If bodv A Ju i i. • 
 co„.ac. with .„.,, B, „„„ A l„.os ,„k, b'L r f„ t, t, t ^A Z 
 a,d to have ha.I o,i,n„an,- „ higher ,nnpcra>.,re thu, 
 m.thor body gai„» or loses the,, both l,ad the sa„,e tcLra 
 u e Temperature is tke state .^f a M, e.^smereU JZfit 
 ence to Us pou,erof c.m„„„{cunni, Ma, lo or r.c«vi„, hen, rL 
 other W«. The direction of , he flow of heat deter,'i„e» wh 
 of two bod,es has the higl,er temi,e,ntn,c 
 
 It may be n.athematieally demo„st,ated that, if tl,e ave,-a.re 
 k,net,e energy of eaeh u,oh.oale of A is eaoil to "° •""■"'«'' 
 kuetie energy of each n,o,ee„,e of B, «.«;":;:„ 7 ..Ta?: 
 brought into contact the inoleeales of A wjii „-.»!■.. '■ T 
 
 those of A. Hence we may say tlmt two bodies have </« same 
 
TEMPERATURE. 
 
 161 
 
 temperature wh.n the average kinettc energy of each molecule of 
 the other' '" ''*' *'"'''^' ^''''^'' '""'^^ ^-^'"''^ '^^^'''^' ^-^ 
 
 heit^^^'-n'^T^^''^*''''^ distinguished from quantity of 
 
 neat. - Ihe tonu temperature has no refeivuee to quantity of 
 
 heat. If we mix together two equal quantities of a substance at 
 
 ^he same temperature, the temperature of the mixture is not the 
 
 sum of the temi)eraturos, - it is not greater or less tlian tliat oi 
 
 either before they were mixed ; but evidently the mixture contains 
 
 vv.ce as nmcli heat as either alone. If we dip from a gallon of 
 
 boilmg water a cupful, the cup of water is just as hot, i.e., 
 
 has the same temperature, as the larger quantity, although of 
 
 cou,-se there ,s a great difference in the quantities of heat the 
 
 two bodies of water contain. Temperature depends upon the 
 
 average lanet^c energy of the individual molecule, while quantity 
 
 of heat depends upon the average Kinetic energy of the individual 
 
 molecule multiplied by the number of molecules. 
 
 XIX. DIFFUSION OF HEAT. 
 
 There is always a tendency to equalization of temperature; 
 tha ,s, heat lias a tendency to pass from a warmer body to a 
 colder or from a warmer to a colder part of the same body, 
 until there is an equilibrium of tempeiuture. 
 
 If you put your hand in contact with a body at a lower tern- 
 perature than your hand molecular kinetic energy passes from 
 your hand to the body, and you experience a sensation which leads 
 yon to say that the body feels col.l. If the body is at a higher 
 ten,i,erature than you, hand your hand gains molecular ki.^tio 
 nierg.v, and you experience a sensation which leads you to say 
 that the body feels warm. 7 h. intensify of the sensation depends 
 upon the rate at which your hand loses or gains molecular L.fic 
 energy. In other words, it depends upon the rate at which the 
 temperature of your hand falls or rises. The rate at which the 
 temperature of your hand changes tvill depend, of course, upon the 
 
 ,m 
 
 U 
 
 il ■ 
 
 til 
 
 '"ii'f 
 
162 
 
 MOLECULAR ENERGY. —. HEAT. 
 
 tnie ofthatpart oj the body in immediate contact wi'h it. 
 
 abouU^°: Io.!?° nfrr*'°^; ~ ^''"^""'''"* »• ^'"-- on. end of a wire 
 Api^yonrZ^^V^';^'""''' ''''''''''' "- «^'- -»I i" tl.e hand. 
 
 Pla'ces ot- Lot 'r' ?""^"' "^ -equally-heated bo.ly, fron. 
 Zton 2t r i^^---f-»-ver temperature, is called c..- 
 
 enld .h !, ""''" ^" *^' ^^""^ '^^^« their motion nuiek- 
 
 ened, th y strike their neighbors, and quielcen their motion the 
 latter .n turn quicken the motion of tl,e next, and so 0,?;. 
 some 01 the motion may be linally communiclte.) to th 'h d 
 and creates in it the sensation of heat. 
 
 Experiment 2. Hold wires of dhlbront uwfnU nf ti, 
 also a glass tube, a pipe-stem ete i h ' ''"'"^ ^'^"^'"'- 
 
 :;^::.Ser-— --— ^ 
 
 probably fin;:;;:",;" a r rs::rrr '" "^^^^"'"' ^-^^ ^^'" 
 
 perature. Place your ban on tl,o ^ ^' ""^'■^•' ""^ '^"^^' *^''"- 
 
 lmveverydiflc.en t ,.Su"s ivTr^"''"^ *'"'^^- ^^^^'^ '- 
 
 wood. Theironfeel codlr than tZ '"T" '"" ""' ^^ '^'"^"^ «^ 
 iron and the other o th woo.l f '^°''''- ^^''^ «'"^ '•'^"^' «» the 
 
 mometertothe n^^c "late^i'T T f^T '^^"" ^^'''^ ^'^ «-- 
 still atthesan.ue;.u" at r:P^:rH:^^^^^^^ '"""J'^- ^'•^' ^"^'-^ 
 
 been raised by contaet with yonZ^o^^^^''''' "T' """' ""^ 
 of the iron, in the same tinw/ i !■ ^! ''''''''''^"''^^ 
 between the «ensatior:::pr:. J^ H^';::^-^;-;'' '''--^ 
 account for tlie fact itself? 1. u -, , bands .^ Mow can you 
 
 dactor, and henc^ h ^^^j,.^:; ^ ^^S ""^V'","""' '^ ^ '^'"' ™"- 
 
 because th.-. heat it receives from b , ""' '' '"''''^^^ ^^'"•'"^'^' 
 
 other parts .>f the bockw ,"" '' ""' '"'"^"'■^''•' '^^^•'^^ ^o 
 
 ndne wlu.ther wl't'L.^ ".;"'"" ;;'"' "" ^"" -.^«-t to d.-ter- 
 . --' —'•••"'?-. !Hut r.-t,pi(iiy or iKjtr 
 
 ftxpcnmeiit 4. Twist toL'otber -it rm,> o.wi • •. 
 
 "i ft' 
 
OOifVECTiON. 
 
 163 
 
 seeing ,.o. far J^n rZioTt wm ^^ "^^^^ ^'^'^^' ^^^^ «^-' ^^ 
 
 tl-gh they cliffer widely a^ongte^^^^^^^^^^^ '"^ ^°'^'"^^^^' 
 
 tube near the surfal of ^hh^^te^ Do ™''' ^ '"' "'''' °' "" 
 you find that the heat is rapidly or slow- ^^«- ^'»- 
 
 ly^^tmnsferred to the lower part of the 
 
 Liquids, as a class, are poorer con- 
 ductors than solids. Gases are much 
 poorer conductors than liquids. It is 
 difficult to discover that pure, dry air 
 possesses an^ conducting power The — - 
 
 y muMug or heated substances, the onerilion 5^ ^„ii » 
 convection • huf fi,;c t- • "I'erauon is callea 
 
 not necesaarilv «« «, , f ^''« <^''^"veyai'ce ; fluids do 
 
 ncceesanly, a. may be seeu by the foUowiug experiments . ■»- 
 
 I 
 
1 
 
 164 
 
 MOLECtTLAR EKERGV. — SEAT. 
 
 Experiment 1. Arrange apparatus as In Fig. 107. Fill tlie larf'e 
 beaker nearly full of water, and elevate It so that the tip of a Bunsen 
 flame may just touch the middle of the bottom. Fill a glass tube B 
 
 Fig. 107. 
 
 Fig. 108. 
 
 With a deeply-colored aniline solution, stop one end 
 with a finger, and thrust the other end into the water 
 to the bottom of the beaker; remove the finger, and 
 allow the solution to flow out and color the water at 
 the bottom for a little denrh. Soon the colored 
 liquid immediately over the flame becomes heated, 
 expands, and thereby becomes less dense than the 
 liquid above ; consequently it rises luu] forms an up- 
 ward current through the colorless liquid. At the 
 saftie time the cooler liquid on the sides descends to 
 take the place of that which rises, and soon the 
 descending currents become visible by the coloration 
 of the water. By this means heat is conveyed to all 
 parts of the liquid, which would otherwise become 
 much hotter at the bottom than at the top in couse- 
 quence of the poor conducting power of water. 
 
 If a glass tube C, bent as shown in the figure, is 
 filled with water, and introduced into the beaker so 
 that the orifice of the short arm shall be 
 just beneath the surface of the colored 
 water, the colored liquid will be seen slowly 
 to ascend the short arm, while the colder 
 water will descend the longer arm. 
 
 Experiment 2. Provide a tightly-cov- 
 ered tin vessel (Fig. 108) and two lamp- 
 chimneys A and B. Near one side of the 
 top of the cover cut a hole a little smaller 
 than the large aperture of chimney B. Near 
 the opposite side of the cover cut a series of 
 holes of about 7""" diameter, arranged in a 
 circle, the circle being large enough to ad- 
 mit a candle without covering the holes. 
 Light the candle, and cover it with chim- 
 ney A, which should be outside the circle 
 of holes Fasten both chimnoys to the 
 cover flith wax^ Iloid siuuki.ig touch-paper C (see § 265)'; near the 
 top of chimney B. The smoke. instea.l of rising, as it usually .loos 
 rapidlj descends the chimney, and in a few seconds will be found 
 
VENTILATION. 
 
 165 
 
 ascending the chimney A. How do you explain this movement of the 
 smolie ? Cover the orifice of B with the hand. What happens after a 
 short time? Why? 
 
 The last experiment furnishes an explanation of many 
 familiar phenomena. It explains the cause of chimney- drafts, 
 and shows the necessity of providing a means of ingress as well 
 as egress of air to and from a confined fire. It explains the 
 metliod by which air is put in motion in winds. It illustrates a 
 method often adopted to ventilate mines. Let tlie interior of 
 the tin vessel represent a mine deep in the earth, and the chim- 
 neys two shafts sunk to opposite extremities of the mine. A fire 
 Ifept burning at the bottom of one shaft will cause a current of 
 ■■■iv to sweep down the otiier shaft, and through the mine, and 
 thus keep up a circulation of pure air through the mine. 
 
 Liquids and gases are heated by convection. (Wliy not 
 solids?) The heat must be applied at tlie bottom of the body of 
 liquid or gas. (Why not at the top?) There is a still m'ore 
 important method by which heat is diffused, called radiation, 
 the method, for example, by which heat reaches us from the sun, 
 which will be treated of in its proper place, under the head of 
 radiant energy. 
 
 § 112. Ventilation. — Intimately connected with the topic 
 Convection is the subject (of vital importance) Ventilation, 
 njasnnich as our chief moans of securing the latter is through the 
 agency of the former. The chief constituents of our atmos°)here 
 are nitrogen and oxygen, witli vaiying quantities of water vai)or, 
 carbon dioxide, sometimes called carbonic acid, ammonia gas, 
 nitric-acid vapor, and other gases. The atmosphere also eon- 
 tains, in a state of suspension, varying quantities of small 
 particles of free carbon in the form of smoke, microscopic 
 
 Oiofanisms. and (hist nf innnmoroJjln o.ih°f'jii'>c'' \" -'• 'I 
 
 constituents, except the first three, are called impurities. Carbon 
 dioxide is the imi)urity that is usually the most abundant and 
 and most easily detected ; so it has come to be taken as 
 
 >i^? 
 «« 
 
 Ht 
 
m 
 
 
 p 
 
 ithi. 
 
 MOLECtTLAil ENERGY. — HEAT. 
 
 tlie measure of the imnnritv ,.f fi 
 
 contams about 4 parts of it by vohnue ia 10 000 T ."i ' 
 
 t.tv rises to 10 parts, the air beeo.es u'vho'lele '" ^^""^"■ 
 
 in the water, and ^00^ h „ 1^"''', ""'^"^ "^^'^ '''' "»^^ ^'^^-Ivec 
 suspended for a timeTn . ' ' '""" ^■'^'•"^"^t'^' ^vluch remains 
 
 at the bottom " "'' ''^""'' '"* «"^"^' «^"1- - a white powder 
 
 Experiment 2. Take a fresh quantity of lime-water iu eaeh of two 
 gh^sses, and i„ any i,oorly-ventilated room wlcl Is 
 been occupied by several persons for a sho t in^ 
 (unfortunately almost auy school-room willausw 
 tJ e purpose), place one gias.s near the floor and w h 
 a he lows blow into the liquid a few pufls of the ow 
 stratum of air. Then place the other glass near 1^ 
 op of the room, and blow with the beUows some 
 the upper stratum of air into the lime-water. 1 ., 
 oases carbon dioxide will be found to be present 
 
 rsho«?bv tr" ''""'"'^ "• "''^ '"^'^- ^^raium, 
 as sho«n by the greater rapidity with which the 
 
 cloudu.ess is produced in the upper stratum! 
 
 Pig. 109. 
 
 pla^>WrrToot- ^" "^« ^-t'^'- «f ^ «nmll circular 
 Plank (J,g. 109) insert an iron wire O0"-Mon- and 
 
 7- in diameter. At intervals of 9™ sold r fo t e 
 
 w.re short pieces of small wire, so as to p oleet 
 
 hor,.outally from the large wire; and to the fr c e" 
 
 trem.fes of these short wires solder small cL^uIar 
 
 platforms spirally around the vertical wire Fix 
 stmnps of candles upon these platforms by means of 
 " l.ttlc melted tallow. Light the candles, and oaref^ W 
 <«>ver the whole with a tall ^Wass u^ jr'^J^^ 
 
 replaced b7ca;bo"r;St'';r'r" '"" '^"^ ^^''^'^''^ extracled' a'nd 
 thetopot-L^r- t^U^S^;,^^^^^^ «--• -> --"-dates at 
 
 ".hlch til,- 
 
VENTILATION. 167 
 
 Carbon dioxide is about one and one-half times heavier than 
 air at the same temperature ; consequently, when both have the 
 same temperatm-e, and the former is very abundant, it (ends 
 to settle to the bottom, as in the vicinity of lime-kilns, in which 
 large quantities of tliis gas are generated. 
 
 The knowledge of this fact has led many to suppose that a 
 means for the escape of impure air need only be provided near 
 the floor of a room. But it siiould be remembered (1) that the 
 tendency of carbonic-acid gas, unless present in excessive quan- 
 tities, IS to diffuse itself equally through a body of air ; but (2) 
 when it is heated to a temperature above that of the surroundin.r 
 an-, as when generated by flames, or when it escapes in the warm 
 breath of animals; it, or rather the air with which it is mixed, 
 IS lighter than the surrounding air, and consequently rises! 
 If this impure air could escape at the ceiling while fresh air 
 entered at the floor the ventilation would be good. But usually 
 this fresh air must be warmed ; and in passing over a stove, 
 furnace, or steam radiator, its temperature will generally become 
 higher than that of the impure air, so that it will rise above the 
 latter, and pass out at a ventilator in the ceiling, leaving the 
 floor cold ; hence, the most impure air is often found iu\i<rh 
 school-rooms half-way up. " 
 
 Experience shows that, with the ordinary means of heating, 
 it is usually best, in cold weather, to provide for the escape of 
 tin; foul air at the floor into a flue, in which a draft is maintained 
 by a neighboring hot chimney-flue, or a gas-burner, while the 
 warm, fresh air is introduced at the floor, on the opposite side 
 of the room, or sometimes at the ceilino. 
 
 The (luantity of fresh air introduced must be great enoucrh to 
 dilute the impurities till they are harmless. An adult nmkes 
 about IH respirations per minute, expelling from his lungs at 
 each inspiration about 500"'" of air, over 4 oer cent, of whioh is 
 carbonic acid. At this rate about 9,000-"' of air per minute 
 become unfit for resniration ; and, to dilute this suflTiciently, good 
 
 !•• 
 
 
 authorities say that about IQO tii. 
 
 -as much fresh air is needed 
 
168 
 
 MOLKCULAK ENTKRG Y. — H K AT. 
 
 ?f r, 
 
 w 
 
 lb t 
 
 Wi 
 
 or, for i)roi)er ventilation, about <, cubic mi'tar of fresh air per 
 minute is neede.l for each i>erson, or, in KnglislMueasure, 2,000 
 caibic fc'L't per hour. 
 
 Jf the heating eouhl be so arranged us to keep the floor prop- 
 crly warnuHl, the v.tiat.dair might puss out at the ceiling, and 
 the quantity of fresh air entering at the floor n.ight be nn.eh 
 less than that just stated. I„ „,il,l weather, when the fresh air 
 does not require warming, the inlet may be at the floor and the 
 outlet at the ceiling. 
 
 QUESTIONS AND PROBLEMS. 
 
 1. How would ya» ventilate the tall jar in Expeyiuieiit 3? 
 
 2. At evening assemblies, i„ lighted halls, what two fruitful sources 
 of carbonic acid are ever present? 
 
 3 Why are gas-burners frequently placed under the orifices of 
 
 \ dltllcli ■'.; ill J 
 
 4. A b. -room is 3'" square and 2.o- high; how long would the en- 
 closed Hi; sm ply two persons on the sui.position that none was to be 
 
 ..^\^ ^f '''""'''"' ''' "'«""''^"^' Per««»S' a»tl its dimensions are 3o X 
 18 X 7". How often should a complete change of air be eflTected that 
 It may not become vitiated? 
 
 6. A silver-mine was situated on an island in Lake Superior so small 
 that here was room for only one shaft. Can you suggest a contrivance 
 by wnclut might have been ventilated? Apply your contrivance to 
 ventilate a tall jar with no opening except a narrow one at the top, and 
 test its efficiency by burning a candle at the bottom of the jar 
 
 XX. EFFECTS OF HEAT. - EXPANSION. 
 
 Havliig learned something of tlie nature of heat, and how it 
 pjisses from point to point, let us examine the effects it pro- 
 duces on bodies : these are expansion and dumge of state. 
 The first gives a means of measuring tmiperature, and leads to 
 a fuller study of gases than we have yet made. Under the 
 second effect of heat we study liquefaction and vap ,rizalion. A 
 
EXPANSION OF SOLIDS. 
 
 169 
 
 third effect that is very obvious, the chmge of temperature, will 
 be found to depend in part on whtit is called specijic heat, to be 
 studied in § 139. 
 
 Fig. uo. 
 
 §113. Expansion of solids, liquids, and gases. —Ex- 
 periment 1. Obtain two short brass t -one of a size that will 
 permit it just to cuter the bore of tli. r. Hout the smaller tiil)c; 
 now try to push it within the other. 
 
 Experiment 2. Fit stoppers tiylitly in the necks of two similar thin 
 glass flasks (or test-tubes), and through eaeh stopper pass a glass tulje 
 about ()0«>» long. The flasks must be as nearly alike as possible. Fill 
 one flask with aleohol and the other with water, and crowd in the stop- 
 pers so as to force the liquids in the tubes a little way above the corks. 
 Set the two flasks into a l)asiu of hot water, and note that, at the 
 instant the flasks enter the hot water, the liquids sink a little in the 
 tubes, but quickly begin to rise, until perhaps 
 they reach the top of the tubes, and run over. 
 Why do the liquids sink at first? When they 
 begin to rise, which rises faster? What do you 
 learn from the experiment? 
 
 Experiment 3. Take one of the flasks used in 
 the last experuuent, dry it well inside and out- 
 side, invert the flask, insert the end of the tube 
 in a bottle of colored water (Fig. UO), and apply 
 heat to the flask. What happens? What does 
 it prove? Remove the flame. What happens? 
 Explain. 
 
 § 114. Coeflacients of expansion. — 
 
 There being generally greater cohesive 
 force between the molecules of solids than 
 between the molecules of liquids, the for- 
 mer expand less than tlie latter on receiving the same increase 
 of temperature, and for the same reason liquids expand less 
 than gases. All gases expand alike for equal differences 
 of temperature, a7ul the expansion is very nearly uni- 
 form at all temperatures. Under uniform pressure the vol- 
 ume of any body of gas is increased by ^f^ its volume at 
 
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 (716) 872-4S03 
 
 
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170 
 
 MOLECULAR ENERGY. -— HEAT. 
 
 the n-ee^ng-poiui of water for every degree Centigrade, 
 oi j,T for every degree Fahrenheit, its temperature is 
 raised. These fractions are called the coefficients of expansion. 
 Not only do the coefficients of expansion of liquids and solids 
 vary with the substance, but the coefficient for the same sub- 
 stance varies at different temperatures, being greater at high 
 tnan at lo», temperatures. 
 
 In the expansion of fluids we have only to do with increase of 
 volume, called cubical expansion. In the expansion of solids 
 we have frequent occasion to speak of expansion in one diroc' 
 tion only, and this is called linear expansion. 
 
 § lia Power of expansion and contraction. -The force 
 which may be exerted by bodies in expanding or contracting may be 
 very great, as showa by the following rough calculation: if aTLn 
 
 500O c ?a d^uirr^dT';; '7f '' '"'" '" ^^ ^'"''^''''^ ^-^^ «^ -<-r) to 
 500 C (a dull red heat), its length, if allowed to expand freely, win be in- 
 
 l^ow, a f.rce capable of stretching a bar of Iron of 1 sq. in section 
 lus amount is about 90 tons, which represents very nealy the "0": 
 Uiat would be necessary to prevent the expansion caused by heat ^ 
 would require an equal force to prevent the same amount of tntra^ 
 t.on ^caused by what?) if the bar is cooled from 600" to 0° C 
 
 Boiler-plates are riveted with red-hot rivets, and tires are* fitted on 
 carnage-wheels when red-hot; why? How may a glass stopper suck 
 fast m a bottle, be removed without breaking the bcTttle? 
 
 §116. Abnorsial expansion and contraction of water 
 -Water presents a partial exception to the general rule thai 
 matter expands oa receiving heat and contracts on 101!^ 
 If a quantity of water at 0° C, or 32° F., is heated t 
 
 about 39 F., when its volume is least, and therefore it has its 
 maximum aensity. If heated beyond this temperature it ex 
 pands, and at about 8° C. its volume is the samJ as at C°. On 
 cooling, water reaches its maximum densitv at 4° C o^d pv 
 pands as the temperature falls below that point. It is'probable' 
 
THERMOMElRy. 
 
 17J 
 
 that crystallization, and consequently expansion (§ 24), begins 
 at 4° C. (What is the temperature at the bottom of a pond 
 wlieu water begins to freeze at the surface?) 
 
 XXI. THERMOMETRY. 
 § 117. Temperature measured by expansion. — The ef- 
 fects of expansion by heat are well illustrated in the common 
 tliermometer. As its temperature rises, both the glass and the 
 mercury expand ; but, as liquids are more expansible than 
 solids, the mercury expands: much more rapidly than the glass, 
 and the apparent expansion of the mercury _ shoion by its rise in 
 the tube, is the difference between the actual increase of volume 
 of the mercury and that of the part of the glass vessel containing 
 it. The thermometer, then, primarily indicates changes in vol- 
 ume; but as changes of volume in this case are caused by 
 changes of temperature, it is commonly used for the more 
 important purpose of measuring temperature. (Will a ther- 
 mometer measure quantity of heat ?) 
 
 § 118. Construction of a thermometer. — A thermometer 
 generally consists of a glass tube of capillary bore, terminating 
 at one end in a bulb. The bulb and part of the tube are filled 
 with mercury, and the space in the tube above the mercury is 
 usually, but not necessarily, c, vacuum. On the tube, ^r on a 
 plate of metal behind the tube, is a scale to show the hight of 
 the mercurial column. 
 
 § 119. Standard temperatures. — That a thermometer 
 may indicate any definite temperature, it is necessary that its 
 scale should relate to some definite and unchangeable points 
 of temperature. Fortunately Nature furnishes us with two 
 convenient standards. It is found that under ordinary at- 
 mospheric pressure ice always melts at the same temperature, 
 called the meltiiig point, or, more commonly, the freezing point 
 
 ■ ' ■ 
 
 m 
 
 
172 
 
 MOLECtTLAR EKERGY. — HEAT. 
 
 Again, the temperntnre of steam rising from boiling water under 
 the same pressure is always the same. 
 
 § 120. Graduation of thermometers. — The bulb of a 
 thermometer is first placed in melting ice, and allowed to stand 
 until the surface W the mercury becomes statioufi-y, and a mark 
 is made upon the stem at that p.int, and indicaies the freezing 
 point. Then the instrument is suspended in steam rising from 
 boiling water, so tliat all but the very top of the column is^in tlie 
 steam. The mercury rises in the ston- until its temperature be- 
 comes the same as that of 
 
 Fig. 111. 
 
 Water bolls . 
 
 Blood licnt., 
 
 Nfax. den.of water 
 Water freezes 
 
 212° 
 
 98° 
 
 100°., 
 
 Abs. 
 temp. 
 
 373° 
 
 Mereury freezes., 
 
 39.2° 
 32°.., 
 
 —37.8°. 
 
 No heat . 
 
 —38.8° 
 
 310° 
 
 277° 
 273° 
 
 234.2°. 
 
 —460°., 
 
 the steam, when it again 
 becomes stationary, and 
 another mark is placed 
 upon the stem to indicate 
 the boili7}g jwint. Then 
 the .space between the two 
 points found is divided into 
 a convenient number of 
 equal parts called ' rees, 
 and the scale is • uled 
 above and below these 
 points as far as desirable. 
 Two methods of division 
 are adopted in this coun- 
 try : by one, the space is 
 divided into 180 equal 
 parts, and the result is 
 , ^ called t lie Fahrenheit 
 
 scale, from the name of its autlior ; by the other, the space is 
 divided into 100 equal parts, and the resulting scale is called 
 centigrade, which means one hundred steps. In the Fahrenheit 
 scale, which is generally employed for ordiiuirv household pur- 
 poses, the freezing and boiling points are marked respectivelv 
 82 and 212=. The of this scale (32^ below freezing point), 
 
 —273°.. 
 
CONVERSION FKOM ONE HUALK TO THE OTHER. 173 
 
 Which is about llie lowest tcMuiH-nitaro that can be obtained by 
 a mixture of snow and salt, was incorrectly supposed to be the 
 lowest temperature attainable. The centigrade scale, which is 
 generally employed by scientists, hm its freezing and boilina 
 points more conveniently marked, respectively 0" and 100° A 
 temperature below 0° in either scale is indicated by a minus si^n 
 before the number. Thus, -12^ F. indicates 12° below 0° (m- 
 44 below freezing-point), according to the Fahrenheit .cale. 
 Under I. and C, Fig. Ul, the two scales are placed side by 
 side so as to exhilit at intervul» a comi,arative view. 
 
 § 121. Conversion from one soale to the other. - Since 
 100°C. ==180°F.,.50C. =0°F.. or TC .-. ^of rF. Hence 
 to convert Centigrade degrees into FiUnenheit degrees, we mul- 
 tiply the number by | ; ,,nd to convi.rt Fahrenheit degrees into 
 Centigrade degrees we multiply by g. In finding the tempera- 
 ture on one scale that cor.es,,onds to a given temperature on 
 the other scab it must be remoniberod that the number that 
 expresses the temperature on a Fahrenheit scale does not, as it 
 does on a Centigrade scale, express (hun.nnber of degrees above 
 freezing-point. For example, :,r on a Fahrenheit scale is not 
 o2 above freezing-point, but u2° — 32' - 20° above it. 
 
 Ilence, if you wish to represent a >?iven temperature on the 
 tahrenheit scale, determine the mnnbcr of F. degrees the jriven 
 temperature is from the freezing-point, and then make allow- 
 ance for the fact that the freezing-point is marked 32° on the 
 t . scale. 
 
 Example 1. How is 13° C. rcproHcnttMl on tl.c V. scale? 
 
 13 C. degrees = U» X « =. ^ l^. dourees. 
 Therefore, tlic i,nve.. temperature 1h 2Hf K. doKrees above freezing 
 and hence is represented by =- »""vi. iieezing, 
 
 231 -f- 32 == or.ii en the F. Mcalc. 
 
174 
 
 MOLEC UL A 11 EN E KG Y HEAT. 
 
 Example 2. How i« 04O R repro.euted on the Centigrade scale? 
 fi4 i. IS J2 F. degrees above freezing. 
 32 F. degrees = 32 X f = 17J C. degrees. 
 
 17 J on the C. scale. 
 
 PROBLEMS, 
 
 Tl,e difference between two ten.peratures is 80 Centigrade degrees 
 . IS the ditterence in Fulireiiholt degrees? "threes. 
 
 ■ "2. , ^: .,^\''''" *''^ temperature of a room falls 30 Fah- 
 
 rclHMt degrees, how many Centigrade degrees is its 
 ^ temperature lowered? 
 
 for,' fuTr "" '^'"P^'-'^t"''^ «f the above room, be- 
 fore tlie fall, was G8°F., (a) what was its temperature 
 after the fall? (.) What were the temperaturToT h" 
 
 z::.^:::;"^^ ^^^^^ ^'^^ ^^"' --^^'»« *- ^-«^-de 
 
 4. Express tlie following temperatures of the Centi- 
 grade scale in the Fahrenheit scale : 100"; 40°- 5fio- ano. 
 0°; -20°; -40"; 80"; 150°. , ^0 , GO , 
 
 6- Exi)ress the following temperatures of the Fah- 
 reulieit scale in the Centigrade scale: 212°; 32°- 90° • 
 77- 20O; loO; _ioo, _2oo. _,oo. ^qo. SO"; 329o ' 
 
 § 122. Air-thermometer. -Prepare apparatus 
 
 one-fourth hter capacity, tightly stopped. Through the 
 stopper extends a glass tube about COo- long, which also 
 passes tla-ough the stopper of a bottle B.^'partly mLd 
 w ih colored water. The latter stopper is j^crc J, by a 
 . hole a, to allow mr to pass in and out freely. A strip 
 
 of paper C, containing a scale of equal parts, is attached 
 to the tube by means of slits cut in the paper 
 Grasp the flasit with tha palms of both hands and 
 hereby heat the air in the flask and cause it to expand Tel ICe 
 through he liquid in bubbles. When several buhhli ha^ e^ 
 remove t^ hands and the air, on cooling, will contr,c , ^^dX 
 liquid wm rise and partly fill the tub^, ^^ 
 
MEASUREMENTS OF EXTREME TEMPERATURES. 175 
 
 does, i„asm„ch aa th^ iXht of L r ." "■"'="'-^-*e™<"»eter 
 ata,.„pbe,.i„ pressure „tt, I bvZn " """ " ""'"''' "^ 
 
 1' r rr --t "— «r^^^^^^^ 
 
 ■ng to the changes of the barometric column r„. ■ 
 
 scientme investigations a good air-ther™ lote; i! et IJr "^h""' 
 one conta miner mpmnmT t-i *i "J'un'ti is better than 
 
 
 r F.), and therefore c It bc^l.s "f ^.X:'-!"" ^•*- '«• 
 above or below these points. K ",',", Wn '""'r'-"""- 
 measured b, the expansion of .^M^T^T:,^ 
 mum, and the instrument uscil fnr i ,r. ' 
 
 Wrcm*.. Alcohol is ,17 „ I, ''":"°'° " ™"''' " 
 measure c.tremel, lo t™ „,■ u,« IT " "7'"-'"' '" 
 
 I 
 
 'I 
 
176 
 
 MOLKCCLAK ENERGY HEAT. 
 
 § 124. Absolute temperatura Tf i, j 
 is hcatci .vhilo tl,. mossier, r ~ ^^ °' ""• "' ""C. 
 
 perat,„.a'„aL* T\t ''■'°. '""' "<■'"'■"= «• "« tern- 
 
 its tem,e,at„ ,^ ,„t! 5 a„d .r"r'' 'I "' "" ''™^ *«- 
 continue to di,.i„i.„Tt:; 1' fe ".T- 7,?c":" "^° '^ 
 
 ove.eo„,e°brrr„rt';:ert,:r "^7^""'; -"'r- '» 
 
 oohesioii in a Tas l|i.„ „,v , , """' *« "Uromolccular 
 
 temperature and nte, " " "'""'"°<' ""'^ »' ^ '•""y 'o" 
 
 tendency re;panr;TV'."'"° " ""'^^ '° -"'«' -y 
 tl.e conlin- 4r ; ' ; ^or" ' "'?" '" "■"' ' P'™^"" "" 
 
 Fo,- tld,%l;o Ij^;"' ^'""' '' 'owe»t possible .empe,.atu,.e 
 tnre reoko "d from ,„ •'?•'" *"'""' ^'■''' """ '™PC'- 
 
 o.. t,ri. sea:i„t:;:::trrjdr:„re':'^ '-~ 
 
 w 
 
 anasolu,.s,.tl/"efor"^h !^^^^^^^ ^'"^^ '^ro convcrtea imo liquids 
 
 "as never been cooled to^l o a, soln^! f ^^ ''^- ''^"«"^'>' '"^ ''«'>V 
 
 -nclusive than tbe one • v" ' "^t^^^ arc reason.s, far n.ore 
 
 further stu.ly of l.eat he ^^o of n ' '"'^ "'^"'' --"•'° ^'- i» ^"e 
 
 great convenience ' ''" ''''^' °^ '^'''^"^"te tenipcrature Ls a 
 
 The ab.solute temperature (..so. on the aWe theory) .ay 
 
LAWS Op gaseous bodies. 177 
 
 Ir/r'c"' tot/ "','" "^ """"' ™ " c-"-^™"^ "-■"'-- 
 
 Fig. n 1 ) ^^ ^"^ "" " P'""'=''''eit tbermomoter. (See 
 
 if 
 1'i 
 
 § 12S. Laws of gaseous bodies.— II folloivs, from thn 
 
 Cc2'r«'"'V* "'-"f '"■'•■<>""' "> U, ul>,olntete,n,eriL Z. 
 .8 called tte £aw 0/ C'/ian'es. 
 
 If, however, a body of gas at 0" C. is enclosed in a vessel of 
 
 ' s "tti-: r'™' :"' "-"^ '°"^"'»' »' "» "C"' 
 
 il M ' "' '■"' ''«<■•■' »'»W i" tl,e l.rcvions sec 
 
 uon, the pressure on the sides is increased l,v ,1, of IL 
 
 cunnn shed jfj for every degree lis temiKrature f.alls; and if it 
 were to continue to decrease at this rati, at -27a»C it wolld 
 become nothing. Hence, >,. pressure „} a glZtiXoftu 
 
 !f 
 
 ,!^^fc/f n» w, /T " """""'^ /-'-"^-'-o™.; ,0 aepressure to 
 
 pii-^f^'iifi una tlie volume in mnQ/n^it tr, ^^ • . 
 
 r,f^r,o ti ■ 7 '"""^ '* '^^"^'^"f- Hence, m a?*« myen mass 
 
 o/,a«, «,.^rod„c, is proportional to t„e aOsolute tel^^raZr^ 
 
 PROBLEMS. 
 
178 
 
 MOLECULAR ENEUGY. _ HEAT. 
 
 % 
 
 m -JcV ^'' """' "'" """» """■»" " ■'" temperature become, 
 
 temp. , the,, 2„3 , mT^'. st^V^. Z' "' '"''" + ''"^ '»^ ""»■ 
 
 4- '^""■'intvolume vvillaliterof cascnnfrnnt if , .. 
 to -]5o C. ? ^ contract if cooled from 30° C. 
 
 w^t::^:r;nin:;^:;:-— - - ati„o.p,.ero.ni .a. 
 
 «00« per square centimeter? '""'''-'''*"'^^' "^^ "••^•««»'-e i« reduced to 
 
 and^nJrr;t«:frr"^^^'"'^ '^^ '^ ^^'"P^'-^'- °^ ^^"C- 
 be its volume^tTtemper , fo 270 P ""^™''"'' '^ ''"^^"' ' ^^'^^'^ "'» 
 «quare centimeter? S r^ 170 r ^1 ""'"' ' ^'"'^'^"^'^ '^^ l"^^"*^ P^'^ 
 
 270c. is equivalent to sooot^s.^p. ^;?::;:'S'.VorT:'^- ''"''■ ' 
 
 1200. Whence x= 344 8ce l^^ ^ ''^» 290 : 300 : .• 600 X 800 : a; x 
 
 will Its volume bo reduced to l~ ,„,,, ' '" " '"" 'e"'l'e™turo 
 
 centimeter, A.s. : ^X^'Z^X -Z^l^c "' ^'"' -»■■ »1"»™ 
 
 na^e/rp*%rSrb„r '"- -^'-"'^ ^=">° »■ "'■'--. -"."es 
 
DIFFUSION OF 0ASE8 AND LIQUIDS. 179 
 
 §127. Pressure of a gas due to the kinetic energy of 
 Its molecules. -Consider, then, what a molecular storm must 
 be ragn,g about us, and how it n.ust beat against us and against 
 every exposed surface. According to the kinetic theory the 
 pressure of a gas (or its expansive power, as it is someLes 
 called) ,s ent.rely due to the striking of the molecules against 
 he s,nfaces on which the gas is said to press, the impulses fol- 
 lown.g one another in such rapid succession that the effect pro- 
 duced cannot be dintinguished from constant pressure. Upon 
 the k.netic energy of tiiese blows, and upon the number of blows 
 per scoud nmst depe„<l the amount of pressure. Hut we saw, in 
 §108, that on the energy of the individual molecules depends 
 that condition of a gas calh.l its Wmperalure; so, it is annaren 
 as statocl above, tlu.t //.;>,..... oA. ,/„«n vian^ ;X^^^^^ 
 a.s a. U.n,eratare. Again, as at the san.e tempera'tureVnT 
 ber of blows per second must <lepend upon the number of mole- 
 cules „i the unit of spacc.it is apparent that the pressure of 
 any particulur <jas varies as the density. 
 
 The following estimates- nmcio for hy,lro«..„ ,„ol„c«lo,s at 0° C and 
 under a pressure of one atmosphere, n.ay pr..ve hitercsting:-- 
 Mean velocity, 0100 feet per second. 
 Mean path without collision, 38 ten-millio„thH of an inch. 
 Collisions, 17,7r,0 millions pe) ond 
 Mass, 210.000 million milHon li-Uon weigh i ^.ram 
 
 The rnsiTv !>V"'""" '"/""" """"" ^'' """ '='""« ^-"^"noter. 
 llie (loiisity of oxygen is 10 times ilmt of hvdroiron nt th 
 
 temperature and pressure; what is, therefor .Tic ranv^ 
 
 oxygen molecules at 0° c, and under a pressnrc\,f i::"::;^ 
 
 ^ §128. Diflftision of gases and liquids. - The kinetic 
 theory o gases explains why g.ses penetnae into a y s le 
 open to them and likewise the phenomenon known as L \^^ 
 
 only de!.ty.. the spread of another gas in the same space by 
 collision between the molecules of the interdiffusi.-.g gases 
 
 ' Muxwoil, 
 
 HI 
 
 SH4 
 
180 
 
 MOLECULAU ENERGY. — HEAT. 
 
 The diffusion between liquids, though not so well understood, 
 is undoubtedly due in part to siniihir molecular motions. 
 
 XXII. 
 
 EFFECTS OF UKAT CONTINUED. —LIQUEFACTION 
 AND VAl'OHIZATION. 
 
 Fig. 113. 
 
 Experiment 1. Melt separately VMo^-, hird, and beeswax. When 
 partially melted stir well with a thernionieter, and ascertain the melt- 
 ing points of each of these substances. 
 
 Experinieut 2. Place a test-tabe (Fig. 113), half lllled -.vith ether, 
 In a beaker contauiing water at a temperature of GO" C. Although the 
 temperature of the water is 40° below its boiling-point, 
 it very quickly raises the temperature of the ether suffi- 
 ciently to cause it to boil violently. Introduce a chemical 
 thermometer' into the test-tube, and ascertain the boiling 
 po'nt of ether. 
 
 Experiment 3. Half All a glass beaker, of a liter 
 capacity, with fragments of ice or snow, and set the beaker 
 into a basin of boiling-hot water. Stir the contents of 
 the beaker with a thermometer until the ice is all melted, 
 observing from time to time the temperature of the contents. 
 
 Experiment 4. As soon as the last piece of ice disappears, remove 
 the flask from the warm water, wipe the outside, and place it over a 
 Bunsen burner and heat. Carefully watch the temperature until the 
 water has been boiling a short time. What do you observe? Place 
 more burners under the beaker; the water boils more violently; does 
 the temperature rise? 
 
 Experiment 5. Place in contact the smooth, dry surfaces of two 
 pieces of ice; press them together for a few seconds; remove the 
 pressure, and they will be found firmly frozen together. The ice at the 
 surfaces of contact melts under the pressure, but when the pressure is 
 removed the liquid instantly freezes and cements the pieces together. 
 It is in this manner that snow-balls are formed. 
 
 Note. — If a thermometer is placed in a mixture of ice and water, 
 and the mixture is subjected to great pressure, some of the ice will 
 melt and the tempcraiure will fall; but when the pressure is removed, 
 a portion of the water freezes and the temperature rises. From this 
 we learn tha,t the melting {orfreezing) point of water is very slightly low- 
 
 « A chemical thermometer has Us scale on the glass stem, instead of a metal plate. 
 BUd IE Otherwise adapted to experimental use. 
 
LIQUEFACTION AND VAPORIZATION. 
 
 187 
 
 ered by pressure. The depression is about -pi-j of 1" C. for each atmos- 
 phere. Oh the other haud, it is found that substances which, unlike ice- 
 expand in melting, have their melting points raised by pressure. 
 
 Fig. 114. 
 
 
 Experiment 0. Half fill a tlilu 
 glass flask with water. Boil the 
 water over a Bunsen burner; the 
 steam will drive the air from th* 
 Jlask. Withdraw tlie burner, (£uickly 
 cork the flask very tightly, ami plunge 
 the flask into cold water, or Invert the 
 flask and pour cold water upon the 
 part containing steam, as in Fig. 114. 
 What is the result? Can you otter any 
 explanation? Suggest an experiment 
 to test your explanation. 
 
 If hot water is poured upon tho 
 flask, the water ceases to boil. 
 (Why?) Under the receiver of an 
 air-pump, water may be made to 
 boil at any temperature between 
 
 0° and 100° C. ; indeed, if exhaustion is carried far enough, 
 boiling and freezing may be going on at the same time. When 
 high temperature is objectionable, apparatus is cc->trived for 
 boiling and evaporating in a vacuum ; as, for instance, in the 
 vacuum- pans used in sugar refineries. As water boils more 
 easily under diminished pressure, so it boils with more difficulty 
 when the pressure is increased ; and the temperature to which 
 water may be raised under the pressure of its own steam is 
 only limited by the strength of the vessel containing it. Ves> 
 sels of this kind are often employed to effect a complete pene- 
 tration of water into solid and hard substances. By this means 
 gelatine is extracted from the interior of bones. In tlie boiler 
 of a locomotive, where the pressure is sometimes 150 lbs. above 
 the atmosphere, the boiling point rises to about 180° C 
 (356° F.) 
 
 <i 
 
 ' 1( 
 
jffi, 
 
 
 182 
 
 MOLECULAR ENERGY. -— HEAT. 
 
 thanO°C Fmm fi t '"^'^'-'"^'^ '^ lower temperature 
 
 aa.,.e, we HZ it t:.-:;r'""°"'^' '- »"-» »' " »^-i'- 
 
 LAWS OF 
 !• ne temperature at which 
 solids melt differs for different sub- 
 stances, but is invariable for the 
 same substance, if the pressure is 
 constant. Substances solidify nsu- 
 ally at the same temperatures as 
 those at which they melt. 
 
 2. After a solid begins to melt, 
 the temperature remains constant 
 until the whole is melted. 
 
 3, Pressure lowers the melting 
 {or solidifying) point of substances 
 that expand on solidifying, and 
 raises the melting point of those 
 that contract. 
 
 4. ^f^e freezing point of \'ater 
 ts lowered by the presence of salts in 
 solution. 
 
 FUSION AND noiLING. 
 
 1- The temperature at which 
 liquids boil differs for diferent sub- 
 stances, but the temperature of the 
 ■oapor is invariable for the same 
 substance if thepressure is constant. 
 ! "P"''' ''1"'fy «< the same tempera- 
 tures as those at which they boil. 
 
 2. After a liquid begins to boil 
 the temperature remains constant 
 until the whole is vnporiztd. 
 
 3. Pressure raises the boilinq 
 point of all substances. 
 
 . *• ^''"' foiling point of water 
 IS raised by the presence of salts in 
 solution. 
 
 REFEUKNCE TAULKS 
 Melting Points. 
 
 Zinc 
 
 o 
 115° 
 
 Alcohol _,,,Qo c 
 
 ^^^rmry -;i,s ,so 
 
 Sulphur", acid __.^^ ^o 
 
 ^^« • .".' "oo 
 
 Phosphorus ^^ 
 
 Sulphur. 
 
 ■^'" about 238° 
 
 ^""^^ " -mo 
 
 Boiling Points under a Pressure of ««. A4 V 
 Carbon dioxide . . . ' n 7 ''^'»^<^sphere. 
 ~', ^ Carbon bisulphide 
 
 -1«" pVator ."."'.'.".'.'*."."' 
 
 ''<">" I Mercury 
 
 about. . . 
 
 Silver w 
 
 Gold .,]][ .. "■ 
 
 Cast-iron .. j(,-(, 
 
 Wr()!i,i,'ht-iron. .. '< j.-i,jj_ 
 
 Iridium (the most 
 
 infusible inetul) 
 
 about 
 
 42r>"C. 
 1000° 
 1200° 
 1250'^ 
 l(i00<3 
 
 Ammonia 
 Sulphur dioxide 
 I'/Uivr 
 
 . 11)50° 
 
 48°C 
 100^"' 
 
which con- 
 inpeniture 
 f a similar 
 
 ' at which 
 [ffereni suh- 
 ttre of the 
 the same 
 s constant, 
 le tempera- 
 "ij hoil. 
 ins to boil 
 
 s constant 
 id. 
 
 le boiling 
 
 of water 
 f salts ill 
 
 . 100()o 
 ]L'()<)0 
 
 -l2.-)(r 
 JiiOOO 
 
 48°C 
 
 blSTILLATION. 
 
 Boiliufj Points of Water at Different Pressures. 
 
 Barometer. 
 
 184° F 1C.G8 inches. 
 
 190" 18.9a «' 
 
 200" 23.45 " 
 
 210° 28.74 " 
 
 212° 29.92 " 
 
 183 
 
 212° F. 
 24r.5°. 
 273.3° . 
 30G° . 
 350.0° . 
 
 Atmospheres. 
 
 1 
 
 2 
 
 3 
 
 6 
 
 10 
 
 Phe temperature of the boiling point of water varies with the alti- 
 tude of places, ,n conse.p.ence of ti.e different atmospheric pressure. 
 Adfferenceof altitude of 533 ft. causes a variation of 1°F. in the 
 boihng point. 
 
 Boiling Points of Water at Different Altitudes. 
 
 Quito 
 
 Above the 
 sea-level. 
 
 Mean hlght of 
 IJaroiueter. Temperature. 
 -. ,„, +9,500 ft 21.53 in 195.8° F. 
 
 TJt'' ''''''" ''•'"" '''° 
 
 Mt. Washington 6,290 " 22.90 " 200^ 
 
 ^^^•■'to" " 30. " 212° 
 
 Dead Sea (below) _ 1,316 « 31.50 „ ; ; ; ; '^^^^ 
 
 §129. Distillation. -Apparatus like that represented in 
 Fig. 115. Figure 115 may be 
 
 easily constructefl. 
 The following ex- 
 periment will be 
 found interesting: 
 and instructive. 
 
 Experiment. Half 
 fill the flask A with 
 waf,er colored with a 
 few drops of Ink. 
 Boil the water, and 
 the steam arising will 
 escape through the 
 nn rn. I .L ^ . ylu^ss delivery tube 
 
 BB. Tills tube Is surrounded In part by a larger tube C, called a con- 
 denser ^yhlch s kept filled with cold water flowing from a vessel D 
 through a siphon S, the water finally escaping through the tube B 
 
 ■ 1, 
 
 
 
 ill 
 
184 
 
 MOLECULAR lONlOUGV. — mUT. 
 
 P 
 
 m 
 
 The Ptpam Is condensed in its passaso throni^h fh. ^ i- 
 
 the resulting li,„ia is eangi.t in t ^^ sS ^^'/^'^^.'J ^"be. and 
 
 colorless. A complete sonlvuJu r „ ^' ^''^ ^'^"id caught la 
 
 HQUid from the otlf " ^ ! ^^ ^V/^ 'r;:"^^"'-^'"'"^" °^ "^ -^^--'^ 
 left in the flask A. "^'""""^^ «' "'« '"^ is effected, the latter being 
 
 The separation h accompHshM on tho miminl. ih , n . 
 
 T[:"Z :; ;;:r 'T''^' tt "" '- -'- -'•^'-- 
 o- .1.; ».tc.;r;::",.,r ';:,rA ;'---"' "■-■-'.o, 
 
 watered his whiskev ton r.. "'''''''''' '"''« ^''^'^ 
 
 mistake hv bl .U.0 n,i ^ '• '''""^^'"' ^^ ^^"'^^^ '- 
 the result? ^ ""'"'■^' "^ '-^^ ^1^^" P«'- What was 
 
 which, rising in bnb.h J . i , ^ '^ '"" "^'" ^"P^^' 
 
 produces a Violent ngita o ";"7/^. -— the surface, 
 
 place quietly and .^^ .'::;; ;''^'^:;;r^r ^"'"^ '^''^ 
 
 laws of vaporization of 1,1 liquid "oi;.,,''.^''''^"^ "'' 
 only the important case of water AI ' '"" '"" '*"'^^ 
 
 the evaporation of water o^;* ./ ? ''"'^'"^"' '^^' '^^•'^'' 
 low nvon i. 7 * '"'•>' tc'nporature, however 
 
 low, e\ en ice and snow evaporate. 
 
 :rAe rai)/(Z//^ of evaporation varies dii-eeth' ,nV/, fho fo 
 ture amount of surfree e.pos.l, ana drj Xet T"' 
 and inversely with the pressure vpnn tlfZui '^'l".'"'"^^'''^'-'^' 
 water .ni.es freely with the air, La mnll.JXZlt 
 acting like another gas (compare §^ 41 and 128V ri 
 does not take up water Mc e u s ,on<n L 7 V'- ^ ""'' 
 
 f«„ ,v *i . ,,*'■ '^I"*"^'' as IS common y iniao-ined • 
 
 for, if the air con h hv rcmov..! f.v.„, . . , ''•sincu , 
 
 lar.rp vo««ol ^f f " '^ '"*""' ^''«''G there is a 
 
 large Acssel of water, everv cubif font r.p ^i . . 
 
Y tube, and 
 I caught is 
 ihe colored 
 alter being 
 
 • the tern. 
 "^er. The 
 ibstancea 
 operation 
 
 ted from 
 e alcohol 
 vlio luid 
 rect his 
 fiat was 
 
 illj ap- 
 vapor, 
 surface, 
 I called 
 li takes 
 ua and 
 I study 
 y heat, 
 owever 
 
 mpera^ 
 'sphere, 
 ipor of 
 ugh it, 
 he air 
 jiued ; 
 B is a 
 room 
 t does 
 
 DEW POINT. jgf 
 
 when the air is present, _ probably a veiy little more. In either 
 case, on y a definite quantity would be found in each cubl oot 
 a quantity uepending on the temperature of the space. Tl u at' 
 C each cubic foot can contain 0.14- at 10°, 0.26« ■ at 20° 
 0.4 ; and at 30°, 0.8o«. Evidently the capLi y i^ LL 
 doubled by a rise of 10° in temperature. ^ 
 
 amLnfnf ""7 P°^^*--When a space contains such an 
 amount of water-vapor, whether it contains other ^ases or not 
 that Its temperature cannot be lowered without som^ of he wat'; 
 
 ^irz^^ ''' '''™ ^^ ^ "^"^^'' ^h^ «p- ^« -i^ : : 
 
 in w . h :. , ^"^P^^'^tuvo ,s called the deto point. The form 
 
 n wh ch the condensed vapor appears is, according to its lod- 
 t on, ce,,^ fog or cloud. The atmosphere is saidt be Z or 
 lum^d, accordmg as the diflerence between the dew pou.t and 
 the temperature of the atmosphere is great or little. 
 
 QUESTIONS. 
 J. Why doe, o„r breath prCuco a cloud to „,„te, and „„. ,„ a„m. 
 a. (a) It air at 0° is warmed to 20<>C how"^!!! i,. i 
 
 "Tu f^ rr ""™' ™" '-^ «<• aThav o VeSLS 
 
 3. If saturated a r at 20^ is blown i.itn n „ n . clothes? 
 
 ture is 10^ what will happen? ' ''""' ^'"^^ *^^ ^^^P^^-" 
 
 4. Wliat is the cause of the sreneral romnini^f e , 
 
 rooms heated by stoves or furnace" ? "^ ^^ ''''''''' "^ ^'' '" 
 
 xxm. my, c^"NyK,.„Br.,.: ,«■,, totkntiat. kkkkov, and 
 
 § "2- Heat unite. -It i„f,.„^„„„t| „„,,,„,, 
 q".u,t,t,v of hoat, a,Kl for tins purpose „sta„.I„r„ „„it o^™ 
 
 jrom (/ (o i c. This uuit is called a calorie. 
 
 *• 
 
 
 i 
 
 Itr 
 
186 
 
 Molecular enetjgy. — heat. 
 
 mi 
 
 
 Let it be required to find the amount of heat that disappears 
 {i^xp. 6, p. 180) during the melting of one kilogram of ice. 
 
 Kiperiment 1. Mix l-^ of water at 0° with l^ at 200; the tempera. 
 
 ure of the mixture becomes lO". Tl.e heat that leave l^of wS 
 
 when .t^falls f ro.n 20° to 10° i,s J„st capable of raising 1^ „f ,vater C 
 
 From this experiment you learn this important truth A 
 body.in coolirn,, gives to surrounding bodies as much heal as is re- 
 quired to restore its own temperature. 
 
 Experiment 2- Put a kilogram of snow or pounded ice at 0° C 
 
 is meZl. "" "'' temperature at the moment the snow 
 
 Let a" be the temp.rature thus observed. One kilo-ram of wator 
 has been cooled (100_0,o„ekilogramof ice has been^^^^^^^ 
 i-esul u.g k.logram of water has been warmed aO. Therefore (lor-2at 
 calones have been required to melt one kilogram of ice ^'^^~'''^ 
 
 Repeat the experiment with different weights of water and 
 snow, and compare the results. Taking the average of your ex- 
 penmenis, how many calories do you find are required to melt 
 one kilogram of ice? <-" meit 
 
 Next, let it be required to find the amount of heat that dis- 
 appears durmg the conversion of 1" of water into steam. 
 
 Experiment 2. Place 1" of water at 0° C. in a beaker and hont thn 
 san,e with a I.msen burner. Note the tin.e that i tkc^ T^^ e 
 
 Ttl.: r ""• '" '''^'- '^'•^^ "•^" ^'"'^ Curing Which .he ten;:.^ t 
 
 f the watc r remauis stationary while the water is boiling away LeJ 
 the latter time be a times the former. ^' 
 
 Now, as tlie water receives 100 calories during the time it is 
 r.s,ng from the freezing to the boiling point, it must receive about 
 100 a calones durn^g the time it i.. converted Into steam ; but Lhe 
 temperature of the water is not changed during the latt;r oper! 
 
Jisappeais 
 of ice. 
 
 le tempera- 
 ^ of water 
 water from 
 
 ruth. A 
 it as is re- 
 
 'e at 0° C. 
 xture until 
 t the snow 
 
 of water 
 id and the 
 
 100 — 2a) 
 
 ater and 
 
 your ex- 
 
 to melt 
 
 hat dis- 
 
 heat the 
 raise the 
 iperature 
 ay. Let 
 
 le it is 
 e about 
 but the 
 !• oper. 
 
 TWO QUKSTIONS ANHWKHED. 
 
 187 
 
 Repeat this experiment, and flud th(* avcraire ,.f th.. . u 
 Experiments made by ,nore r -uraf.- I . . T '''"'*'• 
 
 than the above give the following rcHHl.H J *' """'"'-^^^"^ '"e^-ds 
 
 1. rhe amount of heat that disapptart, or « tn»i 4n /a 
 
 kilogram of ice is 80 calories. ' '" ^^' '"'^^'''^ "/ «"'« 
 
 2. '^^^ <^mount of heat that disappenri, oris lost i„ iu 
 
 one kilogram of ^aterir^to steam is^Z'^!^ '*' '^""''^^^^'^ "/ 
 
 If your experiments are earefidiv miidi. •.•..! ..* 
 
 The answer to the first (lucHtlon 1h All,.*'*! i . 
 melting iee is oo„™„,e<, i d'^ LtL J;; '':'*•."'.''''''"' l" 
 
 similar action goo, on, tl, l,ea T. , " T" '"'» »'<'™ » 
 ™oIec„,es .0 L. t,.at\,.e ^r:,::;'; .iZ ^^fe t 
 longer sensible, all except the sma,l Auction use^l i7„ • 
 
 afnospherio pressnre.- Heat, tl.e c,, « „7°t ITXf 
 instances .oos i„„„rtant worK, „n.l i. 'LX'tZXi i^ ' 
 the energy of position, or ..otential energy, 1 „„„?„; „/," 
 same kmd as that of a raised weight " 
 
 doni'n.TZ-'"/'"' '"."""^ "•"■■"""" '" ''''- -"<"■"' of work 
 done ni both instances is great, as shown by the an.onn. „n . 
 
 consumed in doing the work, HO eal,,rl...',rMr ' ' ' 
 being required in the lirst instance, and f„'17 caiorS Ter L ^ 
 gran, of water in the second, hence It re,,nlres a ™7 i',,' to 
 acqua-c the requisite au.ount of beat, ll i. fortunate S H 
 
 • Tills fructjou i« libout .tg. 
 
 V 
 
 J 
 \ 
 
188 
 
 MOLECUIAR ENERGY HEAT. 
 
 takes a large quantity of beat to melt ice ; otherwise, on a sinalo 
 warm day m winter, all the ice and snow would mdt, creS 
 mos destructive freshets. The heat which <Hsnppears me ."' 
 an,l boihng .s generally, but with our present knowledge of ^ 
 subject, rather objectionably, called latent (hidden) heat Tho 
 error consists in calling that heat which has ceased to b^ heat 
 t.e. , has ceased to be molecular motion. ' 
 
 fh I?*: ^^^^""^^ °f producingr artificial cold. -The fact 
 that heat must be consumed because work is done, in tl e con 
 version of solids into liauids .nd liquid, into vapor , an a I 
 smple expansion of gases, is turned to practicll u^e in ^ny 
 ways for (he purpose of producing artificial cold. They are 
 emoraced under three heads, viz. : Cola prodncea hy soLion 
 by evaporation, and by expansion of gases. The followiuJ 
 experiments will illustrate : — loiiowiug 
 
 § 135. Cold by solution. -Freezing mixtures _ Pv« • 
 ment. Prepare a mixture of 2 parts by wei^hr..? i^T , '^ "" 
 
 nium nitrate and 1 part of anuno'minV H.lS am hs oh' T'"" 
 Of water (not wanner than lOO C). .tirrin, L :i:'::::t^::j::::: 
 
 with a tcst-tnl,e containing a little water.' 
 What js the result? What temperature is 
 mdicated by a thermometer placed in the 
 mixture? 
 
 Fig. 117. 
 
 One of the most common freozin<r 
 mixtures consists of 3 parts snow ov 
 broken ice and 1 partof common salt 
 The aniuity of salt for water causes a 
 lupiefaction of the ice, and the result- 
 iiig liquid dissolves the salt, both oper- 
 ations requiring heat. 
 
 result? '^"'"^'^'^ evaporates, and what is the 
 
 Kxperiment a. Plaeo water at al,out 10° C. in a tlm, . 
 cup, such as is usorl in fH^ n , , *'"" P"''o"s 
 
 J« «.ed in the Grove's battery, and introduce 
 
on a single 
 It, creating 
 in melting 
 tlge of the 
 teat.. Tiic 
 io be lieat, 
 
 -Tlie fact 
 3 the con- 
 and ill tlie 
 
 ill many 
 Tliey are 
 
 solution, 
 foil 
 
 EXPANSION OP GASES. 
 
 OWIU" 
 
 -Experi- 
 
 eU ainino- 
 iu ;J parts 
 llssolviiig, 
 tie water, 
 erature is 
 ed iu the 
 
 froezinjr 
 snow or 
 ion salt, 
 causes a 
 i result- 
 tli oper- 
 
 he pahn 
 at is tlio 
 
 porous 
 trocluco 
 
 189 
 
 Will so hasten evaporation that in the course offl.^ ^ """ ''"'''' 
 wlU bo a very sensible fall in temperatr """^ """^^ '""^'^ 
 
 llZWUh'Z'^;/"";'""'^ '''' •'"^'^ ''^ ^" --^ thermometer (Fig 
 
 1 he water iu the stem will quickly freeze even in a warm room. ^ 
 
 QUESTIONS. 
 I How dir ''''''' *^' ''''*^''''' ^''^^^^'^^ ^'"» '^l^ohol and wa..r ? 
 
 3.- m; dtr^r rir r "" " ^-^ -™^- ^ 
 the^e:prtreirL^-rrCp""^^^"^^ 
 
 6. How does sprinlcling a floor cool the air of a room ? 
 
 § 137. Cold by expansion of gases. - When a beer bottl« 
 
 duS ;[;fj;"«»>^ >"•«'"-<. in the „.. „?.r S 
 
 uue to tne chiil of an expanding gas 
 
 eZnl into ' vl"™ ^"^ ""•■ " ""-""^O "" '' """-^ ^ 
 nT^hl ^ mt ' "° '"'* •' ''''"'=• "»" *<= temperature is 
 not changed. What conclusion does this point .„ conoe^n^- 
 ..Uern,olec„lar .attraction in air? By allowing condc°S,7a^ 
 ont.,n,„g, as it usually does, watery vapor, toLape sudden; 
 from the vessel m which it is conflned, icicles have been 
 formed around the oriBce whence it escapiis. 
 
 dissolve more (hot water will dissolve about twice^irw. „ht " / tht 
 substance). Then set the hot solution in a place where Tt will not k! 
 d.sto-bed, and let it stand for about 24 hours' tit "t ma "aluirtS 
 
 soE Tnd a.'';^ "'"• ^''"' *'^ ''^^ '' * thermor^rt" th 
 solution.i and at the same time drop in a lump of sodium sulphate; 
 
 » The eoluUon is now said to be »t^er$aturated. 
 
 m 
 
 i\ 
 
 ns 
 
 V I If 
 
190 
 
 MOLECTTLAU ENERGY. — HEAT. 
 
 soJidlflcation Instantly sets in nn«i . 
 
 c*».e «.. puce i„ .H„ .e„.pe J, j^'t.t^;:'r :c;;r:; 
 
 frozen, when its temperature a»am Wl/ Th ' 7 ^ " 
 
 the f««.i„g „i,t,„, i, ,„„^„ ,o;rt™'-. .f" ™P«at.ne of 
 
 freezing ; the latt^.,- fi,„.. • ■ ' °' "'" "t"'' "I'ilo 
 
 «.e miftarfre't;: t ' """' '"' '""" '" "" f^-- ''>' 
 
 Fig. 118. 
 
 the mixture receives heat 
 is shown by the continua- 
 tion of the melting and 
 dissolving. But as the 
 temperature of the water 
 while freezing does not 
 <all, it must be that the 
 beat which it surrenders 
 tluring solidification arises 
 from the conversion into 
 heat of the potential en- 
 
 ergy possessed by the molecules of the liquid 
 
 Jn ttCTbegLstir TT " ^" ^^^"^« "«• ^hen water 
 
 ^to a vessel C of ^Itr^ 'o '"^ t: stLrLl *''^ ^^"^^^ ^^^ « 
 tnh« lo ^«.,-j„__-^ ^' -^"*^ f'teain that passes t»>i-"!..'»' rh'* 
 
 -.t« «„.,u.. u I, „a Je:?zr :,iri"s :rc:i' 
 
liquid mass 
 
 time, what 
 
 ange prove? 
 
 111(1 in giv 
 1 wlieu the 
 ious, as u 
 raising it. 
 
 and intro 
 
 salt freez- 
 
 1 it reaches 
 
 ig tlie ice 
 o freeze, 
 water is 
 •atiire of 
 tor while 
 r. That 
 
 n water 
 ■ tube B 
 !|?h the 
 water, 
 igb the 
 
 in COQ- 
 
 SPECIPIO HEAT. 
 
 191 
 
 verted Into steam ; also ascertain tlie temperature of the water In C. 
 and the number of calories which it has received. Compare the resul 
 of your calculation with tlie statement in § 132. 
 
 For every kilogram of water that is converted into steam, 
 5.3/ of water (practically, considerably less than this .inantity, 
 m consequence of loss of heat by radiation and evaporation 
 from C) will be raised from 0° to 100°. As 1^ requires 100 
 units of heat to raise it to 100°, the 5.37^ must require 537 units 
 of heat. But the steam raises the water to its own temperature 
 without having its own temperature lowered. (Whence come 
 the 537 units of heat that raise the temperature of the water?) 
 
 Heat that is consumed in liquefying solids, and in vaporizinn 
 hqmds, is always restored when the reverse change takes place. 
 Farmers well understand that water, in freezing, gives out a 
 great deal of heat, -at a low temperature, it is true, but still 
 high enough to protect vegetables which freeze only when con- 
 siderably colder than melting ice. The fact that steam, in 
 condensing, generates a large amount of heat, is turned to 
 practical use in heating buildings by steam. 
 
 XXIV. SPECIFIC HEAT. 
 
 § 139. Temperatures of different substances raised 
 unequaUy by equal quantities of heat. - Will equal quanti- 
 ties of heat applied to equal weights of different substances 
 raise their temperatures equally ? 
 
 rnifoTf"?' ^' '^"'' ^'*y^ ^^^' °^ '^'^' J^^<J' and make a loose 
 roU of ,t, and suspend It by a thread in boiling water for about five 
 minutes that it may acquire the same temperfture C100oc';a/the 
 water. Remove the roll from the hot water, and immerse it as nuckly 
 as possible m 300k of water atQO, and introduce the bulb of a her 
 
 IT^ ;„ uT ''' ^^^P-'^^t^r^ of the water when it ceases to rill 
 which will be found to be about 3- (accurately 3.30+). The lead coS 
 very much more than the water warms. Lead falls ahontT^^of 
 degree an equal ..eight of water is warmed "' '^''^ 
 
 11 
 
 m 
 
192 
 
 MOLECULAR ENERGY. — HEAT. 
 
 that .y«a^ 5u«„,,7,-,,, ^y. LtaZMfn ' ; w ''' '^"'"'"^'" 
 
 ^snhsfances, raise their tel T ^''"^ '""^^''' ""^ ^^'^«^«"< 
 
 1 ««Nc men temperatKres unequally. 
 
 these »ub,ta„oo/f,U* to" 4^,,;::%; f "'™ °' """" "' 
 alcohol. Since lieat a J.,?!?'. "•'' "" """='' "^ f<"' "'-^ 
 
 F»-<W«« of a ,.nU„fJT ^ ''"'""'''' '" ™« "« '««- 
 
 ^ "'c L/y aunitoj mass of a suh'itntino i° n • 
 
 ^^^'P'^cily for heai of that su^iancl ' '' '"^'^'^ ''"'^ 
 
 § 141. Specific heat definfid _ Tf Jc „ 
 to be able to coinn-iro H,n °^°°f.^- " J* '« » great convenience 
 
 heat. The ;:;r;: ::; ; :;:rr„.if :::r':, ^-"i-- '"■■ 
 
 expresses the co,.,„„,,»o„'i. L,e„ .;4;' T:!'"" '^'"^ '^^'^" 
 
 heat that raises tl,e water Z.'^ZTw tot,T°- '"""'"^ "' 
 96.70° (from 3.3- to 100°1 • l,.,,!?^ * ""'^^' "«' '«'<! 
 
 3£_ to iW ) , henos, to m„e the lea,! V req,me» 
 
 j,g_j_.084+ as much heat as to raise tlie water 1°. 
 
 The specific heat of all solids an^ K„..-i 
 iocreases slightl, with the ^„1 '-rt' '"' "'"'' «"'"' 
 hasaspeciScheatotI,. at W Jo v > -» '^ "' "°^- 
 stances i„ a liquid s.tj usu^^'i^l^hi: e ;e j l^tt T 
 
 teat of steam. '"" """■" """' ""'""<' the spceiflc 
 
heat that 
 to water, 
 
 conchulo 
 ^ different 
 
 inerciiry, 
 i inei-c'iiiy 
 ' ia rising 
 ' each of 
 iicli Jieat 
 for the 
 ess than 
 capacity 
 the tem- 
 illeil till'. 
 
 'eniencc 
 ices for 
 ' which 
 
 for heat 
 
 ' calcu- 
 tity of 
 le lead 
 jquires 
 
 gases, 
 0°C. 
 Sub- 
 L than 
 le the 
 •ecific 
 
 DIFFKinONCE IN CAPACITY VOH HKAT. 
 
 KKKEUKNCK TA1JLK8. 
 Tahle of mean specific heat htlween i)" (',. and lOO^ C. 
 ">'f''"«"^" ;U0!)0 I Iron ' 
 
 (-'opplT 
 
 193 
 
 Air 
 
 Sulpliiii 
 CJlas.s . . . 
 
 .202fi 
 .1770 
 
 Mercury 
 lA>tu\ .... 
 
 .li;58 
 .0!)-,2 
 
 .();5U 
 
 Specific heat of the same s„hsta„ce in different states. 
 
 Water... 
 Bromine. 
 Lead . . . 
 Alcohol . . 
 
 Solid. 
 .0040 
 .0833 
 .0314 
 
 ''''l"''l- ClaBeoiiH. 
 1-""W 4,so.-, 
 
 •1«W 0.555 
 
 .0(02 
 
 •0'5-77 ■■".45 
 
 § 142 Causes of difference in capacity for heat. - Of 
 the wholcMinanUty of l.cat applied to a Holi.l or Iic,nid body, 
 only a part goes to increase the heat of the body and thereby 
 to ra,se its temperature ; the renKiinder perforn.H 'interior work in 
 overconnng cohesion between the n.olecuh,.H of the body, and in 
 fomng them to take np new positions. The greater the portion 
 of heat consumed .n interior work upon a l,ody the less there 
 IS left to raise its temperature, and co„He(,uentiy th., greater its 
 eapacty for heat. Again, considering' tb.t'.rticfn'f heat 
 which does raise the temperature, since the temperature depends 
 upon the average kinetic energy of each molecule (§§ 108 109^ 
 It is evident that the quantity of h.at required to raise the tem- 
 perature of a unit of mass of a substance 1°, i,s greater the 
 greater the number of mnh.cul<.s in a unit of mass. Thus, much 
 nioreheati,srequirnitor..,ise .h. t.npera.ure of a pound of 
 water 1 than t„ ,,u.se that of a pound of lead 1° • (I) 
 
 1pT'%"7.m ;■"'"'"■ "■"■' '^ ''^"^ '" '''« water than in the 
 lead, and (2) be<.,,use tJ.ere are more mohcnU, in a pound of 
 water than in a po.nid of Uad. There are other matters to be 
 considered in connection .itii the subjec-t of this paragraph, but 
 the limits of this work forbid tiicir discu<«iou. ^ ^ ' ""' 
 
 
 ti 
 
194 
 
 MOLECULAR ENKRGY. — HEAT. 
 
 given ra.;o o ; ; :', Z^ "' T" '" ^'^"''"^' ^''-"^''^ " 
 The quantity of ^X at' r ^'^ f- "'""''' ^"^'^''^^ ''^'^'•^g-'- 
 
 .»00°C., or above a rod heat Co i. r: kT '" ^ "^ 
 in coolinjr from 100° to ()°r • ''"'^"^' •"' ^^^ognmi „f water 
 
 gram of Tron in cooling f- T" "'' '' "^"^''' ^'^'^^ ^^ ^ ^ilo- 
 e uuii lu cooling from about 900° to 0° C. 
 
 QUESTIONS AND PROBLEMS 
 at Joo"c:; '""^' "^^^ '^ '''^'''' *« '••-^- 100. or'ieo at oo ,„to stea. 
 
 boiler at a temperatut om'" '^ J X'. ? -;'!'--'tK„. n.urus to the 
 
 ins. ih) ri.e same q.mntity oTJZl^^ ^'''"' "'"^ '" ^'"^ ""'•'<'- 
 of water from 0° to lOo" ? ^ ""' ''""'^' ^'^'^'^ '»"^v "'",.y kilo^a-ams 
 
 «• (a) Apply the same X ;:;tl^^ J^ "-''^ "•'^".' " ^^^ *'> ^"^ C- ? 
 water, each at a temperature oTooc"^^,^*! ''"f 7' «"''* of ice and 
 boiling point What Will be the ten.poratur o" t^o^ ' T'"'"^ "" 
 Will not both have the same temperafaT; '""""^'^ ' ^'^ ^^"^ 
 
 6. What effect on the temperature of the air hn« fh„ f , 
 water of lakes and other bodies of water ? ''"''"^ °^ "'« 
 
 be Ib;:LX:m;:i::r^-^ ^^ ^^ - -- atooc. what Will 
 
 put'-intrif Of '^:::rt^:J':z: it'''--' '' ^' '"''^^ ^* ^««^' -'- 
 
 II ™„. "»(cr, ato raises llsteinpc;atiireto6°c ? 
 
 9. ;;^»^°frae'^>.ryat80<'w,:imcltwl,a.wcisl,toflocatO=L , 
 
 tor, f.?e.c,S ■',,„!" °'"""' ''°'' '"'"""" -"'-■■ -d w,„. 
 
THEKMO-I>YNAMIC3. 
 
 lOS 
 
 XXV. TnEnMO-DYNAMICS. 
 
 M, «,W, ofscence ,l,at treats of tke retation Mween keat „Z 
 mccUumal t^rk. 0„o of tlio most important discoveries 
 «e„c» ,, tl,at of the e^avalence of,.„t Ll ,„rk; thai,," 
 
 'lf<Me ^nantity ,.f fcoi; a,„l conver...l,j, this heatifL conver- 
 non v^ere cmpl^, can perform the original guamUyofJ^k 
 
 § 14a Correlation and conservation of energy -The 
 
 proof of 1,0 facts j„st s.Ued was ooe of the most toportant 
 
 tcps „, the establishment of the grand twin conoeptfons " 
 
 modern scence. ( 1 ) That all l-l,,u of energy are so relaZ Z 
 
 one another that energy of any k!ni can le ckangetl into 1^1 of 
 
 1.NP.RGY (2) Tliat wUn one form of energy disapmars an 
 
 the s,tm. total of energy ,s nnchanged, ~ kmwn as the doctrine 
 of c„.,.„v™ o. ..™„v. These two principles eons 1 
 the corner-stone of physical science. 
 
 iJ^*^ 5°""^,'° experiment. -The experiment to ascertain 
 the . mechanical value of heat," as perform«l by Dr. Jonte^" 
 KoK.and, was conducted about as follows. He caused a paddle 
 wheel to revolve in water by means of a falling weight a[^hed 
 to a cord wound aroun.l the a.xle of a wheel. Th! resi^oe 
 offered by the w.ater ,„the motion of the paddles w^ t^n 
 l.ywh,cl,themochanieal motion of the weight wa. co^vertrd 
 .n to heat wh ch raised «,o temperature of the wat^r. Ctal 
 a body of a known weight, e.g., 80^ he raised it a mea^n^ 
 .Lstanco, e.5,., 53" high; by so doing 4240'«» of w"" ' 
 
 onTr^n"":" '!; ""'*."'"'-1-"'y - equivalent amoMt^f 
 energy was stored np in it ready to be converted, first, into 
 mechanical motion, then in.», heat. He took a deflL J^e^w 
 
 € 
 
 !i 
 
'Sf^. 
 
 196 
 
 •11 
 
 MOLECULAR liNEltGY. HEAT. 
 
 Ill 
 
 Of w. er to be ag.tnted, r.g., 2\ .t a temperature of 0° C 
 ^f er the descent of the weight, the water wn. found to ha . 
 Uempera ure of 5° C. ; consequently the 2^ of water must have 
 cived 10 units of heat (careful allowance being made for 
 all bsses of heat), which is the amount of heat-energy tlutt is 
 equivalent to 4240''ff^ of wnrV ^7 u ^ i ^^ 
 
 7,^04..". ./• 7 ; ' ^ '"''^ ''•^ ''^«* *^ equivalent 
 
 o 424 ^^ of work (more accurately 423. ySa^^"') . 
 
 § 147 Mechanical equivalent of heat. -As a converse 
 
 t e t,: r ofT 'r ^^^"^^'f -^-^ ^^^ -^-^l --Tonment that 
 he qua tity of heat required to raise 1^ of water from 0° to 
 O will If converted into work, raise a 424^ wei-Wit 1"' hicrh 
 or 1^ weight 424" high. According to the English s^ltemt 
 
 ;r Mb W ror^"T- ^''^ ^--^^^/onJ thTt\:il 
 aise ] lb. of water ]°F. will raise 772 lbs 1 ft hiah Ti,n 
 
 lent oj heat, or Joule's equivalent (abbreviated, simply J.). 
 
 XXVI. STEAM ENGINE. 
 
 § 148. Description of a steam engine — A «f«or« « , • 
 i. a machine i„ w„ioh the Castio .Wo o'lanATZ:: 
 power na^ueh as the clastfc force of rteam is en reTy d .^ 
 
 tlansloimcd wto work or mcclianical motion 
 
 The modern stea.n engine consists essentially of an arran<.e. 
 raent by wh.eh steam from a boiler is eondnetil to botHfes 
 of a p,ston alternately; and tl.on, having done its work in dd 
 mg he i^ton to or ft«, is discharged "from both sMe a i 
 natcly, either n,to the air or into a condenser. The dialll 
 
 o^^Tri '° '""■'"'" '"" '-""^^ foaturetTd'" 
 operation ot a steam eug no. T!>» dof-^u- -c -i 
 
 ^cLameal contrivances are purposely omitted, so .■« to pr Zt 
 the engine as nearly as possible in its simplicity. 
 
 f 
 
THE STlOAISt ENGINE. 
 
 197 
 
 In the (lfa«n-am. B reprnsents tlio hoilrr, F the furnaep, S the -^team 
 pipe through which steam passes from the boiler to a small chamber 
 VC, called the valve chest. In this chamber is a slide valve V, which, as 
 It is moved to and fro, opens and closes alternately the passa-^cs' M 
 and N leading from the valve chest to the ojlindn- C, and thus .admits 
 the steam alternately each side of the piston P. When one of these 
 passages is open the other is always closed. Though the passage 
 between the valve chest and the space in the cylinder on one side of 
 
 Fig. U9. 
 
 I 
 
 the piston Is closed, thereby preventing the entrance of steaui into this 
 space, the passage leading from the same space is open throuoh the 
 nterior of the valve so that steam can escape from this space through 
 the exhaust pipe E. Thus, lu the position of the valve represented in 
 the diagram, the passage N Is open, and steam entering the cylinder 
 at the top drivc:^ the piston hi the direction indicated by the arrow At 
 the same timr the steam on the other side of the piston escapes through 
 the passage M and the exhaust pipe E. While the piston moves to the 
 left, the valve moves to the right, and eveutuaUy cloaca the passage 
 
 , : 
 
 I!, -fl! 
 
 m 
 
198 
 
 MOLECULA ft KNEUG V HEAT. 
 
 
 N" leading from the valve cho^f «n i 
 
 and thus the order of m^t ZZa' '"' '""^^ ^ ^"^° ^'^^ ««-«. 
 
 the shaft by means of ,„. era, k H t{ '^''T^'^'^^- Connected with 
 valve V, so that as the shaf ro' Ues tl' r"" ''""' ''""^^^« ^^^^ the 
 
 sr ^'-^ ^'^ - oppos.t:t:;;2: t:^ x::^ ^, r 
 - -snrr :/t^t:;:j^ ;^^:: ^ -^e, ..av^ ..., ...., 
 
 serves as a reservoir of enerffvvhr, "'^^ ''' «i'-^»mference ; if 
 two point, (called the ^ J. S n oaeJr^^T' '.'^ ""''^ "^^ ^^^^^ P- 
 the power communicated direct v ZT T^^"""" ^^ «>« shaft, where 
 the Shaft. It also assists to make fe ro n'. "" I' ''''^''''''' '" '-°v"'^' 
 "jachinery connected with itliform so H '? "' ?' ^'"" ^"^ ^'" «"-r 
 city resulting from sudden el an4 of tl^ •'"'''" '"""^"^^ °^ ^^J- 
 aje avoided. (Why should the wLouI tavvTx?."'^''' "'' ''•^^^'^^^^^^^ 
 Why should the rim be heavy ? Se n 02? n^ '^^"''^ '* ""' '^'^e' 
 Ing over the wheel W motion nmv h "^ "^ ""'''"' °^ « '^^It pass- 
 to any machinery desirable ' ^" communicated from the shaft 
 
 Sometimes steam, aftoTit ha^ "-"n-oondensing engines.' _ 
 conducted tbrough t^e elhl 2 f ™* "' "" "y'taO^'. - 
 »«*„«.., where? by laL of ' ^ ." '■'""""""• <^ '""'=<' « 
 «h-oug.. a pipe t! uTS™ /e Err'rir"'' ■"'™'--' 
 condensed steam must be pumpeTou ofM "?'" "'"• ""= 
 special pump called tecluiicalTv tlLl °°"*"""' ^^ " 
 
 vacuum is maintained. Sud, an ° ."'T'"^-' *us a partial 
 er^gine. The adva„ta<,e of such a, * " °'"'"' " "''*"»«» 
 exhaust pipe, instead o o,k i , ° t^T"." *""'"• '■°^' '^ *« 
 with the outside air as in tl I ! """"""^r, communicates 
 
 i» obliged to move ^e pist; rr*"™''''"^--. «'« ^^am 
 
 -Wng tau atn,ospher c' ,re :T ""' T'"' " "^'^'""« 
 inch of the surface of the ^Z"']^ „\P""""' '■-,«-J- square 
 
 -stance arises from afnospherie ::::::,T„ ^'r?' ::■ .^ 
 
p. 
 
 i M into the same, 
 
 le piston rod R to 
 Counected with 
 connects with the 
 le to slide to and 
 he motion of the 
 
 vy wheel, luiving 
 rciimferenoc ; it 
 T the shaft past 
 tlie shaft, where 
 Jctiial in movin" 
 aft and all other 
 hanges of velo- 
 r or resistances 
 •uld it be large? 
 of a belt pass- 
 from the shaft 
 
 engines.' — 
 e cylinder, is 
 r Q called a 
 er introduced 
 i^ater and the 
 denser by a 
 'US a partial 
 a condensmg 
 Si for, if the 
 •mmunicates 
 3, tlie steam 
 1 resistance 
 very square 
 ig engine no 
 dth a given 
 
 Uve as appUed 
 
 
 m 
 
 4 < 
 
 ■'■ I. ml 
 
i 
 
 THK L0(!0 MOTIVE. jgg 
 
 ".o!ii^c^;J,^"i,^?r.^°*^^ ^-^"- "f the loco- 
 
 -u-ily required of it, fro.n tla^e :: xt r 1, " "k ""'' "•^''- 
 iuto steam per liour. This is m^comnli ■ '""'* '''' converteil 
 
 a rapid comhustiou of I Zf :;;'''' '^ ""^"- ^■'"' "'•'^*- "^^ 
 per hour), second, by bri ngi^ „" 'C. ?? ' ''" '" "^ *«" "^-'^l 
 
 extent (about 800 sq.a) of,'; ted ,Zt; 'r'^'^' ^^'•"' '^ ^^^'^^ 
 A (see cut ou the oppo. te pa J ,, 1 T . ^ '" '"'^ "' «'« " "''e-box " 
 
 powerful draft vviud'is i^, t ■ ;,'' ;'""" ""'^^ '^ "^^'^^^ "^ '^ 
 steam, after it i.as doue i,s ^o; Z :^^:!: ^r = i'""" ^^'"^"•^' 
 exhaust pipe C to the suiolve bov h ' .' ^' ^""''"^'^'-'^l by the 
 
 The steau. as it escapes ,Vo tl . blasi p""' ' "" "'"'"' ^^'^'^^ ^^• 
 
 and drags l,y friction the air „ o ' " , T'''' ^'^ "'»• '^^>ove it. 
 vacuum in the smoke box. Tl" tx ' '• ' '''°""''' ^ ''^'^"^^ 
 
 then forces the air throu-h the fun. , 'T"'"'"' ''^ "'" atmosphere 
 thus causes a constant daf. Th tr. xV'";' '7'^ '""'"^ ^' ^"^ 
 secured as follows: Thewitemr h f, "^^^''"^ "^ ^^^^ted surface is 
 
 tact With the heated 1!:^:::^/^::^ ':!::t' "" r^ '" ^■"°- 
 
 G (a boiler usually coutains about «^) T '' .""'''"""^'^ ">« P'P^^ 
 the lieated gases and smoko Xnf ' . '"'' '''""" "'"'^ ''^P^ ^»«t by 
 the smoke blv and smoke s:^ek ""'' '"'"^ ^"^^ "''•""«'' ^^-" ^' 
 
 steam pipe J, etc., to its exit ft-o n o nm k ^ A I" ""''"""■^' 
 neer to explain from the object tho oluZl^^ huT: t '""'' '"^''■ 
 understand. "^ ""^'^ P"""^** «« you do not 
 
 The steam engine, with all its merilH and with nil fi. • 
 •nents whieh .nodcrn n.eehanieul ur I a dl^s ' 'VT"' 
 exceedingly wasteful mnehine. Tie b^t tZ t .V" Y '" 
 
CHAPTER IV. 
 
 BLECTBICITT AND MAGNETISM 
 
 was to be i,U,o.l„cecI So let , , I f « '^^ ^'" "" """-'' 
 
 ".at w:„ ,eaa ,. „,„.. .^t t'.i :::tru:aT;;r""-'' 
 
 I': 
 
 i;f-: 
 
 Fig. 120. 
 
 XXVII. CURRENT ELECTRICITY. 
 
 ami 4et f ^ «l'oet-zmc, oacli about 10- i 
 ....... and 4e." wide; carefully weigh both. Take also a 
 
 ujnbler two-thirds full of water, and to it add about 
 
 ^tup ,„ the liquid. What do you observe? Leave 
 the zinc strip in the liquid for a few minutes tl, 
 remove, dry, and weigh it. What do you find' ' '"' 
 
 a lifrf "T* *• , ^'^''" *'•" "'^^''' '^t'-'l' '" the liquid 
 
 Watcl, the surface of the copper. Now bring the 
 
 "quid intocontacr^-fE;,:''^" "ZlTZ:!:^ '"""'T '^^""'^ 
 
 copper. What ao .ou obse^vop Le^tij:::;::;::^-: ^tS: 
 
CtTRRENT ELE<;TKICITY. 
 
 201 
 
 CTflSr-"' "■°°- ^"'""""■""'."'y-.weteh.hem. What 
 
 elea.,. Cut t„e con,,:; ; , ,," ' T, i,i^ '"""" "' ~"'"'^' "'"« 
 zinc or copper. ^ ''°""' >"' ^Parato It from tlio 
 
 wha..e„era,oo„eu,s,o„fr;::':rri;r,f:;;^^^^^^ 
 
 It appears that there must be a connection n,„i « . . 
 <• particular kh„:, between the two ZTZ ol T\ ! 
 may occur. The connecting wire Ihe, i,' 1 """^ 
 
 m the eLan..es that nee,,,. „ i ' ""portant factor 
 
 some iu«ueC i e^™ ^d 'bv I"' '"."""""■■'' """""" "'-" 
 tinough the wire ■ in oH 1 ^ T '' "'""' """ ""«''"=' 
 
 going°o„ in t,::":i;.e"ic:„ :::* ■ '"-' ^°™"''"=" — < '^ 
 
 ing"rs'„1:r""'"°" "'" "°'^"- ^■'^- — > l..-oportieM„r. 
 
 
 
 liCjJ 
 
 M, 
 
 "ii'l 
 
 ™w.tea .,. t,n.aci,an,u;;L;!i;r;.::,:;.;,, r:';;.i!:!' ;: ;;>■ - «■- 
 
 "."■ '™"'=' "I'ei, „t rest, points „„„!, „„(, ™t, tT ='"'■"• 
 
 w,rc helns over the needle, and par j ,. Ir n i . "'° ,™""==""s 
 tremltteof ,!,„ „,r„ „,to contact VI a 'I ',""»' ""' "™ <^''- 
 twoext..„Utl«»orth„wi«,si what,»r-"Lw ' ' *""'»■■"'«"'„ 
 
 m 
 
 ill 
 
202 
 
 ELECTRICITV AND MACTNETISM. 
 
 Experiment 7. Brine the ends of th.. , 
 
 h ends of the .vires together, as »,efore, 
 
 Pi, ,2, f't^'-Po^i'iy a piece Of paper be- 
 
 *"-<^^'" »'e.n. Ls the needle 
 moved? Does this result cor- 
 respond with tliat of Experi- 
 ment 5? What ('o the two 
 cxperiinonts indicate? 
 
 Experiment 8. T a Ice a 
 large iron nail, and plunge one 
 end of it into iron filings, and 
 then remove it. Next, wrap a 
 piece of paper around the nail 
 leaving the ends exposed, and 
 turns of copper wire, taking pains that'll ''T"," '' ^^ ''' "'"••« 
 other. Now connect he J re wit, th "T ''" ""* *^^"^'^ '^'^' 
 
 that there will he a c„„ti„ . 1« T ?■ "' ^'"^ "''''P''''" J"*** "«^tl, so 
 through the coil n ^^^ ^T ""^ «trip to the other 
 
 raise the nail. WluU is t e 1 f " '^"" "*' "" «""^"' 
 
 «^l| 
 
 in acid iy:2::j:^'':^T2r^''--''f-^-^^ 
 
 usual properties. The cause of' f \ ''^'"^^*' ""' 
 
 allied Phenomena is Xa7^r^L ^ u" "'^"^ ^'^- 
 the wire are attributed to th rslte " . ''"'^^ P'"!^^'^'^'- '- 
 througJi it. passage of an electric current 
 
 Almost from the dawn of the science of eleetricitv tU... , 
 been many who h-ivc bol,-n,.„i • ^, t^ieciucity tliere ha\e 
 
 Of it» existence, and ', er "'' 'al? 'r'^^"^ P""' 
 
I 
 
 DmECTION OF THE CURRENT. ^OS 
 
 §152 Some deflmtions. - Experiments (not easily per- 
 formed 1^- the pupil) show that the eurrent traverses the lia.^ 
 between the metallic plates in the battery at the same time tha 
 It traverses the eonneeting wire, so that the eurrent makes a 
 c-omplete eu-c.nt The term circrnt is applied to the entire path 
 Mong winch eleetr.c.ty is supposed to flow, and the wire along 
 h.ch . flows .s called the conauctor. Bringing the two extreme 
 
 r.ll V-"" ^T"*^^^' ^"^ separating them, is called, tech- 
 
 nxc^X^.maUng and breaking, or closing and opening, the circuit. 
 
 Our arrangement of acidulated water and two metals is called 
 
 a voUa^c cell element, ov pair. A series of cells, properly con- 
 
 Ta^ltn' '' '"""^' ^'^"^^^^ '''' '-'- '' -meLe^^pHed 
 
 § 153. Direction of the current. -It is evidently neces- 
 sary, m defiuxng a current, to know its direction ; b.ft alTo 
 
 Fig. 122. 
 
 Fig. 123. 
 
 phenomena known serve to indicate the direction, electricians 
 have umversally agreed to assume that in such a cell a desXd 
 l!.o electricity flows /.o,. the copper to the zinc in the ^oire 
 
 Experiment 1. Place the eoncluctin^ wire over and parallel with a 
 niagnet,c needle, in the n.annor represented in Fi^. 122 n^^hat 
 U.rectiou is the north end of tl.o needle deflected? Turn the cell half 
 way around so as to have the position In Fig. 123. In wl at direc fon 
 is the north end now deflected? m what direction 
 
 ofllTuatHef" '"'"''' *"• ^*«"-'-''° '»-.sed the voltaic pna.whlchUthep«.„, 
 
 !5 Ji 
 
 . I' 
 
8''n ■ 
 
 
 204 
 
 KI.EOTRrciTV Am> MAGNETISM. 
 
 § 154. Poles or electrodes ti 
 
 T-ntly called the ..^a//.. ,,/,,,;,;; Ir ''^^'". '"''P ^^^ '''^■ 
 ;>/«^^ and the end of an/co^.'.t!^ ""f /^''^ ^''^ ^--'^-^ 
 o.-..ogative plate Is called r't T'^ ^''''' ''^'^ ^'^I'l'^'^ 
 tl.c end connected wit .0 ''J ""■'!''''' "' '^'^^^"^^^^ -hile 
 
 together the + and - electm / ""^''i««"»'Ptio», if we bring 
 former to the lttte"acroi^^'^t''K' "'■'""' '""^"^ ^^^ ^h^ 
 -Kl that electrode s" ! w i^'^^^^ ' "' °"""'^^"^ ^''-^^ I'^"^^'^ 
 Pl-te and that electr;;!; is !_"''', "f ?"""'"' »""^' ^"^' "'^^ 
 current flows ^vithm the cell tVo.n 'Ir'^*''"' ^"•'''^'"t goes. The 
 
 cm, tioni the z,„c to the copper. 
 
 § 155. Potential — If o 
 one vessel A to another B throuT ""■ ''''^'' '' *'^ ^"^^ ^''^m 
 -nst be a greater pressure f „:, ^tl "' '""^ ^^^"^' "'"-' 
 
 at the other end ; i.e., iu ordinnrv ' '"'" ""'^* ^ "»''^» 
 
 i" A is higher than in B 80 n ^ "?' ?^'' "'^ ^''^'' «' ^vater 
 t«-o bodies in different com tir^^ - ^^ 
 tWcity flows from one (A) to "1 '''nJ''' '^ -"-"^ of elec- 
 a l.igl.er,,o...,,«, than B '"j 'J,^"^^'' («)'-"^J we say that A has 
 +e]ectrode, or the wire connected '''.f'T"'''' ^^^'^^^h' trioa the 
 potential (according to our .ss,m.nH . ' '?^^'''' '^^^^ "" ^'^^hev 
 current) than the -elect ode "? '' '''' '''''''''^^' "^ the 
 
 zinc. "'""^'^^^ «'• t^e wire connected with the 
 
 ^''^^Z:^^:^^::^;;: ^^- ^-^^^ ^.m t,. center Of 
 
 f-'^- it is to fill; Whit :; ;i jT' ' "^. '■""■^"■'•' '^-^ "- 
 
 ''ij,'l't between the two J l/l ."'''' '' "'" ^^^^''^'^^^ m 
 y detennfnes the direction VL^^^^^ ""'^^T'' of potential 
 electricity that is to now thro^L '^ ' ''"'^ '^*' '?"^"'''^'/ of 
 ^-- Sometimes th! ot fn f of f T"/^."^-^-' - ^' r;^-n 
 »any units above or be ow that \ .? ^ '' '"^'^^^^^^ '"^^ «- 
 zero. ''"'^"^ that of the earth, assumed as 
 
 Piaced over the current, its deflection 
 
OALVAN()H(;()I»K, 
 
 205 
 
 Fig. 124. 
 
 ^^ 
 
 « the reverse of that produced wher. placed beneath it Th- 
 tends to conAiso • l)iit ..n .,..(:« "« "tneatn it. This 
 
 Of the current is known, and to deter- 
 inn.e the (h'rection of the current when 
 tl'-'tt of the de/I .tion is known. Il" 
 suggests that .;« ,.„a,jine ourselves to be 
 swinminrj U the current, and with the 
 current and faeln^ the needle; in which 
 case the north end of the needle will 
 always he dejler.ted towards our left 
 
 EXERCISES. 
 
 1. Let the current be above the Meedle, u.ul ^o from N to «? w>. . 
 will be its deflection? ** J> to S; what 
 
 2. Let the current be below the needle ftii.l ,,,. r- . a ^ ,r 
 deflection will it cause? ' ^ '^""" '^ *° ^5 "'!'»' 
 
 4- Let the needle be below the current, and thu deflertir n f 
 east; what is the direction of tlie current? ^^fl^-^"^" toward tlie 
 
 6. What is the effect when the current In at the Hide of the needle? 
 
 4^.^::::;:^:.^!^^^^^^^^ -^^ the 
 
 and evaporate it, there wi.i er,sta.ii.e ou::;,::;!;;--:-^^^ 
 > G»W»noBcope. ru^ne^^^r Oalvani, on. of tN cw.y .,l,..vc,ro« ,„ ..ectricity. 
 
 r'ul 
 
206 
 
 ELKCTllUTrv ANf) ^rAGNETfSM. 
 
 iri. 
 
 ^''i 
 
 wm 
 
 solid ill ucedle-lilce crvsfAl« ti.; u . 
 
 .«. It exi.., i„ „ ■„„.,„„„, ■ ,: '* : 't';:^:'' ,t'"- 
 
 this (lormanf sf-ih. )>,, i • • ** aioused from 
 
 into mc.l,a„ u ,;^ t"','" " " '""^ ""^ "■""»f™-".«l 
 
 by ti.e u„. .of ,,;.,; i: ''■'';,"'""""■'"''" '^ -' - •»»«- 
 
 »= i" the „,„ina,.v h, ;,„ T." '":'■! '"; "•■"■°"' '■"» ""•■". 
 
 -''."• "» '■■ •-» ■'.."■ :; "of 1':: '; r;;:^;;. "'"" """ ^'"^'^ 
 
 acid ],v,l,„.„„ , Cm ' f° f '"""" '-""' •■""' ^"'1' ■''^ 
 
 bubble. „e iz^^::i:^xry "r, '"^ '-""-■• ^'^ ■■" 
 
 As a plausible but i,.,^.flr =X ^ of t"" f "'"'"• 
 the well-known l,yix,tl,',is of rJ "'' pbeuomena 
 
 what ohemirt., 1 J.vc t !f ? ""'" °«™''- " "'«'"»'•'« 
 
 «.!,... to outc... into c.„,bi„a,„/:;";;'„;,';:f 1"::- "'"" 
 
WHY FIVniUXJKN AFI'KARS. 
 
 207 
 
 Rg. Iffi, 
 
 Let the circles 1, 2, 3, etc. (Fig. 125), represent a series of 
 rnolecules of II,SO, connecting the two phites. At the instant 
 the cn-cnit is closed, 
 the SO4 of molecule 
 1 miiles with an atom 
 of zinc, setting free 
 its two atoms of hy- 
 drogen 2H. These 
 2H instantly iin i te 
 with the SO4 of mole- 
 cule 2, forming a new 
 molecule, 1', of 
 ILSO^, and setting 
 free the 211 of mole- 
 cule 2. These 2H 
 unite with the SO, of molecule 3, forming molecule 2'. This 
 decomi)os,tion and re«)n)positi<m continues till the 2H of mole- 
 cjUe 6 are sot free. These 211 unite with each other forming 
 H,or a nmlecule of hydrogen, which unites with other mole! 
 cules of hydrogen, and finally rises in a bubble to the surface • 
 so he molecule <.f l.vdrogen that .scapes is not con.posed' 
 of the same atoms of hydrogen that were first set free at the 
 zmc plate. 
 
 m 
 
 §160. Electro-chemical series. - If two phitos of zinc 
 were used in a cell instead of a zinc and a coppc.. there >vo„ld be 
 no difference of potential between the two i-httes, an.l so no 
 current It is, therefore, important that the two soli.ls should 
 be acted upon by the liquid in different degrees. T/>e qr.aWr 
 the disparity between the two solid dements, with reference t; the 
 action of the liquid onthem, the greater the difference in f,',te>dial ■ 
 hence, the greater the current. In the folknving electro-chemical 
 series the substances are so arranged that the mostelectro-posi- 
 t.ve, or those most affected by d.Iute s.dplun-ic acid, are at the 
 beginning, while those most electro-negative, or those least 
 
 iiiiifl 
 
208 
 
 I- 
 Hi 
 
 !»>! 
 
 ELECTRrciTY AND MAONHTISM. 
 
 affected by tlie acid, are at the end Ti 
 
 <'i-tionof t,.cun-ent..,.o^rl;,.^:^ '"^" '''''''''' ''' 
 
 Flp. I2(!. 
 
 '"' »"°'"" -'■«■ « -.»lU ,•„ the It leHer'"' "■■"^- 
 
 plate IS immersed in dilute «„.,i " ™'='' " 
 
 Circuits, and a transfer of ere ," I'""™" ™""'^ 
 will take niace Thi. „ "7"' "'"J' "lojig; the snrface 
 
 tween the . ,e ami the "° ■"■'""■• "' '' ""■"■ •">- 
 
 diverts so „n,ehZ,;r;;.''2t°"'''™*-- 
 
 «-ei.y weakens it. r„ adjitt, tottl! Tr'' "" 
 a great waste of chemie,!. 1 ' "'■'-"sions 
 
 circnit is broken 1,1^' "' "'"'" ""= «S"lar 
 
 eontinnes. If ".'n :?:r' '""^"' "^ '' « «'"cd. -'till 
 
 -™ at any tl',: ' Xe" TlnW •,;'" '°""' -«"" "''""' 
 choraieals, except at times vhel "" «'"»™l'"''" of 
 
 ■"crenryisrnlJdovertrsn ™„Jh ""■"" " '"""■"■ " 
 '■»" hce.. dipped in add to cle r-. I """' '"*''' "'- '""" 
 "Olvcs a portion of te^,, °1*. ™'-"-- "'" n.erenrj. dis- 
 amalgam whieh coverri ' "«' "'"' ''' " «<""i-li'l.ml 
 
 -no thea con,por.« itoe I'^k pZX"' """ "'" ""■"l«»"'ateU 
 
VARIOUS BATTERIES. 
 
 209 
 
 XXVIII. VARIOUS BATTERIES. 
 § 162. Polarization of plates _ Whn., 4.u • 
 per elements are first plaeedia the dilt ^ ''"' '"^ ""P" 
 
 current of electricity is iroducei but tho ''1' " ^'"'^ ^^^^^^ 
 fppl.lA 'PK • ^ """^^^ ' ^ut the current soon becomes 
 
 feeble The cause is easily discovered. The liberated hZ 
 
 visibly covered with bnhhin. , . , '^'^^''^^^^^ P'^te is very soon 
 
 :rs„rr:: :a: r::;r 'if- f 
 
 coated with h,d,.„ge„ Bmore.tro ^IvZ, !'-p„^ h':!' " ""? 
 
 Fig. 127. 
 
 ^.o,.o/^7,«^,to,,. Very many methods have 
 ,^ been devised lor remedying these evils. Thov 
 are all included in two classes: mechanic^ 
 and chemical methods. 
 
 tJvff' ^"Jt^.b^*^^'^--'^^^ Smee bat. 
 te^y (Fig. 127) IS an example of the former 
 
 eass. A silver plate, or sometimes a lend 
 P ate IS coated with a fine, powdery deposit 
 of platmum, which gives the surface a rouo-h 
 
 readily adhere to it 'DHu'to '^'''•' ''" ''''''''^''' ^^'^^ -^ 
 forv Th 1 ! ^ sulphuric acid is used in this bat- 
 
 th.. „eoess,t„te, a c„,.ta„t ,„,.,, to U^ >^e ^^TL,^ 
 
 if 
 
 1 
 
 i 
 
210 
 
 
 I I 
 I 
 
 ELECTRICITY AND MA(;NETIsM. 
 
 No mechanical method can w)mii,. 
 
 Mrogen on the electro nc'tivo^^^^^^^ "" ''''''''''' ^' 
 
 Pletely accomplished by Ssll J . "• "" "^'^' ^^^ -«- 
 
 hydrogen, as soon as 1 be / ^ ^ "' '""'''"' ''''^' ^^^'^^ t^« 
 on as iibeiated, may go into combination. 
 
 Fig. 128. 
 
 § 164. Grenet battery. _i„ the 
 C.reuet or bottle battery the hydrogen 
 - ^Lsposed of by chemicaf action. 
 1 he chemical action is quite complex, 
 and will therefore be omitted. The 
 
 'qtnd used is a mixture of potassium 
 l^iehromate and sulphuric acid dis- 
 solved in water. The zinc plate Z 
 (J^ig. 128) is suspended between two 
 carbon plates, C, C. The carbons 
 remam in the liquid all the time. 
 (Carbon is now largely used in 
 X7r' ^''' "'' *^l««^'-«-"egative 
 
 This battery gives a very energetic — 
 
 only to ,„.a„ the .i„o o„t of ^l,o ta „" , '" ,™''' "" ""^ 
 «nd, „„ pushing ,h. zinc back i to t L,^ "l "'™' "™ "• 
 immediately. It is well u, „l „ . .^ ' '""'°" '"""menws 
 
 -nallybylithdra„i:r,ht.tL 'uh r^','? " "''" "«""- 
 With ODB Grenet cell Lriv ""' '^'' " »''»'■' 'im"- 
 
 ».I, bt.es^r™';Xttr'^ batteries. - There is, 
 Mia batteries. T| c ic fs 2''""'V'. '"'■'■"" """""■■■• "' '«">- 
 
 imposed by it, wi.il ^ „• irV",,'';" "^:" ^ ^ "■'■ 
 
 and the condn.ti„„ ,,,.,tp ,, '"='1""''^ ■» dil„te sulphuric acid, 
 
 be decomposed b;i;ro„,'Z''' "'' T"' " '"""" '"-■" »" 
 J nydrogeu. 1 |,e t„o liquids arc usually eep- 
 
IS, 
 
 ^BtmSEN's AND GEOVE's BATTEEIES. 211 
 
 arated by a poroiis partition of niurla,.,,! r.,-A„ 
 
 '<'•<■ l-vent t„o pa.s,age of 4^4 '« o co'fri t^''^ T 
 l«Uoo- (Fig. 129) i,a., a bar of ca,bo„ mZtau!'J ""f" 
 a™l contained in a porons cup. This I Z "l'^ T"° 
 "Hotber vessel containing the dilute , ,1 Zr td-'^-'" 
 
 ;;:;r„:':.r ^•™° 'rv " '■■"'"- '^■'' '-'^^^^^^^^^^^^ ti;:: 
 
 "Inch „ea ly sniTounds the porous cup. The by.lrogen tr.v 
 ■ses, by decomp„.si,i„„ and reco„,positi„„, the s, IplXl acH 
 
 ::;'":"'"™'Sl'''-P-™"»l.ar.itio„,andin,„ediatly te^^^^^^^^^ 
 oheunea. act.on with the nitric acid, so that nono',t;,::'l" 
 
 carbon. Thcrp nm ,...^ i ^ i 
 ^ndi, aie prodiieetl by 
 
 this action, water — which in time di- 
 lutes the acid -unci orange-colored 
 fumes of nitric oxide, whicli rise Irom 
 tlic battery. These fumes are very 
 offensive, corrosive, and poisonous. 
 I- the nitric acid is first saturated 
 witli nitrate of ammonium, tlie acici 
 will hist longer without dihition, and 
 tlie fumes are ulmost entirely pre- 
 vented. Strong sulpluiric acid will 
 not answer in an3- battery. Usually, 
 
 j;;..-. by «ig,,t or .0 by ZZ 7::^^^,;^^ i!: 
 Grove T.sed a strip of platinum instead of tlie onrl^on m i ' 
 
 Mmmm 
 
 iji 
 
 rill 
 
 I 
 
 .jlil 
 
212 
 
 ELECTiaciTr AXD MAGNETISM. 
 
 FiK. 130. 
 
 § 166. Gravity battery. — rii.. l,..+f • • 
 
 this country for toiog,..phiL is cabled ,7 ''''"'''"^' "^"' '" 
 copper plate V, ImThmv 1°o is ^''"'"'^ ^"''^"^Z- A 
 
 placed ou tl.e l,ott(.ni oi a vessel 
 and covered with crvstals of cop- 
 per sulphate (hh.e vitriol), and > 
 tlio whole covered witli water 
 As the vitriol dissolves, its s,,e- 
 c.fic gras-ity causes it to reuiaiu 
 
 at the bottom, iu contact with the 
 
 copper plate. The zinc plate Z 
 
 18 suspended in the clear li^jnid 
 
 above. To start the action quickly 
 
 a teaspoonful of common salt or 
 
 o. zinc sulphate is dissolved in the 
 
 ^vater. As the cheniical action 
 
 P'-oceeds, the vitriol is decom- 
 
 rosed, its sulphuric acid constitu- 
 
 p-e. The.incd::;^--;;j::^^ 
 
 the electr'odes'of ?|,f ^o; Gr^'::^T- '' "'''■"'•"•« ""''^'-^^^^ 
 
 S;;;--. -out ...,.,,-!-- a. ^^^ 
 
 ..^LSr'""^'"" ^' ^"^'-^^ ^^'^ P^-^ - the above 
 
LUMINOUS ElfVKCT. 
 
 213 
 
 § 168. Luminous effect. -^W.. h„vo ..i. i 
 
 "• vvc nave nlrcady seen one illus- 
 
 ^i«-i3i. tmtion of tliis ofTeet 
 
 in the glowing of the 
 white-hot platinum 
 wire. 
 
 Experiment. Attarh 
 
 otic polo of tlu! hattrly 
 
 to a flic (Fijr. 132), aiitl 
 pass the other pole over 
 its roii^rli surface. Tlie 
 /11(! forms part of the 
 circuit; and as the wire 
 passes over it, the (;ir- 
 ciiit Is rapklly inailc and 
 
 causes a spark at the pouit where the elrmit ll"i,?","' """U''"'' '''"^'^ 
 Of sparks that flies fron. the flie l^du^ 1.! rl:,;;.;; '''" ''''' •'^'""^" 
 
 particles of Iron tliat ^'^>^- '•''•''• 
 
 are pn.Jecttul Into llie 
 
 air. 
 
 Fig. 132. 
 
 § 169, Chemical 
 
 effect. Kx|M«ii. 
 
 mcnt I. Kt(!cp Home 
 , ., . , leaves of jiiirple cah- 
 
 bap; the infusion lias a deep purple ,.,|or. i,is. 
 solve a llttie caustic soda, and pour u f<.w drops 
 ot the solution into a iiortlon of flw. ii.c.wi, ., i ., 
 
 tery-wires two narrow strips of plaU,,.,,., ami „l- re o,u o n 
 strips iu each branch of the t.-he, a Ilt,le way rt ' fh H 
 wm be obliged to traverse a part of thl ^ ll f' n,^ M^'^S: 
 bubbles of gas arctaeUlatcI^dlHcngagcafioiu the i-lltlnl ^Ir^s j 
 
 II 
 
 f. 
 
214 
 
 ELECTEICITY AND MAGNETISM. 
 
 «">Phate Has ta.eu Place; - aeiurrari^XT^; -J^J^ -"'- 
 
 or fusion. A largo m,mtr of itf '"' "'"'" "^ "<""«"" 
 sodium 8„lptote, Sf an acid a, ,1 ">f "'"'' "''' ""'"P''^'"', like 
 
 ™bsta„oo t„at will ,„;: ; ::'„ r";"'f r "-'^ ''"■^■■ 
 
 neutralize an aeid is enll.,1 ., ; , •* ""''«'«'■«'= *at will 
 
 -1 a base i. called T^^ Xk. " T""'°""" °' ™ -"< 
 
 I'latlnum „„ which eo,„.cr has ,o . 1+.';"'""""'' """•. ™""ect the 
 
 per sulphate (CnSO.) wl, o , ,' ",'^°'7 " ""'^ '""'eeule of eop- 
 -conn., for the ^t^UZZ Tt"! "', '"" "•■"*■• ■''"- 
 does not lose its stronoth foTl I ^'""'- '^''"= ^°''"»" 
 
 »;pha.e i^ decomposed,'! er/wr If"'" "?««- 
 poles are used, the SO, docs „n, „ i . '""' Plat'iun, 
 
 «.ters into chemical .a tt w t th "w'lT '""n"" '"■■'""™' ""' 
 with the hydrogen of the «^r r ^^'° ^< "'""Wncs 
 
 o..genof theUr ^::::::-';^Tiotu:^-:; 
 
le that around 
 'f the sodium 
 3sults. 
 
 »- in the bat- 
 battery. A 
 
 is called an 
 
 lyte must be 
 by solution 
 
 fiposod, like 
 some other 
 
 ce that will 
 of an acid 
 
 'olyzed, the 
 
 the -j-pole. 
 
 iilphate, and 
 collects on 
 connect the 
 ire, and the 
 in the solu- 
 e the result 
 
 a copper 
 
 atom of 
 e of cop- 
 'r. This 
 
 solution 
 f copper 
 platinum 
 mm, but 
 ombines 
 
 and the 
 ^ + 0.) 
 
 CHEM1(;al effect. 
 
 215 
 
 The liberation of the oxyoe„ is t|,n m^uU ^f 
 
 eal action, subsequent to' the clecLi;;::!!!^"^"^^^ ^'--^- 
 
 I'^loctrolyze this salt i„ sl^n'uu^Z-'''''''- ^'' ^ '^"'^' ^^'^'^- 
 ..•outh of tin crystals .1,1 slu.: Z^^^l^''- ^ '-^'^"I 
 
 In a Similar manner, silver and 1.^ 
 
 from then- salts, silver nitratl and 
 k'a.1 acetate. Each metal has its 
 
 own peculiar form of growth; and 
 sometin,es the same metal, par- 
 t'C't.larly silver, exhibits different 
 forms, according to the stren-th 
 ot the solution an,l tlie powc.'of 
 tlic current. lu Fjo-,,,.^ j.^^ ^ 
 ;;q)i'osents a silver tree deposited 
 *'om a weak solution of silver 
 •"trate, and B a tree fornu-d from 
 :i still weaker solution of the 
 same. 
 
 Kxperlment*. Re.nove the bot- 
 tom of a glass bottle having? a wide 
 "•outh flt a cork to the n.onth. and 
 waterproof substance such as .uftTl T "7''''' '""^"'att^d with son.e 
 "atln,. l„ piatinun. strl^. Fit '"' FUl'; T''' "" ''''•''' *«""'- 
 the inverted bottle witldil.uc", In „nr- "'° ''''•''''''' '^'"l Part of 
 the platinum poles. ' a 'e 1 1 .f h '•'"''."'"' '''''''' '^' t"^^«« over 
 "f gas innuediately ar e f ^.n t. "T"" "*' "" '^'^"^^>'- ^^''^les 
 '» the tubes. About ;wlcermcV:^^ I^n '^'^^'^^^ "'« "^"^^» 
 as over the +pole. Thrust •, L,TT ,^ "''''*'' °''^*'' "'« -pole 
 
 the former i., the 1 1 .a^^ ^f '';:^'-;"n T' ^^ "''^ ^^^^«' 
 rapidly than it burned in the a^r Tl ,Vi T '"""" ""'^" '"«••« 
 I'yUrogen gas and the latter o vgen . Tif " .'""' *'^' '"""^•' '« 
 "e used in making this experin en ''' '^""•' '^"^ ""^''^^ to 
 
 
21G 
 
 KLECTUrciTV AND MACiNETrSM. 
 
 Fig. ,3.. clcsolv lik-e tint Xo r '''*'^" •« ^^^n- 
 
 . '"^»- uiat already ff veil fSt^o\ <. 
 .'iction iirti.e simple cell T. , ^ ^'"' *'^^ 
 
 tf"it wat,.r is ultiunteh 1 '' '^''■^^^'" 
 
 -Ipluu-io acid is o V, !'^^7"P«'-^'' for no 
 
 t"'^t -tor is eon, oil oM "''"'^"" ^''«^^« 
 or l.vd.o.en to on . /7 ^'"'"*^ ''J volume 
 . u„Ln lo one pjiit of oxvo-pn tin 
 
 ^"•gl^t not copper poles to be used ^ f^^'f ^^ 
 
 PO-- wi.ati:t'rrii:r^"'"^^"^-p^- 
 
 !""1-- tia. poles. The serous iluid t ' o r '"" ''^f^'^^"' 
 
 'eles under the positive pole is ad I T-7 ^'"'''' '^'^ ^^^«- 
 """- the negative pole is alkaline '"" '^ ^'" ^--^- 
 
 tongue „,ay forn, part „f ,1,0 ci,4m '"'^'^' "^'"' '^^ "^^t the 
 
 Panied by a peculiar acrid taste. 
 
 When a battery is knowu not 
 to be very powerful, the tongue 
 sorves as a very eonvenient gal- 
 vnnoseope, to determine whether 
 
 tl'c circuit is in Making eondition, 
 
 'ind approximately the strength of 
 
 tne e.„Tent. If the erural nerve 
 
 (n white cord next the backbone) 
 
 , ''' ^'""S, '-^"^^"tly killed, is laid ' .| 
 
 i-tautly eouvui!; ^^^^^ iT d"^' ^^^'^' '^ ^^^^^ ^ 
 
 ie„ (11 awn up, as represeuted by the 
 
itor of eloc- 
 iion is verv 
 58) for trie 
 •"■ic acid is 
 ^O^; then 
 ' is certain 
 2(1 > for no 
 ysis shows 
 1>J volume 
 n. Why 
 this exper- 
 "g copper 
 
 ne apph'ed 
 
 I's appear 
 
 the v<is- 
 
 ' vesicles 
 
 3e the tip 
 ' that the 
 
 MAGNETIC EFFECT. 
 
 tonch- 
 les are 
 by the 
 
 217 
 
 'lotted lines in Fi"'in-p nr. ti, 
 
 -ta„e .,,0 ci,.c„ir is b,t 0,,™ , T„:,r:"'r "™'-' ■"*= 
 
 pieco of zinc •md tli,. m., i ■.; ' """"S '"» norvo «• th a 
 
 it ...ay ,.o,„ai„'fo,' J^' 1^^™ t'"' '■" -' r' '"""""■"""■'■• 
 is armed with a ,„f, ., ' ' '"'"' '■'""'^^- " one iwie 
 
 l«.'k of lh„ „„.,,-, „,,,„;.. '',"'"'• '■" <>l«.- .^ appli«l at tho 
 
 --' -' oa.,.; s.:ii:':;.r;f ™:i:ir '- '"= 
 
 tlio circuit; wl,„t lakes pltL'-V"'" "'"" ".""t 
 
 of H.l» experiment „,t,«,"f<°"''""'" '"" '■""'" 
 Whv wo« fi.„ .. -I exponment 8, S ir>l 
 
 -I'^Cl^r W^'r^^f :;r'--^ P^Porin th^^ 
 
 --PP-' in pap ' If th " ;,,r '" ""^ ""^' ""' 
 paper, wo„iaitUeiJ^^/l:r""^^^^^^ 
 
 OM.th„.acertain limit, themorepower- 
 in y .s It niagnetized. This arran..en ent . 
 called an electro.nagnet, becanse itis " m J 
 net produced by electricity. The ro of T^" 
 IS called its cor,, and tho coil f ""^ 
 
 ' ttio coil of wire the helix. -^ 
 
 lusulatod, covereil ./..•// 
 
 ' I. 
 
 
 i 
 
218 
 
 ELECTRICITY AND MAGNETISM. 
 
 ! 
 
 I- 
 
 In order to tako advantage of the attraction of both ends or 
 
 poles of the magnet, the rod is most frequently bent in a U-shape 
 
 (A, Fig. 13H), :i,jd then it is ^j^ ^3^ 
 
 called a horse-shoe magnet. 
 
 Sometimes two iron rods are 
 
 used, connected by a rectan- ,_ ^^ 
 
 gular piece of iron, as a, in ^H ^H^ B MB 
 
 IJ of Figure t38. The metiiod 
 
 of winding is such that if the iron core of t!ie horse-shoe wero 
 
 straightened, or the two spools were placed together, end to 
 
 end, one would r.ppear as a continuation of the other. A 
 
 piece of soft iron, 6, placed across tlie ends, and attracted by 
 
 them, is called an armature. The piece of iron a is called a 
 
 back armature. 
 
 XXX. ELECTRICAL MEASUREMENTS. 
 
 The wonderful developments of electric 1 science in recent 
 years are almost wholly due to a l)etter uiulerstanding of what 
 electrical measurements can and ought to be made, and how to 
 make thera. Most of this increased knowledge has been gained 
 since the first Atlantic cable failed in 1858. Let us learn how 
 to make some of them. 
 
 § 172. Strength of current. — It is evident that the ther- 
 mal and luminous effects of electrical discharges, electro-chemi- 
 cal decomposition, the deflection of the magnetic needle, the 
 magnetization of iron, and even physiological effects, or any 
 external manifestation, may be employed to detect the presenc-e 
 of an electric current, in a circuit however extended. Since 
 the magnitude of any effect varies as its cause, it is also 
 obvious that the magnitxule of these effects may serve to measure 
 the strength of the current. Now, as the quantity of water that 
 passes through a given pipe in a minute or an hour indicates 
 the strength of the current, so by the strength of an electric cur- 
 rent is meant the quantity of electricity that passes through an 
 electrical conductor in a unit of time. 
 
>oth ends or 
 in a U-shap« 
 
 GALVANOMKTKIt. 
 
 219 
 
 ie-shoo were 
 her, end to 
 
 other. A 
 attracted by 
 
 is called a 
 
 e in recent 
 ng of what 
 md how to 
 )een gained 
 learn how 
 
 t the thor- 
 citro-cheini- 
 ncedlo, the 
 its, or any 
 le presence 
 ed. Since 
 it is also 
 to measure 
 water that 
 r indicates 
 lectric cur- 
 hrough an 
 
 § 173. Voltameter. _ The quantity of electricity that passes 
 any cross section of any conduct.>r in the Harnc cinunt, 1 owe" 
 U>ng ., unless there is a leakage at so.ne point, neces's ri^ h 
 
 part of U,e cncuit, and measure the strength of a current by the 
 temperature to which the wire is raised ; or we mav T.V 
 water and collect the gases resulting theroLm /^ '^^SZ 
 
 oj time. The lattc-r ar.angement, called a voltameter, is easily 
 
 Fiff. 139. constructed sufllcientb- u..Mn-ate for many pur- 
 
 poses, and should be constructed and used by 
 
 •?very pupil. '' 
 
 In Figure, m, a Is a «,a„„ ♦,,„,, ,ocn. j^ng and 3=™ ia 
 diameter (a ...uch .short^-r tube- will answer; for X 
 ample., a largo sized tost-tuho), cloHod at one end 
 and graduated i. cubic centimeters (this may o 
 cloncby means of a paper scale pasted on one side 
 of he tube) ; Ms a bottomlcHs glass b<,ttle of al)„ub 
 Iter capacity. Through the Ht<„>per of the bottlo 
 pass two wu-es, msuluted with gutta-porchU or seal- 
 »g wax, terminating in platinum .strips, which are 
 ■"•<xh.ced a Uttle way into the ,ube. ' The tube s 
 «1 ed NM h water .slightly aclduhUod with sulphuric 
 acid, and Its oriflce is Immersed in the same kind 
 of li.iuid, which partly IIIIh the bottle. When the 
 colls in series a IH-^^T '""' '''"'"i'^;^'^ ^»"' >* 'x^ttc-ry of two or more 
 
 §174. Galvanometer.- The i„8tn,ment In mo»t common 
 
 besides ,ta ordmary use a, a gulmno^o,,,,, ,,„rr„rn„ t ,e stil 
 more unpoi-tout offlee of a g,.ha,^u,-r. Tl,' ,i„,p|„ ...a^ne " 
 r.e.dle, used as ah-eady described, answer, lolcraby wcirwil 
 lir T, "™^' "'" " ■' ■"" »"'"l"™ enough to be 
 flowmg m the same duectlou, are ,,h.e«| one »bovc aud tU« 
 
 )V 
 
 
 '1 ' 
 
 ( •■ 
 
 
 *!ti 
 
220 
 
 ELECTRICITY AND MAGNETISM. 
 
 ill 
 
 I 
 I 
 
 II 
 
 ih 
 
 Other below a magnetic needle, they tend to produce opposite de- 
 flections, und to neutralize one another's efFect, so that no deflec 
 t.on occurs Evidently, if they flow in opposite directions, they 
 tend to produce a deflection in the same direction, and the result 
 IS a deflection twice as great as that produced by u single cur- 
 rent. Ihe same result is accomplished if the same current is 
 nuule to pass both above and below a needle, as in A, Figure 
 140. If the wire were carried four times uround the needle, as 
 
 Fljj. 140. 
 
 I'l B, the mfluence of tiie current on the needle would l)e about 
 four tunes that of a single turn. Very sensitive galvanometers 
 constructed on this priudple, often with thousands of turns 
 of wire, are sometimes called loi>>j.coil galvanometers, in dis- 
 tmction from those having few turns, which are called short-coil 
 galvanometers. 
 
 § 175. Tangent galvanometer. — Th.. arrangement de- 
 scribed above is more commonly used as a gaivanoscope than a 
 galvanometer, though it may be so calil>rated as to answer the 
 atter purpose. The law connecting the current strength with 
 the (lertection of the nee.Ue of this galvanometer is not known • 
 but m another form, called the tamjent galvanometer, the rela' 
 tion 18 expressed in a simple tangent of the angle of deflection 
 This apparatus is constructed on the principle that the strength 
 of currents are proportional to the tangents of the angles of 
 deflection, when the needle is verv short in eompurison with tiie 
 diameter of ^ circle described by a current circulating arouna 
 
opposite dc- 
 lat no deflec- 
 ections, they 
 lul the result 
 single cur- 
 ie current is 
 a A, Figure 
 c needle, us 
 
 ^D). 
 
 1(1 1)0 about 
 anonietcrs, 
 ■< of turns 
 >'■% in dis- 
 d short-coil 
 
 c'inent dc- 
 >pe than n 
 mswer the 
 Migth with 
 >t known ; 
 , the rela- 
 3eflection. 
 e strength 
 angles of 
 \ with tile 
 ig around 
 
 EXPERIMENTS IN MEASITREMEKTS. 221 
 
 A magnetic needle, about 2.5- long, is suspended fVeely by an un 
 twl^sted thread „, Figure Ul, In the center of a copper hoop „ ",out 
 .50- in dm,neter, which tor.ninates 1„ the wires ,nr, ; and l"e e a^e Z 
 nee ed wiU. the battery whose current is to be measured I ccul^ 
 ca d-board r.. containing a circle divided to degrees to Indicate t^ 
 exten of deflection, is placed beneath the needle. The rW ^^^^ 
 
 the scale. When a current passes through the ring a, the needle i, 
 Uetlected. The tangents of the angles of deliection'm^ be found by 
 
 Fig. 141. 
 
 reference o a Table of Natural Tangents in Section Dof the Appendix 
 and the relative s.rengths of currents nmy be determined hy the tiw 
 g.ven above. A tangent-galvanon.eter is indispensable to Z ( ..lent 
 of electricity and one may be made by any one haviiig only ordinary 
 mechanical ability. " - >"uumry 
 
 § 176. Experiments in measurements. _ Inasmuch aa 
 the magnitude of the effects that can be produced by an elec- 
 tric current, or the amount of work that can be done bv it 
 depends upon the strength of the current, it is of the utmost 
 importance to understand the principles by wlnVh it is remi- 
 xuLcu. ^ raw experiments will make this apparent. Provide 
 four coila or spools of insulated wire. Mark the coils A li C 
 and D. Let A contain 100 ft. (about 1 lb.) of No. 16 copper 
 
 I 
 
 II:! 
 m 
 
222 
 
 ip 
 
 ELECTRICITY AND MAGNETISM. 
 
 Wire ; B and C respectively 80 ft. and 40 ft. of xNo 24 conner 
 w.re ; and I) 40 ft. of No. 24 German silver wire '' 
 
 V "rs. ';o.^^; TTT-; ^;X: t'^;r ''- - 
 
 Plar. 142. 
 
 Experiment 2. Place coil C in tlio circnit xsnth n «„ i 
 
 each case in tlie above exneriiru>nf« t>.L , «lenectioii in 
 
 §177. On what strength of current denenrts u 
 appea« t ,at the strength of the o,„.e„t varies „„'t„Tvw-t,7t,e 
 
 o>'cieot that „| co,Kh,ct«™ do not allow the c„„e„t'to It 
 wth equal fac,l,ty , f„ other words, so^e conductors omZl 
 
FORMULA OF RESISTANCE. 
 
 24 copper 
 
 , in the same 
 
 tlio miniber 
 
 5 for A, and 
 
 :impare the 
 it. Talic li 
 
 mparo the 
 
 vc or Bim- 
 lie circuit, 
 flection in 
 Jrtional to 
 elusion do 
 'lie third? 
 
 is. — It 
 with the 
 
 kind of 
 ■' It is 
 
 to pass 
 er more 
 
 223 
 
 resfsiance to the passage of a current than others. The larger 
 conductor offers loss resistance than the smaller. It is 3 
 by expenment that (1) the str.n.jik of currents varies airerJUjas 
 the areas of the cross-sections of tU conductors^ or the squares of 
 the diameters of cylindrical conductors, inasmuch as areas Jy 
 as the squares of their diame -s. (2) It varies inversely as til 
 Ingth of the conductor, i.e., if vire one mile long offers a cer- 
 tam arnount of resistance, a wire two miles long will offer twice 
 as much resistance. (3) U varies inversely as the specific resist- 
 ances of the substances used for conductors. The measure of the 
 conduc tmg power of a substance is the reciprocal of the meas- 
 ure of Its resistance. 
 
 Resistance is expressed in units called ohms^ (see §181) 
 The student can easily provide himself with a standard having 
 approximately a resistance of one ohm, by obtaining 40 feet of 
 No 24 ordinary copper wire 0.5-- in diameter. The student 
 wi now be able to see that the strength of the current t a 
 cncui IS less when a galvanometer is in it than when it is not 
 in, unless the resistance of the galvanometer is very small 
 
 Tr\ "'1^ T '' '"' "^* ^' *^^ ^'--^- The resistance' 
 of the tangent-galvanometer of § 175 is almost nothing, while 
 that of tlie voltameter of § 173 is very considerable. 
 
 § 178. Formula for Resistance. - Having found that re 
 s^tance vanes directly as the length and inversel^ as theZale 
 ofj^^aneter of a conductor, we may include all its laws TI 
 
 m which R==the measure of the resistance, ^ = the measure 
 
 the length, and d the measure of the diameter "f. 
 C3lndnca conductor. K is a constant, such that when the 
 inaterialof thewireis Vnown and the denominntion t wh ch 
 
 1 and d are expressed, a value of K taken from a table ma^ be 
 
 
 i) 
 
 ■a 
 
 Tfli 
 
224 
 
 ELECTRICITY AND MAGNETISM. 
 
 US to find the value 
 
 substituted in the equation, and thus enable 
 ot K ui ohms 
 
 thous«udto,R = 0.72xB!;»=„.i,;a„,™. 
 
 When I . e.p„.e<, io f„.. „„, , „ ZZ:L I:^! ZT'" 
 
 REFERENCE TABLE OF RELATIVE RESIST 
 
 S"^er.- ^Qoc, 
 
 Copper „ 
 
 Zinc ,, 
 
 Platinum ,, 
 
 Iron ' " ,, 
 
 German silver n 
 
 Mercury ,, 
 
 Nitric acid — commercial. 
 
 ANCES, 
 
 ETC. 
 
 
 R«l. Resist. 
 • I.OO 
 
 K. 
 
 o.ir, 
 
 . 1.06 
 
 
 9.72 
 
 3.74 
 
 
 34.2 
 
 «.02 
 
 
 55.1 
 
 (lie, 
 
 
 59.1 
 
 13.ai 
 
 
 127.3 
 
 G3.24 
 
 
 578.(J 
 
 
 Rel 
 
 • Resist. 
 
 • ('i\ 16 to ''8 O 
 
 Sulphuric acid, 1 to 12 ,, arts water "" 1.100,000 
 
 Common salt — saturated sol • ■ 2,000,000 
 
 Sulphate copper << ',, 3,200,000 
 
 Distilled water 1«,000,000 
 
 Glass 6'''H)i)0 p """^ ^'''■^'' *'""' "^'"«f''000,000 
 
 Gutta percha ... rinar, 1'',<JOO,000,000,000 
 
 ^ C. . . . 6,000,000,000,000,000,000,000 
 
 risI'"bifti:ro7ii 'T^ 'r?""^ ^^^^^^^ ^^ ^'^^ ^-"i--tu..o 
 
 seeond 1st 1 ^'^l^'^^^ '"'<! the other poor eond„ctor.s in the 
 
 second list decreases very raoidiv wif). ., ..;. ■ * 
 
 Tlie resistinpo ^f . r - ^ " ^'^^ '" temperature. 
 
 th.n r.f '''"'''^' ""^'"'" "^^t^^« i« <^t-teu much higher 
 
 than that given in the table. ^ 
 
 § 179. Formula for internal resistance. - Re^istanoo in 
 a vultaic circuit may be divided, for convenience, intx> two Z 
 VI.., internal re.stance (.), which the curreu encolte^ n 
 
nrl the value 
 f 1000 ft. of 
 the value of 
 ill feet, and 
 1 equals 100 
 
 distances of 
 *'e equation 
 n inch. 
 
 ETO. 
 
 t- K. 
 
 !).!-> 
 
 !}.72 
 
 34.2 
 
 • • • . 55. 1 
 
 • • . . 59. 1 
 .... 127.3 
 .... 578.(J 
 
 Kel. Resist. 
 
 • .1,100,000 
 .•2,000,000 
 •..'i, 200,000 
 . 18,000,000 
 )00,000,000 
 )UO,000,000 
 )00,000,000 
 
 iiI)oratur<! 
 •'.s in the 
 l^erature. 
 -h higher 
 
 ^t.anno in 
 o parts ; 
 uters in 
 
 ELECTRO-MOTIVE FORCE. 
 
 225 
 
 passing through the liquid between the two plates in the cell 
 and exlernal resistance (R), which it suffers in the renuainder of 
 Its path. The internal resistance is governed by the same laws 
 as the exteinal resistance. In this case 
 
 ineasiire^otjlistJince_ofJhe j^ apart (/) 
 
 r=K 
 
 measure of areas of the plates submerged (rF; ' 
 
 QUESTIONS AND PROBLEMS. 
 
 1. What length of copper wire avIII have the name resistance as a 
 mile of iron wire of tlie same diameter? 
 
 2. How can yon reduce the resistance of an iron wire to that of a 
 copper wire of t);.i .same lengtli? 
 
 3. Abo-^ h , -, much is tlie conductivity of water affected l)y addinij 
 a little snlji ;; . acid? " 
 
 4. How many times greater is the resistance of dilute sulphuric 
 acid than tliat of copper? 
 
 6. Upon what does the resistance otfered uy the liquid part of a 
 circuit depend, and how may it lie diminished? 
 
 6. What is the resistance of 500 feet of copper wire .014 inch in 
 diameter (No. 30 B.W. gauge)? Ans. 24.7 + ohms. 
 
 7. What lengtli of copper wire .00« inch in diameter (No. 38) will 
 offer a resistance of 1 olim? 
 
 8. What is tlie resistance of 16 yards of German silver wire CNo 
 30) .014 incli in diameter? ^ " 
 
 9. What Is the resistance of 1 mile of iron telegraph wire the 
 usual size being .175 iiicli in diameter? 
 
 10. Express in ohms the resistance of 1 mile of copper wire, O.OG 
 inch in diameter? Ans. 0.72 X ^'^ = 14.25r, ohms. 
 
 11. In Experiment 2, § 17(!, why is tlie current when C is in the cir- 
 cuit alone not twice the current when 15 is in the circuit alone? From 
 the results of tins expiTiment. an<l tlie laws stated in §177 estimate 
 the internal resistance of tlie cell u.sed. In .solving (1^; prohlem vou 
 may assume that tl.e electr. .-motive force (§ 180) of the cell remains'the 
 same, tliougli the resistance of the circuit is changed. The above as 
 sumption is not strictly ti-uc (§ isi). 
 
 §180. Electro-motive force. -The experiments de- 
 scribed n. § 151 show that electri.-ity constautiv flows in a 
 closed circuit containing a voltaic cell; hence the^ cell has the 
 
 3 
 
 iisr 
 
 !!'!! 
 
 ill 
 If 
 
ELECTRICITY AND MAGNETISM, 
 power of Sottino- eleefrioifv ,„ . ^- 
 
 (..sualivabbrevk d E m'fT uT T "" *«"-°-''-'™ force 
 depends solely „„„ ^f.-^:*- "has been f„„„d that E.M.P. 
 
 about the K.M.F. „, o„e grtit J,' ^Th 1 V "°"-' " '' 
 exh,b,t. the .,e„t.o-,„„tlve force h^vo^S^fdi;™?.!:'''^ 
 
 Buasen and Gmve ^'^^ *° ^-^^ volts. 
 
 Leclunche, at first l"i> to l/Jo " 
 
 Grenet <« ' 1-48 to l.fio " 
 
 Smee 1-80 to 2.3 
 
 • p^ 
 
 oU^^^Z:\Z!::^ "S^^r ^^^^^^ ^^^'^"'^"the'circuit is 
 Will require 195 Since cells t ,'h c ho "^""^^'' '"'" ^"'**^»^^' ^^at it 
 ln«h resistance) as would be fe^ive . h fi5 gTo': '"T"' *" '*^ ^'^^"** (^^ 
 sary, i„ order ^,hat this statement mav ho ^^ ''"'■ ^^^^ '^ ^' »«^'^«- 
 .^^.ma-f r«t.^anc. should be high? ^ approximately true, that the 
 
 § 181. Ohm's Law Tbr. i 
 
 strength of the current and is (hn ,' • '1'' '-^P'""''^« ^^'^^ 
 calculations on curr nt ' ^ ei T"' ^^ "^"^' n^athematical 
 
 0^>n,s La.. Call ; ^^Zfc tl" K M ^"""^ '^"^^" ^ 
 the whole -istanco^a hT " ,t R t,w T''' '^' ^"'^ 
 the law is ^ ^ ' *'^^ ^o™"la expressing 
 
 H' 
 
 In words, this means that the strenuth of fh. . 
 
 the electro-motive force nf //>« a "^"^ ..''•^ '^'^ c«'-'-e«« w e(?/«i/ /o 
 
 the ciraut; i.e. C ^^{elr'^^ f ""^'^ '^ ''' '•^«'^^'- «/ 
 
 but will b; less wLr nr'' r ^ "" ^ ^'■^^*^'- -^-' 
 
 ^" " '^ &• "^^t-"'-. ^"J greater when R' is less. 
 "~ ''°^ E "'''^" ^"^ ^-'--ternal resistance is considered 
 
 1 Voit,/rom the name Volta. 
 
Oim'H LAW. 
 
 notive force 
 lat E.M.F. 
 ' substances 
 dependent of 
 5 unit em- 
 iolL' It is 
 wing table 
 cells : — 
 
 raits. 
 
 227 
 
 le circuit is 
 »ce, tliat it 
 circuit (of 
 is it neccs- 
 ie, tliat tlie 
 
 3Pes the 
 lematical 
 :nown as 
 y E, and 
 ^pressing 
 
 equal to 
 
 tance of 
 
 or less, 
 
 13 Jess. 
 
 isideied 
 
 S-me'rf™ T^ ";*^"'"^'' --^' »'« --erted thus: calling 
 the foimer R, and the latter r, tl.o uxpresBion becomes 
 
 C 
 
 R-fr 
 
 .-rrent has a va.ul ' T^^l "■;';^ •- »reg„„,ec. t„e„ the 
 I.- a resistance, B, eq„a, to', „:,„,"tl'l;' "" """""'""^ -^ 
 
 between the two ends of r ll " '« «^"^'tb' the same as that 
 
 - pc^ts at ^t:rz:\;::ti::r^ 
 
 .o„r,a„ge„t.,aL„o™ter7o Ci^tT ' """^"""^ 
 
 as we expressTnTmou:t ofTo "1 rdT""T'-7^"* 
 or a „,ass of coal in the dZmC on ^""7"^'"" ''"'^^''^^ 
 electrical a^uomlnut.on pounds, we express 
 
 ii^;f^ 
 
 M 
 
! 
 
 000 
 
 ELEGTRlorTY AND MAGNETISM. 
 
 Potential, P (commonly difference of P) . . . . i„ volts 
 Electro-motive force, E , 
 
 RosistanccR '''''*•''• 
 
 Stren-tli of current C " **^'""'* 
 
 Quantity of electricity .! '''''^^'''^• 
 
 rp. -, „ . coulombs. 
 
 Ihe foIlovviDg will give some idea of the maffnitndp .f ,u 
 |lenon.,nations. A g.-avity cell produees a diSce of ,^ot n' 
 .a o,. an eleet.o-motive force (lor these are onl diff ;e rwl;" 
 of viewing the same quantity) of nearlv 1 volt ^"'"^"* ^^^^ 
 «P-k 1- long requires f romVoOO l\ o 'olts T^Zl 
 ordmary copper wire, 250 feet long (diameter 05 in/ 
 2 lbs.), hns a resistance of about Llir A t 1o fTef of 
 copper w.re,l™™ in diameter, has a resistance of l oC a' 
 
 rent whose strength is h aJpLt '' ''''''' "'" ^ ^-" 
 
 ^^w.lupon.eeentimUgran::;td::^^^^ 
 
 called C.G.S., or absolute units, has been devised A r- 
 s.on of these units is altogether beyond the ir^tr 'f ttis bo^ir 
 but any student wishing further instruction concer in.^K : 
 system may read Maxwell's Electnoitn nnrl nr "^''"""^ ^^'''s 
 collected reports of the Com^t ft ISTsr' T "^^ 
 Electrical Standards. Association on 
 
 The absolute units have approximately the following values : 
 
 Absolute unit of E.M.F. =J,vnif 
 
 '* I^esistance = Jp ohm. 
 " Current = 10 ampcVes. 
 
 Tf„-ii, , , Quantity = 10 coulombs. 
 
 It will be observed that the formula 
 
 holds true also when these units are used. 
 
ARRANGEMENT OF UATTEKIES. 
 
 229 
 
 3S. 
 
 lbs. 
 
 ide of the 
 
 of poten- 
 rent ways 
 produce a 
 A No. 16 
 h, weight 
 >0 feet of 
 hm. An 
 f i ohm ; 
 ire a cur- 
 id, which 
 electrical 
 »d, and 
 ^ discus- 
 lis book, 
 ing this 
 
 and the 
 ation on 
 
 values : 
 
 I 
 
 Fig. 143. 
 
 PROBLEMS. 
 
 1. What current will ho obtained from a gravity cell when E =. 1, 
 r = 2 olmis, and R r. lo ohms? 
 
 ^istancc js 3 ohms, and external resistance is 8 ohms? 
 
 3. What cnrrent will a Grove cell furnish, having the same Internal 
 and external resistances a,» tho last? internal 
 
 § 183. Arrangement of batteries. _ The internal resist- 
 ance may be diminished by placing the plates as near to each 
 other as practicable, and by employing large plates, and thereby 
 increasing the size of tho liquid conductor. But it is not always 
 convenient to employ very large plates, or we may have occasion 
 to employ a battery for certain purposes, as we shall see pres- 
 ently, m which Inrge cells would be of little or no advantage. 
 The same result that can be produced by a single pair of large 
 plates, may be obtained by connecting the similar 
 plates of several pairs in separate cells, thereby 
 practically reducing several pairs to one pair 
 having an area equal to the sum of the areas of 
 the several pairs. Figure 143 illustrates a method 
 of connecting cells for the purpose of reducing 
 the internal resistance. This is called arranging 
 cells parallel, in multiple ore, or abreast. 
 
 This arrangement is very effectual in increasing 
 the current-strength when the internal resistance 
 is the principal one to be overcome. For instance, 
 call the electro-motive force (E) of a single cell 
 1 volt, its internal resistance 5 ohms, and let the 
 plates be connected by a short, thick wire, whose 
 resistance may be regarded as nothing; then 
 ^ = 7 = 5 = -2 ampere. Now connect 10 similar 
 
 cells abreast. The size of the liquid conductor ■ 
 
 being increased tenfold, the internal resistance is one-tentb 
 as large, and we have = 5=1 ^-^=2 amp6re.. So that, 
 
 ' iiii 
 
 m 
 
230 
 
 ET.ECTRICITV AND MAGNETISM. 
 
 ^hen there is no external resistance, the curr^f ' 
 the size of the plates ,s increased tL 1 '^"''''' ""' 
 
 fuo in ease the external r^JZ^', '^ZryZl ;;PP-xin.ately 
 with tlie internal resistance ^ " co.npariscn 
 
 . y^'^ '-' ''-'^'-' ^^"-' - '^bove, hut the external re- 
 ^'""^"^="^^^'-'^^-^-^-.0048.a.p,re.rf 
 10 pairs are connected abreast, C = __l 
 
 Fig. 144- 
 
 4. .f^ ~ "^049+ ampere. 
 
 lessening r has little effect ; 
 80 there remains only one 
 way, viz., to increase E, the 
 electro-motive force. How 
 may tliis be done? If the 
 current from a cell, instead 
 of passing immediately out of 
 the cell on its journey, is made 
 
 the KM,K. ^:.:2:::::^zi2:z::i^r 
 
 one, and that fhp r \r jp • ., ^'itLer resuic is the true 
 
 U.0 clootro-motive force a esridt b in ^ '' "" *" '""'''''<' 
 
 T.>e method of conuect tag .^1^^ Z '" """•" "•"*»• 
 l''igure 144. ^ ""^ '™ Purpose is shown in 
 
 It will be seen that in the multiple are (Fi» I4S^ »II ,k„ • 
 
 een is connected wit^'tiiixrortrs.T i::;c. °' °- 
 
ARRANGEMENT OF RATTERIES. 
 
 Fig. 145. 
 
 231 
 
 In the last example given above, let us see what wonkl be the 
 effect of conneeting the 10 cells in .eries. In this case E is 
 increased tenfold ; and, as the current is 
 obliged to pass t])rough tlie liquid of 10 
 colls inst(!ad of one, the internal resistance 
 will also be increased tenfold; hence 
 r _ 1 X 10 _,^ 
 
 (T-^^^^y-^;^ = .0400anip6re, more 
 
 than eight times as much as before. Tims 
 it appears that, ivhen the external resistance 
 is large in comparison icith the internal 
 resistance, the current may be largely in- 
 creased by multiplying the cells in series; 
 m other words, by forming a lattery of 
 great electro-motive force. In long tele- 
 graph lines the battery is made up o'f hun- 
 Pig. 140. d r (! (1 s of 
 
 cells joined 
 ill series. 
 
 Large cells are used simply be 
 cause the fluids last longer, and 
 so tlie cells need less attention. 
 
 Sometimes a coml)ination of the 
 two ai-rangements gives a stronger 
 current than either alone. The 
 cells may be grouped together in 
 pairs (as in Figure 14')), or in 
 triplets (as in Figure 14G), so as 
 to increase the electro-motixe 
 force; then the several groups 
 may be connected abreast, to reduce the internal resistance. 
 Uie strongest current is obtained by such an arrangemont of 
 the cells as makes the internal resistance as nearly as possible 
 equal to the external resistance. 
 
 ^11 
 
232 
 
 ELECTRICITY AND MAGNETISM. 
 
 I! : 
 
 PROBLEMS, 
 hattor,? "" ■" "'""• "'"" '""■'■••' "ill i.» iiol from the 
 
 HiJ^inSr;:;;:::;::::: ::„: ^;'^«.7'.-"'- , o, ..„«, ,vhc.„ 
 
 cm-nt of ''«!;:;■:";:;;:: '■"■"!''''"' '■■' - ""-■ >» (ou„u t„ rc„„,rc ,. 
 
 If the cirailt coSsof r, """' """'y»'™"y «■-■"» "i]Utm,ul,,, 
 smvity «.11», oaoh of 3 oh,„, reik w ^ '" °'' ""'"'^^"'> "' »" 
 
 w,,>;,/aXr'rrs';r-',o",:r^ "; ""■" '=■«" • 
 
 «■ By mca„, „f v„„r tan"™, „7i "''"" " = •'«' "l""»? 
 
 obtalnc, „,. ,Wcrc.„tU,,gS Jf;!""?""'' """""« "'« ■^""-"» 
 
 e..™a> .,..„. . ,0,. (., :r.;:'v«: „r, ,Z;iL> *" •" 
 
 halTe-folfot-riaSr ""-'^ '"'^^""' ""^'-^ -"M 
 1. /( toouUUave a khjh electro-notive force. 
 
 the „se to which i^^^;^t::zi:rr"''"^''' 
 
 about 7,000 oh„.,\v a outlt ,' ' '"" "''^""'™ '' 
 
 geiiemtoU in n ladv's tl 
 
 limbic. 
 
GENEllAL <;ONC'LU8IONS. 
 
 • increased 
 '■ from the 
 
 oil;,', wlion 
 
 each fell 
 
 e, and the 
 
 require a 
 it require, 
 . iliameter 
 i? 
 
 11) of 210 
 esistance 
 
 = .8 ohm. 
 oJims? 
 currents 
 t'hen the 
 igh. 
 
 would 
 
 ince of 
 
 ce. 
 
 actiee, 
 id the 
 which 
 is the 
 
 wire, 
 leedle 
 same 
 been 
 L!e is 
 nble. 
 
 238 
 
 and the signals produced were as dislim.f ,. u 
 
 qua„„ty „f „|eetn„ii.v that |„«,,., ,,,_ , ,. '°, "» '=««'' «!■« 
 h-M.I-. ..f ll,c l„ttc.,.y, a„;„„t U|J,U,-. ''''';" "" 
 
 -o con w„„l„ f„s. the i„eh owi it ,;,"': ""."• ,«'" »'"l" 
 the ujile of wire. strength of current in 
 
 A battery of three cells arranged abreast will f.. 
 length of platinuni wire, but will not rf, " "^ '"''*^'" 
 
 the poles in his l^^nds ; ;hi.e: I^.^ ,?' ;>;.:,|--" '!'^'<'-^ 
 not fuse the same wire b„f win 7 "^ '" ^^^'''^^^ "'H 
 
 <l"cti„g |,„„.er of a ,,ar8o„'» b,„ly" ••■""^""■.g the cou- 
 
 ..po„ t„o „ea,.„„. o^f ;: r,:: : r r^^^^^^ "» -- -" 
 
 »"d to k«.|, the curct near the core IZ ' '•'""l'»°ta««s, 
 
 Butaio,,,. thin wire wo,,,., o;,^",:; ."riZr:'.^^ "";'• 
 
 so reduce tl,e e„rre„t a» to ,„ore tl,..,, , ,«■!„? ' '* '"'»''" 
 wou,.i ot,,e,wi.se be ,r,in,.,l T. [ ''" '"'™n«"ge that 
 
 '" - '^n-cuit with otiL large ;Lto,,e,,",""^' '' '" "" ™"' 
 ^ l.e,ix of ,„a„.v t„r„. of Z^^^Z'J^"^ '""■«"-'»" »' 
 co,n,.a,-ativelv ; so the at,e„.„l, , '"'"'" '■'distance 
 
 core. Fo;t,,e.a™eCo,f :;:; '' '7'-" "'-»™t o„ the 
 
 in c.e..it„ Where n,.:::^::^-^^:;^^;:^: '" r 
 
 few turns of large wire • but if ;* ,• ♦ i ' "" ^^nttim only a 
 
 an., it .,o„u, ^ontai,'; ^^^ .' ii:::^:^^ ""■^^^'''- 
 
 ! 'r 
 
 fil 
 
234 
 
 ELECTRICITY AND MAGNETISM. 
 
 QUESTIONS. 
 
 mL![ '!iV°7'^«»>'"St'^° •« to be rung by tho action of an olcctro- 
 magnc I. The current used comes from a battery u. Now Vn/ir ,r 
 should the olectro-magnet be constructed? ^ ^'"'^- "°^^ 
 
 2. If you wished to njeasure the current bv the \utrnrU,r.n 
 
 S- Would It require a different fc-alvanometer If It were to I,, h r , 
 
 ./.' 
 
 if* 
 
 r... 
 
 XXXI. MAGNETS AND MAGNETISM. 
 ^ ^ ,^- .^^^^anent and temporary magnets. - One of the 
 
 know how It can p.ck up bits of iron and steel. By the aid of 
 a small instrument already st^died, we may uiake a plir of mul 
 magnets, and study their actions and laws. 
 
 Experiment — Take the electro-magnet describod in r 17, 
 couple of sewing-needles or larger steVrods Vm 1 .>,^ ^"'^ ^ 
 
 o? mn h^t '• ^u-o-magnet, possess the power of attra^tin^bi 
 
 "c: i:st rirrd::sr ' "^ ^^^^^ ^^ '-^ *- -- -« --" 
 
 Both of them exerted that peculiar force called magnetic force, 
 or possessed the property called magnetism; that is' both w7r 
 magnets ; but, as the steel retains its power, it is called .perZ 
 nent magnet to distinguish it from a temporary magnet, hke The 
 non w.re or the electro-magnet itself. The o^ialit?. of steel bv 
 which It at first resists the power of magnets, and resists the 
 escape of magnetism which it has once acquired, is cal d coe 
 
Law of Magnets. 03 '' 
 
 "a.*,.c„ i,.„„ po.Le, JioZ^xr^:::^ r""- 
 
 01 eloctro.„,aguet. should be ma.le „f the "LI' lV,7 
 ...ay acquire and part wit,, .a,. .,..„ U.Jl^Z;. "' ""•■' 
 
 and severs: te^t """"'"'" '■<•"-•". ''3' «"ca<la that will „„t „„twlst, 
 distant from each i- ig. 147. 
 
 other (v. Fig. 177, 
 §211). When they 
 <'ome to rest no 
 
 Pcrraanoiit Magnet. 
 
 IC^ 
 
 Inducfd Nfagnets. 
 
 ..ear to the markoTcnd of theU",, '%""" '""'''"»' """' "' ""» 
 one «.„.„». C„rerXt*°,,:rn,.freL''':je"'"°*"' °"* "'''' 
 
 We discover tlie following law of magnets • ;-*. .,„; 
 whTe poles attract one amtlr. ""'""'"• ^^^ Poles repel, 
 
 two mas„et». I)oc" ^e Uele ,» '.ffT''' "5 """X"-''.''".!-' .>« ween ,l,e 
 «ny effect? InterpLaTI^r, , """' ".I"'™™' Produce 
 
 What 1, the St° '■°'"""°" """■»''" ".= '"••> """S»«. 
 
 Substances that are not susceptible to magnetism are like 
 glass, paper, and wood, mag„eticalhj tran^arent. When a 
 magnet cause, another body, i„ contact with it or in itTn W> 
 
 tbat tody, ».e., .t .,yi„e«ce» U to ie like itself. As ftt^acUr 
 
 !l 
 
 III 
 
If 
 
 ''I' 
 
 i>' 
 
 li 
 
 236 
 
 ELECTIIICITY AN1> MAONETrSM. 
 
 netized , ,ece of non or steel, it must be that the ,„a..„etism 
 ndaee.. n, the „t,e.. i, sueh that o,.„osite polea are adle 
 that .s, a N or +,.„,e induces a S or -pole ..ext itself, as'show,; 
 
 in Fig 
 
 147. 
 
 §188. Polarity.- Experiment I. Strew iron filings o„ a flat 
 *" surface, and lay a bar-nKii^nct on 
 
 tlioni. On raising tju. maifnot, liow 
 !in> tlio fllin;;fs wliicli cling to it dis- 
 tributed? What does thl,s prove? 
 
 Magnetic attraction fs greatest at the poles, and diminishes 
 ton:a.ds thec.nter^.here it is nothing, or the center of tke bar is 
 neutral Tl.e dual character of the magnet, as exhibited in its 
 opposite extrenufes, is called ;>o^anVy, and magnetism is stvled 
 
 VT?7^r ' !^7^-^-^-'--^titsn:,tndli„e,as-I 
 i^xp. 1, § 25, It IS found that equd and opposite polarities 
 Fig. 149. exist uiiere there is ordinarily ,h) 
 
 = evidence of them. 
 
 Fxperimeiit 2. I'lacc! a copjior wire, 
 „P , . . tliroiigli whicli a vcrv stroiiir cnrrcnl' 
 
 w,rr«;:';::,:;:r""' ' """'' --^----....r: 
 
 H 
 
 This exper.ment, and those with the electro-magnet, and the 
 deflecfon of the magnetic needle hy an electric J^.rrent, and a 
 "Altitude o others that the pupil will meet with, cannot fl 
 to convince h.m that an intimate relation exists between elertricit,, 
 
 ert.es, yet ahke .„ many, and almost invariably accompanvin . 
 one another, and con < nUly .nerging one into the othe^u p^ 
 as If they were only different manifestations of one and he 
 same agent. "^ 
 
ATTRACTION OF CURRENTS. 
 
 237 
 
 §189. Attraction and repulsion between currents. ,- 
 
 Let us study still further 
 the properties of the cur- 
 rent. 
 
 Battsry 
 
 Experiment 1. Suspend 
 two copper wires (Fig. 
 150), eacli 50"" long, aud 
 about 5""" apart, with their 
 lower extremities dippiug 
 about 2'"m into mercury, so 
 as to move with little re- 
 sistance either toward or 
 from each other. In Fig- 
 ure 150 the current divides 
 itself and flows down both 
 wires to the liquid, so that 
 that part of the circuit presents parallel currents flowing in tlie same 
 dn-ection. Figure 151 is the same apparat.is, with the connections so 
 made that the current flows down one wire and up the other, and we 
 have an example of parallel currents flowing in opposite directions. 
 What IS the result in the first case? 
 What in the second ? What conclusions 
 do you draw? 
 
 FiK. 152. 
 
 Hence, the First Law of Cur- 
 rents : Parallel currents in the same 
 direction attract one another; 2)ar- 
 allcl currents in ojyjwsite directions 
 rejyel one another. 
 
 An interesting illustration of the 
 former part of this law can be ar- 
 ranged as in Figure 152. A bat- 
 tery wire is bent in the form of v. 
 spiral coil. At a the wire is broken, 
 and one ond dips just below the surfnee of morcurv in a <rlaps, 
 while the other end is i)laced in the same liquid at' a little dis- 
 tance from the first. When the circuit is closed the current will 
 be parallel with itself, am] will (low hi the same direction Ju 
 
 iiii 
 
238 
 
 ELECTRICITV AND MAGNETISM. 
 
 the n^ercur, and break Ih^ ^T T ^^ "" ^^^^^ ^"* ^' 
 attraction ceases, a.Kl the coil is Zt \ "''""'^ ^'^^^°' t^e 
 of gravity, and closes the circu.r" "" ''^^"^ '^ ^^^ ^^^^^ 
 vibrator, .notion is l>rodnce<l inthe cf" 'rr"' ''"^ ^'^-^-^ 
 bo -nade with considerable care or it wll, n';':.::^'"^"* '"^^ 
 
 ^.pcH^ent .. Prepare a,,.... ,. .,,resonte. ,„ ,.,,re .. 
 
 a;"l 5™ u,ick, cut a circular hole 
 
 ;;"""t 4- in diameter, and insert 
 ' f^'la^s test-tufje b, about 6cm 
 
 ' lon^'. that « ill j„st fit in the hole. 
 
 iiikcan(No.L^O) insulated copper 
 >vire about 200- long, ,,i„^,tj,^ 
 
 l^entral portion into a coil c, 12cm 
 
 ong and 15.nm i„ diameter, with 
 
 turns about 3mm ^part, leaving 
 
 about 12cm at both extremities 
 
 "nwound. To these extremities 
 
 Holder strips of copper and amal- 
 fc'anuited zinc about 3cm io,j„ g,,,, 
 
 as wide as the interior of the 'test, 
 tube will admit, and allow them 
 to be separated about 5mm. j^. 
 sort them in the tube, 
 and cover with dilute 
 sulphuric acid. In the 
 center of the coil lay 
 a No. 16 soft iron 
 wire ,1, and float the 
 >vhole ill a vessel of 
 water. The apparatus 
 floating battery and electro-magnet Rr.„„ ^°"ft'f"t^« a small 
 inagnet, or a short piece of soft . I".. ^' "'" ""' "^ '^ Permanent 
 stirrup „, near to one of the jioles , f hi! ' "' "i'T"'^''' '" '^ P"^*''' 
 a»d prove by experiu.ent Lat^olonlJ^ "' the Hectro-magnot, 
 respect like a magnet ^ '^'^ '''''■" ''^'^ave in every 
 
 E.peH„.e„t 3. Bo„,„ve «„ „„„ ,,„ ,„,„ .„^ ^^^^,^^ ^^^^^^_ 
 
ATTRACTION OF CURRENTS. 
 
 239 
 
 "enx, as in Figure 154. In this position the two currents flow in 
 
 t'endrtoVf '"^."^ '^ "" """"^^''- l^'-cliately the c'l 1.1!" d 
 ends to talje a position at right angles to the wire above, so hat the 
 
 i" Fig;;:r;55. ""'' '"^ ^° '''''-' ''•■^"^^ -^ ^^ ^^^^ -- ^i-r^ a: 
 
 Hence, the Second Law of Currents: ^«^«?a,- c^rren^. ^em? fo 
 Z'c^come pam^^^Z a»,rf>z« /„, the same direction. 
 
 Fig. 156. 
 
 Observe that the action of the 
 helix in the last experiment is 
 analogous to the deflection of a 
 magnetic needle by a-^ electric 
 current. 
 
 Experiment 4. Place opposite 
 one end of tlie floating helix a second 
 helix Figure 156, in such a manner that the currents in the two helices 
 may have the same direction. The two poles of the helici att mc 
 one another i„ conformity to the Fir«t Law of Currents. Reverse the 
 po es of the helix in your hand so that the currents will flow hi opp " 
 8,te directions, though still parallel; they repel one another. (Why?) 
 
 The two helices appear to be polarised like two magnets, and 
 tor many purposes may be considered as magnets. Observe 
 that at one pole of each helix the current revolves in the direc 
 t.on that the hands of a watch move, and at the opposite pole 
 It revolves ni a direction contrary to the movement o'the hands 
 of a watch Bring the north pole of a bar-m.gnet near that 
 pole of the hehx where the motion of the current corresponds to 
 tlie movement of the hands of a watch. They attract one an- 
 other; but if the same pole of the helix is approached by the 
 south pole of the magnet, repulsion follows. Hence, that is tiio 
 south pole of a iielix where the current corresponds to the motion 
 of the handsof a watch,(8), and that is the north pole where the 
 ourront IS in the reverse direction, (n). B„t the important les- 
 son derived from these latter experiments is, that helices througk 
 which currents are Jlomng behave toward one another, or toward 
 a magnet, in many resjwcts as if they were magnets. 
 
 ! M 
 
 '"Hi 
 
 I i 
 
 m 
 
240 
 
 
 KlECTKIcm- AND MA.i.NE-nsM. 
 
 §190. Ampere's theory, _Thp (v,„t. ■■ , 
 
 a.... thus eve,., moleo^k^e 1 ' ; Z.^T "'"""■°"''^-' 
 
 plane.,, an.,, „,,■' ,o , i v j , ?™* "" '" "" '"-""'^ 
 another, and so theh- fee f,t "■""""' ""•'' ""''''''''<' ""^ 
 of eleetricity or a m JneT , ^ " '" '■'"'■ ''"'"■'' ™™"' 
 
 «a.ne dh^eetion, ilZZXru 'T'"" '"""=^' """ "' ">« 
 If the coercive force isT™ . '""' ^" '"' <^"™'"»- 
 
 attained on the ™ov I of t " 'T"? '' ""' '""""'^''""^ "» "e 
 ".agnct is the relnU '"""""S '""'^'=' ""^ » i'e™ane„. 
 
 Intensity of magnetizalion denentk on fl,„ ,, 
 
 -.. and .he latt..- depends onTi f t,™, ., TtT r°"^'- 
 nnigiiet. When i|„,„. „„.,.„„. . ""ength of the ujilnen M,g 
 
 !»<-.V l.as reeeived al, th,: C „" j;™;";' ""'^^r^^'- ■>" 
 
 "'U, and i, said to he m.^^.tl^^X^Z' T"'"" 
 
 "^'"^"o" '''•« ciu-rents really 
 i^i^.157. ^"•™'''^toarouncUheinaivid.,aI 
 
 molecules, jet the resultant of 
 these forces is essentially the 
 same as if the currents circu- 
 lated around the body as a 
 whole. Figure 157 represents 
 sections of a cylindrical macr. 
 net, and the inchukui circles 
 the circulation of the seve,al 
 currents around the molecules 
 b'"'g in these sections. Itvvii' 
 
 contiguous sides of any two of .-t!!'"." ?'''^ *''^ "'"''""^^ '^^ ^'''' 
 directions. .nn,l tho.-JeT, f ""''''' '"^^'' '» «!>P'^«i^« 
 
 currents that pass a^;::::^ -----= ^^'^'^^' ^"« 
 so affected. •^""utitnte of the magnet are not 
 
AUVkllK'n TIIKOUY. 
 
 241 
 
 The hypothetical c.irrcM.ts that <.|r(,„1ato around a ma^netio 
 molecule we shall call A.,^,r^an currents, to dis L^.rl 
 from the known current that trav.rHen the helix. In strict aocor 
 a..ce with this theory, the p.U. of the eIectro-n.ag: e Tt^ 
 mmed by the direction of the c.UTent in the helix. The inductTvo 
 influence of the electric current can., the Arnp6rian cu . lo 
 take the same direction with itself, uh represented in Figure 158 
 
 Vlu, J/W, 
 
 kuo« pi enomona of magnoti.m, It ,ho„M be home h, mi„.l 
 that physicts of this generati,,,, vah.o the theory rathor" a 
 L Ip to the ™agi„atioua.,d ,„e„,„.,, ti,a„ a, a uL staten^ nt 
 of tl,e faets It ,s nearer the t™th to - - that the moleeules 
 are po anzed as if eurrents were circlat,,,, around them „ 
 the actual e.„te„ee of ,„„h e„rr«„t» we k„„w nothing. 8„ 
 also of the real nature of polarit;,- wo know little or nothing. 
 
 EXERCISES AND QUESTIONS. 
 
 1. Regarding your lead-pencil aH a rod of imn nnH „ * . 
 electric wire, tie a knot at the .,u, lullZrZf.l "^ ^^ ''" 
 
 2. What would be the effect of rcvcrsltiK ♦ho current? 
 
 curreat., . h, K,,„re ,«». ,„,.„„„ :::i:c:2^:::^ 
 
 m 
 
242 
 
 ELECTRICITY wND MAGNETISM. 
 
 .J' .?^ ^'l''"" *'"' ^'''° '""'•' P'^^'^^ "^^'- o"« a»'^"ier, and appertain 
 
 t4 s!;::ia r etr :,:r. ^^^^^ ''-' °- *'^°^^-' -<' ^^- -^^ 
 
 .o'd >nro?^^r"^ '" ' ""'■''"'''^ ""^ ««"*'^^'"^^ direction, .nd sus- 
 
 H^ 1 wiU ^r "^t' ",' '"'t^'"'^ '"""^ "^•^'' -'^' P-aH.l With the 
 
 r^lhTsr /a ^ '^' '""*'"'' """'^ then imagine a current to 
 
 t^e needle '"'"'" '-"'^treniity, and determine its effects ou 
 
 7. Why is a n.agnetic . edio d.;iected by an electric current? 
 
 Of Lllli^t?" " '' ''' 'f '^'^"'^ "^P^"'^^"* "" "- d--«on 
 
 npi^^* '^!'®.®*^*^^^^®at magnet. -Experiment 1. ^r i^. 
 n a cambric needle. Suspend it by a fine thread attached to fs 
 
 m.ddle over a n,.g„et, and midway between its poles. The need 
 
 Trr.n r" • """"^^r!^^"^ ^^^^^ ^ P-'«o" P-aHel With the mag .' 
 The magnet exerts a directive influence on the needle. Remove the 
 vnagnet, and the needle takes a northerly and southerly direcUon 
 
 I) you carry the needle all over your town or State, it will still main- 
 am fus direction. Something, like the magnet, everts a d eXe 
 influence on the magnetic needle. uiiccuve 
 
 Fig. 160. 
 
 E^jperlment 2. Place the needle once n.ore in its original position 
 over the magnet, and gradually move It fron. the middle toward one 
 
 of the c nteTif "; ' "" "^^^'^^ ''''''' '' ^^ ^"••^-"*'^'- ^* ^'^her si"e 
 of the center it dtps; if it is nearer the N-pole of the bar, the S- oi 
 
 dips, and conversely, as shown in Figure 160. If the needle Is pro, : v 
 
 supported, the dip increases till at the poles the Inclination is 90° 
 
 If a magnetic needle P . suspended is carried to .; , nt 
 parts of the earth's sia-iu.., it will dip as it a^ ^aohes 
 the polar regions, and is only horizontal at or near the ■ h'^ 
 
THK KARTH A IVFAGNET. £43 
 
 need to ,. J the N " o.a ^if T '" ^'^'-r^^' ''"^ -"^ 
 Like e,.ets a. eo.. ^ttti'u^r^^'^ "^^^^ ^" ^-^-^^- 
 o -ke causes. These phenomena are 
 just what we shonhl expect if (as is 
 very improbable) a lu.ge magnet were 
 
 u-ust through the axis of rotation o? 
 
 he earth, as represented in Figure 
 161, -having its N-pole near tlie S 
 geographical pole, and its S-pole near 
 the N geographical pole; or if (as is 
 more probable) the earth itself is a magnet 
 
 plate of glass. Sift over "'S o'ametei.v Place it beneath a 
 
 Fig. 162. 
 
 the glass fine iron-filings, 
 
 as ill Exp. 2, p. 28. Geutly 
 
 tap the glass a few times, 
 
 so as to agitato the filings. 
 
 Once in motion, tliey ar- 
 range themselves in lines 
 
 radiating from either pole, 
 forming graceful curves 
 from pole to pole, as rep- 
 resented in Figure 162. 
 These represent what are 
 called lines of magnetic 
 force. They represent the 
 
 resultants of the combined 
 action of the two poles. 
 Now carry the little mag- 
 netized cambric needle 
 aroundthedi.sk. It follows 
 the lines of magnetic force 
 as mapped out by the fit- 
 
 lugs, always assuming a ^ 
 
 position tangent to the magnetic curve, as shown in Figure 162 
 
 It is evident that the space arouud a magnet is the «eat of. 
 
 ftll 
 
244 
 
 KLKCTlilCITY AND .MAGNETISM. 
 
 f 
 
 lii: 
 
 peculiar influenco ; this space, extending as far as the inagncr 
 exerts any efreet, is culled the magnetic field. The hist experi- 
 ment presents a true exhibition, on a small scale, of what the 
 earth does on a large one, and thereby presents one of many 
 |)henomeua which lead to the conclusion that the earth is a 
 magnet. 
 
 § 192. Magnetic poles of the earth. -It will be seen 
 that there are two points where the needle points directly to the 
 center of the disk. A point was found on the western coast of 
 Boothia, by Sir James Ross, in the year 1831, where the dipping 
 needle lacked only one-sixtieth of a degree of pointing directly 
 to the earth's center. The same voyager subsequently reached 
 a point in Victoria Land where the opposite pole of the needle 
 Iack(!d only 1° 20' of pointing to the earth's center. 
 
 It uill l,e soon that, if we call that end of a magnetic needle which 
 pom s north the N-pole, we must call th.-rt magnetic pole of the earth 
 which IS in the northern hemisphere the S-poIe, and vice versa. (See 
 iMg. 102.) Hence, to avoid confusion, many careful writers abstain 
 from the use of the terms north and south poles, an<l substitute for 
 them the terms positive and neyative, or marked aud unmarked poles. 
 
 § 193. Variation of the needle. — Inasmuch as the macr. 
 netic i)oles of the earth do not coincide with the geographicll 
 poles, it follows that the needle does not in most places point due 
 north and south. The angle which the needle makes with the 
 geographical meridian is known as the angle of declination. 
 Ihis angle differs at different places. 
 
 Experhnct. As Columbus found, we can easily flud, the decUna- 
 tion at any place as follows : Set up two sticks so that a string iolniu- 
 them points to the North Star; the string will lie in the geographical 
 merRlian. Place a long magnetic needle over the string; the angle 
 between the needle and the string is the required declination. If great 
 accuracy is required, allowance must be made for the fact that the 
 star IS not exactly over the pole, but appears to describe daily around 
 It a circle whose diameter is about 4°, 
 
VARIATION OF THE NEEDLE. 245 
 
 Let A (Fig. 16.3) represent the nortli magnetic pole an.l H 
 the north geographical pole ; it will he l^Z 
 
 «een that there is a position in which the 
 "oedlewill point cine north. A line pass- 
 ing around the earth 'brough the two 
 '"^ignetic poles, connecting those places 
 
 where the needle points due north, is -^^_^_. 
 called a line of no variation. On the min Plot tt •. •" 
 marked 0. Lines east and west of thf, r ^^ f "" '* ^' 
 parallel with it, represent liZ of '"'' ""^ '^Pi^'-^-^'-'-Hv 
 
 in North A ;*^P^<^sent lines of equal variation. At places 
 in iNorth America east of this lin^ iu i, pt'tn s 
 
 iiKe an island or cape, but are constantly chanrino- ti,. 
 pear to swing, somewhat like a nenttalnm T„ *' J , ^ """ 
 westerly direction, each swing roquTrtot ^""tariesl'" "ir"' 
 Tl,e north magnetic po,e is^no^ onTJ^Z^^S^t 
 
 stances should partake of its magnetic properti f dctbn" 
 oxraes ot this metal, possesses more or less magnetic oowcr 
 
 trnZ^s w ''™''' """"•'" »'^-'«. '»'j-tingirtr:m 
 
 irom the artificial magc<-'.s of steel. 
 
 fhf ^M ^^""^^ °^ '■ • ' ^^^^'« ^lagnetism. -The cause of 
 the earth's magnetism is not known. The thoorv - i 
 
 electro-magnet in virtue of currents "flowing ^^^^undTne " T 
 
 surface, from east to west, explains all tl e° eZs thlt t"n 
 
 duces ou the magn< 'v., needle T5,.t wi../ 7 . P'"''" 
 
 g" - V uteuie. uut vfhut sustams these electric 
 
 4ii 
 
246 
 
 ELECTlilCITV: AND MAGNETISM. 
 
 ¥i 
 
 source ot the earth's magnetism. Those who adopt this tlieorv 
 generally regard the terrestrial . .ruut. .s IkernJelecMc 
 
 certhllf Ltrh r"' "r" '" ''''''''''' "" '"^""-^^^ r^'-tion that 
 ne K " • In 1 t ''";' ' '""'' '°"^"''°° ""'' ^^e earth's ™a^.. 
 neti-,ni In 18o9 two observers remote Irom eacl, other saw si.n,^ 
 taneously a bright spot break out on the fuee of the sun hosel at u 
 was oalv nve minutes. Exactly at this time there was a "eue ml 
 t.n-b.nce of. ma,M.etic needles, and telegraph wires all over the wor J 
 vore traversed with so-eaUed eartk currents. Telegraphers eeeivel 
 shock., and an apparatus in Norway was set on fire. These phenomen 
 were qn.ckly followed by auroral displays. S.>n.etinKvs teleg rhTa e 
 worked by earth cu.xcnts alone, without any battery in the S. ' 
 
 l^f?'?^''^''^^ ^^°^^^^« °^ ^^ --'^ and magnetism. 
 -Artificial magnets, including l)ormanent magnets and electro- 
 i^u^gnets, are usually made in the shap-^ either of a straight bar 
 or of the letter U, called the korse-skoe, according to the us" 
 inade of them. If we wish, as la the experimct^ already d 
 scribed, to use but a single pole, it is desirable to have the other 
 us far away as possible ; then obviously the har-magnet is most 
 convenient. But i" the mapnot is to be used for lifting or l^h 
 ing weights, the h . .,o-shoe form is far better, be-all 
 attraction of both poles is conveniently available 
 and becr.i.e their combined powe, is more than 
 twice that of a single pole. This is due ^o th.> 
 reflex influence of the poles ..> one another throu<rh 
 the armature. Magnets, >vhen not in use, ou-Ii 
 always to be prote- by 'umatures (A, Fi-. 1G4) 
 of soft iron; for, ,tw landing the coercive 
 power of steel, they slowly part with their nague- 
 tism. But when an armature is used, the opposite 
 poles of the magnet and armature being in contact 
 with one another, i.e., N with S, they serve to bind 
 one another's magnetism. .> ^ vc to uiuct 
 
 Thin bar« of steel can h, more thoroughly magnetized than 
 
 J 
 
Plate IT. 
 
 M. 
 
 > the sun as the 
 idopt this theory 
 no-electnc. 
 
 mate relation that 
 the earth's :'nag- 
 other saw siinul- 
 n, whose duration 
 'as a general dis- 
 U over the world 
 fraphers received 
 riiese phenomena 
 es telegraphs are 
 u the circuit. 
 
 d magnetism, 
 ets and cleetro- 
 
 a straight bar 
 ing to the use 
 nts already dt 
 
 liave the other 
 uagnet is most 
 lifting or hold- 
 *, because the 
 ntly available, 
 
 is more than 
 is due to tho 
 iother through 
 in use, ought 
 (A, Fig. 1G4) 
 
 the coercive 
 
 their nague- 
 l, the opposite 
 iug in contact 
 
 serve to bind 
 
 ^etized than 
 
 fifi 
 
 I 
 
!l 
 
 
 ill 
 
BIAMAGNETISM. 
 
 247 
 
 b) .. Ic, with their corresponding polos turned in the same 
 
 ^tro^r xr ""^ti '""'-'''-^ ^ -^^- p-^-^'-i Xe 
 
 IS the result. I his is called n co/»^,o««cZ «i«f7„eL i„ .^^^ ^_ 
 central ones; so a steel tube makes very nearly as stron-^ a 
 
 srtter ' "' ^' ''' ""^^ '^^"^^'^"•' -^^ ^^ --'^ '^^'^te;:ha; 
 
 § 197. Diamagnetism. _ Resides iron and steel, many 
 otlu^r substances, and possibly all substances, both in the iTou d 
 and gaseous, no well as in the solid state, are more or les su 
 cepti ble to magnc-tie inflnonce. Conspicuous amon. these are 
 
 kind. A small bar of bismuth suspended between the poles of 
 . Dowerful electro-magnet, instead of being attracted is epe led 
 1) he poles of the magnet, as shown by its takin^ a no it on 
 w. h Its longest axis at right angles to a direct line beleen h 
 poles. Substances which behave in this manner are caTled^^a 
 magnetic, and they are siid fo nln.n f. , 
 
 botwpon \h 1 o , P "" themselves equatorially 
 
 between the poles, as iron and nickel, are called paramagnetic 
 or simply magnetic. '"-agneiic, 
 
 Paramagnetic liquids placed in a watch-glass between the 
 poles become heaped np at the poles and depressed in Te oen! 
 
 iTquidl Tho 'T"'l f""™'"''^ ""''''' "'"^ cliamaguetic 
 liqinds. The magnetic behavior of gases may be learned by 
 
 ■nating soap-bubbles with them, and noting Uie direction of 
 
 1 eir distension. Alcohol, water, nitrogen, and carbonic acid 
 
 are diamagnetic. Oxygen is paramagnetic. The only snt 
 
 s ances whose magnetic properties can be shown without extra- 
 
 oidmary apparatus are iron and its compounds. 
 
 § IDS. Magnets not sources of energy. - Pernetual 
 motion seekers are easily led into the error of snpposingThlt in 
 the magnet they have an inexhaustible supply oT energy jbl^ 
 
 III 
 
248 
 
 KLECTEIGITY AND MAGNKTrSAf. 
 
 « very little study will serve to exhibit tho . " . 
 
 e>ror. If, for instance, we bri.i. . n • f . ^-''a'-aoter of the 
 
 f i« att-ted, and. if ^Uo.^rtV n^ ^^/t^.r'''' ^ "^=^'^^^' 
 force of attraction will do i c<^vt.u ^ '^ ^^^gnet, this 
 
 another piece of iron sti ar^ o rZt ^^n* ''^'^ ^"^^ 
 -eted, and a certain an.ount ot wo kt .',,'" f ^ "'" ^^ "^^- 
 less amount than that done in til fit P«'-formed, but a 
 
 operation until the n.a n no lo. "\;*"*"^^" ^-^^"""^ the 
 J-s done a definite an^oj .^k :;:^tt; """ '"^ "^^'^^^ 
 
 more. To restore it to its o ,, , ' ^ "^' '^^^^■^''' ^^ ^^^^ng 
 
 allthepiecesof iron; "nt';:' .^"''''^'-' ^ '""st remov^ 
 
 nal work exactly eq , a 1 .''•'•'" '"''^^^'^^'^^''^ «f -^ter- 
 magnet. ^ ^ '^ *''^^' ^''g'^^Hy performed by the 
 
 XXXH. MAaNETO-KLECTUrc AND CURREXT INn.CTION. 
 
 a magnetized steel ro<: i,,,o th. c t /V'^"- ''^ ""' ''"''^'^l^' thru.st 
 Fig ,e5 ^ , ^^ '"^' "^'"^''"^"^ '"^ve you that a 
 
 ^i.-^65. • '-"^ Of e,.,etrieity is i,K,ueecI 
 
 '" I"' ''^■'■-^••^ l^o^'s this current 
 ;;"'t""K-, oris it only uKMuentary? 
 
 t2>iK-klyn.u,ov(.tlu.„,afr„et. wi.at 
 i« tlie result iu this ease? Re- 
 peat the experiment. U'lien tlie 
 magnet ai)proae].es the eoil wliat 
 is the direction of the induced 
 
 current as coinparedwith that of 
 the aniperian currents in tlie ma- 
 
 net as represented in Figs. ir.() ami 
 1'"-^ ^Vhat is tlie direction of 
 the induced current when tlie 
 magenet is withdrawn? if you 
 rectly you will see that in th. fn. '''"*''""'"" ^^'^'^^« <iirections eor- 
 
■acter of the 
 ar a magnet, 
 ^'ignet, this 
 Take now 
 'O will be at- 
 I'med, but a 
 'ontinue the 
 the magnet 
 '^i' of doiujjf 
 ust remove 
 fe of exter- 
 »<?d by the 
 
 LTCTION. 
 
 nt 1. Con- 
 ckly thrust 
 yon Unit a 
 i« induced 
 Ids current 
 lo'iiontary? 
 met. Wliat 
 a,so? Ke- 
 WJien tlie 
 foil wliat 
 e induced 
 Ml that of 
 II tlie niaif- 
 •s. Ififiand 
 oction of 
 vhen the 
 If you 
 ons cor- 
 ue to op- 
 »gs them 
 
 MAGNETO AND DYNAMO ]V[ACHINES. O49 
 
 Kig. 106. 
 
 Fig. 16 
 
 direction must resist 
 the force that sepa- 
 rates them. Hence, 
 the energy shown by 
 the electric current 
 has heen generated at 
 the expense of mechayi- 
 teal energy. 
 
 E?' peri me lit 2. 
 
 I'lace within tlie coil ^.,._^^ 
 
 a core of soft iron, Wave back and forlli over one evt.. •, ~~ 
 ->-■ one of the poles of a povve.-fu, ha,- "n.t N, " "n . T 
 expernnents with the opposite poh- of the u.a^';;! AVh- 1 ' ^ '" 
 nemena observed? Are they such as v..,, ^^ ''«it are the phe- 
 
 would expect? Wliy? " Fig. les. 
 
 § 200. Magneto and dynamo 
 
 machines. - If the pennauent mao- 
 
 nut is stationary, and tlio clectro- 
 
 iiicignot i.s moved back and (brtli, 
 
 the 1-e.siilt is the same a.s when the 
 m^^gnet was mo^ed and the clectio- 
 liiagnet was stationary. Machines 
 constructed for the purjjose of gener- 
 ating electric enirentsin this manner 
 are called maijncto-electrkal nmchmen. 
 
 Fi«:. Ki.S will isjive a general idea of the 
 construction of the simpler kinds ot 
 magneto machines. N S is a permanent 
 compound horse-shoo magnet. K v are 
 coils containing cores of soft iron cim- 
 nected by the back armature, C C the 
 
 whole constituting a sort of armature -— 
 
 to the permanent mairnet. The brass -.vir r. i^ • ■ 
 with the back-armature C C l.hT , '. ^ ' '" '■'^'"*'>' ^•""'«'fted 
 
 Of the crank, A. .j^,!;^':: '^:^2:^;:;^rT '' -'""^ 
 
 the crank to be turned; duriuL' the fi, . I '^'"''' *'"J''^"'^^ 
 
 k 
 
250 
 
 m 
 
 ELECTItlcITV AND MACJNETISM. 
 
 in opposite directions through Cve LaT T'""'' '"^^ "«^ «-- 
 other, but may have a common cUreXn ar ,;"■;'' "'"''""'' """^ ^"- 
 of double the electro-motive fo 'rt^t \2 '^^^ ^ T '"'' " ^"^^^"^ 
 ho ix. During the second quarter-revaTutlontlo oof '""''' '" ' ""^'^^ 
 other, and tlie elTect ivo.ild be to rcverlH ^ ^^ •'^PProach one an- 
 
 the cores also chan.^e as they are now .' "'^?"•^"^ ' l^"* ^he polarity of 
 poles Which they are app rjhi" a "d "f >',"'" "" '"'"^'"^ °^' he 
 rent to flow in the same direc "t'as t d I h r ' '''"^' '^^^'^^ ">« «»-■ 
 revolution there is a reversal of cnL / f*""'- ^' "^« «"'' ^^ ^ I>alf 
 this point. The result KtltdS".;" " "''^^ '"^ "^* ^'-"^^ ^^^ 
 rent half of the time in o ,e irecSnfn , rTr'r"'"*'"" '''''' '« ^ '="- 
 «itc direction. I„ order to secure 'con . "' "'" ""'^' '" "'^ «PP«- 
 a current-reverser I, or .::": , anT Nllirr. '", ^r '^'''^' 
 i« so arranged that during one hah o 1 l f ,' ? ''*' to the axle, 
 connected with G and n ^vith li i I , '•^•volution of the axis m is 
 nected with II .„„ , n^ n', "', .f 1 r'''V''' """'' ''''' '« '« -- 
 rents in the helix-wire nl wc 1 „ 1 ''"' ''''' ''^^'^' «>f^'rnati„g cur- 
 
 ".' the wires G H, wlddr T/ " rV^m;T. ''' '" "'^" ^"""^' ^"^-^'o" 
 circuit. ^owHL, must l.o coiuiccted to close the 
 
 Each of the two currents produced In n c , 
 
 1 ro(uiu u In a single revolution has a 
 
 '"'' '"" "'••'ximum point, or point of 
 
 Kreatest intensity, when tiie 
 cores are nearest the poles of 
 tlio magnet; and a minimum 
 point,orpointofleast intensity, 
 when they are farthest from the 
 poles. Between these two 
 
 points the current is constantly 
 Kioning or diminishing. It is 
 apparent that such a machine 
 fc'ives not only an intermittent 
 current, l,ut one that resembles 
 a succession of waves or a 
 Ntream produced by the strokes 
 of a i)ump. alternately risin-' 
 
 purposes for which electricity is employ^, Tuf ""* '"^ '"«^'* 
 current should be continnou/n,.,, f. /' ^ \1 '' '"'P«rtant that the 
 
 Illustrate the principle by wh'"i tl ir'u'"' f' " ^'''' '"" ^'^^'^ *« 
 Gramme machine. '' *' '"""'"'^ '" "'^ widely-knowa 
 
MAGNETO AND DYNAMO MACHINES. 251 
 
 The armature ns consists nf a ..t^™ „ 
 Irou wires (better sl3n nil /l^?^™^°'f °^ '' '""^^^^ ^^ ««f' 
 an eudless coil of Te T If wi •! in""'""''' '^ "'"^^ ^'^ ^"'^"^"^ 
 coiIs,theln.vvireofZu„ite^to 2 T'-''"' '' ^^^ °" '" ^^P^'-^'e 
 
 each juuctiou abra^h w re fs vdt ° '''' "' "" "'"''' '"' ''"'" 
 -tatiou ... A ho2ere'^„;;'„et\V7oni;f ^'Z ^^'^ «^ 
 
 shown in the cut) is so placed th^t one Lf "f tho "• ""'''"'' '' 
 influence of the N-nolo «n,i fi "*' ""^ "a^^ of the ring is under the 
 
 suppose tiJt?to'::r:;Lt:c-:no7«^ *'" -'''- «-^°'«- 
 
 point of tlie iron core a. 1*7. "'"^ ''''™'^' t''^" every 
 
 magnet. Will success^:,/, e on a'proT''^ ' -f^^" ''°'"' «^ "^' 
 points .• and i' are the neutral n"ints Tf P''°"'' "'™'^' ^^'"^« '^e 
 divided at tl.e points n am fwelfat / ''' ■'"'^''"" "^'^ "°^« *« ^« 
 north poles and wlj 2 ,' ^1 Tes^e^IXr^^'' "^^"^*^ -^°- 
 the two mutually-facing pole on e ther s le^h A '"' "''"'''''■ '" 
 must be in opposite direction. ""'''' '""^' ^'''^' Ampcrian currents 
 
 diagram in th 1 gU of w " von ,7 "" '"•"'"' ^'"^^ ''^ ""« '^-^ 
 the generation of induce '.^rrts win ^T,''"''^ '^'^^'^'^ '''^^'^'-S 
 ring armature rotatetth CO rs^on^^^ '"" ^/^^^ ^''^^ ^ "- 
 
 of the ring will induce cunvnt, i tf ^ "'" "^ "^" ^"'^"^^d poles 
 
 the coils Which afa^y give n om ntV"-' ," ""' " '""'"«'• *^"' -" 
 the magnet poles- L he Wh V" "'" '''"'""'^"^ "^^* ""'^ ^^ 
 
 ^li.-oction. Similari;:C:emTctl;rrer !.^ ^^ 
 
 approaching, or immediately receding fZ he South no, '"^'''''^ 
 same time traversed by a current nf ti P'^'" ^"'^ ^* "'« 
 
 is that currents in the lovver h^f / " ^^'^^^^'^^ ^"^««t'^»- The result 
 and in the upper Li //Z poiu „ S r' "" ^ '"* "^ "^ *"« ^-«. 
 from these points are oZ th.l ' t f^ ^' "'^ ^'^'*^*°S ""^-wires 
 
 nuently op/ose ancTuVuSr eZire^'tutTf "r" "T ^°"^^^- 
 «^' are connected by a wire L we shnlliU ^^'""^^ "^ ^»'' 
 
 ;;;. currenc flowing'throu^h'C^^r? ^^^ IToT ^ J^'^l"^""- 
 
 Xf tct:::r:r^ f --ror:pper;:r :! 
 co.;;ntconn:tS::i«;\;t^rSersr;t^^^ ^^^^^"""^- '^ 
 
 inducing or the so-callod /; / ; ^ extensively used as the 
 
252 
 
 ELECTHICITY AND AfAGNETISM. 
 
 ^ ~;: t:x^::-:zr::i: s- -- - anna.. . 
 
 -inch «oft h-on ahvays ret^Uus Xr t 1 " , '""' "' "'^ '"'^^«etis,n 
 at flivst a M-eak a,.Tc.„t in the wire of t . T '"'•^'^"^'^'■^^-O mrtuces 
 
 i" ig. iey fc. 
 
 ?Hl 
 
 ;'"' «''1<I ma,-r,„.t, and nmif.,oti/es th,- ..,.., 
 
 A ami n, „„ „„k„ ,„ „„ ,,,„„„™';;;;, ■;'"'■■ «■■".««„.„. ,,,0 d,„L„„s' 
 
 '" ' iniiati.ro wilua 11. " ,vt„,., ■.:..'.;;' ;' ''? '"'«" "» many iu|.|,» 
 
 r»te of i-evoUitlf 
 
 I ; 
 
 f'»- Which niJl 
 
I the armature Is 
 tJie magnetism 
 'ictized) induco.v 
 t as a portion of 
 >iigh tlie coil of 
 
 . i6a b. 
 
 CUHREST INimcTrON. 
 
 re stroiiglv, 
 yroacljc'sju 
 
 isod (§ 201) 
 L- tlyuamos, 
 many turns 
 
 253 
 
 ffiA-e the greater E.M.P ? t.. «« 
 
 f '- coil makes it necc.;ary to ^"1//"' T''"' "^ ^""'^ '" the anna, 
 •ted. What eflect has tlus^,^! n T '''* ""'''' '"''' ^"^' ^P-^"'' is li^- 
 cnrrent must traverse? The' doh. . uT' "' "'^ ^''''^'''t the induce, 
 coils Of the fleld-magne ; r ; :'''^r'''''"^'''^^'™'^*"--on"S 
 c."t The fleld-magnlt .oi „ ^^ '"r'' ""^^'- ^'^^ "^ the'ci" 
 .->.netunes arranged in nn.l.ip,. a , t U n"^ ^^ "^•'^*"-""« ^^'re are 
 -nes. This arrangement, L S I ho '^ v"' '"'' '^'""''^''-^ '" 
 Icpens upon the Mork for whic-h tl ' nln "'""^ ^' ""-' '"-"lature, 
 
 ■''^^-^.ne Wire has a very high r.!^ ."l ;;;;,;; ,f-'^-^- U the work: 
 "•th many t,„-„s of fine wire or wit .wt " '""'"''''"'' ""' ^^""'"^ 
 
 constructing dynan.os care nn.stT. / ? 'r''^ "^ — - -ire? m 
 «i/^e that the current passing t, , '^S /V"*''' ^'^^'■J' ""•« of such 
 «peedslull not be sufficient to^u^:,;^^^^^^^^^^^^^ the dynamo is at f uU 
 
 §201. Current Induction. -If i„ ♦, • . 
 m magneto-electric induction (§ vm ' "'' '^'''^''"■'^^ ^^'PeH.ncnt 
 battery is substituted for the Lnn.: \ ««""ected with a 
 
 «ame results are obtained an wit tl rr ,r'''"r^; ^'r '-'^' '^- 
 to expect the same results. (Win t" ^'!'^'''^^ ^^ ought 
 
 ^"'00 Ihe wire A, Fig. 170, 
 
 PiK. 170, 
 
 tlirough which the battcrv-currenf , i . 
 
 case as tl,« ..;...,,. ,. ..-^ ''"'''"* '^"-.^illateH, in known m> f..{, 
 
 or mdt^cm^ cnrrevt. The wh. H ""'"'•>'-^''"-''«"t the primary 
 
 currents circulate, is called the ./ ; ""^ '"'^'''^ "''^^ "'^^"'^«d 
 
 fl'at traverse this wire are frc4en«vtj T''' ""^ '^^ "'^^«"^« 
 
 frequently called ^econrfar^ cum/i^. 
 
 ^|t 
 
254 
 
 EI.ECTRlCiTV AND MAGNETISM. 
 
 Ill 
 
 Ml 
 
 il 
 ■,'i 
 
 It will be observed that in nil 4-u 
 relative motion between a cond 1? 7"™"^*^ "^ ^^^ ^ 
 (rnagnet or current-bearing TkI 2)^^^ T^"^''^"^ ^^^^ 
 
 flows onl^ during the continuance ot" ;elti'' "'■ ''''''''''' 
 cate measurements have proved hat nr^ T*'^^"' ^'^^• 
 lucluced electricity transmittecl t T . "**"' ^"'^^"^'^^ ^^ 
 tl'e total quantity of ehann V. '''"^"''"'" "^'^'^^^ ^n 
 oa the cime occupied in^rchange '^I "'""^' ^"^^ ^^ ^' "^» 
 tlic more rapid the change thf mnrJ- / '''' '* '^ •^^''dent, that 
 t-.y current ; i.e., the S; te.- mn .?' "^"'' '^^ ^^^ ™««^-- 
 
 -"t flowing at tl. moCt clb " r'""^ ^' "^ -•- 
 Ohm's law, rememberin. thlt the " /' "" ^^''*^"^«"* ^^^^ 
 circuit is constant, we derive hff H '"'' "' '^^ ^^'^^"darv 
 In a., inauced current, Z EMp"'7 '''' '^P^^'^'^"^ 'aw^ 
 ^^onal . ae ra^iaUy of tUe r^<^^^.::;^Z ::T^ 
 
 found that making and breaking (ri.?rr ^''''^'" ^"« '^"«r. it Is 
 . arfng and stopping a primary cu ret ' „ "" ""''"^'''y '^"'-^nt, ^•.e., 
 
 '.try wire. Indeed this process L eviden ' ""''''"'' '" "'^ ^^con^ 
 
 "' theory exactly the same thinl 11 ^ '""''' '^' ^'^'"'^ thing (and 
 
 with unbroken circuit, Jm f, ^1'"^"? "'^ P^^™-^ cond'ucto"' 
 
 would be zero, into the ^econlrTcilrthf'rr "^"" '*« -«- 
 a very brief time. A reversal of t , "''^^''^ ^^^"«« occupying 
 
 «Pond to breaking the pr^arT il " C 7""' ^^''^'^""^^ '-- 
 
 § 1««, enable us at once to predi™ dil.r'''"' ' "'^"^"^"'^ *" ^xp. i. 
 
 " aTVT' ^-"-^ate the two clsfs tlfus !^"' ^' "" ^"^"^^^ — t. 
 
 0/ «Ae /)nwaj-y. ""^^"^ ^"^ «« op/Jostie f?eV<»c<w« fo that 
 
 <Ac jDnmory. ""^" fi«newf has the same direction as 
 
 secticu ldnTl^^^!^.^''t~'^^'^ conclusions of the preop^ing 
 
we have a 
 cing body 
 electricity 
 oil. Deli- 
 iai)tity of 
 spc-uds on 
 not at all 
 dent, that 
 e momea- 
 ' the cur- 
 lent with 
 econdary 
 ant law: 
 ' propor- 
 \t. 
 
 econdary 
 tter, it is 
 fent, i.e., 
 le secon- 
 i"g (and 
 nductor, 
 5 nction 
 icupying 
 y corre- 
 I Exp. 1, 
 "urrent, 
 
 'lated in 
 ! to that 
 
 'nt 1% a 
 'Hon as 
 
 (firlir 
 
 it any 
 
 i COQ* 
 
 INDUCTION OF COILS, 255 
 
 ductor gives rise 10 a current. But it is evident that in « • , 
 wire every portion must bo considered as a neil ''°^ 
 
 tor with respect to everv other nZlt^ "^'ghl'onng conduc 
 
 be no change of eU^Z^r^^ T^'r^r^ '''' 
 panymg induction phenomena. If wc s 'ddcnlv ' ""' 
 
 the current does not abruptly assume -tsla^S T ^ '"'"'' 
 there is a current indnoJirf^t mtensity, because 
 
 -0. as to :ztTJXT.:r%:T 'r^r " 
 
 circuit be suddenly brotpn fK ' *'^°' '^ "" ^^o^^d 
 
 --ion Of ^i^z::^:.^^:^:^:-;^ ^^ r 
 
 ^ero. These induced currents are often for L f^^^'^f ,.*« 
 tinction, called extra current. Or? ' „ ^^^ '^^^ °^ ^'•«- 
 
 ductor are kept close tolttrh"?' '' '" ^'^'^ "^"^^ '<^^- 
 or a spiral, thl ^^ ^^S^ ^:::^' ^]:^-J^-> ^ helix 
 direct extra currpnf m„cf '''''^®^s«d. It ,s evident that the 
 
 the latter is subtracted Thk T t ""* ''""""'y' «■'"'" 
 
 on breaking a st™. current ,„, ""T "' ""= ""«•" »'-* 
 or Shocks fxpericne'ed n „ t thV S*'*'"^'^"' '*'»'^ 
 «peri„,ent illustrated b/ pZ'e fyo P;TV'™" '" '"" 
 introduced into a heliv the ItZ „ . ""' "■°° <">''« '« 
 by the action o, the n^^lir^Xr^Xpr"^ '""«-'' 
 
 ooil,itisevidentthita'J^rii;,:Xr''dV''^ ''""■^'^• 
 every tin,e the pri.uar, is n,ade a:^ f The sTT''"'! 
 cessation of AmpWan carr,>nt, i„ "°'""': ^'"' starting and 
 as the primarv u tot .Td ,, """^ '" *" »""■« '««"'»„ 
 
 ment and cndingof the -i' "'"'"'"""'■«'' "'* *« -nimenee- 
 secondary current To IT' T'T' 8'''"'"^- '"'»"'i''es the 
 in^ bv hLd "!■; F " IT-."""."-"'""'' of '"aki-g and b,^ak- 
 
 constrnctio~n'of an a^tolllc' "t""™,'' "'''* '"'''''^'' '» ""e 
 
 i.™ han,mer4 is?onnert^rwi,h7. Tt'""* '"'""• ^ ^°« 
 
 turn connected wZne of the , 7' '"""''' ^' ""'* '^ '" 
 
 "'"= "' ""o ternnnals of the primary wire. 
 
256 
 
 r i4 
 
 4' 
 
 Iff 
 
 m 
 
 ELECTRICITY AND MAGNETISM. 
 
 The hammer presses against the point of a screw d, and thus 
 through the screw, closes the circuit. But when a current 
 passes through the primary wire, the core becomes magnetized 
 draws the hammer away from the screw, and breaks the cii'cuit' 
 
 Fig. 171. 
 
 The circuit br..ken, the core loses its magnetism, and the hammer 
 springs back and c-loses tlie circuit again. Thus the sprint and 
 hammer vibrate, and open and close the primary circuit with 
 great rapidity. An instrument made on these principles is 
 called an induction coil. 
 
 § 204. RuhmkorflP's coil. -This instrument has the impor- 
 tant addition, to the parts already explained, of a condenser Bli. 
 This consists of two sets of layers of tinfoil separated by paraf- 
 tne paper; the layers are connected alternately with one and 
 the other pole of the battery, as the figure shows, so that they 
 serve as a sort of expansion of the primary wire. When the 
 circuit is broken, the v-xlra current would jump across at b, and 
 would vaporize the i)oints of contact, and form a bridge with the 
 vapor of metal that would prolong the time of breaking. But 
 

 THEEMO-ELECTE ICITY. 
 
 257 
 
 
 when the condenser is attached, the extra current finds an 
 escape into it easier than to jump across at 6, so the vaporizing 
 of the contact is avoided, and tlie time of breaking being much 
 shortened, the secondary is much more intense. 
 
 The primary helices of induction coils consist of comparatively 
 few turns of coarse insulated wire ; but the secondary helices 
 contain many turns of very fine wire, insulated with great cai-o. 
 The secondary current is, at breaking, as we ought to expect 
 from the extreme rapidity with which the primary circuit is 
 broken, distinguished from the primary, or galvanic current, by 
 its vastly greater tension, or power to overcome resistances. 
 A coil constructed for Mr. Spottiswoode of London has two 
 hundred and eighty miles of wire in its secondary coil. With 
 five Grove cells this coil gives a secondary spark forty-two inches 
 long, and perforates glass three inches thick. Many brilliant 
 experiments may be performed with these coils which will be 
 indicated in connection with frictional machines. 
 
 and 
 
 XXXIII. THERMO-ELECTRICITY. 
 
 § 205. So far in our experiments we have obtained a current 
 of electricity by using the potential energy due to the chemical 
 afllnity of zinc and sulphuric acid, or by ex[)ending mechanical 
 energy ; can we not also get a current directly from the molec- 
 ular energy that we know as heat? 
 
 Experiment. Insert in one screw cup of a sensitive galvanomotc r 
 an iron wire, and in the other cup a copper, or, better, a Gcriaan silver 
 Avire. Twist the otlier ends of the wire together, and heat them at 
 their junction in a (lame; a dellcction of tlie needle shows that a cur- 
 rent of clectricitj' is traversing tlie wire, riace a piece of Ice at their 
 junction. A deflection in tiie opposite direction shows that a current 
 now traverses the wire in the opposite direction. 
 
 These currouts are named, from their origin, thermo-electric. 
 The apparatus required for the generation of these currents is 
 very simple, consisting merely of bars of tM different metals 
 
 I: 
 
liM 
 
 ! 
 
 It 1 1' 
 
 I i 
 If 
 
 I i in 
 
 258 
 
 ELECTRICITY AND MAGNETISM. 
 
 omed at one extremity, and some moans of raising or lowering 
 the.r temperature at their junction, or of raising tl.c tempem ,r! 
 at one extremity of tke pair and lowering it at L othe,Tfor 
 edectro-n>ot.ve force, and consequently the strength of the cu ! 
 the two "T^ ?''P^f^--' to the difference in temperature of 
 the two extremities of the pair. The strength of the current is 
 dso dependent as in the voltaic pai, on the thonno elect -l 
 motn e force of the metals employed. The following thermo- 
 electnc senes ,s so arranged that if the temperature^ of both 
 junctions are near the ordinary temperatures of the air, those 
 .e^ls farthest removed from each other give the strongest cur! 
 rent when combmed ; and the current passes, when heated at 
 then, junction from the one first named to that succeeding it 
 
 Jnd con" T ''' ''"^^'"^ ^^ '''' ^"--'t '' t^« J-Ited 
 
 and cold ends re::,po,.>tively. At high temperatures the current 
 may be reversed v^uxiuus 
 
 Cold. 
 
 .a 
 pq 
 
 03 
 
 .a 
 o 
 
 a 
 
 g 
 
 O 
 
 ~i 
 
 
 
 u 
 
 <u 
 
 . 
 
 
 0) 
 
 i£ 
 
 •o 
 
 
 Q, 
 
 2 
 
 
 
 o 
 o 
 
 3 
 
 3 
 
 a 
 
 1> 
 
 CC 
 
 •= 2 
 
 Heat. 
 
 -> 
 
 t 
 
 S3 
 <1 
 
 HEAT 
 
 § 206. Thermo-electric batteries and thermo-pile -^ 
 
 The electro-motive force of the thermo-electric pair is v^ry 
 small in comparison with that of the voltaic pair • 
 hence the greater necessity of combining a large ^'^" "'" 
 number of pairs with one another in series. This 
 is done on the same principle, and in the same 
 manner, that voltaic pairs are united, viz., by join- 
 mg the +metal of one pair to the -metal of an- 
 other. Figure 172 represents such an arran^e- 
 mont. The light bars arc bismuth, and the dirk 
 ones antimony. If the source of heat is stroncr ■ ■ 
 
 and near, by either conduction or convection on'e face may bo 
 
or fowering 
 cmperature 
 lor ; for the 
 of the cur- 
 perature of 
 3 current is 
 nio-electro- 
 ng thermo' 
 es of both 
 ! air, those 
 )ngest cur- 
 heated at 
 needing it. 
 iho heated 
 !ie current 
 
 3 -pile. — 
 
 r is very 
 
 ng. 172. 
 
 HEAT 
 
 ^ 
 
 lOLD T 
 
 , J 
 
 THKUMO-KLKCTUICITY, 
 
 259 
 
 heated much hotter than the other, and a current equal to that 
 from an ordinary galvanic cell is ofti'ti obtuiii.-d. Instruments 
 conbtructed on these principles, and uHcd uh a source of elec- 
 tricity, are very convenient and efricient for many purposes, 
 especially when a steady current is requin^d with st xternal 
 
 resistance ; they are called thernio-electrk hallcrieH. 
 
 If the source of heat is feeble or distant, the feeble current 
 may serve to measure the difference of U-niperature between the 
 ends of tiie bars turned toward the heat (as in Figure 172) 
 and the other ends, which are at the tt-mporature of the air. 
 The api)aratus, when used for this pur,.os(,, is called a thermo- 
 pile, or a thermo-muUiplier. A combination of as many as 
 thuty-six pairs of antimony and bismuth bnrs, connected with 
 a very sensitive galvanometer, constitutes an exceedingly deli- 
 cate thermoscope and thermometer. QiiantiticH of heat, that 
 would not perceptibly expand the mercury in an ordinary ther- 
 mometer, can, by the use of a thermo-electric pile, be made to 
 produce large deflections of the galvanometer needle. Heat 
 radiated from the body of an insect several inches from the pile 
 may cause a sensible deflection. 
 
 Some contrivance by which heat energy may be directlv trans- 
 formed to difference of electrical potential, capable of doin^ as 
 much work, or nearly as much work, as tlu! heat itself can"do 
 is very much U) be desired. When discovered, as it probably 
 will be in time, it will supersede all present methods of produc- 
 ing the electrie current. In the thermo pile heat energy is 
 transformed iodifft^ronce of electrical potential ; but the contriv- 
 ance is not efficient ; too much of the heat energy is dissipated. 
 
 .Jiiii 
 
 may be 
 
9u 
 
 ^^^■.,.v 
 
 
 
 
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 260 
 
 ELECTRICITY AND MAGNETISM. 
 
 
 1 1 , 
 
 If 
 
 XXXIV. FRTCTIONAL ELECTRICITY 
 
 placing a square board on Sri and ^ , ''""^ ^^^'^ § '''^ ^^^ 
 
 iegs. Let a person whom TlTZju ^^' *"™''^^'^' "^'''^ «- 
 ^t a second person? '^^^ *^°''" ^*""^ «" ^^^^ «tool, and 
 
 James, strike Jolin a „. „ 
 
 few times with a cat's '^' ''^' 
 
 fur. Then let James 
 
 bring a knuclcle of a 
 
 finger near to some part 
 
 of John's person, for 
 
 instance a knuckle of 
 Iiis hand, or his chin or 
 lioso ; an electric spark 
 will pass between tlie 
 two, aLd both will expe- 
 rience a slight shock. 
 The length of the spark 
 shows that the electri- 
 city is urged by a high 
 E.M.F., like the induced 
 
 currents of the magneto-machine and induction coil. 
 
 § 208. Electroscope. - Experiment. Suspend in 
 
 bo near cocb other «,7 ,1 ° ™"'°'" 1">«'°°« ■™J' lA 
 timeJlthaefor ic.hf,l°I ' " '"" ''°™ """''' " '"'' ^ 
 
 ..en.,,, o. t 'sr.^ r:'?„rt"or/rT',i "°^ "■" "■>"- - 
 
 .to 
 
electrifl- 
 
 § 214) by 
 
 rs, used a:) 
 
 stool, and 
 
 olecular 
 laers an 
 at each 
 r known 
 inner is 
 
 Ig- 174. 
 
 per ox- 
 li verge, 
 sbody. 
 
 ATTRAr;TIONS .^ND REPULSIONC, ETC. 261 
 
 We have already found that this force is due to electricity 
 Bodies in th.s state are said to be charged with electricity, or 
 simply dectrijied. Such electrification in a person is often 
 manu^sted by a divergence of hair on his head. Any ar- 
 mugement, like that of the foil just described, intended to 
 detect the presence of electrification, is called an electroscope. 
 One of the most common and useful electroscopes consists 
 ot one or two pith-balls, made from the pith of elder or 
 sunflower, suspended by silk thread. If an electroscope is 
 brought near to either pole of a secondary wire of an induction 
 co.l, a similar electrification is manifested by the poles Like- 
 wise, by means of very delicate electroscopes, the poles of a 
 eCdld "^■' '^ '' ^ ^^--b^ttery, are'found to'be fel; 
 
 Experiment 1. Poise 
 
 Fig. 176. 
 
 ^ 209. Attractions and repulsions 
 a flat wooden ruler on an inverted 
 bottle or flask, having a round bot- 
 tom, as In Fig. 175. Draw a rubber 
 <;omb two or three times through yonr 
 Jiair, or rub it witli a woolen cloth, and 
 place it near one end of the ruler. 
 What takes place? What does the 
 phenomenon indicate? 
 
 Experiment 2. Hoh; the comb over 
 a handful of bits of tissue-paper. 
 What is the result at first? What 
 takes place in a short time? What 
 do these phenomena indicate? 
 
 § 210. Two states of electri- 
 city. - It is quite apparent that we are now dealing with a very 
 different class of electrical phenomena from any that we have 
 previously observed. It is also quite as obvious that we are 
 dealing with electricity in a very difforont state or condition 
 from that in which we have before studied it. Hitherto we 
 have studied only those phenomena produced by electricity 
 
 i 
 
262 
 
 ELECTEICITY AND MAGNK^TISM. 
 
 E I' 
 
 I' !• 
 
 If 
 
 When in motion ; and, inasmuch as when in that staf . Jfo 
 
 we fl„<, that o,« f, thl' r"! ^. ""' °" *^ *>"'" "'««>• 
 
 decomposition, or ffivinjr shookQ Rnf v. ^ "s, pioaucing 
 
 ehamed vifl. T , "* "^^ observe that bodies 
 
 .31;,"'?'''^^ '''''"^^*^- "^^^^'^•^ extraordinary 
 
 t.om the attractions and repulsions observed between parallel 
 currents (§ 189). ctvYccn paiauei 
 
 This state of electricity is called static, in distinction from the 
 current state, which is often called dynamic. We have seen 
 that under certain conditions, electricity may change from one 
 state to the other, as when the electricity which had accumu- 
 lated m the boy on the insulated stool passed ^ >e other bov 
 producmg, in its current •^' 
 
 state, both illuminating and 
 physiological effects; and 
 again, when a current is 
 broken, the current ceases, 
 but electricity accumulates 
 in <^he wire (see § 208) , We 
 have also learned that elec- 
 tricity of high potential, 
 such as is most readily de- 
 veloped by friction, exhib. u__„,^^ 
 
 s^-ifdX''\ul"r"''^; t"' '^''^''' ^'^^ ^«p"^«--' -St 
 
 SI jungly, but we must be careful (> avoid the notion that 
 these are peculiar to electricity so derived. 
 
 §2U. Two kinds of electrification. -Experiment 1 Rp.^ 
 assail glass tube into thefoHnrepresentedbyA.Fjur^X^^^ 
 
ELECTRinCATION. 
 
 268 
 
 l''ig. 177. 
 
 KItf. 178. 
 
 aTitrbal ^nT'' ''''"'■ '^''^'^^'^ ""■^"^■"^' ^^om tho tube 
 
 Juof, and present it to the ball. Now, rub a stick ol' sealing-wax or a 
 hard-rubber ruler, with rtannel, an.l i,re- * 
 
 sent it to the ball, which has just been 
 in contact with the excited glass rod. 
 
 Experiment 3. Suspend two glass 
 rods that have each been rubbed with 
 silk in -,wo wire stirrups (Fig. 177), and 
 present them to each other. Suspend 
 two sticks of sealing-wax that have been 
 rubbed with flannel in the same niaiuier. 
 Now, in a like mamier, present one of 
 the glass rods and one of the sticks of sealing-wax to each other. 
 
 Experiment 3. Make a pin- 
 hole in eacli end of a hen's eg<r, 
 and blow its li<iuid contents out. 
 Apply, with flour paste, tin-foil 
 smoothly to the surface of the 
 shell, and completely cover it. 
 With a drop of melted sealing-wax 
 attach one end of a silk thread 
 midway between the ends of the 
 shell, so that it may be suspended, 
 as in Fig. 178. i{ei)eat Experi- 
 ment 1 witli tlie shell as with the 
 l)ith-l.all. (Carefully note the phe- 
 nomeuK in tlic above experiments. 
 Wliat conclusions do you draw 
 from them? 
 
 It Is evklciit (1) that there 
 are two kinds or conditions of 
 
 d.clrijtm,ton; or, for eonvenv-ioc, w. »u,„.!ti,„™ M,y («o kimU 
 ofelecncuy; (i) .„at l„ey ne .o .olaW U, ,.„ch otUor a 
 l^ek,n,U repel a-M unlike kUOs „Un,c, o„« anolker. The To 
 kmds are usually distinguished fro,,, o„e a„o,l,er ,,v the names 
 po.«(.«« and ncyalue, or, ,„„,■„ l,ri,„v, „« +,- and-. T|l 
 forruor ,», by dennition, such as is ,Ievelo,„,d o„ ^lass wl,o. 
 
 If 
 
r 
 
 264 
 
 ELECTRICITY AND MAGNETFSM. 
 
 Il: 
 
 rubbed will, flannel, a,„i the latter is the ki„d develoned od 
 Beal.ng-wax ,vl,e„ rubbed with flannel. There inorea«r 
 except custon,, , or calling the one positive rather tbl the X;: 
 
 n..rr,';=.A;etr;raXr,r vvLrr'T-""^ r » "-■ 
 
 pith-ball touch the ttannol wLf t^L , . "'" '*'""'*• ^""^ "^^' 
 Ooekindof electrificatio,, i, nevcrdeveioped alone ; when two 
 
 cSeT ::::"' "t^" -'■'" - -'^^ "-n.'opjit; 
 
 aeve oped o , a lubber comb when it is passed through the hair ■ 
 »lso the lend developed on a person when whipped ;m fnr bj 
 prese„tn,g the bodies whose eleetriflcation i. L be t d to ' ' 
 body having a linown electrification. ) 
 
 Fig. 179. 
 
 .- .« near ,. one e„a ot the shei^'I^T^rtotrril^S 
 
DISCHARGE. 
 
 eveloped on 
 
 no reason, 
 
 a the other. 
 
 with a flaii- 
 ilt? Let the 
 ediately pre- 
 ' the effect? 
 the electrifi- 
 I repeat tlic 
 
 i when two 
 oppositely- 
 •never any 
 tity of the 
 itriflcation 
 1 the hair ; 
 th fur, by 
 ested to a 
 
 265 
 
 rg-shells, 
 
 hi Fi<?. 
 
 ling- wax 
 
 rod excited with flauuel, aud therefore having -.. While the rod is i„ 
 
 bv7sill Jh"' TT " ";^ ''"^ "' ti.s.suo-paper, or a pith-hall suspended 
 by a silk thiead, along the eggs, not allowing it to touch them. Where 
 clo j-ou find the attraction strongest? Electrify your pith-ball negatively. 
 
 pith-ball to the eggs. Are they electrified alike? If not, which is 4-v. 
 and wluch is —? Separate B from A three or four inches while iTis 
 
 fled pith-ball to the eggs. Are they still electrifled? 
 
 We learn from this exi)eriment that without transference of 
 e.eetncity from the exciting body, or, as it is usually expressed, 
 by induction, we may charge at the same time two' bodies one 
 with 4-e, and the other with — e. ' 
 
 «ii^" ^^SO^arge.-ExpeHment. Bring the two shells oppo- 
 sitely charged near one another; when near enough they exhn)it 
 mutual attraction for one another. On bringing them still neare . 
 spark passes between them, tlieir mutual attraction suddenly ceases 
 and on testing them with an electroscope, it is found that both have 
 lost their electrification, i.e. , both have become discharged. 
 
 Wien two bodies containing equal amounts of opposite electri 
 cities are brought together, both become discharged. Durln- 
 the process of discharge, the electricity which was previously in 
 a condition of rest, or a static state, assumes a condition of 
 motion, or a dynamic state, as is shown by a spark passing 
 between the two bodies when brought near one another One 
 of the bodies-tliat positively charged-is at a potential higher 
 ban that of the earth, the other being lower. When they are 
 brought sufficiently near, the tendency for tlie electricity to pass 
 from the region of higher potential becomes strong enougli to 
 penetrate the insulating air and establish a condition of equili- 
 bnum. In this particular case the result is zero potential or 
 no electrification ; but in general botl, bodies wouhl be b^ft at a 
 bke condition of electrification, its sign depending upon the 
 sign of that electricity which was in excess. 
 We may now understand how it is that an electrified body 
 
 I 
 
 iii 
 

 266 
 
 in 
 
 ;^)1 
 
 ! f 
 
 
 Bl^mTUmiTY AND MAfJNETlSM. 
 
 '^ttiacts to it.elf light bodies i„ it« vh,'„|tv P 
 stick of sealing-wax, excited with Z> h. 1 , ^ '' """"i^^^' « 
 induces -f e next itself and repels , \ T^ T' ^ ^^'"^■^''^"' 
 forces arc in action between teLTI ' "'"•^' "'" ^^^^'^ 
 
 attraction between the -eof L > "''" "'"' ^''« P'^'-^'^-^H • 
 Pith-ball, and repnlst be^t: trr^r;;^ "' ^ +^ ^^ '- 
 the ~e of the pith-ball Bnf sin. 7 ^-ahng-wax and 
 
 «-case,ti.el.actione^:ru;e:t^^^^^^^ 
 
 that is, the opposite ends become oppoL " 1l '.'^'""^^ P°'-^-^'- 
 withthe flnger. Tlirough your bodrttf h "''''"'■ ^ouch the shell 
 the earth, while the posttive char^ r '^ ''*"?'"' '"'"''^'^ '« ^h-'^^" *« 
 (Explain.) Remove the ^ .^r and i ! ^ " "^"^''"'^>' ^o the rod. 
 the shell, and you will Had « a^ 1 Is Sm wli TV' "''^ '''''' *««' 
 
 -or+e?) Touch this Shell With L.othrhH^^^ ^'^ " 
 
 Test them, and you find that tiiev huvn h 7 .' "'" '^'^Pa'-ate them. 
 
 It is evident that the flr«t si el be ame ttHn" "' ?' ^'-^^^A-tion. 
 
 .irriSrz:;.:;:;r:::;;^:;-;r 
 
 that the electricity could not W 1 t^ Hnl" ' I' \"'''^"* 
 we could not have charged the shell Mn. '"•^' "'^"'^'^^ 
 
 alW electricity to pai reac^,;'^ ou-d h rr'^n^ ?^ "^^ 
 conductors or ui.t^^ator«. A bodv that fJ f • ''''"'^ '^^^*- 
 
 charge of electricity niust be hs , : ' '' '"T'^ " ^^ -^ 
 -th the earth through a conducti g ; ' ^ta o "s """f"'^ 
 best insulating substances are clr, a/., .^^2^ "" 
 giass (free from lead p n n^ V . ^'^^' «'«e//ac, resins, 
 
 b.''tterv is the source of electrioitv th! , ° " ^'''™"'° 
 
 wood, for mBtaooe wh r f ^' ' ""''»'""«» -sueh as dry 
 
 Jatter case «; „ot s7r™ , T- T'T"^' •*""" '"»"'"'»- » «•" 
 injures the iusrio, Xdios!" '"™"'' *"■'"'"'■« ^'-''y 
 
ELECTRICAL POTENTIAL. 
 
 267 
 
 or example, a 
 ill- a pith-ball, 
 st side. Two 
 tlie pith-ball, 
 the -f-e of the 
 ling-wax and 
 8 less iu the 
 
 electrified seal- 
 tnes polarized, 
 'ouch the she'll 
 ' is driven to 
 ''}' to the rod. 
 the rod; test 
 "icity. (Is it 
 'parate them, 
 lectriflcation. 
 ^Mon and yic 
 he shells; it 
 
 ctric charge 
 t is evident 
 I, otherwise 
 hich do not 
 -ailed wow- 
 permanent 
 connection 
 >me of the 
 "c, resins, 
 silks, and 
 state, the 
 I galvanic 
 uch .as dr" 
 tors in the 
 re greatly 
 
 § 215. Electrification confined to the external surface. 
 — Experiment 1. Place a tin fruit-can on a clean, dry, -lass tunil)lfr 
 (Fig IHO). Fasten a circular disk, a, of tin, l.V"" iu diameter, to one oml 
 of a rod of scalinjj:-wax. Charge the can heavily with el.;ctricitv from 
 anelectrical machine (s.-c §217). Through an orifice, c, in tl'ie can 
 introduce the disk, and touch the interior surface of the 
 can. Withdraw the disk and present it to an electroscoi)e. ^'^- ^*'*- 
 Is the disk electrincd? Now touch the exterior surfai-e of 
 the can with the disk, and i)resent it to the electroscope. 
 Is the disk electrified tills time? 
 
 Experiment 2. Attach to the can a gold-foil, or double 
 pith-ball electroscope, and put into the can a few feet of 
 metal chain. Fasten the outer end to a rod of glass, or 
 some other insulator, and charge the can till the leaves of the electro- 
 scope diverge widely. Then draw up the chain by the glass rod- the 
 leaves come together somewhat. What does this indicate? Drop the 
 Cham mto the can; the leaves separate again, showing that the charge 
 had not been lost. 
 
 These experiments show (1) that no electricity can be found 
 inside of a hollow-charged body ; or, roughly stated, electricity 
 at rest resides on the exterior surfaces of bodies; (2) that 
 token the exterior surface of an electrified body is incr ..fd with- 
 out increasing its mass or the charge, the amount of elect icity at 
 any point is diminished. 
 
 § 216. Electrical potential. — We have seen that the 
 passage of electricity from point to point sometimes causes a 
 spark ; so, conversely, the spark indicates the passage of elec- 
 tricity. The passage of a current from one shell to the other 
 (Fig. 179) might be proved, and its direction determined, by 
 connecting the sliells by wires joined to a suitable galvanome- 
 ter. The current would flow from A charged with +e, to B 
 charged with -e, thus showing that A had a higher potential 
 than B. A body charged with -\-e is understood to be one that 
 has a higher potential than that of the earth, and a body 
 charged with -e is one that has a lower potential than that 
 of the earth, the potential of the earth being regarded for con- 
 venience as zero. 
 
 ;i 
 
 II 
 
 ifi! 
 
268 
 
 II If: 
 
 mi 
 
 ELECTRrciTV AXD MAONETfSNf. 
 
 "W"ith a very sensitive oleotroscono ,> « u , 
 wires connected with the nhtes T '^" """' "'"' ^'^'^ 
 
 ^'i«-erent potentials when h^^ L is^:t""" '"^"^••'^' ^^ '^^ 
 ence of potential is so small T '"' ^"* ^^e difTer- 
 
 <'"eed by friction, that a U o, f ^ ""' ''" ^^'"^^^^^^ P'^o- 
 a^Parlconlyabo.:;::,:;:^;!^^^^^^^ 
 
 Potentili, l;! t\t:7'"'^"""^ "-'^'^ ^'-*-''*y o. hi.h 
 «on coil or I^ch"^™r"""'^^^^-''^'- 
 -achinedependin^eithero^:^ci""'""""^■' "^" ^'-^---I 
 charge of electricity. Brief de 1, ,' '' "" ''' "-luction-of a 
 
 -- bo given, followed b7a ^e^of ^ ''-"^ '^" ™^^^"^^^ -'" 
 performed with them. experiments that may be 
 
 Fig. 181. 
 
 made of two cushions of leatTr 1^ \ ^ '' P'''^'^ ^' ^ ^"^ber D 
 •atin,^ supports E, p'g a.Kl h ^ T' '""' "" ^'"'^'^'^"^' ^«»r Insu 
 c'>ain K, used to congee; ettr. 1 '"'"''^""» ^'"^^ ^' «"^J a brass 
 
 side of the plate; their pointed teeth sr?, 1 ''°"'''"' ""« ^^ either 
 ft pith-baU electroscope '"'^ ^'« *"^°«^ t«'-^ard the plate. Mis 
 
«vii tliat the 
 ttery are nt 
 ' the difTer- 
 'ercnce pro- 
 serios give 
 
 IS, ETC. 
 
 ty oi 'iigh 
 
 an iiiduc- 
 
 eloctrical 
 
 ction'of a 
 
 Aincs will 
 
 It may be 
 
 ^ 
 ^ 
 
 r prime 
 bber D, 
 ur insu- 
 a brass 
 I exten- 
 1 either 
 s. Mis 
 
 KLECTUOPHOltUS. 269 
 
 When the plate is turned in the direction indicated by the 
 arrow, i passes between the rubbers, and the friction .venerates 
 
 of the p ate then passn.g through the silk bag comes opposite 
 the comb when .t polarizes the prime conductor, attract! - 
 and rq,elhng +.. But the -e escapes from the points of the 
 comb (see § 22G) to the plate and neutralizes the +e of the plate 
 and thereby leaves the conductor charged with +e U both con; 
 ductors are msulated at the same time, the mutual attraction of 
 the two kmds of electricity would prevent their becoming heavily 
 Fig. 182. charged, so one of the con- 
 
 ductors is always connected to 
 earth by a chain. If +e if, 
 wanted, A is insulated ; if -e 
 is wanted, B is insulated. 
 
 § 219. Electrophorus. -» 
 Experiment. On a circular dis^ 
 of shcot-iron or tiu 2G™ in dianic- 
 ti'r ccineut a circular dislj of vul- 
 canite 22c'n in diameter. To the 
 center of another circu- 
 lar disk of tin 18<;"' in 
 diameter (Fig. 182) ap- 
 ply with heat one end of 
 a stick of sealing-wax 
 for a handle. Strike the 
 surface of the vulcanite 
 a few times with a cat's 
 fur or a fox-ull; it will 
 become electrified with 
 -c. Then place the tin 
 disk ou the vulcanite; 
 -e of the vulcanite will 
 poi.xrize the disk, indue- 
 
 ~, WIU eacpc though your body l„ the c-arll.. tat tho IT^mvmX 
 
 Fig. 183. 
 
 i 
 
 :B, 
 
 I 
 
270 
 
 BLECTRICITV AND MAGNETISM. 
 
 on the disk, bound bv the —^ r.r ti. i 
 
 linger and ral.se the dii ,n it LZn^Tu' ''"'"^^' ''"^"^'^ ^''^ 
 influence of the -. „„ the , t H. T " '''''''''''' ''"^'^ *'•« 
 and if a k„„ekle .,f .,„. of V l! 2" /^? +' "' *'"' ^"'^•^ ''^ ""^v free, 
 
 bright spark wil, ,k.ss fn n t^ vom/f "'-/''V' "'"'"^'''^ "-'' '^ « 
 charged. ^ * ^°"' ^"^"'l' an^ it will become dls- 
 
 n Jl;:;^^^ S: :-uS:Sn1;^ -ll^^^^ "^-- — a .reat 
 A U,v.len Jar (page ..0) .na^ e^u h U '''' "'"' "'^ '"'•• 
 
 '""intcs. (Is the disk charged by contl f ' /'"•"^'•''tus in a few 
 
 the proofs?) ° ^ conduction or induction? What are 
 
 § 220. Continuous electrophorus \'n..; 
 have been aaoi>to(l for clevoloni„.r .k^.?, 7 ^ ''''^''' '"^^^''^^^^ 
 the olectrophonts, unci Zro ^^^^'"^•'t^' -"^"-ously from 
 
 Pig. 184. 
 
 the electrophorus, und more 
 
 ••ai)idly and with less manipu- 
 
 hition thiin can be done with 
 
 tlie apparatus above described 
 
 I'^igurc 184, from which the 
 
 supporting parts are omitted 
 for the sake of simplicity, will 
 serve to illustrate the general 
 principle of such machines. 
 
 A is a vulcanite or glass vj -<tA ^ 
 
 wheel, which can be rotated bv means of o . 
 CC. About 2-" back of \ i • i '^'''''"' ^^ ^^^'^'1« 
 
 as an inducer, or tL sal nurnn' "";?' "'^'^ ''' ^^"^'^ ^-ves 
 electrophortts O os^ n ^^^ T ''' ""^'^"^'^ ^'^^^ «^ "^e 
 
 ^:--'Hiscon^r:iS:b:o:r^^^ir^ 
 
 cited on D with a cat-skin Ti, 1 ^^ ~^ ^"^ *^-^- 
 
 J^. Ihen, smce electricity escatDcs ronrliil f 
 
 port,: J; th! „;?:!"" ^'"^'"s ," ""--"''-'«'■. o.i/th: 
 
illy, remove the 
 novetl from the 
 lisk is now free, 
 oii^'Iit near It, a 
 ill beconio Uis- 
 
 mauncr a great 
 e with tlie fur. 
 iratiis ill a few 
 on? What are 
 
 )us methods 
 miouslj from 
 
 I of wheels 
 ''hieh serves 
 iisk of the 
 tallic comb 
 — e be ex- 
 system NIJ 
 ven to its 
 ' the comb 
 Joints, the 
 Its by the 
 only that 
 |-« that is 
 being de- 
 rotate the 
 
 TOEPLKU-HOLTZ MACHINE. O71 
 
 disk A When it has accomplished half a revolution tint nor 
 fon o :t which is charged with +. eo.nes op " te l ; 
 comb H, which is also connected with a conduct^ P tC 1 
 <lMctor B' P becomes polarized. Its _. nassos off fL 
 Loints of B to the disk A, and discharges T!^ ^t di ^ 
 wlue the conductor VIV is left char^.d with free 4^ jfl 
 ovdent that a constant rotation of A would c' so it tt bo 
 
 conducto. B N is constantly receiving a change of n in nr. 
 q.ience of the loss of its -Ce • and for • In. ~ '" 
 
 dupfor n> i» • . ^ ' ^ '^ similar reason the con- 
 
 «l<«ctor B' P ,s constantly receiving a charge of +e Wit ' 
 ra,Hd rota.0.1 of the disk the two condncto^ will tso l^; 
 . K 1"^^'' y electrided, the one with -e and the other wit + 
 
 incessant flon of sparks will p,iss between them, even when th. 
 extremities, P and N, are several i.iehes apart. " 
 
 § 221 The Toepler-Holtz Maohii. Fig. 185^ is inntho. 
 
 -- o. .e o. „::;o„ i:,r. "Tir ::ri:r 
 
 imsscs a Bpinc e to wliicli is fiv,.,l l.„ „ • , ''^'^ 
 
 a revolving .lass ilteO .'.^ ,',"''''"''"« •■'«'''^''""'. 
 
 plate 0„ the h.„ r "'"""'■''' """' ""^ «""ionary 
 
 with . a,„, *, :; ;,„ o\; tt ; r'o"™;;,,! "■": k\ """""•'"" 
 Lrri^^::r™"'"'''-^^^^^ 
 
 rass wire. Bs turning these screws the brushes may be -u} 
 justed so as to anniniph o~ » , • '* 
 
 •ovolviu^ nhto 'P ?p "'■ "' ""'^'' ^" ^'^«''-^^» to the 
 
 eae.0^/:::..::— '-r;::;:-:--^ 
 
 m 
 
 ifii 
 
 
272 
 
 KLECTRICITY AND AfAGNETISRf. 
 
 f'-ont of O and opposite tl,e strips of tiu foil d and a Tn .. 
 
 a with eadi other by a conductor passing uuder the 
 
 Fig. 185. 
 
 '^'^'>^^TrafntSi:so*AS^^^^;^; 
 
 
TOEPLER-HOLTZ MACHINE. 
 
 273 
 
 Vw^ 
 
 ofy y and c c'. By means of the wheel W, provided with a 
 belt passing round the spindle, the plate O may be rapidly re- 
 volved. '' 
 
 The action of this machine is in the main the same as that of 
 Ihe Continuous Elkctkophouus (§ 220). It will be observed 
 that It IS provided with mo inducers, d and d* . The L-sli'iped 
 arms y and y' enable the revolving plate O (by what means?) 
 to increase the charges on the inducers d and d\ so th.t, though 
 these charges may be small at first, they rapidly increase to°a 
 maximum as the machine ' vorked. If tlie brushes y ,/ and cc/ 
 are adjusted so as just to couch the metallic buttons on O the 
 infinitesimal difference of potential always existin-r between d 
 and d' ,s enough, when the atmosphere is in a favorable con- 
 dition, to start the machine. When the condition of the -it- 
 mosphere is very favorable, the machine works equally well 
 with the brushes not touching the buttons. The two Leyden 
 jars act as condensers, causing a greater quantity of electricity 
 to accunmlate before the difference of potential between the 
 two electrodes is sufncient to cause a spark to pass between 
 them Ihe rod c C has a ten.lency to prevent a dischar<xe tak- ' 
 ing place across the face of the plate O. This discharge may 
 be observed by working the machine in the dark witii the rod 
 cc' removed. This rod also makes it easier to stnrt the action 
 of the machine without first charging d or d' from some external 
 source. 
 
 If the atmosphere is warm and very dry, and the machine in 
 good order, sparks may be obtained of a length equal to five- 
 sixths of the radius of the reviving plate. ^ The inducers or 
 armatures, as they are generally called, must be very carefJlly 
 insulated from the air. For this pu.pose it is necessary from 
 time to tune, to renew the coating of shellac-varnish at certain 
 points, particularly at the edge of tlie reyolviu- plate By 
 working the machine in the ,lark, any leaks from the armatures 
 may be easily seen. 
 
 m 
 
 iMii 
 
 From what you know about electrical attraction and 
 
 repul- 
 
274 
 
 ELECTRICITY AND MAGNETISM. 
 
 
 IH' 
 
 n 
 
 , t 
 
 It 
 
 .Ion can you see .any reason why this machine shonid be harder 
 to tmn when ,t ,s working than when it is not? That yon may 
 be able to answer ti.is question, e..ann„e very careful y any at 
 
 p™ eZ, that'""":;""" '"'"■^^" '"' »''«'™»^ °" «- -°'v4 
 
 plate and that on o(her parts of the machine. 
 
 called a cuneHl-irmker, by means of which the induced cur 
 l-e'nt between the outercoatings . an.l , of the two Cden as 
 
 out and he ends jomed to two sliding electrodes, which may be 
 l>l...'ed at any distance apart. If the electrodes of the machine 
 
 aTa,?::: trr "°r "^'"^ cnr..„t.brea.er are notZ 
 apa.t, „hen the maclune is worked sparks will pass between 
 
 bleaker It the electrodes of (he eurrent-breake. are held in 
 
 he hands and the m.aehine is worked with its own electrode, sen" 
 
 aated a senes of shocks will be felt whose inteusi rwUl 
 
 apa t Wthh,s attachment the Holtz machine may often be 
 made to take the place of an induction coil. 
 
 .ri!,f^^' I .°°°'»™«'"-- - A very in.portant adjunct to an elec- 
 tr cal „,achn,c ,s a coMlens.. of some kind, by means of whte ■ 
 a large quantity can be collected on a small surface 
 
 piac?,r=\„:i::s"r:i:r"^'r;;;sri«^ 
 
 triH,!'iri''"'n "r', ""^ ""' ''"""^^ ••'° ™"»"»1 q'-^'-'ity of elce- 
 
 IS snnple The hand of the first |.erson, charged with +c. .a,-,, 
 
 inV "tthl""?" "'%«'"^' """" "■' ^«^""<' l-"-"- a, ,:^. 
 mg -c to the surface of the gl.ass with which his baud is in 
 
LEYDEN JAR. 
 
 275 
 
 contact, and repelling +e to the earth. Thus, thrcgh their 
 mutual attraction, the two kinds of electricity become as it 
 vyere, heaped up opposite each other, and yet are prevoiited, by 
 the insulating glass, from uniting. 
 
 §223. Leyden jar. -The most convenient form of con- 
 Fig. 186. *^enser is the Leyden jar. Coat a green glass quart 
 fruit-jar (Fig. 186), within and without, for about 
 two-thirds its hight, with tin foil, using flour paste. 
 Close the mouth with a cork saturated with hot 
 parafflne. Through the cork pass a stout brass wire 
 till It touches the inner foil. Cast a lead bullet a on 
 the exposed end of the wire. Clean, warm, and var- 
 nish the exposed glass surftico of the jar, and when thorouakly 
 clri/ it IS ready for use. ^ 
 
 The jar may be charged by connecting one of its coatings 
 w.tii the +conductor, and the other with the -conductor of an 
 electr.cal machine, or by connecting one of the coatings with 
 Fig. 1.7. one of the conductors, and the other with 
 
 the earth. Or it may be charged by con- 
 necting the outside coating with one of the 
 poles of the secondary coil of an induction 
 coil, and bringing the other pole near to the 
 ball leading from the inner coating. To 
 „ .„ ,, , , discharge the jar, connect the outer coating 
 
 with the knob of the jar. To avoid a shock in so doing, prepare 
 a discharger as follows: Tlirough the cork of a bottle (e.r., a 
 soda-water bottle, Figure 187) pass a stout brass semiclrc4iar 
 wne. Cast f,n each of its ends a lead bullet. Use the bottle 
 as an insulating handle. 
 
 The effects are greater in proportion to the number and size 
 
 rFi!" ^Z r" ""T'f ^""»^^^'-^- Let any number of jars 
 (1 ig. 188) be placed on a sheet of tin foil, by which their 
 outer coatings are connected. Connect also their inner coat- 
 ings with one another by a wire ru.uiing around their projectino- 
 
 m 
 
 iLiil 
 
 ill 
 
276 
 
 ELECTRICITY AND MAGNETISM. 
 
 into one latgelln' ^'ZTclll^^'^'T P''^«*'«''^"y converted 
 
 be fused, and eten vol t li.rd V? "^' ^^''^'^ ^^'^ « ^^^ 
 
 through it, cards a.^'ln f^ 1^ '^ 7^ "^ ^^ ^^^^^ 
 gas or ether ignited. ° ■'^ Pwlorated, and 
 
 Pig- 188. 
 
 5 224. Bleotrioity not in the coatings -If n,„ , 
 sons m the experiment (S -I-i-n k„»t °*^- " '"" two per- 
 the glass plate after hit , '',°"' "'"°™ "">'■• hamls from 
 
 replaee their hands o^, tS a , Z "" '""^''- """ '^ '"^^ 
 they reeeivo a shoek. TO show, ^.i^ """""""^ '■"°<'«' 
 their hodies, but on the surfae ^n,, It ThT T ""' " 
 r-eyden jar serve the purpose of oonlf . """'"«' "' " 
 
 on the glass at the tfme'Tf „ha " ntt^ f T"' .**-''^- 
 «-om al, parts oHts eleetri« sS^ t '^JtT.ZZ^ 
 
 QUESTIONS. 
 1. ^°J"«»'ated jar ca»uot receive a great charjre Whv? 
 
ATTRACTIONS AND REPULSIONS. 
 
 277 
 
 Fig. 189. 
 
 «. If the knob of i second jar be held near the outer coafJno. ^f a« 
 
 ovulated jar sparks will pass from the coating to t. Lkn^ and bo" 
 
 jai 8 will be charged. Suppose that the ,nner ctatlng of the flr t jar 1« 
 
 « 5^; Attractions and repulsions. -Experiment i. Sup- 
 P^rt a plate of window glass (Fig. 189) about S-n from a tabic. Rub its 
 
 upper surface with a silk handkerchief 
 
 and place plth-balls, or bits of tissue paper,' 
 
 on the table beneath the glass. They will 
 
 dance up and down between the plate and 
 
 _, . _ table in a lively manner. (Explain.) 
 
 Experiment 2. Place a handl\il of bits of tissue paper on a tin disk 
 
 ■ upported by a prime conductor of an electrical nmd.lne. The papers 
 
 t, come excited, are repeUed into the air. and fall on all sides. gMng 
 
 tt, e appearance of a miniature snow-storm ^ 
 
 a . Sw"'°f frl ^?^^ °"' '"^ °^ * discharger to the conductor of 
 a V .achine, and the other end to the inner surface of a glass tumbler 
 oithbar' *f interior with electricity, and then place It overtme 
 
 The electric whirl consists of a cap of metal resting upon a 
 pointed wire, which serves as a pivot. The cap lias pointed 
 wires branching out from it, like tlie spokes of a 
 wheel, and bent near their ends at right angles, 
 and all turned in the same direction, as shown in 
 Figure 190. When this apparatus is placed upon 
 the conductor of a machine, the air particles 
 around the highly electriiled points become ex- 
 cit«d, and are repelled, producing a current of air 
 issuing from the points. Tlie reaction causes 
 t^ wheel to revolve in the opposite direction, as indicated by 
 the arrows m the figure. A candle flame placed near the point 
 ot a rod attached to a conductor will be extinguishod. 
 
 §226. Effect of points. -We might reasonably expect 
 that a current of excited air-particles issuing fl-om points on 
 m excited conductor would serve to carry away with them elec- 
 
 Pig. 190. 
 
 il 
 
 ;■•'■ 
 liji' 
 
f' 
 
 278 
 
 ELECTRICITY AND MAGNETISM. 
 
 will pass to your knuckle, or. at most, very feeb e o^esin l ""T" 
 
 seconds after the operation of gener- ' ^°'^ '" ^ '^''^ 
 
 ating electricity ceases, the conductor 
 
 will be found completely discharged, 
 
 although it Is thoroughly insulated. It 
 
 Is apparent that the electricity escaped 
 
 from the points. 
 
 Fig. 191. 
 
 r*,^"-^ '^""^v, 
 
 We conclude, therefore, that the 
 effect of points on an elect nfied in- 
 sulated body is greatly to facilitate 
 the discharge of its electncity. 
 
 § 227. Luminous effects. — 
 Figure 191 represents a glass shade 
 having circular bits of tinfoil pasted 
 spirally around it, from top to bot- 
 tom, and about 1™™ apart. If the 
 poles of an induction coil, or the ^up 
 
 conductors of an electrical machine, are connected, one with 
 each extremity of this spiral line, an intermittent li le of l" 
 will be produced in the path of the current by the sparkrwhT h 
 appear at the intervals between the bits of foiL All ex^ei inT 
 mustraung luminous effects should be performed in a dLCom 
 Beautiful luminous effects may be produced with apparatus 
 arranged as follows : Apply to one surface of a mica dT F i^ 
 192), about X 10-, a sheet of silver leaf or tin foil, 8 x 5- 
 
 within 1 of the foil, and as far apart as the power of the 
 macl-o w. 1 admit. Sparks will leap from the poles to the foi^ 
 and travel m tortuous branching lines between the pole«. ' 
 
LUMINOUS EFPECtS. 
 
 ^79 
 
 tmrge it. Do 
 
 peratiou, hold 
 irks will pass 
 points on the 
 ■her no sparks 
 and in a fe\Y 
 
 , one with 
 le of light 
 arks wJiich 
 fperinieiits 
 lark room, 
 apparatus 
 disk (Fig. 
 i, 8 X 5«''. 
 machine, 
 er of the 
 o the foil, 
 es. 
 
 If the air is partially exhausted from the glass tube used in 
 Illustrating the law of falling bodies (Fig. 87), and the 
 poles of a coil or machine are applied to the opposite extremi- 
 ties of the tube, sparks of electricity passing through the rarefied 
 air spread out ia sheets of bluish white light resemblin- the 
 auroral lights, hence this tube has received the name Awora 
 tube. 
 
 Fig. 192. 
 
 If a circular disk is divided into black and white sectors, as 
 In Figure 193, and rotated very rapidly in ordinary daylight, 
 the colors blend, and the disk appears of a uniform gray color. 
 Fi.;. 103. But if the disk is illuminated in a 
 
 darkened room by tlie electric spark, 
 each sector appears separate, and the 
 disk appears to be at rest. A rail- 
 road train in rapid motion, and even 
 its wheels, appear to be at rest when 
 illuminated on a dark night by a flash 
 of lightning. This shows that the 
 duration of an electric spark must be 
 ^'''i'3' ^I'ief, inasmuch as it fails to 
 illuminate these objects in two successive positions. 
 
 The remarkable beauty and brilliancy of the discharge is 
 perhaps best exhibited by me.ins of the well known Geisder's 
 tubes. These tubes contain highly rarefied vapors and gases of 
 various kinds. Platinum wires arc sealed into the glass at each 
 end to conduct the eliH-tria current through the glass. The 
 light, instead of appearing, as ii the Aurora tube, like a stream 
 
 01 
 
 If 
 
m 
 
 ttECTBtOITV AK,, MAONCTtHM. 
 
 
 I^«. 194. 
 
 "*"' ^--« "-' When „i. X;„ :Sttr"'' ""'^'' 
 
 dmtely beneatli the cloud L, "''J"""* *''"«'™ tame- 
 
 opposite kind of eiecSy Tll 7" """'"''™'^ -"" '"» 
 spond to the coatings, aid the ■?, " "'"^ ""^ ""«'' ""^ 
 l""go Leyden Jar. The Ztch ■"'",'* "''' '^ *'"= S'-''^' of a 
 
 ^ "old one another prirn r^v ""' """ """ "" "■«<"o"d 
 
 ".e charges aeen™„',a^ thed;;";""" ""-""''°"' ■"""• - 
 overcome the -istance^f tht ntt .itZ™ f ™' ™""'"'" *" 
 takes place. It is the acenmnlation oM ' ' W'™. "discharge 
 elevated objects, sneh as bnildt "''"''"«'' o'ectricity on 
 «ttraetio„ for the' oppo itT eWrS Tl "?' ."'"' "«-' "" 
 «.em especially ,iab,r,» be strueT;!; ligtotaf ""' """"" 
 
 § 229. Lightning-rods — Ti a u 
 of least resistance. A good liXll l^ ^"''' '^'""^ ^he line 
 
 ful .eans of co.n,„nicrti n ttZ'Z.T ^^^^^ ^ ^^-- 
 leaJs the electricity of the ZT ^ ''"*' ""^' '^ ^^^"d : it 
 and allocs H to -r 1 ^''"^'>' "P townnl tho clnn.^ 
 
 ^ ^""'"^ "'^^ ^^« «H-ite wlthout'disttbtef: 
 
GENERAL OB8EUVATIONS. 
 
 281 
 
 thereby so far dischargmir the clonrl «« f 
 
 stroke; or, if the tension ts too t to ^^^^ ? ''""'"-^ 
 
 of, the flash strikes downwardtlnd I led h ""^ ''""^^"^^ 
 earth by the conductor in ? / harmlessly to the 
 
 ducting material, so lar^ thaTt ^'J""^^*^ ^^ ^^^^^ ^«»- 
 
 fnnn loose join s Tiflo , u''^' ^' ""'^'''"^^ ^^^'^ f''«« 
 
 t'- ^s aiwaj-s tist"::dt:\rp ; r^^^^^^^ .^" r 
 
 several sharp points. terramate in 
 
 when we coosider that a n,rf „f ., ^7 """= •"""='■.>• ■? evident 
 the ah-, for in«t ,ee ta T T . ""•°"" '^ "'«>'' ""•"■'Sl> 
 the p,aL and'rr " 'l^ZZT:' t"!"l "? ''""°^" 
 yield a strong cnrrent is, ordTnarUy ofl 1 1 T '' ■'"" """"'" 
 tlie amount of „ork th^t T , ^; "■"'""' "'»«m"cli as 
 
 "ona, to thes,r„rtLrn;ro: Ltr„^^'-rf--- 
 rs:irt::;:rrra^""^^ 
 
 tio. a certain eiairelXtiXrrnr"^"^ " '"™''- 
 
 .-ThiaTirr^^^^^^^^ 
 
 tainedthat the duration of the snark Tin . T "*""- 
 
 the .niiiionth part of a seeon^, and h'voeLoT; ''^? T 
 discharge from a Lovrlon ^n. f- , »«iocicy ot the electric 
 
 is 280,0°00 mires i,!^!''™?:! "^ " ''"'^' """" <=°PP- -■■« 
 
 The phenomena of elecMcihi /« r, .* *• , 
 
 magnetic, pkysiolo,ic„i, cl^e^ica't^ZdJl^t '""'•'"'-. 
 prodwid by electricit,, ,•« ,h. \, mtsclumml effects can be 
 
 former Je TleeZ Z ! .'""'"'"" '""^ '""i'- '" «« 
 «r?«,Stt, Jy/'* '"' '"'■^'^''- •» '*« '««»'■ .■< travels 
 
 fit' 
 
 '^.'1 
 
 till I 
 
282 
 
 ELECTRICITY AND MAGNETISM. 
 
 M 
 
 Much 18 known of electricity, its nature, its laws, and its 
 capacity for worii ; miicli remains to be learned. The question. 
 What is electricity? is so far unanswered. But we may r^asoil 
 as follows concerning it, and the conclusion answers all practi- 
 cal purposes. For example, the energy of the chemical combi- 
 nation of coal and oxygen in the furnace is transformed mto 
 heat, heat works an engine, the engine rotates a coil of wire in a 
 magnetic field, the motion of this coil in the vicinity of a magnet 
 induces currents of electricity in the wire, these currents pro- 
 duce an arc, and thereby heat and light. So the energy of the 
 coal is transformed into heat and light, through the intermediate 
 agency of electricity. Hence, it is certain that this intermediate 
 agency, this so-called electricity, whatever it is, may receive and 
 impart energy. 
 
 § 231. Transformation of energy. - We have found that 
 every contnvance for the development of electric enevgy is simply 
 a machine for the transformation of some other form of encrnn 
 into electric energy. In the voltaic battery the chemical poten- 
 tial energy of the combustibles is transformed into the kinetic 
 energy of the electric current. With the magneto and frictional 
 machines, mechanical energy is transformed into electric enercry 
 In the thermopile, heat is changed directly into electric energy 
 By means of an induction coil, the fnll of a large quantity of 
 electrui'y through a small difference of potential raises a 
 small quantity through a great difference of potential The 
 kinetic or current form of electricity may. under suitable 
 conditions, be converted into the potential or static state, and 
 vice versa. Not only are these various forms of energy trans- 
 formable into electric energy, but electric energy may be'^changed 
 into any one of these. Thus electric energy may be transformed 
 uito heat, magnetism, light, chemical action, and mechanical 
 motion. These forms of energy are all interchangeable : as in 
 fact, all known forms of energy are mutually convertible. 
 
TRANSFORMATION OF ENERGY. 
 
 283 
 
 Lws, and its 
 lie question, 
 may reason 
 's all practi- 
 nical conibi- 
 forined into 
 of wire in a 
 of a magnet 
 irrents pro- 
 ergy of the 
 ntcrmediate 
 iitermediate 
 receive and 
 
 found that 
 VJ »s simjily 
 I of encn/y 
 lical poton- 
 the kinetio 
 d friction al 
 ;ric energy. 
 rie energy, 
 [uantity of 
 1 raises a 
 -ial. The 
 r suitable 
 
 state, and 
 ;rgy trans- 
 )e changed 
 ansformed 
 nechanical 
 ble ; as in 
 ble. 
 
 The woik done by a pound of matter in falling through a 
 distance of one foot at the surface of the earth is called a foot- 
 l)ouu(I (§ 89). It has been determined that the work done by 
 one coulomb (§181) of electricity in falling through a differ- 
 ence of electrical level or potential of oi.e volt (§182) is 7373 
 of a foot-pound. Hence, the work done in any part of a circuit 
 in a second may be calculated when we know the stren-^th of 
 the current in arapi^res and the full in potential, in that p'Lrt of 
 the circuit, in volts. 
 
 - Example 1. Find the work ilone in one second in an arc lamp (8 23o) 
 he stn-ngth of the current i.olnj,. 10 ampere, and the resistancx> of tl e 
 lamp 5 oliias. 
 
 To n.ahitain a current of lo amperes throujrh a resistance of 5 oinns 
 will re(iuu-c a fall of potential of 50 volts (§ isi) 
 
 Hence, hi this case, we have a fall of lo" coulombs through 50 volts 
 each second. Therefore, the work done per seconil is 
 10 X 50 X .7,373 foot-pounds 
 368.C3 " 
 Therefore, the work done per minute is 
 
 3(J8.(i5 X CO = 22, Hi) foot-pounds. 
 
 Now, one horse-power is required to do 33,000 foot-pounds of work 
 per nnnute (§<)0). Therefore, it requires about two-thirds of one 
 horse-power to do the work done in this lamp. 
 
 Example 2. What horse-power is reciuired to maintain a current of 
 5 amperes through a resistance of 100 ohms? 
 
 To maintain a current of 5 amperes through a resistance of 100 
 ohms will re(iun-e a faU of potential of 500 volts (§ 181) 
 
 mZlts "' *'"' ''''''' '''^ ^'^^^ "" ^^'^ ""^ ^ coulombs per second through 
 Therefore, the work done per second is, 
 
 5 X 500 X .7373 foot-pounds 
 1,843.25 " " 
 Therefore, the work done per minute is, 
 
 1,843.25 X GO foot-pounds 
 
 Therefore, it requir 
 
 110.595 " 
 
 'es 
 
 110,595 
 
 TiT;;; = 3.35 hor.se- 
 
 do,m 
 
 power. 
 
1^1 
 
 284 
 
 ELBCTRICITV AND MAGNETISM. 
 EXERCISES. 
 
 2. The m...,ll,. ..r V^' ' *" ^''^' ""'" '"'-'^^u't- 
 
 I 
 
 
 It! 
 
 XXXVr. tlSEPITL APPLICATIONS OF ELECTUICITY. " 
 § 232. Medical and surgical oDfirflfir.r,o f^ 
 .» iuductiou coil have ,ir.Jl^^lT\^°^-~''YT'' '""^ 
 
 iunum b«Ij. and ,„.„<1,k. vi„l„ ,t 1 ^ t f J T"^ °' ,«- 
 rents induced Ijy a sin,>lc volt-uc ,,. n ',"''"""""«'• tur. 
 an indncticn co^l, .na/p::,^':^ i' jr:! v Lr't" 1 
 ta.o current l,us a sinnlar effect al ti.e iLt-." , ,f > • ' 
 brcalcing tl.e oireui' l,vl,vJ^■ l,„f i , ""'""§ •■""' 
 
 impunity. (Explain ^ Tho nl • ; , ^^ ^ P"'"^'^ ^'^^ 
 
 -». ...u, ;;.x,rs.;ri:* ;';•:■*;:• -■ 
 
 uol current produces .i 1,enui„bin.r infl„e„ • *'''" 
 
 pain. A to-and-fm n,„tion „ e e , "ut' i; 'T '"^"'-""lity t„ 
 agitation of the part tln„u,-l, „ ic , "s 7 " ■""''^"'"'' 
 
 stimulating cffccti of whicir ar^ "i .ij^ ^ ,'°"" ""' 
 
 n"lr-..i^nnr^"'n-^'""-^^^^^^^^ 
 

 ELECTRIC IJQHT. 
 
 285 
 
 »les, filled with a 
 
 galvanic current, 
 
 10 battery, stale 
 
 suit. 
 
 a thermopile j j 
 
 rned. 
 
 u electric llyht. 
 
 THICITY. 
 
 Currents from 
 il t'loetricity, 
 issues of the 
 :jtions. Cur- 
 mediation of 
 3US. A vol- 
 making and 
 i a mild ctw- 
 th, a current 
 I^erson with 
 duced by an 
 tlian at the 
 ir,h one pol( 
 
 The gra(' 
 fusibility to 
 a muscular 
 
 tonic and 
 f muscular 
 crful elee- 
 '• has been 
 
 sed like a 
 advantage 
 
 over the latter in that it sears the extremities of the blood vea- 
 sols and t o..by prevents hen.orrhage. Enough has b". Id 
 to show that a medical practitioner who canTtpply the laws of 
 e..tnc,ty has at his com.naud a powerful therlp'utic ag't 
 b.. except m expc-rienced hands it is likely to prove useless W 
 not positively dangerous. ^ ^' " 
 
 S 233 Electric light. -If the tei-minal.s of wires from a 
 ":::;; ^^^';";r-^-^-' ".aclnneor gmvanic batterv are 1 Tght 
 ogether and then separ.-.ted 1 or 2 n.illimoters, the cu, rent loJs 
 
 not cease to flow, but volatilizes a porti.m of the tern i, Us 
 he vapor formed becomes a conductor of high resist nc^fd 
 
 ^annng a a vei^ high temperature prodt^es intens^^; , 
 
 pt nty. Ihe heat is so great that it fuses the most refractorA 
 substa-ices, mclu.ling even the diamond. Metal terminals quiokh 
 
 that the electro-motive force is no longer sufficient for the in- 
 creased^resistance, and the lightis extinguished. Hence pencils of 
 carbon (prepared from i^c'i«^ns or 
 
 Fig. 195. 
 ^ B 
 
 coke deposited in the 
 distillation of coal in- 
 side of gas i-etorts), 
 which are less fusible, 
 are used for 1 terminals. 
 For simple experi- 
 
 Thm rods slide ,n brass heads A and B, ,„pportod h, insu- 
 IntlCaW. " "" """""^ """""' '"' -^'-"'"'"'^ 
 
 § m Voltaic aro.-Tho light is too intense to admit of 
 examination with the naked eve ; but if an image of the teli 
 
 
286 
 
 ELECTEICiTY AKD MAGNETISM. 
 
 Fig. 190. 
 
 aals is thrown oo a screen bv means nf , l„„. • . 
 
 a car,), an arch-shaned hIm ' °'' " '""■'=°'° '" 
 
 •o polo, as show, Tf f ,0C "r,"- "V",""? '""^ "* 
 "- -,„e ot the .„,J:;, \Jl':. "'"" '-»» ™--od 
 h-,rl,f I, '"'="'' l""''"" "f llle 
 
 light, however, emanates fron, the tips of the two 
 carbon terminals, whieh are heated to an ,te„™ 
 wmeness, h„t some from the are. 'ne ;por 
 ■otter than the -pole, as is shown by itstCin,^ 
 
 the +p„le beeoraes volatilized, and the ligh,-<,ivin,. 
 part,eles are transported from the +po°,,. To "h^ 
 
 t.,epoies. wl::;^:re''^n;t2;l;"'::r™'''>■■•^--" 
 - w. apa,^ i,.^ =r::^:;~:rr;;::;o: 
 
 § 235. Bleotrio lamp. — The -4-nnl„ „„ . 
 twiee as fast as the -note At +''"'" .''"""''^ ""•■'J »'>out 
 conicaUhaped cavity is ?„ Ld thi a,'"' , »'' "" ""'"""" 
 
 latter warty protnberanees ,^ 'a v ^'in eo" """" "' "'° 
 the wearino- awnv r.f fi . ' '" consequence of 
 
 penciiX-omTL ;:: f^" "tl.e let '""" '"''''"' "^ '«o 
 
 li^ht goes ont. Nnmioitifi i' ;: irz'f:: ,:r; ■"° 
 
 ing a nniform distance between the noles , , """Ham- 
 Such an arrangement is called L, I "? r"" "","■ 
 carbons are ,„oved bv clock w,„l TT-^ ■ "°""'' "'" 
 
 -asionally, in otlJs ;!:':. ;,r,T,"r r"'""= "" 
 complished autonratieally by the a^r If '';,;:;,'.:;:. ;:,-; 
 
 vialf ali n!t::;rna;:rJ' ^:/-r -'""'''' "'"- 
 
 s.ae, aand b (F,g. 19,), separated by « thin insulating septum! 
 
BLECTIIOTYPING. 
 
 287 
 
 I" a pin-hole in 
 ing from pole 
 
 has received 
 M-tion of the 
 ips of the two 
 
 to an intense 
 The -|-poIe is 
 y its glowing 
 I'lie carbon of 
 le light -giving 
 +pole to tlie 
 k'apor between 
 iinoiiti matter. 
 cr, as may l)e 
 ■tallic poles, a 
 k can be pro- 
 
 ■ awaj about 
 the former a 
 point of the 
 asequence of 
 ^een the two 
 to span, the 
 or mainlaln- 
 Jen devised, 
 n some, the 
 winding np 
 bons is ac- 
 irrent itself. 
 
 -Hudle " ob- 
 
 instead of 
 
 ced side by 
 
 ng septum, 
 
 c, of kaolin. The current passes up one carbon, across the 
 space between the points, and down the other. In its passage 
 between the points it forms the luminous arc. The 
 heat of the arc fuses and volatilizes the kaolin, and 
 it wastes slowly away like the wick of a candle ; 
 hence its name. 
 
 The electric light is of the purest white. In it the 
 most delicate colors retain their noonday purity of 
 tint, while a gas light appears of a sickly yellow hue 
 in comparison. 
 
 § 237. Blectrot3rping. — This book is printed 
 from electrotype plates. A moulding-case of brass, in the shape 
 of a shallow pan, is filled to the depth of about one centimeter 
 with melted wax. A few pages are set up in common type, and 
 an impression or mould is made by pressing these into the wax. 
 The type are then distributed, and again used to set up other 
 pages. Powdered plumbago is applied by brushes to the sur- 
 face of the wax mould to render it a conductor. The mould is 
 then flowed with alcohol to prevent adhesion of air-bubbles, and 
 afterwards with a solution of copper sulphate, and dusted with 
 iron filings, which form by chemical action a thin film of copper 
 on the plumbago surface. The'ease is then suspended in a bath 
 of copper sulphate dissolved in dilute sulphuric acid. The —pole 
 (why the —pole?) of a gnl van ic battery or dynamo-electric ma- 
 chine is applied to it ; and from the -f pole is suspended in the 
 bath a copper plate (why?) opposite and near to the wax face. 
 The salt of copper is decomposed by the electric current, and 
 the copper is deposited on the surface of the mould. The sul- 
 phuric acid appears at the -fpole, and, combining with the 
 copper of this pole, forms new molecules of copper sulphate. 
 When the copper film has acquired about the thickness of an 
 ordinary visiting card, it is removed from the mould. This shell 
 shows distinctly every line of the types or engraving. It is 
 then backed with melted type-metal to give firmness to the 
 
 HI 
 
 m 
 
288 
 
 ELECTRICITY AKD MAGNETISM. 
 
 -■"io, with the iatte., It^^Z^t Z ""'"" '' " "'"°^"=''' 
 ces»os arc, in tl,o main, the JZ r ''"'T""'"'- Tl>« Vo- 
 
 Fig. 198. 
 
 1,11 
 
 m . 
 
 I 
 
 battery, and then a Dhtp ,.f(i,„ 
 
 leposited on the giv™ arti t, i""' '"" "'"'''■•" »"' - 'o be 
 
 to bo deposited. The ^^1^ 'T °^ " ^»" »' «h» metal 
 
 -a for gilding and .ui ': m1 rri,'";" "^ ^'=-™"^ 
 
 qUTe to be eleetro-copnered fir f i,! i "= ''""' """"b re- 
 
 of .1.0 gold or silver. Tre"™', "f"^ '" 'o^-o *« adhesion 
 
 completely replaeed the vol a "Ta;'; '"r" """"'"" ""» """o^t 
 
 •electio-plating purposes ""'■' '°'' ^''=<=""typing and 
 
THE TELEGRAPH. 289 
 
 § 239. Electric motors le ^ , - , 
 
 passed through the ZuofTa! T"'-"^ '^'^^""'^ '' 
 
 so,,,e exter,...,l so,„ce, the armature of the m.aeh"o toIZ 
 
 cngme. A djnamo th„s used is called an electric motor 
 
 s earn or water power, and the two dynamos may be a lon^ 
 distance apart, so long as the cirouit ionnecting them is w ll 
 .nsnlate,! uud has not too high resistance. Thus a p'wev for 
 example a waterfall, may be used to perform work at a d stae 
 of many m.les, while, by placing several small dynamotsed 
 arm *""" "' '"' large dynamo use'd aTage ' 
 m y be de Ter";: '"'' '"' -«^'"''"'»<' ^ a-^ e.tent^hat 
 
 ™Lm:r:ppi,rith~-ra;ir- - — 
 
 "dually it Is applied only to electrical methods * ' "' 
 
 First, ,t should be nnderstood that, instead of two lines of 
 w.re, one to convey the electric current far awav froL »T, ! 
 tery, and another to return it to the batterv Tth T, ! 
 is connected with a large metalli'o' 1^ ^ h n'm^ ^ In ° 
 andf! H '"' 1'" ' ^" "' "'^^ f'^ *»' l^ads tT cart' 
 
 r^.t^h,r;,-tr-~r:;-n 
 XX,i:'ro;t^;;ri-^^^ 
 
 an- of the ground connections, there is a Tav g^f LZS 
 
 I 
 
290 
 
 ELECTKICITY AKJJ MAGNKTISM. 
 
 is broken at B Jot thn ^.. , ^^' " ""^^ '^e seen that the circuit 
 
 key. He cToL i^^^Z^^TZ^' '°'"'' "" "'^ '^^^^^ ^' "'« 
 wire from Boston to Ne v Yor^ It t I '""'"* *"^*"""^ «"« t'"' 
 lever 6, and presses tlfe point 'f a ty^'^n fs?- %'""^ '"^'" *'^ 
 drawn over a rolI.T The nZvlL '*"P ''^ P'^P^''" « ^^^t is 
 
 Circuit is broken, and !^ZZ r^sTd fZ'Z IT t '■^^' "'^ 
 spring d. Let the operator press udo tt i ^ i^ ^^'' ''^ * 'P^"''^ 
 
 long enough to count nn« „ ,^, *'"' ''*'^' **"^y for an instant, or 
 
 tl.eWer''B:tTrrepres;e;Z^^^^ indentation wiU be made' in 
 the point of the style vXeinain in 7 f "^' "'^""^^^ *^ ^^"'^^ tb''^^. 
 length of time- Ind aTtL n^n ^,«"*'^«t ^'^h the paper the same 
 
 short straig,™ii'nes producer t/- 'TY'''"'' ''^^^"^ '^^ ^^^^' - 
 dots and dLhes consti u?o tho" ; . ; '"'"' ^"'' '' ^""^^ ^ ^««''- These 
 part of a me age * ^an is if" t?"" '^'T^'^" ^"^ "^^^--> * 
 graphic character's 'on Te strip ^f paper ^Th 1 " '''',"'^' '' ^^'«- 
 interpret their meaning. ^ ^ ^''™*" ^'''"^^^ above 
 
 Of i,.t„„„eoL inX^c^ r 'i:„r.r'i,r "" "-'-'"■'■ 
 
 that would move a sinc-ln «^ ^"^'^-^-^^^l- Hence, a current 
 Wo u> reuder the message aadlWe. Resort I, V mw 
 
sender, or oper- 
 
 that the circuit 
 tlie knob of tlio 
 iistaiitly fills tho 
 Iraws doAvn the 
 
 paper c that is 
 on the key, the 
 iper by a spiral 
 r au instant, or 
 vill be made in 
 
 to count three, 
 paper the same 
 ith tho pomt, a 
 
 a dash. These 
 For instance, a 
 riuted in tele- 
 I letters above 
 
 emoved, and 
 sharp click is 
 piral spring, 
 the figure), 
 the first, is 
 a dashes by 
 tween these 
 iug heed to 
 tie hammer, 
 rm of which 
 
 the current 
 the number 
 , a current 
 short line, 
 h sufficieat 
 1 to relaps 
 ■ this diffi. 
 
Plate 111. 
 
 y 
 
 :l 
 
 J(| 
 
Relay and repeater. 291 
 
 culty may, perhaps, be l.est explained by analogy. In days gone" 
 bj, posts for couriers were stationed a day's journey apart. 
 At each post were a courier and a horse at all hours ready to 
 start Ihe courier, bearing a dispatch, rode all day, and at 
 .ght reached a post where fresh houses were saddled ready for 
 l.e next stage of the journey ; he himself was exhausted^ hia 
 force was nearly spent, but he could awaken a courier who 
 was stationed there and deliver the despatches to him, and he 
 with fresh strength instantly took ui) the journey. In a similar 
 manner we picture to om-selves the electric current arriving at 
 a station so nearly exhausted that it cannot deliver intelligible 
 signals, yet it may still have strength to wake up another bat- 
 tery and set in motion a fresh current which shall receive and 
 announce audibly, or carry forward the message which the 
 exhausted current has just strength to whisper. 
 
 In Figure 2, Plate III., the letter R represents a relay and S a sounder 
 Suppose a weak current arrives at New York from Boston aTd ha" 
 
 This as may be seen by examination of the diagram, will close another 
 short circuit, called the local circuit, and send a cun-ent from a local 
 
 Ihe sounder bemg operated by a battery in a circuit of only a few fee 
 
 aiT'iH T *' ""''"^' ""'"''^- " '' '' ^^«'^^d that the mes! 
 
 age should go beyond New York, -for instance, to Philadelphia,! 
 then we have only to suppose the local line at New York to be length- 
 ened so as to extend to Philn,' Iphia, and a powerful line battery to be 
 substi uted for the small local , then the message that leaves Boston wHl 
 
 ^ilf . , l"^"" ""' '"■'"^* *^ "'" '''^'' ^* New York, and be delivered 
 ni Philadelphia without the intervention of any oper'ator on the route 
 In this case a relay is called a repeater. The electro-magnets in relays 
 
 Z^Z^ T '""?■ '^"' ""■" "'^^' "^"^ '''^'^ '' --'-« -e wound 
 vih short, large wire. (Explain. The main battery consists of many 
 <ells ; how should they be connected? It may be located at either ter- 
 minus, but It IS generally split in halves, and one half placed at each 
 terminus ; how should the two halves be connected ?) 
 wlVt^ v''^?'"' *^' '^''"^^ '' represented as open at both keys. 
 
 ZZln^nf" .T '" ""• *'' '''''''' «"^^' *^"*>'« t« be left Closed 
 by means of switches connected with the keys (not represented In the 
 
 , ^ i-f 1 
 
 I ii 
 
 
1^92 
 
 
 ELECTftlClTY AND MAGNKTlHM. 
 
 (liagram), so that, when the line Is not "Rf «,« ., .. 
 -s constantly traversing the wire trJlT" "" '^'''''' «""««< 
 consists in interrupting this current » y m a„ o'f 77'' T"^^'!"^"".^ 
 Boston wishes to communicate with NewYnrU uT ^"^^^'^ *hat 
 switch on his Icey, which brPRk^Vh! ,'^,^'''^- "e first removes the 
 the circuit With hi; k^; H^th n'mal^^^^^^^^ f'^'^'' '"™ *« -"«^«' 
 an understood signal, which^^llatX^vv'''': "^"^ '"^ "' *« P'-^'^^ce 
 time that Boston presses on h^.\ '' ^'"^'" «"«ntion. Every 
 
 and in the New York office an 1 «^ ''"''^ """"'"' ^ '" ''" ^^'^ °^^« 
 
 n-age .ay he r.^T:e^:::,:::^z:-^^- '' -- *^^ 
 
 A 
 
 
 sr. 
 
 IP 
 
 G 
 M 
 
 8 
 
 I. O "p ' 
 
 S 
 
 w 
 
 L 
 R 
 X 
 
 T 
 
 z ' &~ 
 
 - — _ 
 
 
 " • 
 
 • - • _ > 
 
 ? 
 
 
 
 -^ . 
 
 
 • 
 
 
 TELEGRAPHIC PIGURRg, 
 
 
 
 — - ■ 
 
 ..!.. ._ 3 4 
 
 6 
 
 6 
 
 
 ---- I'-l.." ""'^""^ 
 
 
 
 
 '!! 
 
 "itted over a wire and appear atTd^rj',''"' "r"™"^' '™- 
 hand-writing of the sender a„7ll^ *"'"""'•' '" "■« "^"e' 
 prineiple o/whieh i operateTll, b^ f ""T' ""'""^- ^he 
 in Plate I., in whieh alfdTwL „nto „ ,""" "''"" ""^ *'«^''"' 
 ^implieity of mustratbn T^a it t ^ 'r''7-r" '"""«^'' *■- 
 message to be seat is written ^itt , '"'"''' °" ">'''''' ""o 
 
 -ali„g.wax in aleobo, The loho "uicr""™' ' •'• """"""« 
 the lines of sealing-wax adhetg t't I f^rT"'"' '^'"^ 
 paper moistened with a solution of prnssia^ „, J I' I """" "' 
 ^f^e pens is Simply asmail, .K-inted^rirdr T':,^' 
 J^at b„„ o. the pens are mo.ed at the »«.„«„„„,;' Z'Z 
 
THE ELECTRIC FIUE-ALAltM. 
 
 093 
 
 needle in New York to tr Jo ' . ''°'""'' "'" """^e the 
 
 «.e «eed,e i„ Boston tj^^ t ^ 'T" ,"'"" '"^ ™ ^'. ""«' 
 tl.e c-ircuit is broken JTZ ^eol'i'S-WK on X, when 
 
 '^e there is a bre^ " UTr "'^J '"" ''■""• ^' "'" -">= 
 If, farther, each n^ l" t 7 ^"^^ "' ""^ ""° "™«' »" Y- 
 'tae it traVe,.esttsl«v:ler/ '"'' '""'"■ '"■'" 
 exaet fae-si™i,e of the^ ," ^o 't^","" T ""^ "" 
 chemically-prepared paper e^ee^ ,7, / V<>^"<xA on the 
 written in darkSetters'o'a , W 1 11' T""' ">» ""S"-' - 
 in light letters on a darkgro ,„d Pe "f .T'""" '" '■"'<^<^'™' 
 tographs and other pieffrTs ^ v br,""" * ''""""''^ "' Pl^"- 
 -vay. The pens are not, ofcoZ t, ,"7"'"', '" ""* '™« 
 "ands, bnt by complex Ja I" ;'''x""''.S"''l^<> "^ ""man 
 ■equisite in the movements otZi ?'"'°'" "'^"■■^tnes^ 
 
 absolute synchronism i ■ the vib atil™ S'T ""7"' "^ '"^ 
 "t each (.rrninns. controlled bl. t'eZt,;' ^rLr'"'""'' """ 
 
 Pig. 109. 
 
 § 244. The electric flr'* •?! , n^u- . 
 
 of the electro-magnetic teleirarh - /n "" "^^^^^^^^on 
 
 -^.^te the ,eneL plan ^^.:.^^-^ ^ 
 
 
 'If 
 
 m 
 
 
294 
 
 ELECTRICITY AND MAGNETISM. 
 
 Prof. M. G. Farmer, and by him first introduced into Boston fn 
 the year 1852. 
 
 From some central station wires radiate to every part of tlio 
 city. At suitable intervals there are inserted in these circuits 
 small cottage-shaped boxes, usually attached to buildings at the 
 corners of streets. On opening one of these boxes, a person 
 who is to give an alarm finds a crank A, which he is directed to 
 " pull down once and let go." This winds the spring il, which 
 sets in motion a train of wheels, and causes a makt-aiid-break 
 wheel C to revolve. This wheel bears upon its circumference 
 notches corresponding to the number of the box. Two terminals 
 of the line are so connected, one with C and the other with a 
 lever 6, that when the lever touches the wheel the circuit is 
 closed. But when the wheel revolves, and a notch passes under 
 the lever, the circuit is broken. The eflfect of breaking the 
 circuit is to demagnetize the eleetro-m.>2 ,et F at the central 
 station, and release the armature which is attached to the 
 tongue of a bell. The tongue then being drawn forcibly by 
 the spring G in the opposite direction, produces one stroke on 
 the bell. By pulling the lever down once, the spring is wound 
 up just pnough to cause C to revolve three times, and thus the 
 number of the box is struck three times in succession. The 
 watchman at the central station, being thus notified of the 
 existence and locality of the fire, at once an . in a simUar man- 
 ner notifies the several fire-engine companies. 
 
 XXXVII. TELEPHONE AND MICROPHONE. 
 
 § 245. Bell telephone. — Figure 200 represents a sectional and 
 a perspective view of this instrument. It consists of a steel ma-net 
 A, encircled at one extremity l,y a spool B of very flue insulated wire 
 tiie ends of whicii arc connected with tiie binding screws DD. Im- 
 mediately in front of the magnet is a thin circular iron disk EE. The 
 whole is enclosed in a wooden or rubber case F. The conical-shaped 
 cavity G serves the purpose of either a mouth-piece or an ear-trnmpet 
 There is no difference between the transmitting and receivin.^ tele- 
 phone; consequently either instniment may be employed as a'trans- 
 mltter, while the other serves as a receiver. Two magneto tele-honea 
 
o Boston Jn 
 
 part of tlie 
 lese circuits 
 lings at tho 
 s, a person 
 
 directed to 
 g »1, which 
 >aii'l-break 
 cumference 
 o terminals 
 ther with a 
 3 circuit is 
 isses under 
 caking tlie 
 tho central 
 led to the 
 forcibly by 
 
 stroke on 
 g is wound 
 d thus the 
 jion. The 
 ed of the 
 [uilar man- 
 
 BELL TKLEPHONE. 
 
 295 
 
 ctional and 
 eel magnet 
 ilated wire, 
 1 DD. Ini- 
 EE. Tlie 
 ical-shaped 
 ir-tnimpet. 
 iving tele- 
 is a trans- 
 telephones 
 
 machino, of course no hattorv , ''''"'' ''''^^"*'*'^ a diminutive magneto 
 
 a tuning fork or the head of a drum vibrations ,.f 
 
 BelUelelTno ' \fT"''''"" "' ''' "'^'"^^ ^"^ «'™P>-t f-'" o^ the 
 
 ^^tiied^^uirs^i:^,::;:,^^- --■ -^ -at of 
 
 formations and especially during ^^tuSio IT tr'S 
 electric energy through largo rettetaaces. become very n^n? en' 
 
m '•■ I 
 
 i t * 
 
 296 
 
 ELECTRICITY Ax\D .MAGNETISM. 
 
 consists' in inL:, !;;\.,V:.v it^t^ ';"i'rovcn.ont on the ori,Mnal 
 the voice instead of l,ei 1 o >Ii ? ' " ""^ '" "'"''' ^'""'^ '''■'^^ 
 
 Fig. 200 a. 
 
 Fig. 200 6. 
 
 ii 
 
 disk of the receiver The flu. t^r "'"'''"'"^ "'''■'^"«"« ^" t^t 
 
 ance in the eircuU The p^i ^ 71"' ?"""?." ^ ^"^^'"^ ^^«'«^ 
 this that the effect of a loo I Ztac 50/^''" '^ '"P^"'^^"^^ ^^« 
 circuit is to increase thorlT. "''''''' ^"^ *^^« P^^t^ of a 
 
 buttheeffec of a i" vart r^^ '"' *'"''^ ^^^'^'^^ "^ <=--"t; 
 when either or bot of the pans are" ' h""'V' '^''^^'^"^ "^*'^^^b^<^' 
 
 -Pie telephonic circ t n'" Lh Vr t^^^^^ '''n "'""'"'^'^ ^ 
 
 transmitter T a mnirnpfn Jo , T '" , "''*^'^ '^ variable resistance 
 
 e.oc.„Ues, a pU.IS'tVtr.f; r c^^oT L ?"; °' ^ 
 disk; the other electrode, a carbon button a TJ *^,\*'*°«™'"er 
 
MICROPHONE. 
 
 F 
 
 ■fl 
 
 297 
 
 R puJls Its disk Th 
 
 The next iinpi-oveim.nt of . f<;'"^o''"ce sounds. 
 
 ™" »,:'«■ ""<! 'I.« Bell LtmmJt ", ] '°™ '""' " ""» '"M- 
 
 Pig. 201. 
 
 § 246. Microphon« — r.. i,- 
 carbon; tire form^^S^^^XaZr^'' t '"' ^ "^ ''»"-« of 
 the latter to a steel sprfn.. r old I ' ^ "''"''"^^'•^' ^^ t^in nine wood 
 
 ^^^r'^'""'^ any slight jar will cause aTo^^^^ '^'■^"» P'-^^^^^s B 
 
 -===:=S5;:::::; 
 
 I 
 
 '1 
 
298 
 
 ELECTRICITY AND MAGNETISM. 
 
 ^.1' 
 
 as its name indicates, sucli as the ticking of a watcli or tlie footfall of 
 an insect, may be reproduced at a considerable distance, and be as 
 audible as though the original sounds were made close to the ear. 
 
 n |l 
 
 it" )« 
 
 1 
 
 t:!]!! 
 
 § 2466. Storage batteries, or accumulators. — Place 
 two strips of sheet-lead, about six by eight centimeters, at a distance 
 of six centimeters from eacli other, in a vessel containing dilute 
 sulphuric acid — one part of acid to ten of water. Pass the current 
 from one or two cells through the liquid from one plate to the other, 
 placing your short-coil galvanometer in the circuit. You Avill observe 
 that the current grows weaker and weaker, and finally ceases. When 
 the galvanometer shows no current, disconnect the battery, and connect 
 the lead plates with the galvanometer. A current will be observed be- 
 tween the lead plates in a direction opposite to that ,enerated by the 
 battery. This is a polarization current due to the generation of oxygen 
 on one of the lead plates and hydrogen on the other, by which a diiler- 
 ence of electrical potential is produced. 
 
 The lead cell described above is called a Plaute cell, from the name of 
 the inventor. The Faure cell consists of lead plates covered with a 
 layer of red oxide of lead, which increases the polarization. It is 
 evident that electricity is not stored in these cells. A difference of 
 chemical constitution is produced, which in turn produces a difference 
 of electrical potential. What form of energy is stored? 
 
 These cells, though greatly improved by many inventors, have not, 
 so far, proved as satisfactory as was at tirst hoped. Suggest some 
 practical applications that might be made of these cells, if so con- 
 structed as to prove efflcient and cheap. 
 
 il « 
 
'ootfall of 
 and be as 
 the car. 
 
 3. — Place 
 a distance 
 inj; dilute 
 he current 
 the other, 
 ill observe 
 es. When 
 nd connect 
 )served be- 
 ted by the 
 of oxygen 
 ch a ditt'er- 
 
 lie name of 
 red with a 
 tion. It is 
 ift'erence of 
 I difference 
 
 , have not, 
 
 jgest some 
 
 if so con- 
 
 CHAPTER V. 
 
 SOUND. 
 
 The subjects of Sound and Light, which we have now to 
 study, have two important characteristics in common that dis- 
 tinguish them from tlie subjects already studied. First, eacli of 
 them affects its peculiar organ of sense, the ear or eye, and 
 \ory many of the phenomena to be studied under each subject 
 are of importance only to one or the other of these senses ; 
 while the most common, and many of the most important appli^ 
 cations of heat and electricity have no direct relation to any 
 organ of sense. Second, both sound and light, we shall find 
 originate in vibrating bodies, and reach us only by the inter 
 vention of some medium capable of being set in vibration. 
 
 Here, as in all other kinds of motion, energy is involved ; the 
 ear or eye absorbs energy whenever a sensation is produced, but 
 the amount absorbed is so minute, and the difficulty of measuring 
 it so great, that usually other points better deserve the student's 
 attention. 
 
 Let us begin with the study of such vibrations as will neither 
 produce sound, nor in a dark room affect the eye. 
 
 Jl 
 
 XXXVIII. VIBRATION AND WAVES. 
 
 §247. Vibration. — Experiment 1. Repeat the experiment with 
 the pendulum 1'" long, page 111, and note in what respects its motion 
 differs from most other motions. 
 
 Experiment 2. Take a pendulum SO"" long, hold it with the string 
 just touching an edge of a table, having the hand about 38™ above the 
 table, and set it vibrating; the ball will be seen to vibrate faster in the 
 portion of its arc that Is under the table than In the other portion, and 
 so mci-e vibrations are made In ten seconds than if the string swing 
 freely without touching the table. 
 
 .ii 
 
300 
 
 SOUND. 
 
 side to side every two seconds, turning instantly at on.- side, and wait- 
 ing at the other till the two seconds are up. ^na^wait 
 
 These three motions, though very different, have this in com- 
 mon : the motions in each case occnr at equal intervals of time 
 This interval of time is called the period of vibration. In Exp 1 
 It was two seconds ; in Exp. 2, about one second ; and in Exp 3 
 two seconds. The motion from one side to the other and back 
 IS called a vibration. If n = the number of vibrations in one 
 second, and t = the period, t =L The amplitude of pendulum 
 vibrations is assumed to be very small. In Exp. 1 the motion is 
 called a .tm;>/e (or pendular) vibration; in the other cases the 
 v^brat^on of the ball or the hand is cample.. Do not confound 
 period with duration of the vibrating state ; in Exp. 1 the 
 pendulum may have vibrated ten or one hundred seconds, 
 before coming to rest, but the period was two seconds. Con- 
 sidered mathematically, other periodic (and therefore vibratory) 
 motions are, the movements of the hands of a watch, the regu- 
 lar trips of a stage-coach, ete. A vibration is a recurrent channe 
 of position. '' 
 
 § 2«. Direction of vibration. - A small rod, like a yard- 
 stick, fixed at one end, may be set in vibration by pullincr the 
 o her end to one side ; a tree vibrates in the wind ; the stdncrg 
 of a piano swing from side to side when vibrating; in all these 
 cases the motion is at right angles to the length of the body, 
 and so the body is bent. These are all cases of transversi^ 
 vibrations. 
 
 hl't '""'^Z'"'' '"^' ^'^' '"'' ^^^'fe'ht' ''^"*'' ^iropping it, notice 
 that the cord vibrates, lengthening and shortening rapidly. 
 
 The motion of the body is in the direction of its length, and 
 so It IS not bent ; this is a case of longitudinal vibration. Twist 
 the strmg, and see that it is possible to set up torsional vibra. 
 
PROPAGATION OP VIBRATION. 
 
 301 
 
 
 tions. Compare these kinds of vibration with the kinds of 
 elasticity studied on page 30. 
 
 § 249. Propagation of vibration. - Waves — Exn..H 
 meat. Take a soft cotton rope, a few meters lon^Tay ft* stral^lftou 
 a floor, sot one end in vibration by quick mov^^ents of th? and 
 No ice any po.ut in tixe rope, and see tliat it is set in vibration that 
 IS, it moves up and down, or laterally from side to side tl ough -^ 
 ongxnal position of rest. Make a single movement of the hand, which 
 is better called a pulse than a vibration; it is easy to see thai he 
 pulse does not reach all points of the rope at the same t ^e. Send 
 quick ucces^sion of equal pulses along the rope; at any instant differ- 
 ent pulses affect different parts of it, and you get n.ore o^- less perf ctll 
 the famihar form that we call a wave-Une. Notice that any S of 
 the rope only moves up and down, while the for^n of the wave moves 
 on. Vibm te the hand in a longer period, and notice that the distance 
 from crest to crest is longer than before. ui^rance 
 
 §250. Wave-length and amplitude. - Imagine an in- 
 stantaneous pliotograph taken of the rope along which tlie waves 
 Fig. 202. are passing. It would appear 
 
 much like the curved line CD, 
 Figure 202. This curve repre- 
 sents wliat is known as a simple 
 wave-line. The shortest of the 
 similar portions into which a 
 wave-line can be cut is called a wave-length, as wx, uv, or en. 
 The greatest distance of any point in a wave from the axis, as 
 ou, IS called the amplitude of the wave. 
 
 meitTsf^o!?f^*'°'' °f ^aves. -Interference.' -Experi- 
 
 and nlu^k it vt 't ."'^ Horizontally between two elevated p I.ts, 
 and pluck it witli the hand or strike it with a .stiek near one end and 
 send along it a single pulse, forming a crest on the rope \t?l 
 
 laverted (B).^"^^ ' '" "" '^'^^ '^"•' ""'^ "^^^ -« -« " reflected a^a 
 » See aectlon G of the Appendij, 
 
E I 
 
 302 
 
 STATIOXARY VIBKATIONS, ETC. 
 
 r.rf«r?H""? ^•~'^"'" *' '^' '''''^''' "^ ^^fl^^"o" «tart a second 
 crest; these two. the crest and the returning inverted crest or trough 
 
 (C), are now traveling 
 along the rope in oppo' 
 site directions, and must 
 meet at some point. This 
 point will be urged up- 
 ward by the crest and 
 (io\,nvvard hy the trough, 
 and so its motion will be 
 due to Mie difference of 
 tile two forces. 
 
 This action on a single point of two pulses, or two trains of 
 waves, no matter if f.cui differe.it sources, is termed interfer- 
 ence. Tlie resulting mo, ion may be greater or less than that 
 clue to either pulse alone, or it may be zero. 
 
 § 252. Water-waves. If you have a long and rather 
 uarrow box or trough, nearly filled with water, you can pro- 
 duce much the same effects as with the rope. Water-waves 
 furnish important illustrations of the fact that energy may be 
 transmitted by vibration as truly as by the actual transfer of the 
 medium, as in the river's current or the wind. 
 
 § 253. Longitudinal waves. Experiment. Procure a brass 
 wire wound in the form of a spiral spring,' about 4'" long. Attach one 
 end to a c.gar-box, and fasten the box to a table. Hold the other end H 
 of tl.e spiral firmly in one hand, and with the other hand insert a knife- 
 Fig. 208. blade between the 
 
 turns of the wire, 
 and quickly rake 
 it for a short dis- 
 tance along the 
 
 K^^ 4.1 u ,. s^piral toward the 
 
 box thereby crowding closer together for a little distance (B. Fi- 
 
 h M 1 * TT "* "■"'' '" '"•'"' "^ ''•« '•'^•^d- ^"^ '-aving the turns 
 behind pulled wider apart (A) for about an equal distarce. The 
 
AIR AS A MKDIUM f)F WA VIS-MOTION. 303 
 
 cromlcd part of the spiral may be called a rondonmtion and the 
 stretched part a rarefaction. The condensatio,, folio v!.b": the rare 
 facon, runs wUh ^^reat velocity through the nplral, ntrikes the box pro- 
 
 a d ffo^f^'' V ""'' " "'^^^"^ ^^'^ *"« '-'^ ^'-'^ to t rha'd, 
 and from the hand again to the box, producing a second blow and by 
 
 cssion. If a piece of twme be tied to some turn of the wire it will 
 be seen as each wave passes it, to receive a slight Jerking , o^ n 
 forward anrf backward In the direction of the le.^th of the spiml 
 
 How is energy transmitted througli these 4- of spring so as 
 to dehver the blow on the box? Certainly not by a bodily 
 movement of the spiral as a whole, as n.ight be the ease if it 
 were a ngul rod. The movement of the twine shows that the 
 only motion winch the coil undergoes is a vibratory movement 
 or Its t.u-ns. Here, as in the case of water-waves, ener^n" is 
 transmitted throngh a medium by the transmission of vibratbns. 
 
 There are two important distinctions between this kind of 
 wave and a liquid wave : the former consists of a condensation 
 and a rarefaction ; the latter, of an elevation and a depression ; 
 m the ormer the vibration of the parts is in the same line with 
 the path of the wave, and hence these are called longitudinal 
 tvaves; ,n tue latter, across its path, and they are therefore 
 transverse waves. 
 
 A wavQ^cannot be transmitted through an inelastic soft iron 
 spiral. Elasttcitj; is essential in a medium, that it may transmit 
 waves made up of condensations and rarofucflor.s; and the greater 
 the elast^cuy, the greater the facility and rapidity with which a 
 medium transmits waves. 
 
 §254. Air as a medium of wave-motion. - May not air 
 and other gases, which are elastic, serve as m.-dia for waves? 
 
 20r^LT*r,r*:,^''''' " '*"''^° ''"'"" '^^ *''° '''^^'' ^ "f" the tube,- FWnire 
 20G, and strike the table a sharp blow with a book near the orlQco I 
 
 made .U.htly tapCng. .o that tl.ey „«,• b^ pi.t to'^tb , '. o "JpV"^' "''''' '^ 
 
 i : 
 
S04 
 
 SOUND. 
 
 Instantly the candle flame Is quenched Th« h a . 
 serves as a medium for transmhsW^n nf" . ""^^ "^ ''^' *" ^^^ t"he 
 Was it the motion '''"''""■^■^'"'^ of motion to the candle. 
 
 blown throu,hr;;?i .rr:^;/;T ""-'^^'' ''' tu.e,.s.Ue. 
 touch-paper, at the oriflce h, so a to flfl ^'"''7' """°"' «»- 
 smoke, and repeat the last experiment ''^ '' *'^° *"^« ^^^^l^ 
 
 smSitiutlSed:;^:',:^;;:;"^;:'^ ^'t "^^"^'^ «»« *'^^«' «- 
 
 but no smoke issues fron^th o^flce ^ Tt" i f ""V^ ^"* "^^ '''^^^' 
 
 onnce a. It is clear that fchere is no 
 
 Fig. 208. 
 
 »pnug ,s ,0 p,.„,u,ce in a certain section (flt 20 oC 
 M>..al a crowding together of the t,„™ o wi ■ °™, I a 
 xei)avat,ou ; but the elastioitv of the soiral ZJZ' - ^ " 
 
 wa. .no^nent orritirr^^^^^^^ 
 
 "Otall: the folds in the sect o, « > ."""" "^"=- ''''''' '» 
 when they have recoveJt L on 'ir loT '" f'" ^^"' 
 pendnlu., swing heyond the po itl^ f oT .e'^ 'tZ' Z' '"' " 
 .-faction at B, where, immediately befor:!' thT: T, HoV 
 
in the tiibe 
 le. 
 
 be, as when 
 ;ion? Burn 
 ; tube with 
 
 e tube, the 
 t as before, 
 *liere is no 
 
 —nothing 
 by some- 
 
 ledium. 
 e spiral 
 ) of the 
 at A a 
 ies B to 
 3ther of 
 3 a for- 
 e same 
 Jtion at 
 rhis is 
 
 swinir 
 
 like a 
 
 icing a 
 
 a con- 
 
 eacnpfhJ 
 'per pro- 
 
 SOtTND-WAVES. 
 
 305 
 
 densation. Thus a forward movement of th. . ■ 
 
 made, and thus a nnl^p «.. ^'^^^^"'^ °^ *he rarefaction is 
 
 XXXIX. WAVE-SOUNDS. 
 
 sen...,. J, :::l' - ;,i:r i::z n:r" '"^ 
 
 of the impression mado on tl.o ear But 2 Tn ""'""^ 
 
 distanee that it cannot itself act on tl,!ear vet 1 ".r ""■■" " 
 act o.^,. .,., and it .nst . the .e,, Sir ttt^ - 
 
 Hrr::r^i:^:i----j.„i,,., 
 
 against t^ebeV wtft^ tbe r I ^tV%'?,:;;^^^ Press the hand 
 
 fork, press the stem against a abl vn ' ^"'"'^' ^^ "^ *""'«-'- 
 
 the Cheek with the .naT::^^^^^ VCt th^^"f ^^'""^ 
 prongs just beneath the surface of water wZ'fl" T^' °' '^' 
 piano, guitar, or viohn or the tnn.,L 7 ■ "'*" ^^""^'« ^f a 
 
 From the above exoeriments ,: ^J :eii;^;;^^^^^^^ ^^^ -""^i"..- 
 in motion? What kind of motio.^" IIow loo. t. • '''""'""" "^"^'^ ^' 
 that which we caU Ixeat? Ho v does it\lt'; ""« '"^*^«« differ from 
 projectile? ^^"^^ ** ^''"^'^ f»*om the motion of a 
 
 ^o:::::^:t;;,:':trnX: t:-t:^ t r -' 
 
 V bratinn • wb-.-. - ' - a, state of contmuoijs 
 
 part,rf;.etd;reT::-;::— ;:r 
 
 that »o„«i <,„v,«a,e» i, a .ibrating'l^!^!^ ^'' ™"°"«"' 
 
 J 
 
 
 Ill 
 
 IN 
 
306 
 
 SOUND. 
 
 il a H 
 
 So,md, that proceed from the tM„i„K.|brk and tbe violin 
 «tn„g are example, of ,„„nd produeed l.y t„u„ver,e vihratior 
 
 a.ul gra^p ,„„ tube tightly „,th thi., ha,,.,, „,„ '"Z ZZ'lZ' 
 
 J'ZTZu ^iJ""" ' '""• "' ""'*""" "' '"■'"' '»•" Ions and 
 o iMUc, make a hole near one e,i<l, and »ii»,>i.m,i h, ,, ,„,„„ ,„ , 
 
 from the hand, and rotate It rapidly ahont th, l,a„l a , f " 
 
 o a ,l,„g The string will rapldly'twl,. a„d',;„ ," ."J ^^ ^^ZZ 
 
 will result from the tovsional eitralloai. 
 
 §257. How sound travels. - IIow „„„ „ l,„||, ^^„^- 
 at a distance, affect the cur? If the 1,„|| „hilo ,o„, din. po" 
 sesses no peouhar propc-t,- except ,„„ti.,„. ti„.„ it „„, ZthZ 
 to coramuinca c to .he ear l,nt motion. n„t „,„ti„„ ean b°c 
 corn,n„„,cated by one body to anotb,.- „t a distance only thron.d! 
 some medium. -^ """"»" 
 
 Does sound require a medium for it. co.umunicution ? If so 
 what IS the medium ? ' 
 
 Experiment. Lay a thick tuft of cotton-wool on fhn nlnf„ ^f 
 a.-pu,„p and on this, face downward, piace alo a?t c tg"^^ ch a" 
 cover with the receiver. Notice tliat the receiver UttlrZJnT. 
 the watch and your ear. greatly diniiniHlLrl r^oSo^ „ : ^r:: 
 with the passage of somHhing to the ear. Tako a few stLes of th. 
 
 lb he ;rrhe::^ ^t^f^^ Progre^Hen. „„tn citherL Im d 
 cau De heard when the ear i.s placed close to tlio receiver or nn nv 
 tro^ely faint one, as if coding from a great dl«ta„" T.'.c relv':; 
 of air from a portion of the space botwee,, tho watch and vom-lr 
 . estroys the sound, although the watch continues to t^k let ^ he 
 air again and the sound is restored. 
 
 Thus it appears that soM/i(^ cannot travel th mm,], n ..ocw^n ■ ir 
 oti^er words .«/^Ao«; a mer/m,n, and the mcdn.m in this case is air 
 Jiy which of the two methods described in § 254 is mo- 
 
AIR-WAVES. 
 
 807 
 
 If 
 
 so. 
 
 tion transmitted from the sounding body tlirough the nir? Take 
 an extreme case : A cannon is discharged at a distance of one- 
 fourth of a mile from you. You not only hear the sound, but 
 fee tiie shock communicated by the air ; the windows are 
 shaken by it ; at tl)e same time, you easily perceive that it is 
 not the motion of a wind, but the motion of a pulse It can 
 easily be shown that the pulse travelled at a rate of about 800 
 miles in an hour, or with nearly the velocity of a rifle ball 
 whereas the wind of a hurricane seldom exceeds 75 miles an 
 hour. What, think you, would be the result if you were ^o be 
 struck by a gust of wind of such velocity? Yet the softest 
 Avlusper travels with very nearly the same speed. 
 
 §258. Air-waves. — Boys amuse themselves by inflatins 
 paper bags, and with a quick blow bursting them, producing 
 with each a single loud report. First the air is suddenly and 
 greatly condensed by the blow, the bag is burst ; the air now, as 
 suddenly and with equal force, expands, and by its expansion 
 condenses the air for a certain distance all around it, leaving a 
 rarefaction where just before had been a condensation. If many 
 bags were burst at the same spot in rapid succession, the result 
 would be that alternating shells of condensation and rarefaction 
 would be thrown off, all having a common center, enlar^ring as 
 they advance, like the waves formed by stones dropped into 
 water; only that, in this case, the waves are not like rings, but 
 hollow globes ; not circular, but spherical. 
 ^ As a wave advances, each individual air-particle concerned in 
 Its transmission performs a short excursion fro and to in a 
 straight line radiating from the center of the shells or hollow 
 globes. A particle begins to move when the front of the shell • 
 of compression touches it, and completes its motion when the 
 back of the next shell of rarefaction leaves it. Accordincrly an 
 air-wave travels its axon length in the time that a particle occupies 
 in going through one complete vibration so as to be ready to start 
 again. 
 
 I 
 
 Sll 
 
 m 
 
 Mfl 
 
 1 
 
308 
 
 SOUND. 
 
 „Jif ; Y^'"'''^ is. -The term «o««rn8 sometimes 
 .ntnf > ' ^^"••'^^^•'^"'^^'"^"tiraes to denote the external 
 
 cause of I.e sensaU.,,. ; it is in this latter sense that the word is 
 nsed ni Piiys.es, and that we have to define it. 
 
 If the ear replu'O the cai...lle ii. the experiment (§ 254), the 
 a.r.pnlse prodnees a loud sound. Air-waves started by the voice 
 may affect a flame. In fact, the relation between the cause of 
 oursensation and a vibration is so uniform that we may say 
 6onndu viljrnUon that may be appreciated by the ear. Accord - 
 •ng to this definition, is the vibration in the metal of a ringii.. 
 
 {ii 
 
 M 
 
 §260. Sohds and liquids as media transmitting sound 
 - Expenment 1. Lay a watch, with its back downward, on and near 
 
 ?old f" /: '• r-^ '"''' ^"' '''^''^' -^"^ --''• t'- vvat h With loose 
 folds of cloth tdl us ticking cannot be heard through the air in am 
 direction at a distance equal to the length of the board. Novv plac • 
 the ear m contact with the distant end of the board. 
 
 Experiment 1. Place one end of a long pole on the sounding-boani 
 of^a p.ano, and apply the stem of a vibrating tuning-fork to the oth^! 
 
 Experiment 3. Place the ear to the earth, and listen to the rumbling 
 of a distant carnage; or, put the ear to one end of a long stick of tim 
 ber, and let some one gently scratch the other end with a pin. 
 
 Experiment 4. The following experiment will be found verv in 
 structive and satisfactory : Let two persons stand about fifteen rods 
 apart, and one of them strike two pel,ble-stones together .so as to be 
 scarcely audible to the other. Then,, when at the same distance apart 
 let one of them d've to the bottom of a pond of water, or hold one I; 
 for a few seconds beneath the surface of the water, while t]ie other 
 ex ..nding his hands into the water, strikes the stones togethe as 
 before. Describe the result in each of as many of the above and 
 
 Solids and liquids, as well as gases, transmit sound vibra. 
 twns. 
 
VELOClTir O'^ SOUND. 
 
 309 
 
 > sometimes 
 
 he external 
 
 the word xs 
 
 ;§ 254), the 
 >y the voice 
 lie cause of 
 
 may say, 
 . Accord - 
 
 a ringing 
 ve call the 
 
 ngr sound. 
 
 on and near 
 
 li with loose 
 
 e air in any 
 
 Now placo 
 
 idlng-boanl 
 the other 
 
 le rumbling 
 ick of tim- 
 1. 
 
 id very in- 
 Ifteen rods 
 so as to be 
 xnce apart, 
 )Id one ear 
 the other, 
 ogether as 
 ibove and 
 elusion do 
 
 
 XL. VELOCITY OF SOUND. 
 § 26L On what velocity of sound depends. - The flash 
 of a gun however distant, is seen by an observer at the instant 
 It IS made But the report, if the distance is several hundred 
 yards IS heard a little later. If the distance is a mile, an in- 
 terval of nearly five seconds will occur; so that sound mu.t 
 occupy that time in traveling a mile, or it must travel about 
 1100 feet m a second, -a velocity somewhat less than that of 
 a rifle ball. 
 
 It is apparent that sound must travel more slowly in a dense 
 than m " rare medium, inasmuch as in the former there is a 
 greater mass to be moved; on the other hand, it travels 
 faster m the medium that is the most elastic. Densitu 
 retards and elasticity increases the velocity of sound. The rela- 
 tion of velocity to the density and elasticity of gases, as ascer- 
 tamed by careful experiment, is as follows : the velocity of sound 
 in gases ts directly proportional to the square root of their elasti- 
 city, and inversely propoHional to the square root of their 
 respective densities. 
 
 ssl^'nm^t,"' ""' i° 'r '* '°^- '^^ ^«^" ^--^ ^ b« 
 
 333 (1093 ft.) per second. Its velocity increases nearly six- 
 tenths of a meter for each degree centigrade. At the temper- 
 
 Tt . S'^'r ^''^^-^ "^ "^^ ^^^^^^ *^« -I«-ty of sound 
 at about 342™ (1125 ft.) per second. 
 
 The greater density of solids and liquids, as compared with 
 gases, tends, of course, to diminish the velocity of sound ; but 
 their greater elasticity^ more than compensates for the decrease 
 of velocity occasioned by the increase of density. As a general 
 rule, solids are more elastic than liquids ; hence, sound generally 
 travels faster m the former than in the latter. For example 
 
 
 sound travels in water about 4 times as fast as in 
 
 "The question will very perUnently ariae here, inwrauch as gases are nerfprtJ 
 eUuitlcHowcao solids and Ilqulda be regarded as having greater elSu' iTshmd 
 ^understood that whUe gases completely recover thL%olume aftl ^ coinp^^^^^^^ 
 force is rerooved, they do it more ■luygi.bly thao .olidi and UquJdi. *=*""P««»°8 
 
 '}| 
 
 'If 
 
810 
 
 SOUND. 
 
 (r 
 
 4 times; m gold, 5 times; iu brass, 10 times; in copper, 11 
 times; m uon, 16 times; in glass, 16 times; in wood aiong 
 ihe fiber between 10 and 15 times; in wood, across the fiber! 
 between 4 and 6 times. 
 
 QUESTIONS. 
 
 affe^ted?^ /see Til' f 5m It ^'^^P^^^^^^'' ^«^ '« ^^s density or volume 
 anected? (See § 12o.) (b) How is its elasticity affected? (c) How is it 
 affected as regards the velocnty witi. which ,t w.ll transnnt^sou^.T 
 
 2 Hydrogen Is sLxteeu times lighter (or rarer) than oxygen under 
 he same pressure. (.) m which will sound travel fasterpT^) Why 
 (c) How many times faster? (.«; wnyr 
 
 3. When sound travels in air with a velocity of 331- per second it 
 travels .„ carbonic acid gas at the rate of 262™ per second (a) Which 
 IS the denser gas ?(&) How many times denser? 
 
 4. When a confined body of air is heated, it has its elasticity in- 
 
 sTn of sound?"' "' '''""" "' ''""'^- ^''' ^"^ '""'^ ^«"-' *^^-™»"- 
 6. If air is heated and allowed to expand freely, as on a warm 
 summer day, its elasticity is unaffected, but its density is diminirhed 
 how will this affect the triusmission of sound^ cluninished . 
 
 XLI. REFLECTION AND REFRACTION OF SOUND. 
 
 §262. Reflection.— In the experiment with the spiral 
 spring, waves wore reflected from the box to the hand, and 
 from the hand to the box. When a sound-wave meets an 
 obstacle in its course, it is reflected ; and a sound heard after 
 being thus reflected is often called an echo, or reverberation when 
 many times reflected, so that the sound becomes nearly con- 
 tinuous. 
 
 § 263. Sound reflected by concave mirrors. — Experi- 
 
 ment. Place a watch at the focus (page 286) A, Figure 208, of a con- 
 cave mirror G. At the focus B of another concave mirror H. nlaop th- 
 .argcupcn.ngof a small tunnel, and with a rubber connector attach 
 the bent glass tube C to the noM- of the tunnel. The extremity D 
 being placed iu the ear, the ticking of the watch can be heard very 
 
in copper, 11 
 I wood, along 
 •osa the fiber, 
 
 isity or volume 
 ' (c) How is it 
 lit sound? 
 oxygeu under 
 er? (6) Why? 
 
 per second, It 
 Id. (a) Which 
 
 s elasticity in- 
 flect transmia- 
 
 is on a warm 
 is diminished ; 
 
 I the spiral 
 e hand, and 
 e meets an 
 
 heard after 
 sration when 
 
 nearly eon- 
 
 s. — Experl- 
 
 108, of a con- 
 r H, place the 
 lector attach 
 extremity U 
 e heard very 
 
 REFLECTION AND REFRACTION. 311 
 
 distinctly, as though it were somewhere near the mirror II. Though 
 the mirrors be 5n> apart, the sound will be heard much louder at B than 
 at an intertermediate point E. 
 
 How is this explained? Every air-particle in a certain 
 radial line, as Ac, receives and transmits motion in the direc- 
 tion of this line ; the last particle strikes tlie mirror at c, and 
 being perfectly elastic, bonnds off in the direction cc' in con- 
 formity to the law of reflection (§87), communicating its 
 motion to the particles in this line. At c' a similar reflection 
 gives motion to the air-particles in the line c'B. In consequence 
 of these two reflections, all divergent lines of force, as Ad, Ae, 
 
 rig_208. ^^*^"' ^''^*^ ^^^^ *'^^ mirror 
 
 Q {/ vj ^'» Jire tliere rendered 
 
 A^," parallel, and afterwards 
 hi rendered convergent at 
 ' the mirror II. The prac- 
 tical result of the concen- 
 tration of this scattering 
 ^ . . force is, that a sound of 
 
 great intensity is heard at B. The points A and B are called 
 the foe. of the mirrors. The front of the wave as it leaves A 
 18 convex, in passing from G to H it is plane, and from II 
 to B .oncave. If you fill a large circular tin basin with 
 water, and strike one edge with a knuckle, circular waves with 
 concave fronts will close in on the center, heaping up the 
 water at that point. t- o f 
 
 ciJr^p"''^''^'""^"^'"'''''''" ^^^« ^^«" constructed on this prin- 
 ciple Persons stationed at the foci of the concave ends of theZ. 
 
 fw eTcarotr^^"": conversation in a whisper which persons be"- 
 tween canno hear. A most notable instance was that of the " Ear of 
 Dionysius," in the dungeon of Syracuse. The roof of the p-ison was 
 
 to the ear of the tyrant, even the whispers of the victims tliere con- 
 
 behTnH .?*'™^l'^' ^" "■ "''""'^ condenser. The hand held concave 
 behind the ear, by its increased surface, adds to its efficiency. An ear 
 
 iF 
 
 tii 
 
 ■ §1 
 
312 
 
 SOUND. 
 
 
 rig. 209, 
 
 4 distant, and thea introduce a collodion balloon B filled with 
 earbomc acid gas between your ear and the watch, and " y 
 near the latter, the sound becomes much louder. ^ 
 
 The cause is obvious ; for, let the curved lines a b r ^to ..r. 
 sections of sound-waves with convex frontrand « «^ ^■' Tf'"^ 
 of carbonic acid gas which is dcn^c^- Z^Z^tTXll h^aT 
 owing to the slower progress of the waves 1^ the den er 2 U.ev 
 
 Zts 'r Te T"'. °° ^"^""'-" ^^'^ ''^'' -^ *^« ^^ZoTlo^Z 
 
 ironts may be changed to waves of plane front<i A,roin , '"®* 
 the .«re™i«es of the wave., having Ici .Ustan™ fo ,™t "„' .Sfd L" 
 gas than points near the center, would emor-e (tr«f «„h „ , ■ '"V""''" 
 
 as It progressed, Is so ehanged In direction In passing mto and o, fot 
 
 :ir;Lrrtr;:?;>:r;rtei^^^^^^ 
 
 Any change in direction of sound, caused by passing from a 
 
ite, at the small 
 Iter at the large 
 
 the small end 
 of a watch A, 
 
 B filled with 
 3h, and very 
 
 itc, represent 
 pherlcal body 
 is clear that, 
 ser gas, they 
 es of convex 
 in, points at 
 
 in the denser 
 it in advance, 
 in the dense 
 
 the form of 
 ning (iimised 
 
 less intense 
 ' and out of 
 icentrated at 
 
 sing from a 
 nt density, 
 
 
 .LOUDNESS. 
 
 813 
 
 XLII. LOUDNESS OF SOUND. 
 
 § _65 Loudness depends on amplitude of vibrations 
 Gentry tap the prong, of a tuning-fork and dip them i.t wate7 
 
 tTo bir .r T" ■ "'^"^ ''' ^'^^"'" ' "---« the fo "of 
 the blow, — the vibrau..,s become wider ind thn ^nf 
 
 thrown with greater foree and to a grelt^ dll;:^^^^^^^^ 
 lung occurs when the fork vibrates in air; thou^^h we do Tot 
 s e the a,r.partieles as they are batted b; the moving Jo" 
 yet we feel the effects as a sound sensation, and we u^d.e o^ 
 their energy by the intensity of the sensat on T ' h ° I 
 sound is really the measure of a sens.t o" b^n '^"''' "' 
 
 suitable or constant standard of ::::^^^\: Z^^:: 
 we are compelled to measure rather the intensity oMe^un ' 
 wave, knowing at the same time that the loudness n'r " 
 portional to this intensity; unfortunately the expres Lns / " 
 ness and intensity of sonnrl are often intorchan '^c 7^ ? 
 s.ty of a vibration is measured by the en^ty^o "the ,/?"" 
 particle. It is clear that if the\amplitI^'or vitat n;"? 
 V^ IS doubled while its period roniins const^,t"l^t: 
 
 ^^..ssor intensity of ^)^r^-^ Z!^^^ 
 of the amphtude of the vibrations of the sounding body 
 
 dinm ^^' Loudness depends upon the density of the me- 
 
 dum.-In the experiment with the watch undei the ite^'r 
 
 the air-pump (§257), the sound grew feebler as he air 
 
 no e to make their conversation heard when they reach are-it 
 nghts than wh.. in the denser lower air. Fill a'g aTsbefl H 
 with hydrogen ga., and place in it a small alarm clodc tj^ 
 sound is exceedingly weak and thin „a ''"" ^^^ck , the 
 
 „„ J , . °^ ^ "^^^ '^"'"i as compared with thp 
 
 sound when the jar is filled with nir ri. - 
 
 us (9\ thr,t ii ' . "^^' ^^"'" ^"- ^''ese experiments teaoh 
 us, (2) that the vntensity of sound depends upon the density of 
 
 lll'l 
 
 ll;i 
 
314 
 
 SOTTND. 
 
 111 
 
 it 
 
 I 
 
 if ■ 
 
 m 
 
 
 the medium in which it is produced. In a rare medium a vibrat- 
 ing body during a single vibration sets in motion either fewer 
 particles, as in the case of the partially exhausted receiver, or, 
 as in the case of the hydrogen gas, it sets in motion lighter par- 
 ticles than in a dense medium; consequently it parts with its 
 energy more slowly, and the sound is consequently weaker. 
 
 (In which ought the vibrations of a body to last longer —in 
 a dense or in a rare medium ? Why?) ' 
 
 § 267. Loudness depends on distance. -It is a matter 
 of every-day observation that the loudness of a sound dimin- 
 ishes very rapidly as the di^ance from its source to the ear 
 increases. The ear is not, however, able to compare very accu- 
 rately the loudness of two sounds ; for instance, it cannot 
 determine when one sound is just twice as loud as another. 
 This, however, so far as it is affected by distance, can be very 
 accurately determined by calculation. For it is evident that as 
 a sound-wave recedes from its source in an ever-widening sphere 
 a given amount of energy becomes distributed over an ever- 
 increasing surface ; and as a greater number of particles par- 
 take of the motion, individual particles receive proportionally 
 less energy ; hence it follows, -as a consequence of the geomet- 
 rical truth, that the surface of a sphere varies as the square of its 
 radius, — that (3) the intensity of sound varies inversely as the 
 square of the distance from its source. For example, if two per- 
 sons, A and B, are respectively 500 and 1000 meters from a -run 
 when It IS discharged, the report that reaches A will be four 
 times as loud as the same report when it reaches B. 
 
 § 268. Speaking tubes. — Experiment. Place a watch at one 
 end of the long tin tube (Fig. 200), and the o u- at the ot'rr end tL 
 tickmg is heard very loud, as though the watch were close to the ear. 
 
 Long tin tubes, called ^.aJ^-ing tubes, passing through many 
 apartments in a building, enable persons at the distant extremi- 
 ties to carry on conversation in a low tone of voice, while per- 
 
ium a vibrat- 
 either fewer 
 receiver, or, 
 lighter par- 
 arts with its 
 weaker, 
 longer, — in 
 
 is a matter 
 5und dimiu- 
 ! to tJie ear 
 G very accu- 
 !, it cannot 
 as another, 
 can be very 
 lent that as 
 ling sphere, 
 er an ever- 
 irticles par- 
 )portionaIly 
 the geomet- 
 quare of its 
 rsely as the 
 if two per- 
 f rom a gun 
 ill be four 
 
 DISTINCTION BETWEEN NOISE AND MUSIC. 315 
 
 IZZ'""" ^f '"' '°°'"' '''°"°^ ^^^^^ ^^« ^"^« P^«««« hear 
 
 notbin 
 
 Ihe reason is the sound-waves which enter 
 
 - -- ._ ™„ .^»^i..i«-»T«vca wiucn enter the 
 
 tube are prevented from expanding, consequently the intensity 
 of sound IS not affected by distance, except as its energy is 
 wasted by friction of the air against the sides of the tube. 
 
 Tf !w^'i ^;^*^°,°.^^°° b«*^««^ noise and musical sound. 
 If the body that strikes the air deals it but a single blow, like the 
 discharge of a firecracker, the ear receives but a single shock 
 and the r.,ult is called a noise. If several shocks a're ^1 
 recr .,y the ear in succession, the ear distinguishes them 
 
 as ..^ay separate noises. If, however, the body that strikes 
 the air is ui vibration, and deals it a great number of little blows 
 m a second, or if a large number oi '^.re-crackers are dischar^^ed 
 one after another very rapidly, so that the ear is unable to dis- 
 tmguish the individual shocks, the effect produced is that of one 
 continuous sound, which may be pleasing to the ear ; and, if so 
 It IS called a musical sound. But continuity of sound does not 
 necessarily render it musical. The sound produced by a hun- 
 dred children beating various articles in a room with clubs 
 might not be lacking in continuity, but it would be an intoler- 
 able noise. There would be wanting those elements that please 
 the ear; viz., regularity both in periodicity and intensity of the 
 shocks which it receives. The distinction between music and 
 no.se IS, generally speaking, a distinction between the agreeable 
 and the disagreeable, botween regularity and confusion. The 
 charactensucs of a musical sound are regularity and simplicity. 
 
 vatch at one 
 r end. The 
 to the ear. 
 
 )ugh many 
 
 it extremi- 
 
 while per- 
 
116 
 
 SOUND. 
 
 m I 
 
 XLIII. PITCH OF SOUNDS. 
 
 § 270. On what pitch depends. — Draw the finger-nail 
 slowly, and then rapidl}', across the teeth of a comb. The two 
 musical sounds produced are commonly described as low or 
 grave, and higJt or acute, and the higlit of a musical sound is 
 called pitch. What is the cause of a difference in hight or 
 pitch of two sounds? 
 
 Fig. 215. 
 
 Experiment. Procure a circular sheet-iron or pasteboard disk A, 
 Figure 21.",, SO-^'" in diameter. Prom tlie center of the disk describe a 
 circle with a radius of 12'='". Iq the circumfer- 
 euce of this circle, with a punch, cut holes S"™ 
 in diameter, leaving equal intervals of about 2="" 
 between the holes. Insert in a rubber tube a 
 piece of glass tube B, of l^m bore, drawn out at 
 one eud so that its orifice is about i"^ jq diame- 
 ter. Attach the disk to some rotating apparatus, 
 hold the small orifice of the glass tube opposite 
 the holes, and blow steadily through the tube, 
 and rotate the disk at first very slowly and then 
 with gradually increasing rapidity. The breath, 
 as it makes its exit f i-om the tube, cannot escape 
 continuously through the holes, but is cut up by 
 the passing obstructions into a series of puffs, 
 which at first are heard as so many distinct 
 sounds; as the speed increases, the number of 
 puffs in a second increases, until the ear can no 
 longer separate them, when they blend together 
 ?n a deep sound of a definite pitch. 
 
 The peculiarity of tliis instrument is that it does not produce 
 sound by its own vibrations. Every time the air is driven 
 through a hole, it produces a pulse of condensation in the air 
 beyond; and during the interval between the successive dis- 
 charges, a pulse of rarefaction will be caused by the elasticity of 
 the air, so that the result is the same, so far as the effect on 
 the air medium is concerned, as if a body were vibrating 
 in it. As the velocity increases, the pitch constantly rises, 
 
LIMITS OF THE SCALE AND IIIOAItlNO. 
 
 ai7 
 
 i finger-nail 
 
 L). The two 
 
 i as low or 
 
 3al sound is 
 
 in hight or 
 
 oard disk A, 
 ik describe a 
 13 circumfer- 
 cut holes S™" 
 1 of about 2="' 
 ubber tube a 
 drawn out at 
 tmm in diame- 
 ig apparatus, 
 ube opposite 
 gh the tube, 
 tvly and then 
 The breath, 
 anuot escape 
 ; is cut up by 
 les of puffs, 
 any distinct 
 e number of 
 le ear can no 
 3nd together 
 
 
 until, at the greatest speed conveniently attainal)l(;, it boeomes 
 painfully shrill. Varying the force of the breath affects the 
 loudness of the sound, but does not affect itH pitch. 
 
 So we liave discovered the important fact that pitch depends 
 upon Vibration-frequency or wave-length, i.e., the greater the 
 number of vibrations per second, or the shorter the wave-length, 
 the higher the pitch. If the number of vibrations per second 
 18 doubled, the pitch is raided one octave. 
 
 QUESTIONS AND EXERCISES. 
 1. Why does the same bell always give a sound of the same pitch? 
 
 f '^' , ^"l^.^^^" '•' "''' ^^^^* "^ '^"'''"« "^ '^^'" "'ll'' <"i"'"-<-nt degrees of 
 force? (6) What change in the vibrations is productcd? (c) What 
 property of sound remains the same? 
 
 8- (a) Strike a key of a piano, and liold It down ; what is the only 
 
 frSn ^^^ "1^'"^^ "' **"' '"""^ P''^^^"'"^^^ ^'"'« "' '•^"•"^»n« audible? 
 (6; What is the cause of this change? 
 
 4. Rake tlie teeth of a comb with a flngcr-iiall, at llrst slowly, then 
 quickly, and account for the dittbrence in the characfr of the sounds 
 produced. 
 
 «• (a) On what does pitch depeiul? (i) ()„ ,vhat, loudness? 
 
 i I 
 
 lot produce 
 r is driven 
 in the air 
 iessive dis- 
 ilasticity of 
 16 effect on 
 ! vibrating 
 .ntly rises, 
 
 § 271. Limits of the scale and hearing. — The lowest 
 note of a 7J octave piano makes about 27i, tlio highest, 4 2->4 
 vibrations per second; but these extreme notes have little 
 musical value, and the lowest notes are only used for their har- 
 monies. The range of the humaii voice 11'.-. bf-twoen 100 
 and 1,000 vibrations per second, or a little more than three 
 octaves; an ordinary singer has ah--^ the compass of two 
 octaves. 
 
318 
 
 SOUND. 
 
 il > 
 
 The ear is capable of hearing vibrations far exceeding in 
 number the requirements of music. It can appreciate sounds 
 arising from 32 to 38,000 vibrations - per second, i.e., a ran^e 
 ofabout eleven octaves, and a corresponding range of wave- 
 length between seventy feet and three or four tenths of an inch 
 These numbers vary, however, considerably with the person, 
 it-xceptional ears can hear as many as 50,000 vibrations. Some 
 ears can hear a bat's cry, or the creaking of a cricket ; others 
 
 i 
 
 XLIV. VIBRATION OF STRINGS. 
 
 § 272. Sonometer. -Experiment. Take a piece of violin- 
 stnng or piano-wire a little longer than your table. Fasten one Ind to 
 
 p. 218 ^ "*" '" °"*^ ^"'^ ^^ *^e 
 
 table, and pass the other 
 
 end over a pulley fastened 
 
 to the other end of the 
 
 table, and to this end of 
 
 the string suspend a pail 
 
 containing sand, the two 
 
 weighing just a pound. 
 
 Place under the string, 
 
 near the ends of the table, 
 
 A and B rFijr 91s^ a . , <^^vo ^vedge-shaped bridges 
 
 Pln.wi, ,■' ^.lu ^ apparatus thus arranged is called a sonomefer 
 r uck the stnng with the fingers near the middle, causing It to vibrate 
 a^d note the pitch of the sound, and the length of the string between 
 the bridges. Movc the bridge A toward B ; the pitch rises as tie v ^rat 
 
 S7s r; T ^*'"^ ^^ ^'"'^"^'^'- ^^'•y''- position o A uSa 
 
 ound ha^ tT . T ""'""' "''"' "" P"^'^ ^'''"^ ^' «r«*' «»d it Will be 
 found that the string is just one-half its original length • i p A« h., 
 
 the string its vibration-number is doubled. ^ ' ' *^ '^"''''*"^ 
 
 Now, increasing the weight in the pail, the pitch rises, till, when the 
 
 tension IS four pounds, the pitch has risen an octave. Let^he ten! 
 
 sujnbe the same ; try another string, weighing, for the samelen.^ 
 
 results.) "^ experiments wiU not give very accurate 
 
 » Preyer place, the lowe.t limit for some e«n at 18 vibration, per woond. 
 
exceeding in 
 reciate sounds 
 
 *.e., a range 
 nge of wave- 
 ihs of an inch. 
 ti the person, 
 ations. Some 
 icket; others 
 
 iece of violin- 
 iten one end to 
 ne end of the 
 jass the other 
 Julley fastened 
 ^r end of the 
 to this end of 
 mspend a pail 
 !iand, the two 
 ist a pound, 
 r the string, 
 s of the table, 
 ihaped bridges 
 d a sonometer. 
 : it to vibrate, 
 tring between 
 as thevibrat- 
 n of A until a 
 and it will be 
 e. , by halving 
 
 till, when the 
 Let the ten- 
 same length, 
 that given by 
 ery accurate 
 
 Moond. 
 
 QUALITY OF SOUND. 
 
 319 
 
 These conclusions may be summarized by saying : T7ie vibra- 
 tion-numbers of strings of the same material vary inversely as 
 their lengths and square roots of their weights, and directly as the 
 square roots of their tensions. 
 
 1. 
 
 ing? 
 
 GocSTIONS AND PROBLEMS. 
 
 Why does a violinist finger the strings of the violin when play- 
 
 2. Examine the strings of a piano, and ascertain the different 
 methods by which a wide range of pitch is effected. 
 
 8. How does the length of the string that gives the note F compare 
 v/ith the length of the C-string below it, other things being equal? 
 
 XLVL QUALITY OF SOUND. 
 
 Let the same note be sounded with the same intensity, suc- 
 cessively, on a variety of musical instruments, e.g., a violin, 
 cornet, clarinet, accordion, jews-harp, etc. ; each instrument 
 will send to your ear the same number of waves, and the waves 
 from each will strike the ear with the same force ; yet the ear is 
 able to distinguish a decided difference between the sounds, — 
 a difference that enables us instantly to identify the instruments 
 from which they come. Sounds from instruments of the same 
 kind, but by different makers, usually exhibit decided differences 
 of character. For instance, of two pianos, the sound of one 
 will be described as richer and fuller, or more ringing, or more 
 " wiry," etc., than the other. No two human voices sound 
 exactly alike. That difference in the character of sounds, not 
 due to pitch or intensity, that enables us to distinguish one 
 from another, is called quality. Two sounds may differ from 
 one another in loudness, pitch, or quality; they can differ in no 
 other respect. 
 
 
 nt 
 
 ii", 
 
'I 
 
 320 
 
 SOUND. 
 
 Pitch depends on frequency of vibrations, loudness on their 
 amplitude ; on what does quality ,/epm,l? 
 
 § 273. Analysis of sounds. - Tl.« u.mkled ear is unable, ex- 
 Fig. 220, •'«'l"' <«"i very limited extent, to 
 
 diHtliiKuish the individual tones 
 tliiit <;onipose a note. Ilelmholtz 
 «n-tti)«t;(l a series of resonators 
 fOHMlutlng of liollovv spheres of 
 l>niM», each having two openings : 
 <nw (A, Fig. 220) large, for the 
 
 reception ofthe sound-waves, and 
 tli« other (B) small and funnel- 
 shaped, and adapted for inser- 
 tion Into the ear. Each reson- 
 ator of the series was adapted 
 
 aTlr ; JT' "' T'" ^'^^'""^^"'•«' t"« ^•'^r, placed at the orifice of 
 
 any one ,s able to single out f rou. a c-olh-.tlon that overtone, if present 
 
 to Which alone this resonator is capa..,« of rcHpouding. t is found 
 
 hat, when a note is produced on a give, Instrument, not only is there 
 
 Sip-— isrr--^,r=i£ 
 
 note can ,;e formed at that point; <-on«.'r,uontlv fhc two im.,^ I . 
 ovmonc. produced b, 2 .„., 4 «„» C; ,, So/o ™ 
 
 g " rX'" „r;,e: '™""r', .'""'«" "' <•"«-• vlcCetc 
 fcencrall, .ti utk uoar <„io of tlii-lr e„,l«, aii.l lliii, thoy are doprivid of 
 only some of their hlsLei- aud feebler overtone.. "'•Pr'vstI "1 
 
ess on their 
 
 is unable, ex- 
 ted extent, to 
 lividual tones 
 e. Helmlioltz 
 )f resonators 
 vv spheres of 
 wo openings: 
 arge, for the 
 Kl-waves,and 
 
 I and funnel- 
 id for iuser- 
 p]ach reson- 
 was adapted 
 
 II powerfully 
 cal sound is 
 he orifice of 
 e, if present. 
 
 It is found 
 3nly is there 
 , but all the 
 
 Which are 
 the point of 
 3 middle, no 
 ) important 
 ;ions of the 
 3, etc., are 
 deprived of 
 
 CHAPTER VI. 
 RADIANT ENERGY. -LIGHT. 
 
 L. INTRODUCTION. 
 
 § 274. Lightaformof energy. — Exposed to the sun, the 
 skin is warmed, and thus the sense of touch is affected ; it is 
 iUuminated, and thereby the sense of sight is affected ; it is 
 tanned, and thereby its chemical condition is changed. It is evi- 
 dent that we receive something which must come to us from the 
 sun. To the sense of touch it appears to be heat ; to the eye 
 it is light ; to certain substances it is a power to produce chemi- 
 cal changes. But what is it that we receive 
 from the sunf 
 
 Fig. 234. 
 
 Experiment — Blacken one-half of one side of a 
 slip of glass with candle-smoke. With a convex 
 lens, sometimes called a " burning-glass," converge 
 the sun's light upon the blackened portion so as to 
 produce a small luminous spot on the black surface. 
 This spot quickly becomes very hot, but the lens 
 meantime remains comparatively cold. Move the 
 luminous spot to the unblackened portion of the 
 glass. The spot becomes only slightly heated. 
 Place a piece of paper behind and in contact with 
 the glass, and It quickly burns. > 
 
 Whether we receive heat from the sun or 
 not, it is evident that we receive something 
 that can be converted into heat. 
 
 Figure 234 represents an instrument called 
 a radiometer. The moving part is a small vane resting on the 
 pomt of a needle. It is so nicely poised on this pivot that it 
 
 ■ill 
 
322 
 
 RADIANT ENERGY. — LIGHT. 
 
 5 ■ 
 
 rotates with the greatest freedom. To the extremities of each 
 of the four arms of the vane are attached disks of aluminum 
 which are white on one side and l)lack on the other. The whole 
 is enclosed in a glass l.nlb from which the air is exhausted till 
 less than ^r^^ of the original (luantity is left. If the instrument 
 is exposed to the sun's light, or even to the light of a candle, 
 the wheel will rotate witli the unblackened faces in advance. 
 
 In just what manner it is caused to rotate does not concern 
 us ; but the fact that it does rotate, and that it is caused to 
 rotate directly or indirectly by something that comes froin the 
 sun or the candle, is pertinent to the question before us. When- 
 ever a body is caused to move or increase its rate of motion, 
 energy must be imparted to it ; hence energy must be imparted 
 to the radiometer-vane by the sun or candle. 
 
 Bell, the inventor of the telephone, has succeeded in produc- 
 ing musical sounds by the action of sun-light and other intense 
 lights. But sound always originates in motion, and motion 
 springs only from some form of energy. So, then, that which toe 
 receive from the sun, tvhether it affects the sense of touch and is 
 called heat, or the eye and is called light, or produces chemical 
 changes and is called chemism, is in reality some form of energy. 
 
 § 275. Ether the medium of motion. — If light is motion, 
 wiiat moves ? Our atmosphere is but a thin investment of the 
 earth, while the great space that separates us from the sun con- 
 tains no air or other known substance. But empty space can 
 neither receive nor communicate motion. It is assumed — lY is 
 necessary to assume — that there is some medium filling the 
 interplanetary space, in fact, filling all otherwise unoccupied 
 space (i.e., where matter is not, ether is), by which motion 
 can be communicated from one point in the otherwise empty 
 space to another. This medium, has received the name of ether. 
 Ether is supposed to penetrate even among the molecules of 
 liquid and solid matter, and thus surrounds every molecule of 
 matter in the universe, as the atmosphere surrounds the earth. 
 
tTNDULATORY THEORY OP LIGHT. 
 
 323 
 
 nities of each 
 of aluminum 
 '. The whole 
 exhausted till 
 le instrumenli 
 ; of a candle, 
 advance. 
 3 not concern 
 is caused to 
 les from the 
 e us. When- 
 te of motion, 
 1 be imparted 
 
 id in produc- 
 >ther intense 
 and motion 
 'hat which toe 
 touch and is 
 ices chemical 
 1 of energy. 
 
 it is motion, 
 ment of the 
 the sun con- 
 y space can 
 iraed — it is 
 I filling the 
 unoccupied 
 lich motion 
 wise empty 
 tne of ether. 
 lolecules of 
 Tiolecule of 
 3 the earth. 
 
 No vacuum of this medium can be obtained; an attempt to 
 pump it out of a space; would be like trying to pump water 
 with a sieve for a piston. We cannot see, hear, feel, taste, 
 smell, weigh, nor measure it. What evidence, then, have we 
 that it exists ? You believe that a horse can see ; you have no 
 absolute knowledge of the fact. But you reason thus : he be- 
 haves as if he could see ; in other words, you are able to account 
 for his actions on the hypothesis that he can see, and on no 
 other. Phenomena occur just as they would occur if all space 
 were filled with an ethereal medium capable of transmitting, 
 motion, and we can account for these phenomena on no other 
 iiypothesis ; hence our belief in the existence of the medium. 
 
 The transmission of energy through the medium of ether is 
 called radiation; energy so transmitted is called radiant energy, 
 and the body emitting energy in this manner is called a radiator. 
 Sound is another form of radiant energy transmitted through 
 solid, liquid, or gaseous media. ^ 
 
 § 276. Undulatory theory of light. — Is motion commu- 
 nicated by a transfer of a medium or by a transfer of vibrations, 
 I.e., by undulations ? All evidence points to one conclusion! 
 that we receive energy from the sun in the form of vibrations or 
 wave-action; that these vibrations, inaudible to our ears, cause 
 through the eye the sensation of sight, and through the hand the 
 sensation of warmth. This is known as the undulatory theory of 
 light. To learn what the special evidences of the correctness 
 of this theory are, the pupil must wait for further development 
 of our subject ; but it should be borne in mind that the strongest 
 p>oofof the correctness of any theory is its exclusive competence 
 to explain phenomena. Light is vibration that may he appre- 
 ciated by the organ of sight. 
 
 § 277. Ray, beam, pencil. — Anv line. R R. FiaurP 935 
 which pierces the surface of a wave of light, a b, perpendicularly 
 IS called a ray of light. It is an expression for the direction 
 in which motion is propagated, and along which the ocessive 
 
 
i I 4 
 
 S24 
 
 .; I 
 
 UADIANT ENEHdV. — LIOTrT. 
 
 ii: I 
 
 effects of liglit occur. If the wave-surface a' b' is a plane, the 
 rays R' R' are parallel, and a collection of such rays is called a 
 
 beam of light. If the wave-surface 
 a 6 18 spherical or concave, the 
 rays R" R" have a common point at 
 the center of curvature, and a col- 
 lection of such rays is called r 
 pencil of light. 
 
 § 278. Seeingr an object. — 
 When a pencil of light enters your 
 eye you experience a sensation 
 which leads you to the conclusion 
 that a particle of matter lies at the 
 point of intersection of the rays of 
 the pencil, and you say that you see 
 a paiticle of matter at that point. 
 The reasoning by which you reach 
 this conclusion is the result of ex- 
 perience ; and, since you go through 
 this process of reasoning uncon- 
 sciously, it is called unconscious reasoning. If your eye is 
 turned towards a caudle-flame, a pencil of light enters your eye 
 from each particle of the flame, and the sensation experienced 
 leacj you to conclude that a particle of matter lies at the poin- 
 in which the rays of each pencil of light intersect. The ag- 
 gregation of these particles constitutes the flame, and you arc 
 led to say that you see the flame. From this explanation it 
 will be seen that, when you say that you see a certain object in 
 a certain position, you have experienced the sensation arising 
 from pencils of light entering your eye, just as if they had 
 emanated from all the points in an object so situated, and had 
 travelled m straiglit lines to your eye. If a uniform medium 
 such as the air at the surface of the earth usually is, lies be- 
 tween your eye and the luminous object, the ra\8 of light 
 
SEEING AN On-FECT. 
 
 825 
 
 do travel .n straight lines, .and you sec the object where 
 it actually m. I5ut if by any means the rays of li^ht 
 emanating from a luminous object have their directions 
 changed before entering your eye, of course the sensation is 
 changed and yon do p . ..e the object where it actually is. 
 If, nfter the change oi dir.aion, the rays of light enter your 
 eye just as they woul i ..t.r it " they emanated from the points 
 of .some object m a .h .ad po .tion, and travelled in straight 
 Imes to your eye, you s. ,;>. object in this second position, or, 
 asjt ,s commonly expressed, you see .nn image of the object. 
 Hut If, after the change of direction, the rays of light enter 
 your eye as they could not enter it if they emanated from the 
 pomts of any possible object in any possible position, and 
 ravelled in straight lines to your eye, you do not see anything, 
 that IS, your experience does not enable you to draw any con- 
 clusion from the sensation. 
 
 i. . ■ 
 
 I 
 
 ine particles of the flame on the side towards your eve give rise to 
 
 rnence in straight lines to your eye. lu consequence of the sensation 
 
 the Sor' *'sle:"'^^^' T ^^" ^^^ ^'^ ^^^^'^^ the Le S nd 
 the mirror. Sometimes, guided by other senses, or judginff from 
 
 previous experience, you may be able to determine wl ether you ar^ 
 
 I 
 
 RnHiVfl o.« * ' -^-^■"^""S"". aud opaque bodies.— 
 
 Hodies are transparent, translucent, or opaque, according to the 
 manner in which they act upon the Iuminife;ou. wavet wh ch 
 pass thix>ugh them. Generally speaking, those objects are 
 
 m 
 
326 
 
 RADIANT ENERGY. — LIGHT. 
 
 transparent that allow other objects to be seen through them 
 (hstinctly ; e.g., air, glass, and water. Those objects are trans- 
 lucent that allow light to pass, but in such a scattered condition 
 that objects are not seen distinctly through them; ea fo" 
 ground glass, and oiled paper. Those objects .^.^p^Z Z 
 
 r r^rz.'^" ''- '''' -^ ^-- ^^-- ^- ^ 
 
 § 280. Liuninous and illuminated objects. _ Some bodies 
 are seen by means of light, which they generate and emit ,e^, 
 he sun, a candle flame, and a -live coal"; they are called 
 lurninous bodies Other bodies are seen only ^y me'ans of ^ 
 which they receive from luminous ones, and when thus rendered 
 visible, are said to be illuminated; e.g., the moon, a man, a 
 cloud, and a "dead "coal. 
 
 § 281. Every point of a luminous body an independent 
 
 source of light. -Place a candle flame in the center of a 
 
 darkened room; every wall and 
 
 every point of each wall becomes 
 
 illuminated. Place your eye in 
 
 any part of the room, i.e., in any 
 
 direction from the flame ; it is able 
 
 to see not only the flame, but 
 
 every point of the flairs; hence 
 
 every point of the flame must emit 
 light in every direcaon. Every 
 
 point of a luminous body is an in- 
 dependent source of light and emits 
 light in -very direction. Such a 
 
 point is called a luminous point. In Figure 2Sfi thorn 
 represented a few of the infinite number ^"s of liZ 
 emitted .y three luminous points of a n„ndle l^Tt w 
 
 of^an illuminated object ab receives light frtreverTl^rr 
 
IMAGES FORMED. 
 
 327 
 
 !♦ 282. Images formed through small apertures. - Ex- 
 P«Kment.--Cut a hole about Sen square in one side of a box; cover 
 the uole with tin-foil, ana prick a hole iu the foil with a pin. Place the 
 box ,ri a darkened room, and a candle flame in the box near to the pin- 
 hole. Hold an oiled-paper screen before the h.xe in the foil. What do 
 you observe? Can you account for the phenomenon? 
 
 If light from objects illuminated by the sun — e.^., trees, 
 houses, clouds, or even an entire landscape — is allowed to pass 
 through a small aperture iu a window shutter and strilce a 
 wlilte screen, or a white wall in a darlc room, rays carrying with 
 them the color of the points from which they issue will imprint 
 their own color on the screen, and inverted images of the objects 
 in their true colors will appear upon it. The cause of those phe- 
 nomena is easily understood. 
 When no screen intervenes 
 between the candle and the 
 screen A, Figure 237, every 
 point of the screen receives 
 liglit from every point of 
 the candle ; consequently, on 
 every point on A, images of 
 the infinite number of points 
 of the candle are formed. 
 The result of the confusion of images is equivalent to no image, 
 lint let the screen B, containing a small hole, be interposed ; then, 
 since light travels only in straight lines, the point Y' can only 
 receive an imago of the point Y, the point Z' only of the point 
 Z, and so for intermediate points ; hence a distinct image of tlio 
 object must bo formed oa the screen A. Tliat an image may b ; 
 distinct, the raj/ s from different points of the object must not mix 
 on the image, but all rai/s from each point on the object must 
 be carried to its otvn ' - ■' • 
 
 t^JOO 
 
 image 
 

 
 (« 
 
 ■Hi: 
 
 ! i. 
 
 328 
 
 EADIANT ENERGY LIGHT. 
 
 QUESTIONS. 
 l' wu^ ^e images, formed through apertures. Inverted? 
 2. Why ,8 the size of the image dependent on the distance of fho 
 screen from the aperture? uihtance of the 
 
 8. Obtain the dimensions, respectively, of an obiect anri if= . 
 and their respective distances from the fntervening sere" ZT^"^^ 
 tarn the law that determines in aU cases the size o?anTma"; ""- 
 
 5 Whv T" "" T?'" '"'""^^ ^™"^^^ «« '' becomes larger? 
 
 7 wTatTr^"^''' '''''' °"'^"°*^^^^*''^«»t interfering? 
 7. What fact does a gunner recognize in talking sight? 
 
 i 
 § 283. Shadows. — Experiment 1. Procnrp tw« «.^ 
 or card-board, one ISc^ square, tl. other 3^^ s^^ p^^ ^^^^^ 
 between a white wall and a candle flame in a darkened ron^ IT 
 opaque tin intercepts the light that strikes it and then-hv , I" 
 light from a space bcliind it ' ^"^"^^ ^''^^"^^'^ 
 
 This space is called a shado^.o. That portion of the surface 
 of the wal that is darkened is a section of the shadow Xl 
 eprcsents the form of a section of the body that intereep s the 
 hg It A section of a shadow is frequently for convenience 
 cal ed a shadow. Notice that the shadow ll made up o tw" 
 distinct parts, -a dark center bordered on all sides by a mudi 
 lighter fringe. The dark center is called the «..6raf and t 
 hghter envelope is called the jienumbra. 
 
 Experiment 2. Carry the tin nearer the wall, and notice that ih^ 
 penumbra gradually disappears and the outline of the mnbri become 
 .nore distinct. Employ two candle flames, a little distance apartl^ 
 no^^ice that two shadows are produced. Move the tin toward ^wa 
 
 Lthis? "''^ "''''"P '''' ^''"^'^^ ^"^ deepest. Why 
 
 Just so the umbra of every shadow is the part that gets no 
 hyht from a luminous body, tohile the penumbra is the part 
 
 I 
 t 
 b 
 c 
 e 
 ii 
 
SHADOWS. 
 
 829 
 
 that gets light from some poHion of the body, hut not from 
 the whole. '' 
 
 Experiment 3. Repeat the above experiments, employing tlie 
 smaller piece of tin, and note all differences in phenomena that occur 
 
 Hold a hair in the sunlight, about a 
 centimeter in front of a fly-leaf of this 
 book, aud observe the shadow cast by 
 the hair. Then gradually increase the 
 distance between the hair and the leaf, 
 and note the change of phenomena. If 
 the source of light were a single luminous point, as A, Figure 238 the 
 shadow of an opaque body B would be of infinite length, and would con- 
 sist only of an umbra. But, if the source of light has a sensible size, 
 the opaque l)ody will intercept just as mauy separate pencils of light as 
 there are luminous points, and consequently will cast an equal number 
 of mdependent shadows. 
 
 Fig. 239. 
 
 n 
 
 bodv Th' ^'^" ff ^' ^^P'-^^^"* « lu'^^nous body, and CD an opaque 
 body. The pencil from the luminous point A will be intercepted Z 
 tween the lines C P and D G, and the pencil from B will be ntcrcepted 
 between the lines CE and DP. Hence, the ll^ht wHl b' Tc^T"^ 
 eluded only from the space between the lines CF and DP which 
 
 included between the lines CE and CP, and between DP amlDG 
 receives light from certain points of the luminous body, but not from all' 
 
 ;!H 
 
330 
 
 EADIANT ENERGY. — LIGHT. 
 
 QUESTIONS. 
 
 m^rfS^"" '^" ""''"■'' ^°^ ^'""™^''* '^* ""^ '''' «P*1»« body HI. 
 
 2. Wht 5 will a transverse section of an umbra of an opaque bodv h^ 
 larger than the object itself? ^^ ^^^ ^^ 
 
 3. When has an umbra a limited length? 
 
 4. What Is the shape of the umbra cast by the sphere C D, Figure 239 ? 
 6. If CD should become the luminous body, and A B a non-fuminous 
 
 ^aque body, what changes would occur lu the umbra and the sTZw 
 
 6. Why is it difficult to determine the exact point where the umbra 
 of a church-steeple terminates on the ground? 
 
 7. What is the shape of a section of a shadow cast by a circular disk 
 Placed o^,hquely between a luminous body and a screen? What is its 
 shape when the disk is placed edgewise? 
 
 h„?„ '^f' f «««° of the earth's umb.a on the moon 'n an eclipse always 
 
 Fig. 240. 
 
 LI. PHOTOMETRY. 
 
 §284. Law of inverse squares. - Experiment l. Arranjre 
 apparatus as follows : Lay a silver half-dollar on the center of a cS 
 lar p,ece of stiff, white, unglazed paper of 15cm diameter, and rub the 
 entire surface, except the portion covered by the coin, with a sperm or 
 a tallow candle. Hold the paper in a warm oven for a minute. When 
 the paper is placed between two lights in a darkened room, the un- 
 greased spot will appear light on a dark background on the side which 
 receives the more light, and 
 dark on a light background 
 on the side which receives 
 less light ; but the spot be- 
 comes nearly invisible 
 when both sides are cqu.il- 
 ly illuminated. Draw a 
 straight chalk line across 
 a table, and place at right 
 
 . , „ n,njjic iignced caudle. Ra^f-v/av hpfw..Pii fhio 
 
 m It ,s endem thai „-e M, „f tu paper receives fo„ tLe^tto 
 
ue body HI, 
 ^ue body be 
 
 Figure 230? 
 3n-luminous 
 the shadow 
 
 3 the umbra 
 
 Lrcular disk 
 iVhat is its 
 
 ipse always 
 le shape of 
 
 I. Arrange 
 of a circu- 
 ad rub the 
 a sperm or 
 te. When 
 'm, the un- 
 hide which 
 
 the same 
 rveeu this 
 in Figure 
 times the 
 
 PHOTOMETRY. gc^j 
 
 light that the other does. M..ve the row of lights slowly awav from 
 
 c foun in either case where the spot will nearly disappear. Wha^ is 
 the p:,s,tion of the paper with respect to the two sources of light when 
 this occurs? What do you infer? "a iifcni wnen 
 
 Thus, by doubling the distance, the intensity of illumination 
 18 diminished four-fold. In a similar manner it may be shown 
 that at three times the distance it takes i.ine lights to be equiv- 
 alent to one light. Hence, the intenaity of light diminishes as 
 the square of the distance increases. Tliis is callea the law of 
 inverse squares. 
 
 Experiment 2. Introduce the paper disk, as above, between a 
 candle light and a kerosene light or a gas flame, and so regulate the 
 distance that the central spot will disappear, and calculate the relative 
 intensities of the two lights in accordance with the law of inverse 
 squares. 
 
 Apparatus arranged for this irposo is called a photometer. 
 'The candle power, which is the unit of light generally em- 
 ployed m photometry, is the amount of light given by a sperm 
 candle weighing one-sixth of a pound, and burning one hundred 
 and twenty grains an hour." The relative brightness of the com- 
 mon sources of light are approximately as follows ' : — 
 
 Sunlight at the sun's surface loo.ooo candle power. 
 
 Most powei 'ul electric arc 55,000 " '« 
 
 Most powerful calcium light I'soo » <« 
 
 Light of ordinary gas-burin r 12 to 10 '• «« 
 
 Standard candle , ,, „ 
 
 ''The total quantity of light emitted by the sun is equivalent 
 to the light of 6,300,000,000,000,000,000,000,000,000 (six thou- 
 sand three hundred bUlions of billions) candles." Of this enor- 
 
 -J .J _. "s-J!- viiv caiiu mtciuupts oD extremeiv small 
 
 fraction. 
 
 ' 0. ▲. Young. 
 
 
 
ii 
 
 332 
 
 RADIANT ENERGY. — LIGHT. 
 QUESTIONS. 
 
 1- Suppose that a ligiitod candle );; niamfi j,, fi,„ „„ , 
 three cubical roo:ns re^ectlvely 10. 2 X^t^.^T'"^ '"'^ ^^ 
 siugJewall Of the first roo., rL v ;n ^or iLl"; ' "'T'' " 
 
 wall of either of the other rooms? ^^* *'"" * ''"^'^« 
 
 ;.st .0. . rf no. What ^r.j:z:^ ^:i:^:jt^.^ 
 
 of '^he^i:.!;^' i'lT"?'^'"' '''"" fromacandle flame, the area 
 Indl w M "'*'■'* "'^'^ ''^ ** s^'-^e" 75e™ distant from the 
 
 candle wu. ,': .,o^v many times the area of the boar.? Then t™i^hl 
 nterceptod , the bo«rdwill illuminate how much of the surface of 
 the scicen i^ the board is withdrawn? 
 
 4. Give a reason for the law of Inverse Squares. 
 
 6. To wnat besides light has this law been found applicable? 
 
 6. The tvvo sides of a paper disk are iUumiuated equally by a candle 
 «ume 50- distantonone side and agas flame 200cn. distant on the o^^ 
 
 theiJsrcr ''' ''^*^""^" "'"^^ *"^ ^^^^*^ ^' ^^-' ^'«^-- from 
 
 Fig. 241. 
 
 LII. VISUAL ANGLE, ETC. 
 
 r.«rH^^^" ^^^^^1 angle. - Experiment. Prick a oi„-hole in a 
 card place an eye near the hole, and look at a pin ab. . =0^ dista"it 
 Then bnng the pin slowly toward the eye. What do j serve? 
 
 Why IS this ? v-e see an object by mean, 
 on the retina of - eye, and itP, apparent 
 mined by the extent of the retina covered b. 
 
 ^ fTaage formed 
 
 lUuie is deter- 
 
 'oiage. Rays 
 
METHOD OF ESTIMATING SIZE. 
 
 333 
 
 proceeding from opposite extremities of an object, as AB, Fig- 
 ure 241, meet and cross one anotlier in the window of tlie eye, 
 usually called the p?ipi7. Now, as the distance between the 
 points of the blades of a pair of scissors depends upon the 
 angle that the handles form with one another, so the size of the 
 image formed on the retina depends upon the size of the angle, 
 called the visual angle, formed by these rays as they enter the 
 eye. But the size of the visual angle diminishes as the distance 
 of the object from the eye increases, as shown in the diagram ; 
 e.g., at twice the distance the angle is one-half as great, at 
 three times the distance the angle is one-third as great, and so 
 on. Hence, the apparent size of an object diminishes as its dis- 
 tancefrom the eye increases. 
 
 QUESTIONS. 
 
 1. Why do the rails of a railroad track appear to converge as their 
 distance from the observer increases? 
 
 2. Why, in looking through a long hall or tunnel, do the floor and 
 the ceiling appear to approach one another? 
 
 8. Why do parallel lines, retreating from the eye, appear to converge? 
 4. Why can a book, held in front of the face, entkely conceal from 
 view a house? 
 
 § 286. Methods of estimating size. ~ Let a man stand beside 
 a boy of half his hight, and to an observer, twenty feet distant, the for- 
 mer will subtend a visual angle twice as great as the latter, and will 
 appear twice as tall. Then, let the man move back twenty feet farther 
 from the observer, and he and the boy will then subtend equal angles, 
 but they will not appear to be of equal hight, nor will the man's hight 
 appear diminished in a very perceptible degree. The sun and the moon 
 are about 4,000 miles nearer to us when they are in the zenith than when 
 near the horizon, but in the latter case they appear much larger. 
 It makes a great difference in the variation of the apparent size of a 
 pin, as it moved to and from the eye, whether it is seen through a 
 pin-hole in a card or whether the card is removed ; and, again, whether 
 it is seen with one eye or both eyes. Tlie fact is, that In estimating 
 tbe s\w of objects, our judjjment is influenced by many other things 
 
884 
 
 BADIANT KNKHGy. — uoHT. 
 
 besides the visual angles which they subten^l n , 
 real size of au object, also of the fact th« ttt i 7 ^"««^>«^g« ^^ the 
 n distance is to ,Ii„,„ish the apparent .eo /^nr'" "' '"^ '"^^^««« 
 ect does not bccono shorter .'^ it rit al" ',f/' '^"^ "'«» an ob- 
 toward correctin-' an estin.ate based on tho«t T "'' ^^^« '""^h 
 Our estimate of the size of obje L whdo « ?, "','' '*'"^' ^"^'«- 
 enced n.uch by comparison witi obje^ «!« t .1 ,"?'°"^" ^^ ^°«"- 
 islcnown.as in tlie case of the sun an f / '''""'^^ whose size 
 
 nmge with other objects in the I,o,..7 /"""" '^''*'" ^^^y are in 
 
 Whether it is seen alone throu. 1, a Ik ,;"' ? '" "'' ^'^«« «^ ^^^^ P^"- 
 objects. Again, when we lo^k at " 7, "l •''^"J""^tion with other 
 obliged to turn the eyes in ^ , "■'"''^ ^'^'' ''"^h ^yes we are 
 
 upproachesorrecedes.ro rLr;i.r^:''''\r''^'''''"''^^ - - object 
 to enter the eye. The effort nec^ "^ .y t a.,:"; T "''^ff ""^'^ ^^""°- 
 so as to see objects at different cHsta.lr ?' i ^"''"°" °^ ^^^ ^y««. 
 estimate of their size. Hen e 1 e L ''^ '" ^''''"''"^' ^ ^«"««t 
 
 appear to undergo so grea a hiu^eir,^^^^^^^^^ '"^'^ ^>'^« ^«- -* 
 the observer, as when seen by one eye We I T'' *" "^"^ '^^'^ 
 scions of going througl, ll.e processe„ o7t f''* '** *^^ "™« «o°- 
 because it has becom^ a matter 0^1 ,/!;«''•'"* """^'«*^^ «bo^«. 
 readily make these allowances ^he/vTw, ,!!.":; f"'''^' "^ "°^« 
 direction than in a vertical direction, t^m^lt^T "" ^ ^^^'^-"^-^ 
 man seen at a considerable hight lool^s small ..f * '''" P"*^*'"*'- ^ 
 equal distance in a horizontal lec«o„S; '''''''" '"''' ^' ^^ 
 m.on near the horizon may, perhapt "e e^^Tr ^' *'^ ^^^^^ 
 man born blind suddenly acquires tL power of 1 j , "'^^ T^' " * 
 ludicrous mistalies in judging of si/e «,ui n ^' ^"^ ''* '*''*'* '"akes 
 
 hohas not acquired thL meS:. ,: "^,^^ ;^^^^^^^^^ because 
 
 out its hands to seize a bird that ^^^^.^ XZ:^^'' 
 
 so groat that no ordinary ^eansTn m^ll^lV'''' !^ 
 so short. But the distances of the iZvZThll ' '' '' 
 
 -t.t.e that their Ii,htre.nire;r^ 
 
 To illustrate nnp mofhr^H !a* T • -n- 
 
 "trfklng a sl„gK,srr;kT;veryl ?„;■'",!'''''•? ^12 „pre,e„, a clock 
 
 w.c„ a pe.„„ w .... , r °:«r,;':;:s;.f ;.:er rr 
 
VELOCITY OF LIGHT. 
 
 335 
 
 ledge of the 
 an increase 
 that an ob- 
 does much 
 sual angle, 
 vn is influ- 
 whose size 
 hey are in 
 of the pin, 
 with other 
 '^es we are 
 
 an object 
 y continue 
 f the eyes, 
 
 a correct 
 
 does not 
 and from 
 time con- 
 id above, 
 
 we more 
 orizontal 
 ctice. A 
 en at an 
 the large 
 ay- If a 
 St makes 
 
 because 
 ill reach 
 ve. 
 
 t light- 
 peed is 
 e, it is 
 o great 
 ) easily 
 
 1 clock 
 around 
 is four 
 
 miles So long as W remains at E, the strokes come exactly once an 
 hour by his watch; but as lie moves away, the intervals become slightly 
 longer, so that, however long he is on the road, if the watch and c'ock 
 run accurately, when he has reached E' the sound of the bell reaches 
 mn about twenty seconds after the hour. As he continues back to E 
 the sounds come more and more nearly on time, so that at E they are 
 just at the proper time. Similarly, at regular intervals in the heavens 
 
 an eclipse of ono 
 of Jupiter's moons 
 takes place ; the 
 average interval 
 being known, add 
 it to the time at 
 which an eclipse 
 is observed when 
 the earth is hmr 
 E, and thus we may 
 predict the times 
 of an eclipse for 
 years ahead. All 
 the eclipses, ex- 
 cept when the 
 earth is at E, are 
 observed to be a 
 little behind the 
 predicted times ; at 
 . ^ E' as much as ICi 
 
 minutes. But at E' the light has had to travel 184,000,000 miles 
 farther to reach the eye than at E. 
 
 Hence, light must travel at the rate of 184,000,000 -5-(lG^ x 60) 
 = about 186,000 miles (about 300,000'^'") in a second. 
 
 Sound creeps along at the comparatively slow pace of about 
 one-fifth of a mile (or i^"^) per second. The former is tiie ve- 
 locity wHi- which waves in ether arc transmitted ; the latter, the 
 veloci^) V ith which waves in air move forward. This great 
 difference can be accounted for only on the supposition that the 
 rarity and elasticity of ether are enormously greater than that of 
 air. 
 
 I' 
 
 im 
 
nm 
 
 liADiANT ENERGY. — t 
 
 rOHT. 
 
 I-in. REFLECTION OF LIGHT. 
 
 AI, Fi.M„.o 243, is u .>oarrI ...: square 'f /"P'*"*"^ «« follows: 
 fastened to one of its si.I.s. e is a rod 2'' .n,""^' ^ ''''''"' ''"' «a"are 
 c ose to the ..,c,dle of one of the ed'es of he ™V""""' ^" "'^ '^^-'J 
 I th "'V"'-^^'^« °^- the board. D F isan a v nf '"'°''' ""^' P^'-P-ndicu 
 - the rod. Tl,e outer edge of the arc Id T"''^''"'""^ ^^PP^^ted 
 .^' the length of the rod, and Is divided, f ^f'^^''^'^^^ by a radius equal 
 '^r, orifice of the tube C of ul L.Z ; "'"^ '^''^'■^««- ^over the one„ 
 in its center by a elrculaS^.T^";-;? ' -J^'^ ^ ^'-"'- tin ieTc.:;- 
 beam of sunlight mc. ' '" ^i'araeter, and admit a slender 
 
 H St::^ ,^:- - r:;;- - - -m of „,. m. st.. 
 
 A bea™ Of light as it approaches „ obta • ""'? '' '' ""« P^'"*- 
 
 The beam, unable to pass th ough 1 ' ' T''^ '" '■""■^^' '^ *^«'«- 
 
 m.rror, ,s reflected by this surface i ^ ' 'V ''''''^ '"'•'^^''- ^^ ".0 
 
 of the perpendicular oc. A beam nf 1 . ;'''.' ''^ *^^ opposite sldo 
 
 r.;fec.ec. Z.C .«. The «pot of ll^t ''' "'''' ''^'^'^'^ ^^ termed « 
 
 on the arc produced by the re- F i;..24.r 
 
 fleeted beam will be found to be 
 
 the same number of degrees dis- 
 
 tanfromtheperpendiculr^-asthe 
 spot produced by the incident 
 
 the a«^ . 0/ .,;?, '^' 1« '-qual to 
 the angle „,co, called the a„^/.o/ 
 tndence i^eline the mirror s^ 
 
 that the incident be.,,. ay striK-e 
 ^,"'"*^'-"'°''« or le.. obliquely, 
 f " "'^ ^^fl'^<=ted beam will leave 
 
 Ifl ";« /••»th of the incident . 
 
 reflected beam luminous by int • ^ 
 
 ^ ' So,no n^eans of introducW , bo,™ ' u ' ''''"'^ "^ "''"''"' ^"°'" *«"■ '' 
 
 t" experimenting with light Th^ ' '"'"^''' '"'° * '^'"•kened room . In.' 
 
 •ition that the pupil i^ iff'' ^'"'^'^Periments on this subject J^nZ T ''"'"•I^ensablo 
 
 f tooting appaS ' uCll t'hl '' """^ °' -compiis n^ 1 £;::»" "'! '"^P"" 
 'n Mayer and I'lrnanl . m.? v P^'P"'"' "^^aUy called a bL^; ^"^'^"'"'^ ■>^'- con- 
 
 'lescriptfon of an Inexpensive '''""J'"'"'^''." Published by £«; & Shi'^l'"'" ^ '''°- ^"^ 
 »' the Append!,. '^^^"^'^^ "^P-'- ^-i.ed hy the autL^ry^etu^d'-iX^on H 
 
mFFl'SEl) LKiHT. 
 
 337 
 
 *8 follows : 
 8<"" square 
 n the board 
 'erpendicu 
 I supported 
 iilius equal 
 t* the open- 
 tin pierced 
 t a sleuder 
 
 nay strike 
 Jne point. 
 '■^ •< beam. 
 ice of the 
 3sito side 
 termed a 
 
 ;)<>nsablo 
 i suppo- 
 'oi- con- 
 s found 
 )., New 
 on. A 
 ition H 
 
 paper, and the angles formed with the perpendinilar will be onit* 
 
 rrcL it:^ "^ ^'^ ''^ ^^""'' ^^-^^^-^ ^^ ^^^ ^-" ^^- '/ ^i!^- 
 
 of^n!?M ,^^^^?^ "ell*- E'^Perimentl. Introduce a small beam 
 of light into a darkened roon, by means of a parte lumierc anZ'Z 
 n its path a mirror. The light is reflected in I definite dir cT on' If 
 the eye is placed so as to receive the reflected light, it will see not the 
 •mrror, but the image of ■ sun. and the light will ^e paillly i tens* 
 Substitute for the mirror a piece of unglazed paper The ^^t 
 not reflected by the paper In any definite Irectiorbut is s at r d 1 
 every direction, illuminating objects in the vicinity ;„d rendedng hem 
 visible. Looking at the paper, you see, not an image o,' the sun b"t 
 the paper, and you may see it equally well in all directions 
 
 Pig. 244. 
 
 The dull surface of the paper receives light in a definite direc- 
 t but reflects it in every direction ; in other words, it scatters 
 or d^ffu.'.'s the light. The difference in the phenomena in the 
 two cases is caused by the difference in the smoothness of the 
 two reflecting surfaces. AB, Figure 244, represents a smooth 
 surface, like that of glass, which reflects nearlr all the rays of 
 light m the same direction, because nearly ail the o ints of 
 reflection are in the same plane. CD represent, . surface of 
 paper havmg the roughness of its surface greatly exaggerated. 
 Ihe vai u,u8 points of reflection are turned in every possible direc- 
 ■on ; consequently, light is reflected in every direction. Thus, 
 the dull surfaces of various objects around us reflect light in all 
 (Lrections, and are consequently visibV from every side. Obiects 
 rendered visible by reflected light are sud to be illuminated. 
 
 ,"-■ """" -e^-^is::-, :-o;icctcc-. light wf Mcc Imagcs of obiects in 
 
 mirrors, but only in definite directions; by means of diffused ijht we 
 «ee the mirror itself in every direction. Whether we see the image of 
 the source of the light (the eye being situated so as to receive the 
 
 IS 
 
 it 
 
888 
 
 BADIANT ENEBGY. ~ LIGHT. 
 
 '%) i 
 
 liquids at rest are excellent^, rors tl .f "■""• ^""^''^^ -' 
 
 smooth surface of a pond surroZi;,! , . ««'»«"'°e« difficult to see a 
 as th. oyo is occupied byt "e X eVLr' T T^'"^'""^'^^ ^'^^^-' 
 
 faint breath of wind, sligluiyrSS '^'''' "''J^^^^-' ^"t « 
 
 Kxperlment 2. Place aTsS^ . '"'■^''''' ^'" ''^'^^^l the water. 
 
 flan.e so that Its rays na^fort/iLr 7 ' ''''^' ^"" ""''' « ^-''Je 
 
 and notice the brigLnesJoflTs 1„Z T' ''**!! ''' ^"^"'" ^^''f-^. 
 
 so that the incident and reflected ra?« 7 '^" "'""^"^ '^"^ *^« «:^« 
 
 surface o, the li<,uid. and ,^ L IV' ""Tl ^ P"'^"^'«' ^^=^« ^^e 
 
 comes. Notice how much Ster 2 ^'^ .^'^''''' ^''^ ^'"*^« ^«- 
 
 ture appear when viewed verv 1.1 /"'".^'^''^ ''''^^''' °^ ^»™'- 
 
 rcflected less obliquely Also notlp^r^^' '''"" ^^''^" '^^^^ ^^ "S^t 
 
 light .fleeted frol th'e stlZTofZ tllZt b"ef " '^r""^ '^ ''^ 
 
 waujr, onij IS parts are reflected • nf An° oo 
 reflected; at 80", 833 parts ; aad at 8 iV2" Vart^ ^7 r"" 
 IS not even approximate! v tr„« ^i' . , ^ ^''® ^^ove 
 
 metallic reflec'tL, s^h aVgaLa, T' " "'^^^"^^^ '^^^^'^^ 
 
 Ml?4t^Te:i:L^T^i-^^^^^^^ 
 
 gent rays proceeding from the poSt A nf u- * P"''"" ^'^ ^^^^r- 
 
 Perpendlculars at thf points of Sence or"th?"' ^ "^ ^"^«"« 
 rays strike the mirror, and making th!!' , P°'"*' ""^''^ ^hese 
 
 ^,-o„,.„^ ^-, , , ^ "^ »»»n-or. In hke manner ennfitrn/.t „ 
 construct another diagram, and show that conv4ent 
 
Jls, or both 
 ;hnes8 pos- 
 irfaces are 
 urfaces of 
 It to see a 
 by clouds, 
 cts: but a 
 the water. 
 (1 a candle 
 1 surface, 
 id the eye 
 graze the 
 image be- 
 of furnl- 
 by light 
 ing is the 
 sun sets, 
 ft to our 
 
 ncreasss 
 perpen- 
 s a sur- 
 irts are 
 e above 
 having 
 
 ■fires.— 
 f diver- 
 Irectinjj 
 e these 
 qua! to 
 id rays 
 
 after 
 
 VVtOt. ft 
 
 ' ajler 
 ergent 
 
 KEFLECTIOX FUOM I'LANE MlUItOKS. 339 
 
 incident rays arc convergent after reflection. To an eye placed 
 at C, the points from which the rays appear to come are of course 
 
 in the direction of the rays as they 
 oMtor the eye. These points may be 
 found by continuing the rays CB and 
 C E behind the mirror, till they moot 
 at the points D and N. Every point 
 of the object All sends out its peu- 
 ciis of rays, and those that strike 
 the mirror at a suitable angle to be 
 reflected to the eye, produce on the 
 retina of the eye an image of that 
 l)oint, and the point from which the 
 light appears to emanate is found, as 
 previously described. Thus, the pencils EC and BC appear to 
 emanate from the points N and D, and the whole body of li-rht 
 received by the eye seems to come from an apparent object ND 
 behind the mirror. This apparent object is called an image, 
 but as of course there can be no real image formed there, it is 
 called a virtual or ' 
 
 an imaginary image. ^^^^^ 
 
 It will be seen, by 
 construction, that an 
 image in a plane 
 mirror ajrpears as far 
 behind the mirror aa 
 the object is in front of 
 it, and is of the same 
 size and shape as the 
 object. 
 
 r- ^ ^^^ Eeflection from concave mirrors. — Let M M' 
 
 iMg. .-.o represent a section of a concave mirror, which may 
 be regarded as a small part of a hollow spherical shell having a 
 pohshod interior surface. The distance M M' is called tho apcr^ 
 
 1 1' 
 
 ii'i 
 
 1 
 
340 
 
 I^AI>IANTKXEU(JV.-L,GHT. 
 
 tH 
 
 ture of the mirror C ia ti,^ j. ^ 
 
 straight line DG, d a [ u til, ^^ "'*" "' *" ""-"■• A 
 the vertex is c.led ti.e Tn" ^ f L^fT " "' '""''"'•' -" 
 mirror may be eonsidered TlT I ""™''- ^ concave 
 small plane snrfaee Ml ",^0 ft,"" °' "° '"""''» """"er of 
 CB, are p.rpendieular to th sm^.r™' f? ^^' °<^- -»" 
 If C be a luminous point it f, !" , I?™ "■'""'' ""y strike. 
 <Vom this point, and sW J tL? *'■" "" "«'" "'"■""■'ting 
 
 its source at C. "^ "" '"'*™''' ""' he reflected back t^ 
 
 ...e^Li^:„'r::^:rx^r:srtT: ""- -«-■ 
 
 .hat 3trme, the n,l™r. S ITv'Sft "if "^^^ '■™'^" '>"'«'■« 
 and B, and draw the JI„e» AF ami BF It- .r'"" "' '""'^n"* A 
 
 pr;;r,2hr;;;s^^^^^^^^ 
 
 h-es .F and BF represent theXt ttn o^ ra^tirS J" 
 
 It will be seen that the rays after refle<.tr„„ 
 and meet at the point P, called ;: ""ThT """r!^™'' 
 
 and BE would repLrrtbrrer^d'^yr^d*; ^^b^ 
 the focus of these nvK «,-„ ^i , -^ ' " ^ would be 
 
 points is sueb that^i^ e^t: nrC^Lrtr" T """^ 
 
 '■-.» Of one JfZJz: zrz 7 "'r """ "^ 
 
 emanating from E aro u.. I- ^ '^^^^ ^^ and EB 
 
 cnanatin ^o^ f ^l^^ T, tTt^ntT ""'l "^ ''"<'^«- 
 striking the same points nZ. "™ ""^ '""">'' ""d 
 
id is called 
 airror. A 
 ature and 
 ^ concave 
 number of 
 CG, and 
 ey strike, 
 manatinsr 
 d back to 
 
 . To find 
 reflection, 
 nting two 
 '1 of light 
 'idence A 
 reflection 
 lines rep- 
 t>n. The 
 Jctlon. 
 
 ^ergent, 
 t is the 
 
 E. It 
 les AE 
 )uld be 
 ro such 
 'I'ought 
 
 conju- 
 lat the 
 Id EB 
 dFB, 
 r, and 
 riking 
 
 it the 
 '■ rays 
 
 FORMATION OF IMAGES. 
 
 341 
 
 rays may be regarded as practically parallel when their source 
 
 is at a very great distance, e.g., the sun's rays. If a sunbeam, 
 
 consisting of a bundle of parallel rays, as E A, D G, and H B 
 
 (Fig. 249), strike a concave mirror parallel with its principal 
 
 Fig. 249. ^^^^-i *^ey become convergent by reflection, 
 
 and meet at a point (F) in the principal axis. 
 
 This point, called the principal focus, is just 
 
 half-way between the center of curvature and 
 
 the vertex of the mirror. 
 
 On the other hand, it is obvious that diver- 
 gent rays emanating from the principal ftitus 
 of a concave mirror become parallel by reflection. 
 
 If a small piece of paper is placed at the principal focus of a 
 concave mirror, and the mirror is exposed to the parallel rays 
 of the sun, the paper will quickly burn, showing that the focus 
 of light is also a focus of heat; or, in other words, that all forms 
 of radiant energy follow the same laws of reflection as light. 
 
 Construct a diagram, and show that rays of light proceed- 
 ing from a point between the principal f,,^ds and the mirror 
 are divergent after reflection, but less divergent than the inci- 
 dent rays. Reversing the direction of the light, the same dia- 
 gram will show that convergent rays of light are rendered more 
 convergent by reflection from concave mirrors. The general 
 effect of a concave mirror is to increase the convergence or to de- 
 crease the divergence of incident rays. 
 
 The statement, that parallel rays after reflection from a concave 
 mirror meet at the principal focus, is only approximately true. The 
 smaUer the aperture of the mirror, the more nearly true is the state- 
 ment. It is strictly true only of parabolic mirrors, such as are used 
 with the head-lights of locomotives. Construct a diagram representing 
 a mirror of large aperture, and it will be found that those rays that 
 strilie the mirror at considerable distance from its center, intersect the 
 principal asis after reflection at points nearer to the mirror than the 
 principal focus. 
 
 § 292. Formationofimages. — Experiment 1. In a dark room 
 bold the concave side of a bright silver dessert spoon a little distance 
 
ip 
 
 I •( 
 
 342 
 
 KADIANT ENERGY. — LIGHT. 
 
 in front of the face, and introduce a candle-flame between the spoon 
 and your eyes. What do you see? Why? See § 078 "« spoon 
 
 Experiment 2. Turn the convex side of the spoon toward you 
 What do you see? Explaiu. ^ 
 
 Experiments. Repeat the two preceding experiments, holdin-' the 
 spoon between the flame and the ryes, but not so as to screen tlie'lace 
 from the light, and you will see f imilar images of yourself 
 
 To determine the position and kind of images formed of objects 
 placed in front of concave mirrors, proceed as follows: Locate the 
 object, as D L, Figure 250. Draw lines, E A and DB, from the extrem- 
 ities of the object through the center j,j 350 
 of curvature of the mirror, to meet the ' 
 mirror. These lines arc called the sec- 
 ondary axes. Incident rays along these 
 lines will return by the same paths 
 after reflection. (Why?) Draw another 
 line from D to any point in the mirror, 
 e.ff., to F, to represent any other of the 
 
 infinite number of rays emanating from 
 
 D Make the angle of reflection CFD' equal to the angle of in- 
 cidence CFD, and the reflected ray will intersect the secondary avis 
 DB at the point D'. This point is the conjugate focus of all ravs 
 proceechng from D. Consequently, an in.age of the point D is formed 
 at D'. This image is called 
 
 a real image, because rays ■ — ■ Fig. 251. 
 
 actually meet at this point. 
 In a similar manner, And the 
 pointE',thc conjiigate focus 
 of the point E. The images 
 of intermediate points be- 
 tween D and E lie between 
 the points D' and E'; and, 
 consequently, the image of 
 the object liesbetweenthose 
 
 points as extremities. 
 
 If, for the second ray to be drawn from any point we select 
 that ray which is parallel with the principal axis, as AG it n-e '5 
 will not be neeessary to measure angles. For this v^^^^. 
 tion must pass through the principal n„.„s F; and con ...,uent y 1 .■ 
 conju^at. i.cus A' is easily found, and so for the point B' an lute 
 
 
FORMATION OF IJIAGES. 
 
 343 
 
 ■ei) the spoon 
 toward you 
 
 f, liolding the 
 
 t'eeu the taco 
 
 F. 
 
 d of objects 
 
 : Locate the 
 
 ti the extrem- 
 
 10. 
 
 ■v F 
 
 mgle of iu- 
 
 lODdary axis 
 
 of all rays 
 
 D is formed 
 
 we select 
 ,:L,nire 251, It 
 fter rc'll(!c- 
 i|ueiitly til." 
 ' iiiul luter- 
 
 Fig. 252. 
 
 mediate points. Both methods of constructing images should be oraa 
 tised by the pupil. ^ 
 
 It thus appears that an image of an object 2')laced beyond thd 
 center of curvature of a concave mirror is real, inverted, smaller 
 than the object, and located between the center of curvature and the 
 principal focus of the mirror. An eye placed in a suitable posi- 
 
 tion to receive the light, as at 
 II (Fig. 252), will receive the 
 same impressiou from the re, 
 fleeted rays as if the image 
 E' D' were a real object. For 
 a cone of rays originally eman- 
 ates from (say) the point D of 
 the object, but it enters the eye 
 as if emanating from D', and consequently api)ears to originate 
 from the latter point. (§ 278.) A person standing in frontof such 
 a mirror, at a distance greater than its radius of curvature, will 
 see an imago of himself suspended, as it were, in mid-air. Or, 
 if in a darkened room an illuminated object is placed in front of 
 the mirror, ;uid a small oiled-paper screen is placed where the 
 image is formed, a large audience may see the image 'projected 
 upon the screen. 
 
 If E' D' (Fig. 250) is taken as the object, then the direction 
 
 pig_253, ^^ ''"^ ^'S'lt in the diagram 
 
 will be reversed, and ED 
 will represent the image. 
 Hence, the image of an ob- 
 ject jylaced between the prin- 
 cipal f)cns and the center of 
 curvature is also real and 
 inverted, but larger than the 
 object, and located beyond 
 the center of curvature. T'le image in this ease may bo pro- 
 jected upon a screen, but it will not be so bright as in the 
 former case, because the light is spread over a larger surface. 
 
 I 
 
»44 
 
 IlAblANT EKERGV. — MGHT. 
 
 6'ii 
 
 a 
 
 1 
 
 II 
 
 Constnict the image of an object placed between the principa. 
 focus and the mirror, as in Figure 253. It will be seen in this 
 case that a pencil of rays proceeding from 
 
 any point of an object, e.g., D, has no ^'''^' 
 
 actual focus, but appears to proceed from 
 a virtual focus D', back of the mirror, and 
 so with other points, as E. The image of 
 an object placed between the principal focus 
 and the mirror is virtual, erect, larger than 
 the object, and is back of the mirror. 
 
 QUESTIONS. 
 
 Ascertain the answers to tlie following questions bv constructing 
 rrStrr^r "*™"^ --^'^ ^-^ concluslonsTer? 
 
 1. When an object is located at a distance from a concave miiTor 
 equal to Its radius, will any image be formed? Why? 
 ^2^ What is the effect of placing the object at the principal focus? 
 
 ui t£i^? ' :)^:^n[ i^^r '^ ' — -- --er 
 
 Wh;n?s\tTfr:ua;? '"' '"^'" '°™^^' '' ^ concave mirror real? (.) 
 
 virLf? /mT -m'^' "" "'^ "'^''* '"''"^^^ "^y ^ ^«"^'«- ""'-ror real or 
 Inverted? ^^ ''^" "' '"^"'^^ """ *^« «'^^'-*^ C'^) I« " o-ct or 
 
 gesS'^aslo^ttteS o" Si" ^fe dllsftiS""? ^"^^'T*^^ -^- 
 e;;.anating from any point, ^^^'^l^^:^:^ ^S'^i ti^i 
 
 rays? '' *"' ^"''"'"' "'''''' "' " '^"""^^ '"^"'^^ *« '^^"^-t or to scatter 
 
the principa. 
 seen in this 
 
 ig.254. 
 
 constructing 
 s by experi- 
 
 icave mirror 
 
 cipal focus? 
 
 •ror smaller 
 
 • real? (h) 
 
 rror real or 
 3 it erect or 
 
 iicntly sug- 
 
 icil of rays 
 
 a convex 
 
 to scatter 
 
 REFRACTION. 
 
 LIV. REFRACTION, 
 
 345 
 
 Experiment 1. Across the bottom of a rectangular tin basin A B O 
 D Figure 255, mark a scale of millimeterH. lnU> a darkened t!m 
 admit a beam of sunlight, so that its rays may fall obUque^ on ti™ 
 bottom Of the ba^in, and note the place o„ the scale where th'edge of 
 Fig- 255. the shadow 1) E cast by the side of 
 
 the basin 1) (J meets the bottom at E. 
 Then, wlthouft moving the basin, fill 
 it even full with water slightly 
 clouded with milk, or with a few 
 drops of a solution of mastic in alco- 
 hoi. It will be found that the edge 
 of the shadow lias moved from D E 
 to I) F, and meets the bottom at F. 
 Hoat a blackboard rubber, and create 
 a cloud oC (lust In the path of the 
 
 that the rflv« r n fKnf .,. ^'''''^'" '" *'" '^'''' ""'' ^o" ^^ill discover 
 
 that the rays G D that graze the edge of the disk at D become bent 
 at the point where they enter the water, and now move in the ben 
 line GDF, instead of, as formerly, in the straight IlnTrlE tZ path 
 of the light in the water is now nearer to the vertical side DC 
 
 Hrxperiment 2. Place a co n (A FI" -ir.iw «,. m,„ k ** i 
 
 empty basin, so that, as you look thl^S' a'l al'h ' in aTard B^C 
 over the edge of the vessel, the coin is Just out of sight Then tiU^ 
 out movmg the card or basin. All the latter with water No v^n 
 lookmg through the aperture in the card, the coin is v slble T e 
 beam of light AE, which formerly moved n the s al4t Hn^^^^ 
 now bent at E, where it leaves the water, and. pJ::^^^:^^ 
 
 aperture in the card, v.nu-rH the eye. Observe 
 that, as the light {.assc's from the water into 
 the air, it is turned farthi.-r from a vertical Hue 
 EF; in other words, /!/*« beam ia farther from 
 the vertical than before. 
 
 Experiment 3. From the same* position as 
 in the last cxperim(;!it, direct the eye to the 
 point G in the basitj fl!l,:d with water. Reacl» 
 your hand around the basin, and place your 
 fluger where that point appears to be. On ex- 
 amination, It will be found that your finger l» considerably above the 
 
 Klg. 256. 
 
 i; 
 
 m 
 
 V ^1 
 
 ' 'fi 
 

 346 
 
 If 
 
 I*; 
 
 I: 1 
 
 RADIANT ENERGY. — LIGHT. 
 
 Fig. 257. 
 
 bottom Ilonco, the. .feet of the hcnding of rays of light, as they pass 
 obliquely out of tcater, is to cause the bottom to appear more elevated than it 
 really is; m other words, to cause the water to appear shallower than it is 
 
 Experiment 4. Thrust a pencil obliquely into water. 
 What appearance does it present? 
 
 Experiment 5. Place a piece of wire (Fig. 257) verti- 
 cally in front of the eye, and hold a narrow strip of thick 
 plate-glass horizontally across the \vire, so tl.ai tlie light 
 from the wire may pass obli<iuely througli the glass to the 
 eye. Wliat do you observe? Why? 
 
 produced depends entirely upon the light which enters the eye Hence 
 Zl"n"^-r'??-''' "^';' '° '°*''" '''' 'y' '■" ^ ^lifferent direction! 
 sensftLn! '' '"'' "' '^ '" '"^ °"^'' ''''''''' ^'"^''-'^^ ^'>« 
 
 When a beam of light passes from one medmm into another of 
 different density, it is bent or refracted at the boundary phxne 
 between the two media, unless it falls exactly perpendieulurly 
 on this plane. If it passes into a denser medium, it is refracted 
 toward a perpendicular to this plane; if into a rarer medium, it 
 IS refracted from the perpen- 
 dicular. The angle GDO (Fig. 
 255) is called the angle of inci- 
 dence; FDN, the angle of re- 
 fraction; and EDF, the angle 
 of deviation. 
 
 Fig. 258. 
 
 § 293. Cause of refraction. 
 
 — Careful experiments have 
 proved that the velocity of light 
 is less in a dense than in a rare 
 medium. Let the series of par- 
 aliel lines A'B (Fig. 258) repre- 
 
 thll'flf "T-'™"'" '-""S » oXJeet C, and passing 
 through a rectangular piece of dasa DE, and ,.^...«.Lv- 
 
 beam of iig„t. Kvery point in awave-ft'ont moves";'i'tren';,aI 
 vdoc, as long as it traverses the same mediun, ; burtl^ ^ 
 
as they pass 
 valed than it 
 'T than it is. 
 
 Fig. 257. 
 
 INDEX OF RKFJlA(.'TroN. 
 
 J sensation 
 yo. Hence, 
 ; direction, 
 lianges tlie 
 
 nother of 
 aiT plane 
 idicularly 
 refracted 
 edium, it 
 
 passing 
 
 «-.,4.: 
 
 th equal 
 lie point 
 
 847 
 
 « of a given wave ah enters the glass first, and its velocity is 
 impeded, while the point h retains its original velocity; so 
 that, while the point a moves to a', b moves to h\ and the 
 result is that the wave-front assumes a new direction (very 
 much in the same manner as a line of soldiers execute a wheel) , 
 and a ray or a line drawn perpendicularly through the series of 
 waves is turned out of its original direction on entering the glass. 
 Again, the extremity c of a given wave-front cd first emerges 
 from the glass, when its velocity is immediately quickened ; so 
 that, while d advances to d\ c advances to c', and the direction 
 of the ra}- is again changed. The direction of the ray, after 
 emerging from the glass, is parallel to its du-ection before enter- 
 ing it, but it has suffered a lateral displacement. Let C repre- 
 sent a section of the wire used in Exp. 5, and the cause of 
 the phenomenon observed will be apparent. If the beam of 
 light strikes the glass perpendicularly, all jioints of the wave 
 will be checked at the same instant on entering the glass ; con- 
 sequently it will suffer no refraction. 
 
 §294. Index of refraction. - The deviation of light, in 
 i-'ig. 259. passing from one medium to 
 
 another, varies with the me- 
 dium and with tlie angle of 
 incidence. It diminishes as 
 the angle of incidence dimin- 
 ishes, and is zero when the 
 incident ray is normal (i.e., 
 l)erpendicular to the surface 
 of the medium) . It is highly 
 important, knowing the angle 
 of incidence, to be ablo to 
 determine the direction 
 which a ray of light will take 
 on eutenng a new medium. Describe a circle around the point 
 of incidence A (Fig. 259) as a center, with a radius of (say) 
 
 m 
 
1 * I 
 
 tlM 
 
 848 
 
 RADIANT ENEUOY.— LIGHT. 
 
 10 ; through the same point draw IH perpendicular to the 
 surfaces of the two media, and to tbi« line drop perpendiculars 
 BD and CE from the points where the circle cuts the ray in the 
 wo media. Then suppose that the perpendicular B D is A of 
 the radms AB; now this fraction ^ i« called (in Trigonom- 
 etry) the s^ne of the angle DAB. Hence, « is the TeZ' 
 ^e angle of incidence. Again, .f we «upp<«e7hat he ^e^ot 
 dicular CE is p of the radius, then the fraction ,V is 1^^. 
 smeof the angle of refraction. The .incs of the two angles 
 
 7L%^ir''^V".Hf'' ''"^"'^- '^'^^ ^-«-* (- this 
 case t) obtmned by dividing the sine of the angle of incidence 
 
 by the sine of the angle of refraction i« called Uie index ofrefrac 
 Uon. It can be proved to be the raiio of the velocity of the 
 incident to that of the refracted light. U is found that, /or the 
 same media the index of refraction in a constant quantity; i.e., 
 the ;°«'^^«^t ray might be more or \mu oblique, still this quo 
 tient would be the same. ^ 
 
 lii^n * n^'''"''^'. °^ refraction. ~ The Index of refraction for 
 hght m passing from air into water te approximately |, and 
 from air into g^ass f ; and, of course, if the order is reversed, the 
 reciprocal of these fractions must be taken as the indices ; 7 
 from water mto air the index is f , from glass into air |. When 
 a ray passes from a vacuum into a medium, the refraclivc index 
 IS greater than unity, and is called the ahmlute index of refrac 
 
 T'fl n T" ?f ''' '-^ ''^''^'''^^ ^'""^ ^^y ^-^^-^^ ^into 
 another B, zs found by dividing tlie ubmlute index of B by the 
 absolute index of A. ./ j <■ ^ 
 
 The refractive index varies with the .vior of the light. (See 
 W 357.) The following table U i,.t«,„led to represent il 
 
 TABLK OF ABSOLUTE WmCKS. 
 
 Air ui 0' C. and 760""» preesure . 1.000294 
 
 Pure water jjg 
 
 Alcohol J 3» 
 
 Spirits of turpentine 1.4$ 
 
 Humors of the eye (about) . . 1.35 
 
 C»rbon Mmt]phMe j 941 
 
 Crown KiMMt (a\mat) ....!. 1 53 
 
 i'ilai gUm (thont) ] j'gj 
 
 DJanwnd (iibout) .' .' 2.6 
 
 Lca4 cbroowUi ........ 3,97 
 
lar to the 
 'endiculars 
 ray in the 
 D is 3% of 
 rrigonom- 
 le sine of' 
 10 iJerpcn- 
 j% is the 
 wo angles 
 it (in this 
 incidence 
 ofrefrac- 
 •ity of the 
 it, for the 
 tity; i.e., 
 this quo- 
 
 action for 
 
 ly I, and 
 srsed, the 
 )es ; e.g., 
 . When 
 ive index 
 >/ refrac- 
 m A into 
 B by the 
 
 it. (See 
 mt mean 
 
 1.641 
 
 1.63 
 
 1.61 
 
 2.5 
 
 2.07 
 
 ISiXERCISES. 
 
 EXERCISES. 
 
 849 
 
 1. Draw a straight line to represent a surface of flint glass, and 
 draw another line meeting this obliquely to represent a ray of light 
 passing from a vacuum into this medium. Find the direction of the 
 ray after it enters the medium, employing the index as given in the 
 above table. 
 
 2. (a) Determine the index of refraction for light in passing from 
 water mto diamond, (ft) In passing from water into air. 
 
 3. Ascertain the index of refraction for water in Exp. 1, p. 3o0, in 
 
 which sine I (sine of angle of incidence) = |g (Fig. 255), and sine 
 
 R (sine of angle of refraction) = |^ . Hence, Uie index of refraction 
 _ sine_I^^E_C ^FC * "" 
 
 sine li El) fd' 
 
 
350 
 
 RADIANT ENEROV. — ur.HT. 
 
 
 LV. PRISMS AND LENSES. 
 §296. Optical prisms. -An optical prism is usually a 
 transparent wedge-shaped body. Figure 262 represents a 
 transverse section of such a 
 prism. Let AB be a ray of 
 light incident upon one of its 
 surfaces. On entering the 
 prism it is refracted toward the 
 normal, and takes the direction 
 BC. On emerging from the 
 prism, it is again refracted, but 
 
 now from tlie normal in the direction C D. The obleet that 
 enuts the ray will appear to be at F. Observe that the ^y A B 
 at both .tractions, is bent toward the thicker part, or hL, of 
 
 § 2SV. Lenses. -Any transparent medium bounded by two 
 curved .tufaces, or one phmo and the other curved, is a lens. 
 
 tha^Ttl"'"",* '• ?°'"''' '' '^"P'" ""^ ^^"«^« ""^'^^r •" the middle 
 than at the edge; strong spectacle glasses, or the large lenses in an 
 opera glass, will answer. Hold one of the lenses in the sun's rays and 
 
 two hi i^h^'^Vnf ''r? '"^ ''"'*^ ""'' ^^"''^ ^« ''y '"'-'^'^^ together 
 two blackboard rubbers) after it passes through the leus; also, that on 
 
 a paper screen all the rays may be brought to a small circle, or even a 
 
 point, not far from 
 
 the lens. This point ^«-'^'"- 
 
 is called the fortis, 
 
 and its distance from 
 
 the lens, the focal 
 
 length of the lens. 
 
 Find the focal ___^^,^^^^ 
 
 YoTflnd'th'l'V'"^'"*^ f the second, and then of the two together. 
 
 th^-i- J "L *''''* ^^f "*''•« P^^^'-f"! a lens or combination of 
 
 Taraildrs trarer;ir' T^*'' ' *^^* '«' *^« ^^-e quickly are' the 
 para^el rays that enter different parts of the lens brought to cross ono 
 
EFFECT OF LENSES. 
 
 85J 
 
 usually a 
 ^presents a 
 
 )lije(it that 
 10 ray A B, 
 )r base, of 
 
 led by two 
 5 a Ions, 
 
 the middle 
 inses in au 
 s rays, and 
 ig together 
 Iso, that on 
 , or even a 
 
 » together. 
 1 of either 
 nation of 
 ly are the 
 
 CrOSB OQQ 
 
 Experiment 2. Trocuro a lens thinner in the middle than at its 
 edge. One of the small lenses or eye-glasses of an opera glass will 
 answer. Repeat the above experiment with this lens, and notice that 
 the light emerging from the lens, instead ming to a point, becomes 
 
 spread out. 
 
 Lenses are of two classes, converging and diverging, accord- 
 ing as they coll(H't or scatter beuuis of light. Eadi class com- 
 prises three kinds (Fig. 203) : — 
 
 Class I. 
 
 1. Double-convex 
 
 2. Plano-convex 
 
 3. Concjivo -convex 
 
 (or mcniscuB) 
 
 Converging or curivex 
 lenses, thicker in 
 the middle than at 
 the edges. 
 
 Class n. 
 4 Double-eoncavo i ^''•"'Sing. or con. 
 5. I'lano-coneave J '="''' ''■"'^■«' ''"°"«'" 
 
 C. Convexo-concave | '" '^^ ""'^'*'" *'"> 
 L at the edges. 
 
 A straight line, as AB, normal to both surfaces of a lens, 
 and passing througii its center of curvature, is called its pnnci- 
 pal axis. In every lens there is a point in tlie principal axis 
 called the optical center. Every ray of light that passes through 
 it has parallel directions at incidence and emergence, i.e., can 
 suffer at most only a slight lateral displacement. In lenses 1 
 and 4 it is half-way between their respective curved surfaces. 
 A ray, drawn through the optical center from any point of an 
 «)l)ject, us A a (Fig. 209), is called the secondary axis of this 
 point. 
 
 § 298. EflFect of lenses.— "We may, for convenience of illus- 
 Fig.264. tration, regard a convex lens as composed, 
 
 approximately, of two prisms placed base to 
 base, as A (Fig. 2G4), and a concave lens 
 as composed of two prisms with their edges 
 in contact, as B. Inasmuch as a berm or 
 pencil of light ordinaril}- strikes a lens in 
 such a manner that the rays will be bent 
 toward the thicker parts or baaes of these 
 approximate prisms, it is obvious that the lens A would tend 
 to bend the transmitted rays toward one another, while the 
 lens B would tend to separate them. Tfte general eJJTect of all 
 

 ^ 
 
 
 IMAGE EVALUATION 
 TEST TARGET (MT-S) 
 
 1.0 
 
 I.I 
 
 11.25 
 
 If i^ IIIM 
 
 " lis 12.2 
 
 us iija 
 
 2.0 
 
 U III! 1.6 
 
 6" 
 
 PnotDgTdphic 
 
 Sciences 
 Corporation 
 
 33 WEST MAIN STREET 
 
 WEBSTER, NY. 14580 
 
 (716) 872-4503 
 
 / 
 
 o 
 
 
 ^ 
 
 
 
 "^'1%^ ^i\\ "^"^ 
 
 
352 
 
 RADL4.NT ENERGY LIGHT. 
 
 'I'M 
 
 ft '•- 
 
 Vb I 
 
 convex lenses is to converge transmitted rays; and of concave 
 lenses, to cause them to diverge. Incident rays parallel with the 
 principal axis of a convex lens are brought to a focus F (Fig. 265) 
 at a point in the principal .-^xis. This point is called the priyi- 
 cipal focus, i.e., it is the focus of incident rays parallel with the 
 principal axis. It may 
 
 be found by holding the "^' ^ - 
 
 lens so that the rays of 
 the sun may fall perpen- 
 dicularly uiKjn it, and then 
 moving a sheet of jjaper 
 back and forth behind it 
 
 until the image of the _^^ 
 
 sun formed on the paper is brightest and smallest. Or in a room 
 It may be found approximately by holding a lens at a considerable 
 distance from a window (why at a considerable distance?), and 
 regulatmg the distance of the paper so that a distinct ima-e of 
 the wmdow will be projected upon it. The focal length il the 
 distance of the optical center of the lens to the center of the 
 image on the paper. The shorter this distance the greater is 
 the power of the lens. 
 
 If the paper is kept at the principal focu^ for a short 
 time it will take 
 fire. Hence, this 
 is the focus of 
 heat as well as of 
 light. The reason 
 is apparent why 
 convex lenses are 
 sometimes called 
 "burning glasses." 
 A pencil of rays 
 
 ^ITJ"'™ ""' "t?"'^ "^"^ ^ (^'«- 2««). "s « luminous 
 pomt, becomes paraUel on emerging from a convex lens. If 
 
 egress, bnf f.. Avergence is loss than before ; it from a point 
 
J^ 
 
 IMAG"ES FORMED. 
 
 353 
 
 of concave 
 lei with the 
 (Fig.2G5) 
 I the prin- 
 el with the 
 
 ' in a room 
 nsiderable 
 ice?), and 
 t image of 
 gth is the 
 ter of the 
 greater is 
 
 ' a short 
 
 luminous 
 
 lens. If 
 
 rge after 
 
 a point 
 
 beyona the pnncipal focus, the rays are rendered convergc-t 
 A concave lens causes parallel incident rays to diverge as^'if 
 they came from a point, as F (Fig. 26G). This point is there- 
 fore Its principal focus. ~ 
 
 point „ 
 It is, of course, a virtual focus. 
 
 §299. Conjugrate foci. -When a luminous point S (Fig. 
 ^?»i267- 267) send.s 
 
 rays to a con- 
 vex lens, the 
 emergent rays 
 converge to 
 another point 
 S'; rays sent 
 from S' to the 
 
 lens would converge to S. Two points thus related are called 
 conjugate foci. The fact, that rays which emanate fron- one 
 point are caused by convex lenses to collect at one point, 
 gives rise to real images, as in the case of concave mirrors. 
 
 § 300. Images formed. — Fairly distinct images of objects 
 may be formed through ve^^ small apertures (page 327) ; but 
 owing to the sraal' amount of light that passes through the 
 aperture, the images are very deficient in brilliancy. If the 
 aperture is enlarged, brilliancy is increased at the expense of 
 distinctness. (Why ?) A convex lens enables us to obtain both 
 brilliancy and distinctness at the same time. 
 
 Experiment 1. By means of aporte lumiere A (Fig. 268") introduce a 
 Imrizontal beam of light into a darkened room. In its path place some 
 object, as B, pa.nted In tran parent colors or photographed on glass 
 
 T, ITT. P^*^^*'""^' ) ««J'°"^' t"'^ object place a convex lens L, 
 
 bpl nH 1 , T ^ '"■'^■" ^- '^^'^ '"'^''' ^^*"^ Illuminated bv the 
 beam of light, all the rays diverging from any point a are bent by the 
 
 lens so as to come together at the point «'. In like manner, all the rays 
 
 proceeding from c are brought to the same point c' ; and so also for aU 
 
 intermediate polntp. Thus, out of the billions of rays emanating from 
 
354 
 
 RADIANT ENERGY. — LKIHT. 
 
 each of the millions of points on the object, those that reach the iens 
 are guided by it, each to its ovvu appropriate point in the image It 
 IS evident that there must result an image, both bright and distinct 
 provided the screen is suitably placed, i.e., at the place where the 
 rays meet. But if the screen is p'.,ced at S' or S", it is evident 
 that a blurred ima^e will be formed. In.stead of moving the screen 
 back and forth, in order to "focus" the rays properly, it is cus- 
 tomary to move the lens. 
 
 Experiment 2. Fill some globular-shaped glass vessel {c.,j., a flask- 
 decanter, or flsh-aquaiium) with water, and place it 1'" In front of a 
 white wall of a darkened room. A little beyond the vessel place a 
 candle flame, and move it back and f^rth till a distinct image of the 
 flame is projected upon the wall by the water lens. Move the vessel 
 farther from the wall, and, on again focusing the flame, its image will 
 be larger than before. Repeat the same with a glass leus 
 
 Fig. 268. 
 
 By properly varying the distances of the lens and flame from 
 the wall, m the last experiment, you may learn that whei the 
 distance of the object is twice that of the princii)al focus, the 
 object and image are of equal size. When the ima-e is within 
 twice the focal distance it is less, and when beyoii.T tliis same 
 distance it is greater, than the object. In all cases the corre- 
 sponding linear dimensions of an object and its image are to one 
 another directly as their respective distances from the optical center. 
 
 § 301. To construct the image formed by 
 
 — Given the lens L (Fig. 269), whose 
 
 a convex lens, 
 principal focus is at F (or F', 
 
reach the iens 
 1 tlie image. It 
 ht and distinct, 
 lace whore the 
 , it is evident 
 ■ing the screen 
 !rly, it is cus- 
 
 Gl (r.fj., a flask, 
 " In front of a 
 vessel place a 
 t image of the 
 ove the vessel 
 , its image will 
 
 )S. 
 
 li flame f.'roni 
 lat whe:.i the 
 
 il focus, thi' 
 i,t?e is witliiii 
 (1 this same 
 es the corre- 
 fe are to one 
 Dtical center^ 
 
 >nvex lens. 
 at F (or F>, 
 
 VTRTtTAL IMAGES. 
 
 S55 
 
 for ray cou.mg from the other direction), and object A B in front of 
 
 t any two ,)f the many rays from A will determine where its image a 
 
 formed The only t.vo M.at can be traced easily are. the one alon^^ 
 
 the secondary axis AO.. and the one parallel to the principal axis A A^ 
 
 Fig. 269. 
 
 • nd V MfTo" T^T"'^ "" "' *" ""''' "''•^"S'^ *h« Prl'^^'Pal focus F, 
 Is 1 r "f;.*^' ^^'';''";"^^"''«^'^t the principal axis at s6me point a ; so this 
 
 oin a rS'e'ar""'-' ".f. ^' '^^"""'^ ''' ^' ^"^ "" -temcdiate 
 ponits along the arrow. Tlius, a real, inverted image is formed at ab. 
 
 Fig. 270. 
 
 § 302. Virtual images. - Since rays that emanate from a 
 pmnt nearer tlie lens than the principal focus diverge after 
 ^^grcss. It IS evicIeuL that their focus must be virtual and on the 
 same side of the leas as the object. Hence, the image of an 
 
 
i — 
 
 i I 
 
 m 
 
 fiADIANT ENERGY. — LIGHT. 
 
 object placed nearer the lens than the principal foms is virtual, 
 magnified, and erect, as shown in Fig. 270. A convex lens 
 used in this manner is called a simple microscope. 
 
 Since the effect of concave lenses is to scatter transmitted 
 rays, pencils of rays emitted from A and B (Fig. 271), after 
 
 Fig. 271. 
 
 refract!' .liverge as if they came from A' and B', and the 
 image will appear to be at A' B'. Hence, images formed by 
 concave lenses are virtual, erect, and smaller than the object. 
 
 LVI. PRISMATIC ANALYSIS OF LIGHT. — SPE'^TRA. 
 
 § 303. Analysis of white light. — Experiment i. Paste tin- 
 foil smoothly over one side of a glass plate about 5<"" square. In the 
 center of the foil cut a slit S^-" long by imm ^ide, leaving smooth and 
 parallel edges. Place the plate with the slit in the aperture of a 
 parte lumiire so as to exclude all light from a darkened room except 
 that which passes through the slit. Near the slit interpose a double 
 convex lens of (say) 10-inch focus. A narrow sheet of light will 
 traverse the room, and produce an image, A B, of the slit on a white 
 screen placed in its path. Now place a glass prism, C, in the path of 
 the beam with its axis (the straight line connecting the centers of the 
 triangular faces) parallel to A B. (1) The light now is not only turned 
 from its former path, but that which before was a narrow sheet is, 
 after emerging from the prism, spread out fan-like into a wedge-shaped 
 body, with its thickest part restmg on the screen. (2) The image, be- 
 fore only a narrow vertical band, is now drawn out into i long horizontal 
 ribbon of Ught, D E. (8) The image, before white, now contains aU the 
 
PRISMATIC ANALYSIS OP LIGHT. 
 
 357 
 
 » is virtual, 
 onvex lens 
 
 transmitted 
 271), after 
 
 J', and the 
 
 formed by 
 object. 
 
 '"TRA. 
 
 Paste tin- 
 ire. In the 
 iraooth and 
 erture of a 
 Jom except 
 ie a double 
 
 light will 
 on a white 
 the path of 
 iters of the 
 Jnly turned 
 w sheet is, 
 dge-shaped 
 image, be- 
 : horizontal 
 alns all the 
 
 colors of the rainbow, from red at one end to violet at the other • 1 
 passes gradually througli all the gradations of orange, yeUow, groon, 
 blue, and violet. (Tlie diftference in deviation l)et\veen the red and the 
 violet is purposely much exaggerated in the figure.) 
 
 Fig. 272. 
 
 From this experiment we learn (1) that white light is not 
 simple in its composition, but the result of a mixture. (2) The 
 colors of which white light is composed may be separated by re- 
 fraction. (3) The cause of the separation is due to the different 
 degrees of deviation which they undergo by refraction. Red, 
 which is always least turned aside from a straight path, is the 
 least refrangible color. Then follow orange, yellow, green, 
 blue and violet in the order of their refrangibility. The many- 
 colored ribbon of light, B E, is called tlie solar spectrum. This 
 separation of white light into its constituents is called dispersion. 
 The number of colors of which white light is composed is really 
 infinite, but we have names for only seven '^f them; viz., red 
 orange, yellow, green, cyan-blue, ultramaHne-blue, and violet] 
 and these are called the primary or prismatic colors. The names 
 
858 
 
 RADIANT ENERGY. — LIOHT. 
 
 of the blues are derived from the names of the pigments which 
 most closely resemble them. The rainbow is an illustration of 
 a solar spectrum on a grand scale. It is the result of the dis- 
 persion of sunliglit by rain-drops. 
 
 The spectrum may be projected upon a screen, or it may be 
 received directly by the eye, as in the two following experi- 
 inents : — 
 
 Experiment 3. Upon a black card-board A (Fig. 274) paste a strip 
 of white paper .Sc" long and 2n..n wide ; and ,>lace the prism and the eye 
 an in tlie flgiire. Now a beam of white light from the strip is refracted 
 aiul dispersed by the prism, and, falling upor the retina of the eye, you 
 see, not the narrow white strip in its true position, but a spectruin in 
 the position A'. This experimenlj is performed in a lighted room. 
 
 Kxperiment 3. Instead of a continuous white strip, paste short 
 strips of red, whi;e, and blue, end to end, on the black card, as repre- 
 sented in Fig. 275. The spectrum of each 
 color is given on the right, the light portions 
 rei)resenting the illumi- 
 nated parts. It will be seen 
 that in the spectrum of the 
 red, the green, blue, and 
 violet portions are almost 
 completely dark, I)ut there 
 is a faint trace of orange ; 
 
 in the spectnim of the blue, 
 
 the red, orange, and yel- 
 low are wanting, blue and 
 
 ^'iolet are present, and a 
 
 small quantity of green. 
 
 (What lessons does this ox[)eriment teach ?) 
 Experiment 4. — In place of the wliite strip 
 
 of paper used in Exix;riment 2, admit light into 
 
 its spectrum. * ^""''^ '"'''"" ""**'"^'' * "^""'''^ ^"*' *"'^ *^^''""»« 
 
 § 304. Synthesis of white light. -The composition of 
 white light has been ascertained by the oroccssof an^i'-,-. . ean 
 it be veriQed by syntheaiaf-^U., can the colors, after dispersion 
 
ents which 
 istration of 
 of the dis- 
 
 it may be 
 ng cxpen- 
 
 )aste a strip 
 and the eye 
 is refracted 
 lie eye, yoii 
 ipectrmn in 
 room. 
 
 )astc short 
 1, as repre- 
 in of each 
 ht portions 
 
 g. 275. 
 
 COLOR AND DISPEllSION. 
 
 359 
 
 each ?) 
 iiitc .strip 
 li?j:ht into 
 1 examine 
 
 sitiou of 
 
 Ola • *^*-» •-» 
 
 ">•:■ i Villi 
 
 persion, 
 
 1)6 reunited? ;,ud, if so, will the result of the reunion be white 
 liglit? 
 
 Experiment 1. Place a second prism (2) in such a position AV that 
 light which has passed through one prism (]), and l)een refracted and 
 deconiposed, may be refracted bacl<, ami the colors will be reblended 
 and a white image of the slit will be restored on the screen. 
 
 Experiment 3. Place a large convex lens, or a concave mirror, so as 
 to receive the colors after dispersion by a prism, and bring the rays to 
 a focus on a screen. What is the color of the image produced? 
 
 Experiment 3. Receive the spectrum on a common plane mirror 
 and rapidly tip the mirror back and fortli in small arcs at right angles 
 to the path of the light, no as to mingle the ditterent colors on the 
 screen. What is the result? 
 
 § 305. Cause of color and dispersion. — The color of light 
 is determined solely hy the number of loaves emitted by a luminous 
 body in a second of time, or by the con-esponding ivave-length. In 
 a dense medium the short waves are more retarded than the longer 
 ones; hence they are more refracted. This is the cause of dis- 
 persion. The ether waves diminish in length from the red to 
 the violet. As pitcli depends on the numb^ f aerial waves 
 which strike the ear in a second, so color depencs on the num- 
 ber of ethereal waves which strike the eye in a second. From 
 well-established data, determined by a variety of methods (see 
 larger works), physicists have calculated the number of waves 
 that succeed one another for each of the several prismatic colors, 
 and the corresponding wave-lengths ; the following table con- 
 tains the results. The letters A, C, D, etc., refer to Fraunho- 
 fijr's lines. 
 
 Dark red A 
 
 Orange C 
 
 I.wigtli of waves 
 iu niilliinetci'8. 
 
 No. of waves 
 per secoud. 
 
 0<^07G0 iW5,000,00(),000,0()0 
 
 ,, „ *^00C5() 458,000,000,000,000 
 
 ^^'"*^'^^' I^ 00058!) 5io.ooo.nnn nno ooo 
 
 ^'^:"" ^'^ 000527 570,000,000,000,000 
 
 C- Blue F 00048G 
 
 U. Blue G 
 
 Violet... H. 
 
 018,000,000,000,000 
 
 ■"•'0^=51 «!)7,000,000,000,000 
 
 ■000397 7G0,000,000,000,000 
 
 5)H 
 
 'ill 
 
360 
 
 RADIANT ENERGY. -—LIGHT. 
 
 Tlioro .H a limit to the sensibility of the eye as well as of the 
 enr. I !,„ l„„it in the n.unber of vibrations appreciable by the 
 ^0 Hen approximately within the range of numbers given in the 
 above table ; U., if the succession of waves is much more or 
 ess rapid tlmn irulicate.l by these numbers thev do not produce 
 the sensation of sight. It is evident that the frequency of the 
 waves emitird h,, « haninous hoOy, and consequently the color of the ' 
 lnj.te,,utl,d,mmt depend on the rapidity of the vibratory motions 
 iphe nolevub'H of that body, i.e., upon its temperature. This 
 has b(.«M. shown in a convincing manner as follows: The tem- 
 perature of a platinum wire is slowly raised l)y passing a gradu- 
 ally nK..rc.aHn.g c.u-rent of electricity through it. At a tempera- 
 ure of alK,nt 540° C. it begins to emit light ; and the light, ana 
 lyzed by a pnsrn, shows that it emits only red IwU. As the 
 temperature rises there will be added to the red of the spec- 
 trum, first yellow, then green, blue, and violet successively. 
 When It reaches a white heat it emits all the prismatic colors. 
 It 18 signl leant that a white-hot body emits more red light than 
 a red-hot body, and likewise more light of every color than at 
 any lower tomperature. The couclusion is that a body which 
 emus white light sends forth simultaneously waves of a variety of 
 
 § 306. Heat and chemical spectra. — If a sensitive ther- 
 mometer ,s placed in different parts of the solar spectrum it 
 will indicate heat in all parts ; but the heat generally increases 
 from the violet toward the red. It does not cease, however, 
 with the limit of the visible spectrum; indeed, if the prism is 
 made of flint-glass, the greatest heat is just bevond the red A 
 strip of paper wet with a solution of chloride of silver suffei-s 
 no change in the dark ; in tiie light it quickly turns black ; ex- 
 posed to the light of the solar spectrum it turns dark, but quite 
 unevenly. The changn is slowest in the red, and constantly 
 increases, till about the region between blue and violet, when 
 It attains its maximum ; from this point it falls off and ceases 
 
ONLY ONE KIND OF RADIATION. 361 
 
 at a point cousiderably buy«n,l tf.o limit of the violet. It thi.s 
 appears that the solar HiHK^tnun is not limited to the visible 
 spectrnm but extends beyond at each extremity. Those ra 
 
 wll ! T^ ,""1 '"^ "'' "''"""y ^'-^"^^^ '^^ ^^^^-red rays, 
 whde those that he beyond tha violet arc called the ultra-violet 
 
 thr;,!. , . ;'f '"^' "•" ^' ^'^'^S^'' vibration-period, and 
 
 the ultra-violet of shorter in^rlcKl, than the luminous rays. 
 
 § 307. Only one kind of radiation. -The fact that radi- 
 ant energy produces three dJHtinct effects-viz., luminous 
 heatmg, and chemical -ban given rise to a quite preva eu Tr.: 
 that there are three distinct kl„d« of nuliation. There is, how- 
 ever, absolute y no proof that those different effects are produced 
 by different kinds of radiation. The same radiation tkat pro- 
 
 frr.r'T ""'^ ^'"''"^' ^''^^ «^*^ chemical action. The fact 
 that the ultra-red and ultra-violet rays do not affect the eye 
 
 th^r W -T. ' y ''' "' " ^"^^^^"' "^*-« f-m tho'se 
 
 bihty of the eye to receive Impressions from radiation. Just as 
 
 felVtn Ttr""" ""^ "^ '^"»' '^"^ «*h^^« ^' too short, 
 period to affect the ear, so there are ethereal waves, some o 
 
 too long and other, too short, period to affect the eye. I 
 IS true, however, that wave« of long period from the sun are 
 more energetic m producing heating effects than those of short 
 penod ; and those of short period are more effective in .vener- 
 ating chemical action In certain substances than those of lon<. 
 period ; while only those which lie between the extremes affect 
 
 uUG 6YG* 
 
 LVII. COLOK. 
 § 308. Color produced by abaomf.i.m — " a ii ^.r^i^cts -p 
 
 ^J^ "" '"!^'" "r' '«^^i"ivale;;t"'to~saying';h7L7:;, 
 hght there ^s no color. U color a quality of an object, or is it a 
 quality of the light which Illuminates the object ?" 
 
I 
 
 862 
 
 RADIANT ENERGY. —. LIGHT. 
 
 Experiment 1. Common salt Introduced Into a Bunsen flame reudcr.s 
 It luminous, and the light, when analyzed with a prism. Is found to 
 contain only yellow. Expose papers or fabrics of various colors to this 
 light in a darkened room. No one of them exhibits its natural color except 
 yellow. , ■' 
 
 Experiment 2. Hold a narrow strip of red paper or ril)bon In the 
 red portion of the solar spectnun; ()l,ser\-e its color. Slowly move it 
 toward the otlier end of the spectrum, carefully observing Its appear- 
 ance as it moves througli the different colors. Uepeat the experiment 
 using other colors. Tabulate the results of these experiments 
 
 These experiments show that (1) color is a quality of the light 
 which illuminates, and not of the object illuminated; (2) in order 
 that an object may appear of a certain color it must receive light 
 of that color; and of course if it receives other colors at the same 
 time it must be cajmble of absorbing them. The enei-gy of the 
 waves absorbed is converted into heat, and warms th'e object. 
 Wiien white light strikes an object it appears white if it reflects 
 all the colors. If red light falls upon the same object it appears 
 led, for it U capable of reflecting red ; or it appears green if 
 green light alone falls on it. If white light falls upon an object, 
 and all the colors are absorbed except the blue, the object 
 appears blue. When we paint our houses we do not apply 
 color to them. We apply substances, cvi\\ei\ pigments, that have 
 a property of absorbing all the colors except those which we 
 would have our houses appear. 
 
 Experiments. By means of mporte lumih-e introduce a beam of 
 hght into a dark room. Cover the oriflce with a deep red (cot.per) -lass 
 Tlie white light, in passing through the glass, appears to be colored 
 red. Does the glass color the light red t 
 
 P..perm>.ent 4. With the slit and prism form a solar spectrum, and 
 between the prism and screen interpose the red glass. What is the re- 
 sult? Ddes the glass color the light? If not, what does the glass do' 
 
THERMAL EFFECTS OP RADIATION. 
 
 363 
 
 I flame rcutlcrs 
 1, Is found to 
 1 colors to this 
 'al color except 
 
 ribbon in tlic 
 lowly move it 
 ij,' its appoar- 
 j cxporiinont, 
 lents. 
 
 / of the light 
 
 (2) in order 
 
 receive light 
 
 at the samp. 
 
 ergy of the 
 
 the object. 
 
 if it reflects 
 
 t it appears 
 
 rs green if 
 
 a au object, 
 
 the object 
 
 not apply 
 
 J, that have 
 
 3 which we 
 
 a beam of 
 l)per) glass. 
 3 be colored 
 
 tjctrum, and 
 It is the re- 
 3 glass do? 
 
 LIX. THERMAL EFFECTS OF RADIATION. 
 
 § 30a Diathermancy and athermancy. —What becomes 
 of radiations that strike a body depends largely upon the char- 
 ucter of the body. If the nature of the body is such that its 
 molecules can accept the motion of the ether, the undulations 
 of ether are said to l)e absorbed by the body, and the body is 
 thereby heated ; that is, the undulations of ether are trans- 
 formed into molecular motion or heat. A good illustration of 
 this is the experiment with blackened glass, page 321. On iJie 
 other hand, the unblackcned glass allows the radiations to pass 
 freely through it, and very little is transformed into heat. 
 Notice how cold window-glass may remain, while radiations 
 pour through it and heat objects within the room. It must be 
 constantly borne in mind, that only those radiations that a body 
 absorbs heat it; those that pass through it do not affect its tern- 
 perature. Bodies that transmit radiant heat freely are said to 
 be diathermanoiis, while those that absorb it largely are called 
 athertnanous. The most diath- nous solid is rock salt. 
 Among the most athermanous solu are lamp-black and alum. 
 Carbon bisulphide, among liquids, is exceptional^' transparent 
 to all forms of radiation; while water, transparent to short 
 waves, absorbs the longer waves, and is thus quite athermanous. 
 
 Expertment 1. Bring the bulb of an air thermometer into the 
 focus of a burning-glass exposed to the sun's rays. The radiation 
 concentrated on the enclosed air scarcely affects this delicate iustru- 
 nient. 
 
 Experiment 2. Cover the outside of the bulb of the air thermom- 
 eter with lamp-black and repeat the last experiment. The lamp-black 
 absorbs the radiant heat, and the heat conducted through the glass to 
 the enclosed air raises its temperature and causes it to expand and 
 rapidly push the liquid out of the stem. 
 
 Dry air is almost perfectly diathermanous. All of the sun's 
 
 '■ -"^ eartn paaa mruugh a layer or air, from 
 
 fifty to two hundred mUes in depth, which contains a vast 
 
 ii 
 
864 
 
 RADIANT ENERGY. — LIGHT. 
 
 ;. 
 
 an»„„t of aqnoom vapor. This vapor, like water, is compara- 
 tively opaque to long waves; Iience it modifies very ITtte 
 el:aracter of tbe radiations whiol, reach the earth, nlfalt 
 together w.th what we have learned from Exp. 2, enablf „' 
 
 1^ T, *"' ""'""^ "^ "'■'■'^ 0" atmosphee become 
 heated. First, a very considerable portion of the radiant re^v 
 which comes to us from the sun, in the form of relat"veirro™ 
 «ves, i, stopped by the watery vapor in the air w^Lti f 
 ^..sequence, heated. Most of that which escape tW ab onT 
 ^on heats the earth by falling upon it. The warmed eartht!, 
 ■ts heat, _ partly by conduction to the air, still more at ll bv 
 
 rre::ht:erd^tra;-2^^^^^^^^ 
 
 waves „Meh are most readily stopped by the a mospLr flic? 
 « e atmosphere or rather the aqneous vapor of the atmCe e 
 ate as „ sort of trap for the energy which comes to us fim the 
 sun Remove the watery vapor (which serves as a " blanket " 
 to the earth) from our atmosphere, and the chill result! g from 
 the r.ap,d escape of heat by radiation would put an end toT 
 
 TuT LT bTT "'" '""'^ "^' ■'°' Lee::i:z 
 
 riat d L™ '""' ""^ '""'"""'"^' ^'"■«'=° "^ "•<»" ">e heat 
 radiated from a stove or any other terrestrial object. Glass is 
 
 diathermanous to the sun's radiations (simply because thev 
 
 T::tXaT :: "^ ^^^^ '--^ -e^byroVe- 
 
 absoipton), but quite athermanous to other radiations. This 
 
 Thrf nth!: '" *^ "" "' ""'-"""^ »»• greeu-hou! 
 1 be sun s heat passes through the glass of these enclosure, 
 almost unobstructed, and heats the earth, but the »us 
 pven out in turn by the earth are such as cannot m s Z 
 thtougb tte glass, hence the heat is retataed wi^i: ^1^ 
 
, is compara- 
 ry much the 
 This fact, 
 2, enable us 
 ere becomes 
 diant energy 
 atively long 
 which is, in 
 this absorp- 
 i earth loses 
 e largely by 
 Br, has been 
 low temper- 
 ransmitted ; 
 short waves 
 
 these Ions: 
 sre ; hence, 
 itmosphere, 
 us from the 
 " blanket " 
 ilting from 
 
 end to all 
 IS from the 
 m the heat 
 Glass ia 
 !ause they 
 tmospheric 
 ms. This 
 en-houses, 
 nclosures, 
 radiations 
 
 pass out 
 the endo 
 
 GOOD ABSORBERS. 
 
 365 
 
 § ^10. All bodies radiate heat. — Hot bodies umally part 
 with their heat much more rapidly by radiation than by all other 
 processes combined. But cold bodies, like ice, radiate heat even 
 when surrounded by warm bodies. This must be so from the 
 nature of the case, for the molecules of the coldest bodies, 
 possess some motion, and being surrounded by ether, they can- 
 not move without imparting some of their motion to the ether, 
 and to that extent losina: some of their own motion. 
 
 § 31L Theory of Exchanges. — Let us suppose that we 
 have two bodies, A and B, at different temperatures, —A warmer 
 than B. Radiation takes place not only from A to B, but from 
 B to A ; but, in consequence of A's excess of temperature, more 
 heat passes from A to B than from B to A, and this continues 
 until both bodies acquire the same temperature. At this point 
 radiation by no means ceases, but each now gives as much as it 
 receives, and thus equilibrium is kept up. This is known as 
 the " Theoi-y of Exchanges." 
 
 § 312. Good absorbers, good radiators. — Experiment. 
 
 Select two small tin boxes of equal capacity, — one should be bright out- 
 side, while the other should be covered thinly with soot from a candle 
 flame. Cut a hole in the cover of each box large enough to admit the 
 bulb of a thermometer. Fill both boxes with hot water, and introduce 
 into each a thermometer. They will register the same temperature at 
 first. Set both in a cool place, and in half an hour you will find that 
 the thermometer in the blackened box registers several degrees lower 
 than the other. Then fill both with cold water, and set them in front 
 of a fire or in the sunshine, and it will be found that the temperature 
 in the blackened box rises fastest. 
 
 As bodies diflTer widely in their absorbing power, so they do 
 m their radiating power, and it is found to be universally true 
 that good absorbers are good radiators, and bad absorbers are 
 bad radiators. Much, in both cases, depends upon the charac- 
 ter of the surface as well as the substance. Bright, polished 
 surfaces are poor absorbers and poor radiators ; while tarnished, 
 dark, and roughened surfaces absorb and radiata heat rapidly. 
 
 ^:| 
 
866 
 
 RADIANT ENEUGY. — LIGHT. 
 
 Dark clothing absorbs and radiates heat more rapidly than light 
 (Which IS better to wear at all seasons? Why? Why are cer 
 tarn parts of steam engines kept scrupulously bright?) 
 
 fZ^^^' ^T' ~ ^^''^"''"^"^ °° •^l^borate experiments to show 
 that some bod.es radiate heat more rapidly than others. AH 
 nature testifies to this every still, cloudless summer night. Dur- 
 .ng the day objects on the earth's surface receive more heat bv 
 radiation than they lose but -m «nnn no fi . ^ 
 
 isrPvPr«,pH T. '°^®' '^"'^ '^« soon as the sun has set this 
 IS rev crsed. Then everything begins to cool as its heat is radi- 
 ated mto space. Objects becoming cool, the air in contact with 
 hem becomes chilled ; its watery vapor condenses, and colli ts 
 in tiny hquid drops on their surfaces. But these dew-drops 
 CO ect much more abundantly on certain things, such as grasse 
 and leaves, than on others, such as stones and earth The 
 reason that it does not collect on the latter so freely, is because 
 of their poor radiating power ; they do not get cool Js rapid" 
 
 Fig. 294. 
 
 LX. SOME OPTICAL INSTRUMENTS 
 
 oLject a.ore than oa,. be done convonic,, y^^':, rf™^:;' 
 "ess by a »,„gfc ,e,„, two convex huso, are ."ed -1?^ 
 H- 2^4) called t.e o.;.c<y«, to fora. a n^I^ZLSL^^ 
 
ASTRONOMICAL TELESCOPE. 
 
 367 
 
 lly than light. 
 »Vhy are cer- 
 t?) 
 
 ents to show 
 others. All 
 night. Dur- 
 nore heat by 
 has set this 
 heat is radi- 
 contact with 
 and collects 
 3 dow-droi)s 
 li as grasses 
 earth. The 
 ', is because 
 as rapidly. 
 
 microscope 
 liagnify an 
 h dfstinct- 
 — one (O, 
 real image 
 
 A'B' of the object AB ; and the other (E) called the eye-glass, 
 to magnify this image so that the image A'B' appears of the size 
 A"B". In the same sense as we look at the object with one 
 lens when we use a simple microscope, here we look at A'B'. 
 
 § 315. Astronomical telescope. — The astronomical re- 
 fracting telescope consists essentially, like the compound micro- 
 scope, of two lenses. The object-glass (O, Fig. 29o) forms a 
 real diminished image ab of the object AB; this image, seen 
 through the eye-glass E, appears magnified and of the size cd. 
 The object-glass is of large diameter, in order to collect as 
 much light as possible from a distant ol)ject for a better illumi- 
 nation of the image. Some idea of the power of some of our 
 
 Pig. 295. 
 
 best telescopes may be obtained from the fact that Mr. Clark 
 of Cambridgeport has made a telescope of such magnifying 
 power, and possessing such distinctness of definition, that a ball 
 two inches in diameter, and two hundred and fifty miles distant 
 (about the distance i)etween Boston and New York), would be 
 difetiuctly visible as a body of perceptible dimensions, if properly 
 illuminated. 
 
 § 316. Photographer's camera. — Tlie ]ihotographer's cam- 
 era, or ca.mp.ra ohscura, of which AB, Figure 20G, represents a 
 vertical section, consists of a dark box painted black on the 
 interior. A screen of ground glass S forms a partition in the 
 
S68 
 
 I 
 
 RADIANT ENERGY. — LIGHT. 
 
 box. A sliding tube T contains a convex lens T if u- . 
 r> IS placed some distance in front and fL !i . ° ""^J"'* 
 
 from the screen is suitably adjS' a dt tt f' 1 • '' '"' 
 image can be seen nnon ,?''' '^'^*'^°*' ^^^1' and inverted 
 
 aperture C. Vhl Zlntl ''"'" '^ ^°°'^"^ ^^^-g^ the 
 grapher replaclte g ur|L:: ZTl '"""'' •'' '''''^■ 
 and the chemical power of thf sun'f ^ ' '''''•*'''^ P'^^' 
 
 of the object on this plate! '" ^""'^ " *^« ^'^^^^ 
 
 Fig. 296. 
 
 Fig. 297. 
 
 called the cornea. A tough membrane 
 2, of which the cornea is a continua- 
 tion, forms the outer wall of the eye 
 and Is called the sclerotic coat, or 
 " white of the eye." This coat is 
 liued ou the interior with a delicate 
 membrane 3, called the choroid coat; 
 the latter is covered with a black 
 pigment, which prevents internal 
 reflection. The inmost coat 4, called 
 «ie mma, is formed by expansion 
 Of the optic nerve 0. The front of 
 the choroid coat ii is called the Ms; 
 Its color constitutes the so-called 
 
 can:d7::trw;oseVut^^^^^^^^^^ 
 
 ment and contraction, the ,uJtV/o7t^:±ZfTr^^^^^^^^ 
 vfiamDcr oi ihe eye. Just hnov ^f *\. 7 , . '"' ""^ "itcnor 
 
 transparent body 6 called .>•, " ^ *°"^''' '^*'"'' "^^^ 
 
 ay 6, called the crystalline lens. This lens dlvldei the 
 
If an object 
 ;e of the lens 
 and inverted 
 through the 
 i, the photo- 
 litized plate, 
 true picture 
 
 HUMAN EYE, 
 
 869 
 
 Z3 
 
 a horizontal 
 le eye, like a 
 
 opening 5, 
 •y enlarge- 
 ic Interior 
 astic, and 
 ivldes the 
 
 eye into two chambers ; the anterior chamber 7 is filled with a limpid 
 Uquid, called the aqnenus humor; tlie posterior chambei 8 is filled with 
 a jelly-lilie substance, called the vitreous humor. 
 
 The eye is a cnmora obscura, in which the retina serves as a 
 screen. Images of outside objects are projected by means of 
 the crystalline Ions, assisted by the refractive powers of the 
 humors, upon this screen, and the impressions thereby made on 
 this delicate network of nerve filaments are conveyed by the 
 optic nerve to the brain. Jf tlie two outer coatings are removed 
 from the back part of the eye of an ox, recently killed, so as tO 
 render it somewhat transparent, true images of whole land- 
 scapes may bo seen formed upon the retina of the eye, when it 
 is held in front of your eye. With the ordinary camera, the 
 distance of the li-ns from the screen must be regulated to adapt 
 itself to the varying distances of outside objects, in order tliat 
 the images may bo i>rop(!rly focused on the screen. In the eye 
 this is accomplished by changing the convexity of the lens. We 
 can almost instantly and involuntarily change the lens of the 
 eye, so as to form on the retina a distinct image of an object 
 miles away or only a few inches distant. The nearest limit at 
 which an object can be placed, and form a distinct image on the 
 retma, is about five inches. On the other hand, the normal eye 
 m a passive state is adjusted for objects at an infinite distance. 
 Curious enough, the retina on careful examination is found to 
 be covered with little projections which have received, from 
 their appearance, the names of rods and cones. These project 
 from the nerve fibres very much like nap from the threads of 
 velvet. It is thought that these rods and cones receive and 
 respond to the vibrations of light ; in other words, that they 
 co-Tibrate with the undulations of the ether, and thereby we get 
 our sensation of light. 
 
 . . _.hromatio aberration. — There is a serious defect 
 in ordinary convex lenses, to which we have not before alluded, 
 caUed chromatic aberration, which has required the highest skill 
 
 
370 
 
 RADIANT ENERGY. — LIGHT. 
 
 Of man to correct. The c6nvex lens both refracts and disperses 
 the hght that passes through it. The tendency, of course, is to 
 bring the more refrangible rays, as the violet, to a focus much 
 sooner than the less refrangible rays, such as the red. The 
 result IS a disagreeable coloration of the images that are formed 
 by the lens, especially by that portion of the light that passes 
 through the lens near its edges. This evil has been overcome 
 very effectually by combining with the convex lens a plano-con- 
 cave lens. Now, if a crown-glass convex lens is taken, 
 a flint-glass concave lens may be prepared that will 
 correct the dispersion of tlie former without neutralizing 
 all its refraction.' The possibility of this is due to the 
 fact that flint-glass disperses more strongly in propor- 
 tion to its refractive power than crown-glass. A com- 
 lK)und lens, composed of these two lenses (Fig. 298) 
 cemented together, constitutes what is called an achromatic lens. 
 
 Fig. 299. 
 
 Fig. 2n8. 
 
 § 319. Stereopticon. — This instrument is extensively em- 
 ployed in the lecture-room for producing on a screen magnified 
 images of small transparent pictures on glass, called dides; 
 also for rendering a certain class of experiments visible to a 
 large audience by projecting them on a screen. The light most 
 commonly used is the lime light, though the electric light is pre- 
 ferred for a certain class of projections. The flame of an oxyhy- 
 drogen blow-pipe. A, Fig. 299, is directed against a stick of 
 lime B, and raises it to a white heat. The light of the lime is 
 converged-^ by means of a convex lens c, called the condensing 
 
 > The refractiTe and dliperiire pow«r. of the two Iot««i are not proporUon.1. 
 
CHROMATIC ABBE RATION. 
 
 371 
 
 Ffg. 208. 
 
 lens (usually two plano-convex lenses are used)— upon the slide 
 D, and strongly illuminates it. In front of it is placed another 
 convex lensE, (or a system of lenses), called the projecting lens. 
 Tlie latter lens produces (or projects) a real, inverted, and 
 magnified imago of the picture on the screen S. The n^ounted 
 lens E may be slid back and forth on the bar F, so as properly 
 to focus the image. (For useful information relating to the 
 operation of projection, see Dolbear's Art of Projection.) 
 
¥?• 
 
APPENDIX. 
 
bnnik: 
 
 jTMIIIInMUn. 
 
 Inehe*. 
 
 J5 |6_ 
 
 CeutUneten. 
 
 Sqiur* 
 Oentimttw 
 
 The area of this figure is a square decimeter. 
 A cube of water, one of whose sides is this area, 
 Is a cubic declmster or a liter of water, and at the 
 temperature of 4" C. weighs a kilogram. The 
 same volume of air at 0" C, and under a pressure 
 of one atmosphere, weighs 1.293 grams. The 
 mm is the weight of loo of pure water at 4' C. 
 
 th< 
 in 
 of 
 mi 
 l)e 
 on 
 ret 
 of 1 
 At 
 mei 
 
 puj 
 
 Square Inch. 
 
J8 l» HI 
 
 S, 
 
 Square Inch. 
 
 APPENDIX. 
 
 BHOTION A. 
 
 measures ^ *"® ™®*®^ *"** o*^her metric 
 
 TABLE OF LENGTHS. 
 
 10 mllllmeten. (mm) _ j centimeter (-cmV 
 OceutimetorH - 1 decimeter (W. 
 
 lOdeclmeterB = 1 meter fm) 
 
 1000 meters ^ , k„ometer (k»). 
 
 TABLE OF AREAS, 
 
 100 square mllllmeter» («mm^ ^ i „n„„r„ „„„« x , 
 100 square centimet«rl i ^ centimeter (icm). 
 
 100 - - cenumet«r« == i square decimeter rqdmv 
 
 — i square meter (•>">). 
 
 = 1 square Jcilometer (Qkm). 
 
 1,000,000 square raeteri 
 
370 
 
 APPENDIX. 
 
 TABLE OP VOLrTMES. 
 
 1000 cubic millimeters (-m) . , cubic centimeter (-'or-) 
 000 cub c centimeters = i cubic decimeter (L^ 
 
 1000 cubic decimeters = 1 cubic meter (cbm;. ^ 
 
 oflllrr^^'Tn^ ^"l»i''« and gases are either expressed in t],. , nita 
 of the above table or in liters. The liter is Icdm, ^r lOOoV 
 
 TABLE OP MASSES OR WEIGHTS. 
 
 10 milligrams (-"«) = i centigram (eg). 
 10 centigrams = i decigram ^<i«). 
 10 decigrams' = i gram (k). 
 
 1000 grams = i kilogram or kilo (k). 
 
 TABLE OF EQUIVALENTS. 
 1 inch = 0.0254 meter, or about 2J centimeters. 
 
 yard! u^r- -'^''-* 30 centimeters, 
 iyard- 0.9144 meter, or ai)out {? meter 
 1 m.le = 1609.0000 meters, or about l^ kilometers. 
 
 1 U.S. ^ "^"^<^ quart = ^ 0.946 liter, „ ,.^^, . igg^ 
 
 1 dry y 1 1.101 liters, "" J'"Ie { JJ^^^ than 1 liter; 
 
 1 U.S. gallon = 8.785 liters, or about 3/^ liters. 
 
 1 J avoirdupois . ^no^.- , ., 
 
 ^ Troy and apothecaries' ounce = <{ 0.(L-;; 1<. o, .^ rather <[ 
 than 30 grams. 
 
 1 avoirdupois pound = 0.46359 kilo, or about ^^ kilo. 
 
 reriTat'"""^ '' "°*"^^^^^^' '' -" ^ ^-"^ convenient to 
 
 centimeters x f = inches (nearly) ; 
 inches X f = centimeters (nearly) • 
 
 5 meters = 1 rod rnearlv^ • 
 
 ^so, kilos XV = pounds (nearly); 
 
 pounds XA = kilos (nearly). 
 
 less 
 more 
 
APPENDIX, 
 
 877 
 
 ["'"'or'"). 
 
 in tlie units 
 
 ). 
 
 eters. 
 eters. 
 
 eters. 
 
 liter; 
 
 ;her <{ 
 
 less 
 more 
 
 iveuient to 
 
 Fie. aoo. 
 
 SECTION B. 
 
 nl.?"*!"'"* firlasB. -Bottoms of glass bottles may be cut off an J 
 f^J r'.""' '"'"^^"* ^" any pattern desirable! by observL the 
 
 xpos d nd be filedt ' Z """i ^" " "°'^'^^" ^'"'^•^ ^' ^"d let the 
 exposed end be fi ed to a smooth surface. With a poi nted piece of soap 
 
 ace a Ime on the glass where you would cut; and if it is a boS 
 
 nt:: vt ZT ' ''-' ^-' ^ (- ^ ^^p^^ -^'-' >vith ;: s 
 
 in the bottle in the 
 
 direction of the 
 
 line drawn. Heaf, 
 
 in a Bunsen flame, 
 
 the free end of the 
 
 rod to a bright red 
 
 heat (the hotter 
 
 the better), and 
 
 apply the heated 
 
 end to the glass, as 
 
 in the figure about l™- from one extremity of the gash for (. v) about 
 
 the other extremity of the gash, as D, and hold it firmly til. ,-ou see 
 a fine crack creepmg toward the rod; then slowly move the re aW 
 
 ^L! !, ^ ^^^'' '' *° ^" ^"*' fil« ^ «"^^" gash E ,1 one 
 
 edge, and, commencmg with this gash as before, you may cut in the 
 
 f r It' Z ""^ ^'"^" y°" '^^^^^^^ T« ^--^ ho>e« in glass, make 
 ; ei S ' ^^ Pf "^^^ '""^^"^ ^^"^ «^-Phor in splits o: tur! 
 pentme, mp off a short piece from the end of a small rat-tail file 
 and keeping he ragged end wet with the paste, you can readily or 
 a^hde by employmg strong pressure, and by a twisting movemenc al 
 
378 
 
 APPENDIX. 
 
 pi 
 
 
 
 ''1 
 
 SECTION O. 
 
 TABLES OP SPECIFIC GRAVTriES OP BODIES. 
 [The standard employed in the tables of solids and liquids is distilled water at4*C.] 
 
 I. Solids. 
 
 Antimony 6.712 
 
 Bismuth 9.822 
 
 Brass 8.380 
 
 Copper, cast 8.790 
 
 Iridium 23.000 
 
 Iron, cast 7.210 
 
 Iron, bar 7.780 
 
 Gold 19.360 
 
 Lead, cast ] 1.350 
 
 Platinum 22.069 
 
 Silver, cast 10.470 
 
 Tin, cast 7.290 
 
 Zinc, cast 6.860 
 
 Anthracite coal 1,800 
 
 Bituminous coal 1.250 
 
 Diamond 3.530 
 
 Glass, flint 3.400 
 
 Human body 0.890 
 
 Ice 0.920 
 
 Quartz 2.650 
 
 Rock salt 2.257 
 
 Saltpetre 1.900 
 
 Sulphur, native 2.033 
 
 Tallow 0.942 
 
 Wax 0.9G9 
 
 Cork 0.240 
 
 Pine 0.650 
 
 Oak 0.846 
 
 Beech 0.852 
 
 Ebony i.i87 
 
 II. Liquids. 
 
 Alcohol, absolute 0.800 
 
 Bisulphide of carbon 1 . 293 
 
 Ether 0.723 
 
 Hydrochloric acid 1.240 
 
 Mercury 13.598 
 
 Milk 1.032 
 
 Naphtha 0.847 
 
 Nitric acid 1.420 
 
 Oil of turpentine 0.870 
 
 Olive oil 0.915 
 
 Sea water 1.026 
 
 Sulphuric acid 1.841 
 
 Water, 4° C. , distilled ... 1 .000 
 
 Water, 0° C. , distilled 0.999 
 
 Air 1.0000 
 
 Ammonia 0.5367 
 
 Carbonic acid 1.5290 
 
 Chlorine 2.4400 
 
 Hydrochloric acid 1.2540 
 
 III. Gases. 
 [Standard : air at 0° C. ; barometer, 76<n».] 
 
 Hydrogen 0.0693 
 
 Nitrogen 0.9714 
 
 Oxygen 1.1057 
 
 Sulphuretted hydrogen . 1.1912 
 
 Sulphurous acid 2.2474 
 
APPENDIX. 
 
 379 
 
 s. 
 
 water at 4* C] 
 
 3.630 
 3.400 
 0.890 
 0.920 
 2.650 
 2.257 
 1.900 
 2.033 
 0.942 
 0.9C9 
 0.240 
 0.650 
 0.846 
 0.852 
 1.187 
 
 1.420 
 
 0.870 
 0.915 
 1.02G 
 1.841 
 1.000 
 0.999 
 
 ... 0.0693 
 
 .. 0.9714 
 
 . .. 1.1057 
 
 n . 1.1912 
 
 . .. 2.2474 
 
 SECTION D. 
 
 TABLE OF NATURAL TANGENTS, 
 
 
 Deg. 
 
 Tangmt. 
 
 Deg. 
 
 Tangent. 
 
 Deg. 
 
 Tangent. 
 
 Deg. 
 
 Tangent. 
 
 
 I 
 
 .017 
 
 24 
 
 .445 
 
 47 
 
 1.07 
 
 70 
 
 2.76 
 
 
 2 
 
 .036 
 
 25 
 
 .466 
 
 48 
 
 1.11 
 
 71 
 
 2.90 
 
 
 3 
 
 .052 
 
 26 
 
 .488 
 
 49 
 
 1.16 
 
 72 
 
 3.08 
 
 
 4 
 
 .070 
 
 27 
 
 .610 
 
 50 
 
 1.19 
 
 73 
 
 3.27 
 
 
 6 
 
 .087 
 
 28 
 
 .632 
 
 61 
 
 1.23 
 
 74 
 
 3.49 
 
 
 6 
 
 .105 
 
 29 
 
 .664 
 
 52 
 
 1.28 
 
 75 
 
 3.73 
 
 
 7 
 
 .123 
 
 30 
 
 .677 
 
 53 
 
 1.33 
 
 76 
 
 4.01 
 
 
 8 
 
 .141 
 
 31 
 
 .601 
 
 54 
 
 1.38 
 
 77 
 
 4.33 
 
 
 9 
 
 .168 
 
 32 
 
 .626 
 
 65 
 
 1.43 
 
 78 
 
 4.70 
 
 
 10 
 
 .176 
 
 33 
 
 .649 
 
 m 
 
 1.48 
 
 79 
 
 6.14 
 
 
 11 
 
 .194 
 
 34 
 
 .675 
 
 57 
 
 1.64 
 
 80 
 
 5.67 
 
 
 12 
 
 .213 
 
 36 
 
 .700 
 
 58 
 
 1.60 
 
 81 
 
 6.31 
 
 
 13 
 
 .231 
 
 36 
 
 .727 
 
 69 
 
 1.66 
 
 82 
 
 7.12 
 
 
 14 
 
 .249 
 
 37 
 
 .754 
 
 60 
 
 1.73 
 
 83 
 
 8.14 
 
 
 15 
 
 .268 
 
 38 
 
 .781 
 
 61 
 
 1.80 
 
 84 
 
 9.51 
 
 
 16 
 
 .287 
 
 39 
 
 .810 
 
 02 
 
 1.88 
 
 85 
 
 11.43 
 
 
 17 
 
 .306 
 
 40 
 
 .839 
 
 63 
 
 1.96 
 
 86 
 
 14.30 
 
 
 18 
 
 .325 
 
 41 
 
 .869 
 
 64 
 
 2.06 
 
 87 
 
 19.08 
 
 
 19 
 
 .344 
 
 42 
 
 .900 
 
 65 
 
 2.14 
 
 88 
 
 28.64 
 
 
 20 
 
 .364 
 
 43 
 
 .033 
 
 66 
 
 2.25 
 
 89 
 
 57.29 
 
 
 21 
 
 .384 
 
 44 
 
 .966 
 
 67 
 
 2.36 
 
 90 
 
 Inflnite. 
 
 
 22 
 
 .404 
 
 45 
 
 1.000 
 
 68 
 
 2.48 
 
 
 
 > 
 
 23 
 
 ,424 
 
 46 
 
 ... 
 
 1.036 
 
 69 
 
 2.61 
 
 
 
880 
 
 APPENDIX. 
 
 m^' 
 
 ^^^i^! 
 
 SECTION B. 
 
 Galvanometer. — A galvanometer that, along with the one de- 
 scribed in §175, will answer sufficiently well all the purposes 
 of this book can be easily and cheaply prepared as follows: 
 Make a wooden frame A (Fig. 301). iQcm square and 2.5o.n thick, 
 joined by wooden or brass pins in grooves; on it wind 50 to 00 
 turns (i lb.) insulated No. ICy wire in two layers, leavin- !<='» 
 space m the center (in the figure this space is exaggerated in-order 
 to show the position of the needles), and insert the extremities in .'-r. 
 brass screw-cups L and K. In this frame insert a copper or brass 
 wire D, carrying a cork E, which supports a silk fibre F and x strip of 
 
 Fig. 301. 
 
 n,T. f h" ?^^^"'*r ^. ^^"■S^ ««^ving-needle II, and insert in the paper, 
 as m the figure; also insert a small copper wire I in the paper for a 
 pointer, and suspend the whole so that the needle will swing freely 
 between the upper and lower windings of wire, and the pointer will be 
 just above the coils. Prepare a graduated circle on a card M. havintr 
 a hole m the center through which to pass the needle, and lay it on the 
 coiL To prevent disturbance from currents of air, cover the whole 
 with a frame N, haying a glass plate O laid over its top. Connect the 
 battery wrres with the screw-cups L and K. The cost of material need 
 not exceed 75 cents. 
 
APPENDIX. 
 
 381 
 
 he one de- 
 le purposes 
 IS follows : 
 2.o"» thick, 
 id 50 to (]0 
 e:ivin,if V" 
 ed in order 
 lities in t'^o 
 sr or br<ass 
 d ,i strip of 
 
 SECTION F. 
 
 i'nportant of these are t"^ !'.>?" "''" ^"^"^ *^« "-«* 
 
 service required L wwV^^ ""^ '"'"'"'"* '"^^""•^J' ^'iJ the 
 i«iiuirea, i.e., whether continuous, temoorarv r.r. ■ , 
 
 currents are wanted. The cost i« «f 'temporary, or occasional 
 
 governed mainly bv the Z^Ta- ««f ^^"^"ce' but that must be 
 
 arrangement preference, 7 ^^ considerations. In the following 
 
 bers, -n the Tdt TwM^Thev ^'^ "^"^^ '^"^"^^ ^^ ""- 
 specified.— ^'^ °''^ ^g*^"«* tbe several uses 
 
 1. Smee. 
 
 2. Leclanche. 
 
 3. Gravity. 
 
 NAJfES OP BATTERIES, ETC. 
 
 4. Dauiell. 
 Grenet. 
 
 5. 
 
 C. Bunsen or Grove. 
 
 7. Magneto or dy- 
 
 namo machines. 
 
 8. Thermo-batteries. 
 
 ;he paper, 
 per for a 
 ng freely 
 er will be 
 I, having 
 it on the 
 he whole 
 anect the 
 rial need 
 
 
 USES CELLS ARE SUITED FOR. 
 Strong, Continuous Currents. 
 
 Electrotypiug or Electro-plating 
 
 Electro-magnets 7, 4, 1, 3. 
 
 Electric light 3, 4, 1. 
 
 Telegraph (closed circuit) ........!...... ! ' ^" 
 
 Temporary. 
 
 Induotion coils 
 
 Medical coils . . S, 6, 4, 3. 
 
 5,1. 
 
 Occasional. 
 
 Annunciators, domestic bells 
 
 Exploding fuses 
 
 Eleetficui measureiueuts (constant current) 
 
 2, 1, 3, 4. 
 2,4. 
 8, 4, 3. 
 
382 
 
 APPENDIX. 
 
 SECTION G. 
 
 Apparatus to iUuBtrate wave-motion. — The most efficient 
 
 apparatus for this purpose that we have seen may be constructed as 
 follows. Procure forty wooden return-balls (sold at toy stores) ; sus- 
 pend them by strings (better, fine wires) about 1" long, as in Figure 
 302, and about 7"" apart. Connect all the balls horizontally by small 
 
 Fig. 302. 
 
 elastic cord (better, small spiral wire coil), and connect the ball at one 
 extremity of the series with a suspended weight B (weighing about 
 1 ) , and from the ball at the other extremity suspend a small weight 
 A, which may be easily removed when desirable. By a simple vibra- 
 tion given with the hand to A, a wave, as CD, will be projectec^ 
 through the series, and on reaching B will be reflected ; though when 
 reflection is wanted, B had better be replaced by a hook attached to a 
 
 WftUa 
 
APfJBINDIX. 
 
 883 
 
 most eflBcient 
 onstructed as 
 stores) ; siis- 
 as in Figure 
 ;ally by small 
 
 e ball at one 
 ghing about 
 iinall weight 
 imple vibrar 
 be projected 
 hough when 
 ttached to a 
 
 SECTION H. 
 
 Porte Lumi^re.- Two half-sections of a tube A and B (Fig 303) 
 may easily bo sawn from a block of pine wood. These glued togeth 
 at their edges make the tube C. together 
 
 Fig. 303. 
 
 This tube is QO^m long and IG"™ in 
 diameter, with a boro of l^" diam- 
 eter. Raise a window-sash about 
 50™, and fit a board I) just to fill 
 the opening. In the middle of this 
 board cut a liole just large enough 
 to receive the tube, and allow it to 
 turn in the hole freely. Attach a 
 bolt E to the board D, and about 
 12<"» from one end of the tube bore 
 a row of holes around the tube, 
 I*"" in depth and about Ic™ apart' 
 to receive the bolt. Uy means of a 
 
 aS:rttir7 
 
 tacks with ia.ge r...^^ !::'!jz:^:T^zt'7ii::^ 
 
 ^eXa^f J; "", f "^: .^"^ '''^' ^' *^« ^-d - -rtd through 
 the tube and fastened to a binding screw H. When the mirror is to 
 
 b adjusted so as to receive the sun's rays and reflect them through the 
 
 tube, rotate the tube, and raise or depress the mirror by means of the 
 
 heavens and then fasten by means of the bolt and string. A win- 
 dow on the soutk side of a building should be selected for elperfmrnts 
 wiii this apparatus The portion of the window not occupierwU 
 the board I), as well as other windows not in use, may be darke^ie 
 
 Tnllf ? ''"' """"^"'^'^ •^^^'h- The whole' cost' of tL above 
 apparatus need iiot exceed «1.00. 
 
384 
 
 APPENDIX. 
 
 SECTION I. 
 
 ro^!fr.«i^''°!, ?f "^ ^^'^^* '^^^^^ ^°^ Cotton and Silk 
 AZr„^2t ^tT ""^.^^^^ Wire. -By Browne . Sh^ e^ 
 Ann ncan Gauge. The resistances are calculated for pure copper wire 
 rhc3 number of feet to the pound is only approximate for' h^suS 
 
 No 
 
 8 
 9 
 10 
 11 
 12 
 13 
 14 
 15 
 16 
 17 
 
 18 
 
 19 
 
 20 
 
 21 
 
 22 
 
 23 
 
 24 
 
 25 
 
 26 
 
 27 
 
 28 
 
 29 
 
 30 
 
 31 
 
 32 
 
 33 
 34 
 35 
 36 
 
 i 
 
 5 
 
 .12849 
 .11443 
 .1018D 
 .09074 
 .08081 
 .07196 
 .06408 
 .05707 
 .05082 
 .04525 
 .0403 
 .03539 
 .03196 
 .02846 
 .02535 
 .02257 
 ,0201 
 .0173 
 .01594 
 01419 
 .01264 
 .01126 
 .01002 
 .00893 
 .00795 
 .00708 
 .0063 
 .00561 
 .005 
 
 Feet per Pound. 
 
 Cotton 
 Covered. 
 
 42 
 55 
 68 
 87 
 110 
 140 
 175 
 220 
 280 
 360 
 450 
 560 
 715 
 910 
 1165 
 1445 
 1810 
 2280 
 2805 
 3605 
 4535 
 
 Silk 
 Covered. 
 
 46 
 60 
 75 
 95 
 120 
 150 
 190 
 240 
 305 
 390 
 490 
 615 
 775 
 990 
 1265 
 1570 
 1970 
 2480 
 3050 
 3920 
 4930 
 6200 
 7830 
 9830 
 12420 
 
 Naked; 
 
 20 
 25 
 32 
 40 
 50 
 64 
 80 
 101 
 128 
 161 
 203 
 256 
 324 
 408 
 514 
 649 
 818 
 1030 
 1300 
 1640 
 2070 
 2617 
 3287 
 4144 
 5227 
 6590 
 8330 
 10460 
 13210 
 
 Resistance Naked Coppeu. 
 
 OhraB Per 
 1,000 Feet. 
 
 .6259 
 .7892 
 .8441 
 1.254 
 1.580 
 1.995 
 2.504 
 3.172 
 4.001 
 5.04 
 6.36 
 8.25 
 10.12 
 12.76 
 16.25 
 20.30 
 25.60 
 32.2 
 40.7 
 51.3 
 64.8 
 81.6 
 103 
 130 
 164 
 206 
 260 
 32S 
 414 
 
 Ohms Feet Per 
 Per Mile. Ohm. 
 
 3.3 
 4.1 
 4.4 
 6.4 
 8.3 
 10.4 
 13.2 
 16.7 
 23. 
 26. 
 33. 
 43. 
 53. 
 68. 
 85. 
 108. 
 135. 
 170. 
 214. 
 270. 
 343. 
 432. 
 538. 
 685. 
 865. 
 1033. 
 1389. 
 1820. 
 2200. 
 
 1600. 
 1272 
 1185. 
 798 
 633. 
 504. 
 400 
 316. 
 230 
 198 
 157. 
 121. 
 99 
 76 5 
 61.8 
 48.9 
 39 
 31.0 
 24.6 
 19..'; 
 15.4 
 12.2 
 9.8 
 7.7 
 6.1 
 4.9 
 3.8 
 2.9 i 
 2.4 
 
 Ohms Per 
 Pound. 
 
 ■ 0125 
 .0197 
 0270 
 .0.501 
 079 
 .127 
 .200 
 .320 
 513 
 .811 
 1.29 
 2.11 
 3.27 
 5.20 
 8.35 
 13.3 
 20.9 
 33 2 
 52.9 
 84 2 
 134. 
 213 
 338. 
 539. 
 856 
 1357. 
 2166. 
 3521. 
 5469. 
 
and Silk 
 Ss Sharpe's 
 )pper wire. 
 * insulated 
 
 PPER. 
 
 Ohms Per 
 
 round. 
 
 -012.) 
 
 .01!»7 
 
 0270 
 
 .0501 
 
 07!) 
 
 .127 
 
 .200 
 
 .320 
 
 513 
 
 .811 
 
 1.29 
 
 2.11 
 
 3.27 
 
 5.20 
 
 8.35 
 
 13.3 
 
 20.9 
 
 33 2 
 
 52.9 
 
 84 2 
 
 134. 
 
 213 
 
 338. 
 
 539. 
 
 856 
 
 357. 
 
 106. 
 
 521. 
 
 409. 
 
 IKDEX. 
 
 [The Numbers repeb to Pages.] 
 
 CHAPTER I, 
 
 Absorption of gases by solids, 41. 
 
 of gases by liquids, 42. 
 Ailhegiou, ^G. 
 Aiiiorplious bodies, 28. 
 Attraction, Mutual, 16. 
 
 Phenomena of, 23. 
 
 Brlttlenegg, 34. 
 
 Capillarity, 37-39. 
 
 Laws of, 39. 
 Chance, Absence of, 2. 
 Changes, Physical and chemical, 12. 
 
 of volume by crystallization, 29. 
 Cleaveage, Plane of, 27, 31. 
 Cohesion, 26. 
 
 in water, 21. 
 Colloids, 44. 
 Crystallization, 27. 
 
 Change of volume by, 29. 
 
 Cause of tendency to, 30. 
 
 of water, 28. 
 
 of iron, 30. 
 Crystalloids, 44. 
 
 Density, 11. 
 nialysls, 44. 
 
 Dimiston of liquids, 42. 
 
 through porous partitions, 43, 4.5. 
 
 of gases, 44. 
 
 fountain, 40. 
 Ductility, 35. 
 
 Slastirity, 32, 
 
 Perfect, 34. 
 
 Limit of, 34. 
 Endosmose, 43. 
 Exosnnose, 43. 
 Kxperlmentatlon, 3, 4, 
 
 Flexibility, 32. 
 Fluidity, 35. 
 Force, 15. 
 
 defined, 16. 
 
 Molar and molecular, 16, 18. 
 
 of gravity, 17, 24. 
 
 Gases, Distinguishing property of, 20. 
 Gravitation, 23. 
 
 Hardness, 31. 
 
 Scale of, 32. 
 Heat, 0. 
 
 Effect of, on solution, 40. 
 
 Impenetrability, 4, 9. 
 
 I'lqulds, Distinguishing properties 
 of, 19. 
 Incompressibility of, 21. 
 
 JRIalleablllty, 35. 
 
 Mass, 11, 23. 
 
 distinguished from weight, 25. 
 Matter, 3. 
 
 An essential property of, 4. 
 Compressibility of, 17. 
 Constitution of, 9. 
 Const^mt quantity of, 13. 
 Crystalline and amorphous, 27. 
 , Minuteness of particles of 6. 
 
 Three states of, 18-22. 
 Molecule, 7, 8, 9. 
 
 IVature, Order of, 2. 
 
 Absence of chance in, 2. 
 
 Law of, 2. 
 
 Law of, not a cause, 2. 
 
 Means of obtaining a knowledge of, 1. 
 
 Osmose, 43. 
 
386 
 
 INDEX. 
 
 Phenomenon, 3, 4. 
 Poreg, Physical, 10. 
 Porosity, 10. 
 
 Repulsion, Mutual, 16. 
 
 Sensations, 1. 
 
 and Inferences, 2. 
 
 and things, 3. 
 Senses, 1. 
 Solids, Distinguishing property of, 19. 
 
 Limit of elasticity of, 34. 
 
 Solution of, 40. 
 
 When wet by liquids, 37. 
 Solution of solids, 40. 
 
 Spring balance, 33. 
 Strain, 34. 
 
 Stress, 34. 
 
 Substanoe, Simple and compound, U, 
 
 Tenacity, 35. 
 
 Vp anil down, 25. 
 
 Viscosity, 34. 
 
 Weight, 23, 2ft. 
 
 distinguished from mass. 25. 
 Ueldlng, 27. 
 
 CHAPTER II. 
 
 it .1 
 
 Action, 128. 
 Alr-Pump, 57, 
 
 Apparatus for raising liquids, 77, 78. 
 Atmosphere, a unit of pressure, 52. 
 
 Pressure of, 52, 57. 
 
 Hight of, 54. 
 Artesian wells, 73. 
 
 Barometer, 53. 
 
 Boyle*s law, 62. 
 
 Buoyant force of fluids, 79-82. 
 
 Center of gravity, 104. 
 
 To find, 106. 
 Center of inertia, 105. 
 Centrifugal force, 109. 
 Centripetal force, 110. 
 Condenser, 63. 
 Couple, Mechanical, 103. 
 
 Density, 82. 
 Direction, Line of, 105. 
 Dynamics defined, 47. 
 
 of fluids, 47. 
 Byne, 137, 140. 
 
 Energy, 131-133. 
 
 contrasted with momentum, 136. 
 
 Fnrn-.uUifor, 135. 
 Potential and kinetic, 133, 134. 
 Transformation of, 140. 
 Unit of, 137, 140. 
 
 Xiqulllbrium, 47. 
 
 Three atixUm of, 106. 
 Stable, 107. 
 Neutral, 107. 
 Unstable, 107. 
 
 of forces acting in one plane, 151-156 
 Erg, 138, 140. 
 Expansibility of gases, 56. 
 
 Ealllng bodies, 112, 114. 
 
 in a vacuum, 115. 
 Foot-pound, 132. 
 Force, Absolute unit of, 137, 140. 
 
 Centrifugal, 109. 
 
 Centripetal, 110. 
 
 Component* of, 94. 
 
 Equillbrant, 103. 
 
 Gravity unit of, 137, 
 
 Measure of a, 135. 
 
 Measure of the effect of a, 138. 
 
 Uesolved part of, 95, 97. 
 
 Moment of, 150. 
 Forces, Composition of, 94, 98, 102. 
 
 Graphic representation of, 93. 
 
 Resolution cJf, 95. 
 
 Resultant of, 94. 
 
 Oases, Compressibility and expanslbilltv 
 
 of, 65. ' 
 
 Law of volume and pressure, 62. 
 
 O 
 P 
 
INDEX. 
 
 387 
 
 iinU compound, 1 1 , 
 
 Tiaas, 25. 
 
 le plane, 151-156. 
 a, 55. 
 
 137, 140. 
 
 f II, 138. 
 
 84, 98, 102. 
 I of, 93. 
 
 d expansibility 
 !B8ure, 62. 
 
 Gravity, Cent<'r of, 104. 
 To find center of, 106. 
 Acceleration due to, 114. 
 Specific, 83-85. 
 
 Horse-power, 13.'?. 
 Hydrometer, 86. 
 
 Hydrostatic hcllowH, 07. 
 presB, 67. 
 
 Inertia, 83. 
 
 Center of, 105. 
 Inclined plane, 149. 
 
 Klloj^rammeter, 132. 
 Kinetic energy, 133. 
 
 I.iqul'i surface level, 72. 
 
 Machines, Uses of, 143. 
 
 Internal work done by, 145. 
 Law of, 144. 
 
 Efficiency or modulus of, 145. 
 ^lechanical units, Summary of, 140. 
 Mariotte's law, 60, 62. 
 Moment of a force, 150. 
 
 Vanishing of moments, 1,50. 
 Momentum, 126. 
 
 contrasted with energy, iS!y. 
 Unit of, 128. 
 Motion, 89. 
 
 All matter in, 89. 
 
 arises from mutual action between at 
 
 least two bodies, 91. 
 Accelerated and retarded, 90, 112, 115. 
 Curvilinear, 109. 
 Formulas for accelerated and retarded 
 
 114, 115. 
 First law of, 90, 92. 
 received by masses of matter gradu- 
 ally, 91. 
 Second law of, 93, 127. 
 Third law of, 128, 129. 
 Uniform and varied, 90. 
 versus rest, 89. 
 
 OscUlaUon, Center of, 122. 
 Parabolleoarve, 118. 
 
 Parallel forces, Composition of, 102. 
 Pendulum, 121. 
 
 Center of oscillation, 122. 
 Center of percussion, 124. 
 Compound, 122, 123. 
 Useful applications of, 125. 
 Time of vibration of, 121, 122. 
 Physics defined, 141. 
 Potential energy, 1,33. 
 Press, Hydrostatic, 67. 
 Pressure, 47. 
 
 in fluids, 48, 60, 51. 
 Transmission of, in fluids, M, 67. 
 in fluids due to gravity, 68-71. 
 Projectiles, 117. 
 
 acted upon by two forces, 117. 
 Horizontal and vertical motions of. 
 
 119. 
 Examples worked out, 120. 
 Path of, 118. 
 
 Range or random of, 117. 
 Pump, Air, 57. 
 Force, 78. 
 Lifting, 77, 78. 
 
 Random of projectiles, 117. 
 
 Reaction, 128. 
 
 Reflection, Angle of, 1.30. 
 
 Law of, 130. 
 Resolved part of a force, 95, 97. 
 Rest, 89. 
 
 Resolution of forces, 93. 
 Resultant of two or more forces, 94. 
 
 Screw, 149. 
 Siphon, 75, 76. 
 Speclflc gravity, 83-85. 
 Stability of bodi- , 107. 
 Summary of mechanical 
 formulas, 140. 
 
 units and 
 
 Tension, 47. 
 
 Torricellian vacuum, 53. 
 Tifauiifonnattou of energy, 140. 
 
 ITnlta, Summary of mechanical, 14C. 
 I GraviUtion and absolute, 137, 188. 
 
888 
 
 tNDE.^. 
 
 Vai'iiuiM, Almoliiff, 50. 
 
 'I'oriim.llliKi, wi. 
 
 KiillliiU ImkIIi.h \n, 116. 
 VtslovMy, \m. 
 
 tli'lllli'il, IH), 
 
 UjiU of, 140, 
 
 H«;iKht losg at equator than at poles. 
 110. * 
 
 Wheel and axle, 148. 
 ^Vork, lol. 
 
 Uate of doing, 132. 
 
 Unit of, 132, 140. 
 
 CHAPTER rri. 
 
 Koillnif, r.iitvN „f, 182. 
 Ii"l)it, 1(11. 
 
 Coiiiliietioii of Jioat, 162. 
 
 Coiiv«<!|| ,t Uma, las. 
 
 CajmcUy for Ik'hI, 192. 
 
 <'""•"'" "f illffort-tico in, 193. 
 Cold, MuthodH of iirodiiclnK, 188. 
 CoiiaerviiUoii of Mu-r^y, Kta. 
 Coi-rulMtlon of cncrKy, 1«6. 
 
 Bew point, IHft, 
 IMflrUiilon of iKMit, 101. 
 DUUUatloii, IHM. 
 
 isotcu of Jioiit, ins. 
 i:uerfgy, Conservation of, 196. 
 
 Corr«l«tlo« of, ]95, 
 Kvaiioratlon, 184. 
 li^ximiiiloii liy »i<mt, 188. 
 
 Abuorinaj, 170, 
 
 ('oomclfiit of, 109, 
 
 Power of, 170. 
 
 Vreeztug point, 171, 
 Funloii, l.iiwuof, 182. 
 iinHtsa, Kliuald theory of, 178. 
 
 Diffusion of, 179. 
 
 I.awK of, 177. 
 
 PreHNuru of, 179. 
 Heat, Ifl7-2<X). 
 
 Capiiclty for, 192. 
 
 CondiKftlon of, 102. 
 
 Oonvo<!tlon of, io;i, 
 
 couvL-rtlblo Into mochanical motion 
 157. 
 
 dertnml, lf,U, 
 DiSlWlOtt 0/, 101. 
 
 Heat, Kffects of, 168. 
 Expansion liy, 168. 
 generated by chemical action, 159. 
 Latent, 187, 188. 
 Liquefaction by, 180. 
 Mechanical equivalent of, 196. 
 Origin of animal, ]o<». 
 Quantity of, 161. 
 .''udiation of, 165. 
 ic.ated to worl<, 158. 
 Specific, 191, 192, 19.3. 
 Thermometry, 171. 
 Units of, 185. 
 
 Jonle'8 equivalent, 19«. 
 
 Latent heat, 187,188. 
 l.aw, Mariotte's, 177. 
 
 of Charles, 177. 
 IiaM-8, r' fusion and boiling, 182. 
 
 of gaaeous bodies, 177, 179. 
 X,lquefactlon, 180. 
 
 Table of melting poinU, 182. 
 I^ocomottve, 199. 
 
 AIariotte'8 law, 177. 
 itiechaiilcal equivalent of heat, 196. 
 
 Quantity of heat, 161. 
 
 Radiation of heat, 105. 
 
 Speclllc heat, 191. 
 
 defined, 192. 
 
 Tabic of, 193. 
 Sun as a source of energy, 360. 
 Steam-enj^ine, lyo. 
 . Condensing and uon-c ...dengiug, 198. 
 
INDEX. 
 
 889 
 
 Temperature, AbnoIiiUi, 1T«, 
 deflned, 100. 
 
 dlHtlii(fuished from (fuikaHty t)( hmt. 
 161. • 
 
 meaaured by expi»n»loD, 171, 
 
 MfUHurcmuiit of ttxtruinit, 17ft. 
 
 ytunduid UMiipc-rutiiritit, 171, 
 Thcrino-nynaialea, Klft. 
 Thfrinometer, 171. 
 
 Air. 174. 
 
 Coimtnirtlon of, 172, 
 
 Thermometer, Conversion from one 
 BCJilo to another, 173, 
 (iriiduatloii of 171. 
 
 Vaporization, 180. 
 
 Uolllng pointe of water, 182, 183. 
 Ventilation, 105. 
 
 Experiments illuatratl.iK, 106. 
 
 Bent means of, 16'( . 
 
 Amount required, 168. 
 
 CHAPTER IV. 
 
 Absolute units, 228. 
 Accumulators, 298. 
 Alpliabet, Telegraphic, 29a, 
 Ampere, a unit of current, i',, 
 Anp^re'8 rule, 204. 
 
 theory, 240. 
 Ampirlau currents, 241. 
 Armature, 218. 
 
 Attractions and repulHlonn, 237,2(11,277, 
 Aurora tube, 279. 
 
 Battery, Electric, 203. 
 
 Amalgamation of zinc in, 20»i, 
 Appearance of hydrogen nt the mp. 
 
 per plate, 206. 
 Arrangement of cellii, 22'.», 
 Bunsen'g, 210. 
 
 General conciuBlonu concurnliiK, 2.12, 
 
 Gravity, 212. 
 
 Gremt, 210. 
 
 Grove's, 210. 
 
 Smee's, 209. 
 
 Local action In, 208, 
 
 Perfect, 332. 
 
 Poles or electrodes of, 204, 
 
 Storage, 298, 
 
 Thermo, 258, 
 
 Coll, Ruhmkorfl's, 256, 
 
 Induction, 255. 
 Condenser, 274, 
 Coulomb, 227. 
 
 Current, Electric, 202. 
 Amperian currents 241. 
 Attraction and repulsion between cur- 
 rents, 237. 
 capable of doing work, 214. 
 Direction of, 203. 
 Extra, 254. 
 
 First law of currents, 2.37. 
 malntjiined by chemical action, 206. 
 Ohm's law, 220. 
 Hecond law of currents, 239, 
 Strength of, 218, 222. 
 Unit of, 227. 
 
 Dlamagnetlsm, 247. 
 
 nynamo machines, 249. 
 Commutator of, 249. 
 
 Karth a magnet, 242, 
 
 Cause, 245. 
 
 Mrgnetic poles of, 244. 
 Electric candle, 286, 
 
 lamp, 286. 
 
 motors, 289. 
 
 light, 285. 
 Electrical attractions, etc., 237, 261, 277. 
 
 machines, 249, 208, 270, 271, 281. 
 
 measurements, 218, 221, 227, 228. 
 
 resistance, 223, 224, 289. 
 Electricity, Chemical effects of, 213. 
 
 Bound, 270. 
 Current, 200. 
 
390 
 
 INDEX. 
 
 Kloctrloltjr, Electro.motlve force, 226 
 Frictlonal, 260. 
 Heating effecta of, 212. 
 Lnmlnoiig effect of, 213. 
 Magnetic effect of, 217. 
 Physiological effect of, 216. 
 Thermo, 257. 
 Static and dynamic, 262. 
 may receive and Impart energy, 282. 
 What is, 282. BJ'.-'-'^- 
 
 Blectrlflcation, 260. 
 
 on external surface, 267. 
 
 Two kinds of, 264. 
 Blectro-chemical series, 207. 
 Electrodes, 204. 
 £lectrolyg|g, 213. 
 
 Electro-magnet, 217. 
 
 Power of, 233. ' 
 
 Electro-magnetic machines, 249. 
 J^lectro-motf ve force, 225, 281. 
 
 of induced current*, 254. 
 Slectrophorus, 269, 270. 
 Electroplating, 288. 
 Electroscope, 260. 
 Electrotyping, 287. 
 Knergy, Electric, How, originates, 205. 
 
 1 ransformatlon of, 282. 
 
 Examples, 28;j. 
 
 Eire-alarm, Electric, 293. ' 
 
 Galvanometer, 219, 220. 
 Galvanoscope, 206. 
 Teissier's tubes, 279. 
 
 Helix, 218. 
 
 Induction, Current, 263. 
 
 colls, 256. 
 
 Magnetic, 236. 
 Insulation, 266. 
 
 Effect of moisture on, 266. 
 
 l«eyden Jar, 275. 
 lieyden battery, 276. 
 I<''ghtnlng, 280. 
 «>ds, 280. 
 
 Magnets and magnellsm, 234, 247. 
 General remarks on, 246. 
 Compound. 247. 
 Law of, 236 
 
 not sources of energy, 247. 
 Natural, 245. 
 
 Permanent and temporary, 284. 
 I'olarity of, 2;J6. 
 Magnetic transparency, 238. 
 Magnetic needle, Ampere's'nile, 204. 
 Declination of, 244. 
 Dip of, 242. 
 
 Line of no variation of, 246. 
 Magneto machines, 249. 
 Measurements, Electrical, 218. 
 Experiment* In, 221. 
 Summary of, 227, 228. 
 Units of, 228. 
 Microphone, 297. 
 
 Olim, 228. 
 Ohm's law, 226. 
 
 Paramagnetism, 247. 
 Potential, Electric, 204, 267. 
 Points, Effect of, 277, 278. 
 
 Helay and repeater, 290, 291. 
 Resistance, Formula for, 228. 
 
 Internal, 224. 
 
 of the earth, 289. 
 
 Table of, 224. 
 
 Storage battery, 298. 
 
 Telegraph, 289. 
 
 alphabet, 292. 
 
 sounder, 290. 
 
 Facsimile, 292. 
 
 Fire-alarm, 293. 
 Telephone, 294. 
 Thermopile, 288. 
 
 Volt, 228. 
 Voltaic arc. 286, 
 Voltameter, 219. 
 
I«ra, 234, 247. 
 1,246. 
 
 XJ, 247. 
 
 porai-y, 2.S4. 
 
 y, 235. 
 
 pere's rule, 204. 
 
 of, 245. 
 
 9. 
 
 irical, 218. 
 
 i. 
 
 INDEX. 
 
 CHAPTER V. 
 
 891 
 
 r, 267. 
 
 '8. 
 
 IVolae dlittngiilahed from musical sound, 
 
 315. 
 
 Scale, LIniltH of, 317. 
 Siren, 310. 
 Sounds 290. 
 
 Analysig of, 32U. 
 
 deflned, 308. 
 
 How, origlniitcH, 306. 
 
 IIow, travelM, 306. 
 
 LoudncHH of, 313. 
 
 Media of, 308. 
 
 Pitch of, 316. 
 
 Quality of, 319. 
 
 Ueflectlon of, 211. 
 
 Refraction of, 312. 
 
 Velocity of, 309. 
 
 Wave-Boundg, 305. 
 Spcakiiig-tulu'H, 314. 
 
 Vacuum, Sound" cannot travel through 
 a, 306. 
 
 Velocltjr of sound, .109. 
 
 deiiends on density and elasticity of 
 
 medium, 309. 
 Vibration, dcHned, :uM). 
 
 Direction of, 3(H). 
 
 Loni^tudlual, 300, 306. 
 
 Propaifatlon of, 301. 
 
 Simple and complex, 300. 
 
 ToiHlonal, 300, 306. 
 
 TrannverHe, 300. 
 
 of strings, 318. 
 
 Wavefi, Air, .307. 
 Amplitude of, 301. 
 Air as a medium of, 303. 
 How, propagated, 304. 
 Length of, 301. 
 Longitudinal, 302, ;503. 
 Keflectlon of, 301. 
 transmitted in elastic medium, 308. 
 Transverse, 303. 
 Water, 302. 
 
 291. 
 r,228. 
 
 CHAPTER VI. 
 
 Aberration, Chromatic, 369. 
 Achromatic lens, 370. 
 Analysig of light, 356. 
 Athermancy, 363. 
 
 Beam of light, 323. 
 
 Camera obscura, 367. 
 
 Photographer's, 367. 
 Candle-power, 331. 
 Color, 361. 
 
 by absorption, 361. 
 
 Cause of, 359, 362. 
 
 Dew, 366. 
 Diatliermancy, 363. 
 
 Dlaperalon of light, 357, 359. 
 
 Energy, Padiant, 321. 
 £ther, a medium of motion, 322. 
 
 Gxclianges, Theory of, 365. 
 Eye, Human, 368. 
 
 Foci, Conjugate, 3W, 353. 
 Focus, Principal, 352. 
 Virtual, 3.">3. 
 
 Heat and chemical spectra, 360. 
 All bodies radiate heat, 365. 
 
 Images, 325. 
 
 Formation of, 341, 343, 344, 353. 
 Real, 342. 
 Size of, 354. 
 
 To construct, 354. 
 Virtual, 355. 
 
 Hicnses, 3o0. 
 Effects of, 351. 
 
892 
 
 INDEX. 
 
 Iilght, a form of energy, 321. 
 
 AnalyBis of, S.W. 
 
 Diffused, ;«7. 
 
 Reflection of, 3.36. 
 
 Syntheses of, 358, 
 1-uml.ious and ill.nninated bodies, 326. 
 
 MJcroscope, Simpk., 35.-.. 
 
 Compound, 366. 
 Mirrors, Reflection from, .338. 
 
 General effect of concave, 341. 
 
 Opacity, 325. 
 
 Pencil of light, 323. 
 Penumbra, 328, 329. 
 Photometry, 330, 3.31. 
 Prisms, Optical, 350. 
 
 Radiation, Only one kind of, 361. 
 
 Thermal effects of, .363. 
 Radiators, Good, 365. 
 Radiometer, 321. 
 Ray of light, 323. 
 
 J(-' 
 
 Reflection, 336. 
 
 from concave mirrors, 339. 
 Refraction, 345. 
 
 Cause of, 346. 
 
 Inde.v of, 347. 
 
 I-.aw of, 348. 
 
 Table of Indices of, 348. 
 
 Seeing an object, 324, 332. 
 Sliadotvs, 328. 
 
 Siie, Methods of estimating, 333. 334 
 Spectrum, 357. 
 Stereoptlcon, 370. 
 
 Telescope, Astronomical, 367. 
 Transluce> oy, 395. 
 Transparency, 325. 
 
 TJmbra, 328, 329. 
 Vndulatory theory, 323. 
 
 Velocity of light, 334, 335. 
 Visual angle, 332. 
 
 ill 
 
TEST QUESTIOJ^S. 
 
 law;^^nrure;\ta^^^^^^^^ (D an observed fact; (2) a 
 
 2.-Classify, under the foregoing headings, the following statements :— 
 ^ (1) The volume of a given body of gas, if the temperature is constant, 
 vanes inversely as the pressure. ^""^i-aiiu, 
 
 (2) If the temperature of a litre of air at OT. is raised to TC. while the 
 
 pressure remams constant, the volume is increased to ?,^ of a litre. 
 
 (3) The temperature of a substance depends upon the average kinetic 
 
 energy of each molecule. g cui. 
 
 (4) If the E. M. F remains constant, the strength of an electric current 
 
 varies inversely as the resistance of the whole circuit. 
 
 ^^^ te^nrTratirrl P'"^'^"'"^' *^^ ^°^""^ °^ ^ S^^ varies as the absolute 
 
 (6) That which we call heat is molecular kinetic energy. 
 
 (7) If a litre of air measured under a pressure of one atmosphere is sub- 
 
 jected to a pressure of two atmospheres, while the temperature 
 remams constant, the volume is recfiiced one half litre. ^'"'P^''*'"'^ 
 
 (8) Equal volumes of any two gases at the same pressure and tempera- 
 
 ture contain the same number of molecules. 
 
 ^^^ ^ct%hl^!Z'^ ""^^f ^\ P','' !?"^'^ ^"''^ *« communicated to any part 
 tUrl^i °^ a fluid the pressure at all points in the fluid is 
 
 thereby increasea by ;j lbs. per square inch. 
 
 mt7Jir^Lff£T * wu ^.' ^^*^^ ''^'".^ suspended for a year, became per- 
 possess 1^ stretched. What property does this phenomenon show gla«s^ to 
 
 tht'^hl'f.'^rtl:^^ °"° ^""l"*^ ^ '""P^ '» ^'^ hands, and another man pulls 
 the other end of the rope with a force of 90 lbs. What force does the latS 
 compel the former to exert in order to retain the rope in his hands ? 
 
 lOo'lh?"^ w"h!^ f l^^^^f^ extremities of a rope pull each with a force of 
 lOOJbs. What IS the force exerted between them, or the tension of the 
 
 from'wMch'theTirl^!?''*""^ *.^ '?^'^t^ ^ P^'"«*^ Magdeburg hemispheres 
 Laches ? ^"^''^^y exhausted, and whose diameter is five 
 
 a Jkle^wS! r°"^'^ be necessary to separate the above hemispheres at 
 a place where the barometrical column is 20 inches ? f » *«< 
 
394 
 
 TEST QUESTIONS. 
 
 11 
 
 Si 
 
 lis aLiLiii 
 
 
 
 
 force of gravity acting on each ? ^' ^' ^^®''* '« produced by the 
 
 effel-'Vrtfty wifl^Xf ^"' *'^ ^*"°^ ''« ™*' -•-* changes in the 
 
 in ia^rh^e? tt hTgSf vS LTat^S^t ?,\f ^ ^ JJe rubber pressed 
 air increases as the vessel is raised ? " ^^"'^ **"" pressure^^of the 
 
 drJprte^fSte^^^^^ and let both 
 
 simultaneously, the boarLeachesXroundlrft/Txplain '"' '^"PP^'^ 
 
 Btop'unSuM^^^^^^^ of the cylinder . and 
 
 applied to the piston to Dull it^f« tL i! ^7^'"^e'-. What force must be 
 the transverse ?ectionStSpstonbe^fi2Scm,°^*''« ^^linder, the area o? 
 at the beginning, is at the middirof the^vlSZ wK'%**'''* *^" P^^^o^' 
 keep It in motion be constant" a kL,*u^^'^' ^'^^ *h« force required to 
 reaches the bottonr:;S tLVelv^'suppSr^^^ force^hen i? 
 
 drawn, what will happen ? Suppose tdnnn it f l^^^ P°^"' '« ^i«>- 
 person were to blow with a forcfof ?of thp^frJf f i° ^' "^^^'•*«''' ^"d a 
 bore of the tube being iQcn^ Xt we.Vht^^? f °^ the cross ser J ^on to the 
 sustained ? If, while thus in^Prtp/tL^ * ^^^^"^ "P*'" *•>« P'^ton might be 
 2m above the lower extrem^^roflhew' "^^'\«'»'*y of the tube is faised 
 tube until it is filled wWwp,v^f 1 ^i**'"' ^""^ "^^t^"^ '" Poured into the 
 by the water ' What name wSd fc''^ "P°".'*^' P'^'<^" ^'" be susSined 
 Suppose thataplug iustTttTncr tHp?\*f?P*''*^l''^^«"'« '° the last case" 
 
 the tube pressin^|S£7at?rr;ttTout\tlt^^^^^^ 
 fourthTof JhlTrtas been're^r *M ^° "V^ P"*"? ^^'^^i^'' '« 20-. Three- 
 
 1.5. What ^t:^:::-:t^-:^t^^z ^^-^ -^^« 
 
 ra^'olirS^n W 1?^?^%?*"^' "'/'? '« '".?^^"^ <^- «-* -* the 
 order that his resultant 'nTt; ^*u ™?^* ^^ "^^^^ due south-west in 
 
 southerly veli^ifr^-^'^vrbr-oS ^^hat will ^£ 
 
 the'tileVaS it's J: rSfast^tre^ra?* fl ^^*,^ '' '' ""^ - '-- ^ 
 wind carries it due north'we8t2;th«^«t ff ^ '"' ^ ^" ^"ur, while the 
 its actual course and velocTtyl °^ ^°"' '"'*'" ^" ^«"'-- What is 
 
 Jh\-;ilXuU%1'tt^Van^^^^^ "^ ^^PV"'^ *- -". one in 
 
 At wiiof ..«•"* ;f - ! *^ - PO'°t between them will th"" •r—t » 
 
 — I'^tiiu ii uuiy one m&n puils ? Why » " "^ '^ ' 
 
 18.-State three causes for the variation of gravity on the earth's surface. 
 19.-Can you move without the aid of some other body? 
 
TEST QUESTIONS. 
 
 395 
 
 B hemispheres at 
 n? 
 
 her stone of the 
 produced by the 
 
 changes in the 
 
 3 rubber pressed 
 3 pressure of the 
 
 ■d and let both 
 d and dropped 
 in. 
 
 cylinder s, and 
 force must be 
 der, the area of 
 ihat the piston, 
 )rce required to 
 B force when it 
 : point is with- 
 nverted, and a 
 3 ser ; 'on to the 
 iston might be 
 ! tube is raised 
 oured into the 
 11 be sustained 
 the last case ? 
 re forced into 
 IS become ? 
 
 20cm. Three 
 iceiver weighs 
 plate ? 
 
 ue east at the 
 jouth-west in 
 t will be his 
 
 iles an hour ; 
 ir, while the 
 ir. What is 
 
 men, one in 
 they meet ? 
 
 th's surface. 
 
 20.— Bodies at rest, with respect to the surface of the earth, are really in 
 motion, and their motion is not constant nor in a straight line. Are the 
 forces which act on them in equilibrium ? 
 
 21.— Upon which will the eflfect of a given force be greater, a body 
 initially at rest or a body in motion ? 
 
 22. — Express the atmospheric pressure at sea- level in absolute units. 
 23.— Why are "top-heavy " bodies unstable ? 
 
 24. — What mechanical advantage may be gained in a copying press in 
 which the hands move through 1 mch, while the end of the screw descends 
 tJ-j inch ? 
 
 25. — How many cubic feet of water will a lO-horse-power engine raise in 
 an hour from a mine 300 feet deep, a cubic foot of water weighing 62^ lbs? 
 
 26. — When a force acts on a body at right angles to the direction of its 
 motion, so as to cause it to revolve in a circle, does it do work on the body? 
 Why? "' 
 
 27. — Does the sun do work on the planets, which revolve about it, by 
 virtue of the mutual attraction between it and them ? Explain. 
 
 28.— Is the energy of the planets increased by this attraction ? Do the 
 planets move in circular orbits ? 
 
 29.— When a force acts upon a body and causes it to move a given dis- 
 tance, in what language would you describe the effect of the force ? 
 
 30. — How does energy differ from power? 
 
 31.— If an engine should raise 55 lbs. 10 ft. in a second, and at the end of 
 a second its energy should be exhausted, could it properly be called a one- 
 horse-power engine ? 
 
 32. — A cannon ball is shot into empty space ; how great a force will be 
 required to deflect it from its path ? 
 
 33. — Can a child sitting on a sled start or stop the sled by pulling on a 
 cord attached to the sled ? Why ? 
 
 34. — Why does not every body move when acted on by force ? 
 
 35.— Why does a body thrown horizontally into the air fall to the earth ? 
 
 • 36. — Is the expression "one-horse-power per second " admissible, as, for 
 instance, when we wish to convey the idea that a horse-power lasts, or is 
 exerted for one second ? 
 
 37. — How many dynes of force are required to set a mass in motion ? 
 
 38.— How many dynes are required to make a gram-mass move with a 
 velocity of 9'81m per second, the force acting constantly for one second 5 
 What, if it act for two seconds ? 
 
 39. — What is the force acting on a falling gram-mass in Ontario? 
 
 40. — Is a spring balance a force measurer or an energy measurer ? Why 
 will it not answer both purposes ? 
 
 41.— How can a force of 6 lbs. raise a ton 10 ft. with a perfect machine! 
 
m 
 
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 P 
 
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 TEST QUESTIONS. 
 
 42 — A pebble stone weipJm f.. n.v oa,» • 
 
 ^tlww iir '"T' "«'""" ^ -»' ^»^^ -- a COM W, . 
 
 ^«- *h™"p^^^^^^ in flannels in the sun^mer ? How 
 
 peS-^e'-be lowfe °' "^""^^ »~ -lorie , how much will the tem 
 ,£-How .any .ilogra.s of ice at 0= C. can be .elted b, . of stea. at 
 
 52;:rt::s^^^^^ 
 
 waterfrom 0= C. VlPhT * '^^ ^^ ^^"^'^ ^^ '-^quired to raise 500k of 
 
 mettuTi^Sll^ee^T^ptntX^^^^^ .T ^^^^*™ -'^ ^'ght. and on 
 "If *^r «t was found to be 45 vdts \VhT *^'!u''^° ^^^''^^^^ by ai elect ° 
 absorbed by this lamp ? °'*'- ^^^^<= ^^^ the amount of horse poier" 
 
 a et7n?7S^4rslTSnS^^^^^^^^^^ - ^-M-F. of 60 volts and 
 horse^^wer. the loL of energylS 'i^^ZS:^'^^^^;^::^ l^X^ 
 
 f ^^ "? ^*^^-:;S^- - I^? a^ -e.^^^ line 
 
 they be c. nnected f ""'"^ '" ^'^^ ^"^^ 'circuit, by which method shouTd 
 ^;[^i^^^l^l^ireator,,intai. ^ current of 20 amperes in a 
 »?Sf ^iT ?;Ln^^t^--,5S r '- ^- ^- «^ ^ volts fnmish 
 
EXAMINATION PAPERS. 
 
 397 
 
 5r It weighs 15g ; 
 'ecific gravity of 
 3? What 18 the 
 
 statement. 
 n when an open- 
 
 i body ? 
 
 Jmperature of a 
 ;o expand ? In 
 
 a body may be 
 e heat in these 
 
 immer ? How 
 
 I will the tern- 
 
 l"^ of steam at 
 
 3f iceatO°C.? 
 ' raise SOOk of 
 
 igbt, and, on 
 ^y an electro- 
 horse-power 
 
 GO volts and 
 ig 15 (useful) 
 t. ? 
 
 II furnish a 
 a such bells 
 
 legraph line 
 
 xternal re- 
 n? If two 
 hod should 
 
 ip^res in a 
 
 ts furnish 
 
 ( 
 
 an^E'lTp^ofV'v^lJu*?' resistance of a circuit in which a battery having 
 aatij. m. tf.oi 2 volts furnishes a current of 0-5 ampere ? J' vmg 
 
 m^l'i^eT^* '' **"" "'°'* convenient test of the E. M. F. of an electrical 
 
 nf +JT^^ * sounding body moves, how will its motion affect the wave-lencth 
 ?/ont ? 5ow ZiUfi •* '^.T'. Ju'*^^"^ • H«^ ^"1 i* affect thosHhrowK 
 
 of ttiSn"Ji;'bodf r"'" '' p'*''' '^ "^'^ "^^ °* *° ^^^°"^^*« *^« -1-ity 
 
 65.— How does one color differ from another color ? 
 thfoVh^'aroitTp^ffi" *'*' «^P^™*^- °^ -1-s when white light parses 
 
 Nam;";r'e'oliio?nif.?''f^ '1-'S°^ '' ''"""^^^ mirror on a beam of light: 
 S! P °^ "P*'"'''^ apparatus that will produce the same 
 
 elong^KrilSi;r«" '' " "«'' ^^ ''''' ''' ^**^^ --"^ -°"nously 
 
 D^s ^^mLa r*oZ oS S Lt*/" """ ^'"'^^ *"^^^ *°^^^^ *^« «^^ ' 
 
 EXAMIISrATIOlSr PAPERS. 
 
 EDUCATION DEPARTMENT, ONTARIO. 
 
 MIDSUMMER EXAMINATIONS, 1887. 
 
 THIRD CLASS TEACHERS. 
 
 4. — What in nnh<itiinn ' uru-t _ u i^ ■ , 
 
 were no cohe'rron 7 T* '„. " .u- '''°"^'^ ^^ '^^^^ oondition of things if there 
 what then? "^ " ovorythmg possessed cohesion to a greS extent? 
 
 hafjoSouT"' °°"'^ y°" ^^°^ ^'^^^ ^**^^ has cohesion ? That mercury 
 
398 
 
 EXAMINATION PAPEBS. 
 
 5.-H0W is a barometer made ? Mention the uses of a barometer 
 specmS 0I Sf '^^* • ^^^^"''^ - -P-iment to show thl great 
 doLl?i;7 t'ef "" '' ""'• ''"^ '^ ^« ««*™-t« the amount of work 
 
 oft;;^eT?s15iftr79rm^^^^^^ 
 
 [Is that which is called latent heat really heS^ J '"'^"^ ^^"^' ^ ^ 
 9— Show the course of a ray of light 
 
 (a) through a flat, thick piece of gla^s ; 
 
 (6) through a piece of glass shaped like a wedge 
 
 intoVatSl-dX^^^^^^^^^ of a blow is changed 
 
 of dec-^m^oslngl^ateT ^°" "''"'' ^'"" *^^* * -°1^- battery has the power 
 
 
 MIDSUMMER EXAMINATIONS, 1887. 
 SECOND CLA S TEACHERS. 
 
 ^':l^rft^'rolZ^^^^^^ at an angle of 
 
 J^-StateNewton'sSecond Lawof Motion, and show howitapplies in the 
 («U Ull thrown vertically upwards from the hand of a person at 
 
 ^'IfotS,*'^""'^ ^^'^^^-^y "P--ds from the hand of a person in 
 ^ (C-) a body projected horizontally from the top of a cliff. 
 
 drawn^uf a":ith pfcSit iXn' S atS1n:^/ T «^* °^ ^« ^^«- >« 
 the horizon? ®°^'^'* *°"* 'helmed at an angle of 30" to 
 
 rwv,»r-' ""^ -"--■!"-".' tu show the trutii of this law. 
 
EXAMINATION PAPERS. 
 
 399 
 
 barometer. 
 
 ' show the great 
 
 amount of work 
 
 whose height is 
 at he may reach 
 
 '■ the latent heat 
 equal to ? 
 
 »Iow is changed 
 ^ has the power 
 
 at an angle of 
 applies in the 
 of a person at 
 >f a person in 
 
 t of 40 lbs. is 
 igle of 30° to 
 
 the relation 
 
 ■e be said to 
 
 6.— State, with reasons, what will take place in the case of 
 
 (a) a siphon in action under the receiver of an air-pump at work ; 
 
 (b) a barometer under the receiver of an air-pump at work ; 
 
 (c) ajjarometer in a closed vessel filled with air to which heat is bemg 
 
 ,r2'~T^ body floats, partly immersed, in4)ure water which exactly fills the 
 
 dronld°?n^T.^''- .A«r'h salt as the water will dissolve is ^carefully 
 dropped in. State minutely what will take place. ^ 
 
 If the body had been floating wholly immersed what would have occurred? 
 
 h.hJ'L^^'l^''}^*'' fV^Pi<^*'ng t'»at air had weight, and wishing to verify his 
 to wP,Vh /^^'"^ * ' • '"u^re* ^'"P^y ^"'J *^^" '"Sated with air ;ind finding i? 
 WhllSi I- ^"^ ?° H*'^ ^?'^^' concluded that air was a weightless fluid." 
 Why did his experiment fail ? * 
 
 9. — Define specific gravity. 
 o..hinf^'%^'^\°^ "^^^u weighs 1 000 oz., find the apparent weight of a 
 iron beS^ 7 2T """^ ""'^ '° ''^^'' *^' '^^^"^ ^^^^''^ °* ''^^^ 
 
 actton~d^pends.^ *^^ ''^^°°' ^""^ *''^^^'° "^^^^'^^ ^^^ principle on which its 
 
 «in?^rj?rP*v7T^^°^^J°°*'''^^!P^°'^*' gravity =-8) can be emptied by a 
 c?!^ be7ng'f3 5l ^^'''''''^' '^^' ** 30 inches, the specific gravity of mer 
 
 11.— State Boyle's Law. 
 
 The mercury in a barometer stands at 30 in., and the sectional area of the 
 tube 18 one square inch. A cubic inch of air is admitted through the mercury 
 intT the vacuum above and depresses the column of mercury through three 
 mches ; find the size of the vacuum. ' ""'''"S" """^ 
 
 JUNE, 1887. 
 
 SECOND CLASS PROFESSIONAL EXAMINATIONS, 
 NORMAL SCHOOLS. 
 
 Note.— 75 per cent, of the value of this paper counts 100 marks — the 
 
 maximum. 
 
 ani'7o^!"*'°?u*°y ° v**"^ ^"^^ w'"*^^ go^e^*^ tl^e expansion of solids, liquids 
 and gases on the application of heat. -"'luo, niiuuis, 
 
 «ho;rf'L*iV-^''K ^"^^ Yi*^ '?° ^'^^^'' '^ ^^^^^ o" a %varm stove. In a 
 Sterwrdsth.'w«r' W*''^* the water does not fill the vessel, but soon 
 ^*lTi„ *^^ "^^^^ '^ ^°"°*i *° ^^ "-"""ing over. Explain the causes in 
 
 ^ 3. -Explain why it is that some kinds of food cannot be cooked by boil- 
 ing in an open vessel on the top of a high mountain. 
 [How would you arrange a vessel in which to cook them by boiling ?] 
 
fM> 
 
 400 
 
 -^ 
 
 EXAMINATION PAPERS. 
 
 posed of a number of differently colored lights? ' ^''^^ ''^'*" ^^8^* " ««'»■ 
 Wanfay tt^S;' "'^'=*'°'^ ^^ ^--^e experiments by which these 
 W^ S^^F:Z^J^:i^^^^y> -^ -P^^^'^ ^ts mode of action. 
 
 11 ' 
 
 I t I 
 
 
 MIDSUMMER EXAMINATIONS, 1886. 
 THIRD CLASS TEACHERS. 
 
 of water and the ice, or the steam, Sto whTcK?'*"!""" "^ 8'^^" q«a°«ty 
 ^c^l^you prove this in -y partic^r^ J^s^ 'I ^hot Xl^^^^^^^^^ 
 
 ex|ti;e^tf UKS^^^ will suffer a loss of weight 
 
 waterv' * ^'" ^^PP- 'f *h^ -bstance be lighter, bulk for bulk than 
 What practical applications are made of the fact which you haJe just 
 
 water. Boiling water EureTonthe^o^^ ''■'"'^'^y fi"ed with 
 
 mediate y the ^ater insidHhe pi> Ldns to'lk*'''^r T^ ^'"^°«* "» 
 After a time (the boiling water stSl Sfnl ^u' ^^^ ^""^^ i* do so ' 
 the p.pe. Why does it do so v ^ "^ '^"^ ^^^ ^^'^^ begins to rise in 
 
 these powers. ^""ou some practical applications of each of 
 
 S^^->^^^^^^^^^ the famous inventor, Mr 
 
 influence which iron undeTgoes a^L t.mn ^1'° '^"'^^P^^^'ty to maguet/c 
 
 eectricenergyfromfuel witK the nliS^^ is c&anged, to produce 
 his contrivance a "Pvrouino-nAHn n '°'*^y,6°won of a steam engine Hb naUa 
 crude stete, it.is abS^a^^Slfif t?"]:-^^.^!.^*^ at prLrnt in^'onlf: 
 
 ^i°I"l^?^^-^/"amo, while it re"^urr'es"no att.^^^r/ii°,e.?i V.'« «team engfne 
 
 Edison read^a P^er on to^^'nV^ffn beL'el^^r ^ *? ^^ ^* '^"X 
 Advancement of Science at itslast session ^«ie"can Association i 
 
 . Mr. 
 for the 
 
g a beam of white 
 vhite light is com- 
 
 its by which these 
 
 1 mode of action. 
 
 liquid, and a, gas? 
 a given quantity 
 inverted." How 
 ould you weigh 
 
 a loss of weight 
 ibe experiments 
 
 for bulk, than 
 
 you have just 
 
 in water ?] 
 otion possesses 
 
 motion of the 
 
 fly filled with 
 ud almost im- 
 loes it do so ? 
 Jgins to rise in 
 
 n do." State 
 ns of each of 
 
 I inventor, Mr. 
 ty to magnetic 
 i, to produce 
 ine. He calls 
 sent in only a 
 steam engine 
 unning. Mr. 
 iation for the 
 
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