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LEOTÜRE ON PNEUMATICS.
The residents of Oxley havo adopted o very de sirablo arrangement by which they havo social gatherings and lectures at ti* time of full moon. Thus tho people aro enabled to spend many an hour in relaxation from tho heavy work of tho farm. On (ho last occasion, Mr. Boyd, of tho Primary School, lectured on " Pneumatics," assisted by Mr. Gray. Mr. Boyd classified his subject ns follows :
Philosophy-This word philosophy mny in duco you to think that I am about to treat of some wiso subject utterly uninteresting to you, hut thero arc various kinds of philosophy ¡ ouo kind relatos to God, mid is called " theology ¡" another relates to man, and ia called " logic and ethics j" the third relates to nature, and is called " physics " (not doctors' stuff, and not nearly so unpleasant to take). It is of this last, called natural philosophy, that I nin about to speak to-night. Kow, if I were able to go ¡uto the whole matter of natural philosophy, a dozen meetings would not suffice, so ive will make one subject suffice nt present-" Pneumatics." Before proceeding, let mo mention that natural philosophy tenches us the gonoral properties of matter, ond the action of innteriol substoncos npon each other. It enables us to investigate and explain natural phenomena, to traco effects from causes, and to ascertain those fixed laws hy which all things are sustained and regulated, called tho "Lows of Nature" Kow, concerning
matter-what is it ? Thoro is another word which moons the same thing, viz., substance. AU know what subBtance is, I imagino. It is every thing or being which octs upon our seuseä, either immediately or by tho perceptible effects it produces. It is composed of a concentration or combination of small particles called atoms, united in regular forms so as to compose a com- pact mass. But in saying a compact mass, I do not mean definitely a hard, solid, risible mass of stono or lead, but a muss of anything which can be massed together. Some kinds of matter are visible, such as wood aud stone ; others aro invisiblo, and we can only ascertain their ex- istence by observing the effect thoy produco on other bodies. Of this invisible nature is the matter which surrounds our globe, and which ?we call tho atmospheric air j but although it is invisiblo, it ia nevertheless as much a substance as the hardest stono. Matter exists under the general forms-solid, liquid, and aeriform.- In the first of these states, the atoms ara fixed and combined ; in the second they move freely upon and among each other ¡ and in the third they are removed to a greater distance from each other. Air, of course, belongs to the last of these, and of all the sensible compressible fluids, air was the first studied, tho most familiar, and the most minutely examined. It is by no means an idle question to say, " What is air ?"
I havo before said that it is a substance na much as stono j but any one who has not studied the subject may easily bo excused for doubting the statement. Ho might say, "Air a substance! Why I can't see it, nor feel anything. I can walk about without any hindrance iu any direc-
tion. Then what is this matter-air-which
surrounds us ?" Now it will be very easy to prove that there is such a thing as air, and that this air is a substance, and possessed of all the properties pertaining to matter, such as impene- trability, elasticity, and mobility, ¿c. Wo ob- serve, if we place a cork on the surface of the water and press an inverted wine-glass on the -water, that the height of the confined water will not coincide with that of tho external water ¡ nnd if we continued pressing this glass down to the depth of 1000 feet, there would still be a space of l-30th, which the water would not fill, and this proves evidently that some impene- trable, compressible substance is confined in the glass, above the water ¡ That something is air. Supposing there were a hole in the bottom, of the glass, and you kept your thumb on this hole "when immersing the glass, and then suddenly Temoved it, you would feel a sensible puff as the air escaped. The water would immediately fill tho glass and thus the impulsive force, and, nt the same time, the mobility of air aro proved. To further illustrate its impenetrability : Most of us know what a syringe is ; the boys may better understand tho word " squirt." A.t all events, here is one. You see there is a hole at the bottom. I stop up this hole, and it íb im- possible to force tho piston or plug down to tho bottom. Equally impossible is it to draw up the piston if it be at the bottom provious to stopping the hole. There is n something which causes this difficulty-it is air. I shall pre- sently show the cause of this.
Next comes Elasticity.-This term applies to matter which, boing compressed or expanded, resumes its former position on tho pressure being removed. If you take on India-rubber ball (hollow) and squeeze it, you cannot flatten it. You may press it out of shape, but it re- sumes ita fair proportions immediately you ro leaso it. The air within was squeezed into smaller hulk, and by its elasticity regained its position at once. In like manner wo demon- strate pressure by means of a pair of bellows. Lastly, it is incorrect to say wo experience no obstruction in passing through the air. Wo do experience an obstruction, and a very sensible one. Take a fan and move it through tho air.
Does it wave backwards and forwards without somo effort on your part ? A very sensible ef- fort is required, and this proves that some sub stauco muet be pushed out of f ha way to pormit of the fan or our bodies' passing along. The sails of a windmill, tho ships moving along, the trees overturned by the wind, aro all proofs of the power of air in motion, of which I shall not have time to speak to-night. In addition to these properties, air has weight, and c.n bo weighed. AVe know that soap-bubbles, balloons, feathers, &c, &o., will float in tho air. But why do they float ? Because they ore lighter than air, and air must have weight to support them. Let us withdraw the nir, and then sec if theso things will float above tho earth. [The lecturer hero explained the principio o£ tho air pump, and demonstrated the fact of air having weight by means of an in verted bottlo of water.] Air may bo accurately weighed by exhausting the air from a bottle and weighing the bottlo ¡ then re-admit tho air, and again weigh tho bottlo. It will bo found tliBt the ditferonco
will bo 1 1-5 oz. for every cubic foot the vessel will hold, and this is surely proof enough, al- though wo might give further proof. The height to which it is supposed (and with very good reason) that' our atmosphere reaches is about 46 miles, ? At all events, at this height it ceases to refract the rays of the sun. It sur- rounds the earth in much the samo manner as an equal bulk of cotton might do ; that is to 6ay, from the suporincumbent weight of cotton tho lower flakes or bales would be pressed closely together, and the higher you went the less pressure you would find, until, finally, tho upper flakes or bales would exporionco no pres- sure at all. Thus tho air is much denser at the
earth's surface than at a considerable height. If wo ascend a high mountain, tho difficulty wo experience in breathing increases, unlU at last wo should die, on account of the extreme rare- faction or lightness of the air. The first 3i miles of tho otmosjihcre contain as much air as all the remaining 42}.
The actual weight of the atmosphere, or the pressuro which it oxcrts at the sea level, ia l8 lbs. on every square inch. The number of square inches on tho surfaco of tho body of au ordinarily sized mau is 2088. The pressure of air on a man is therefore nearly 14 tons. And how is it that this weight does not crush him to a jelly ?-for, you w ill say, it is impossible j no man could carry such a weight. Nor could he, but that there is the pressure of tho air within, which is equal to it, and this prevents any dis- turbance of our animal economy. TIiub, if you place your hand upon an open glass, and ex- haust the air from it, you will find your hand sucked in with groat force, and it will be pain Ali. This arises from tho sustaining fluid being withdrawn from the lower part of the hand, con- sequently the external air presses upon tho ex- ternal surface of the hand, and causes it to bo drawn into the glass.
I have now shown that air possesses weight, elasticity, &o. ; that it is o fluid, and is com- pressible, and by a few simplo experiments I will show the results of llicee properties of air. Firstly, to prove its downward pressure : If I placo a bladder ever a tube opon at both ends, . *nd exhaust the air, tho pressure from outeido
will force down tho bladder with a great report Secondly, to prove tho résistance of tho air A guinea and a ¡bather, not being of equal woight, will not lench the ground in tho samo timo if allowed to fall togothoi in the open air But
allow them to fall m a vacuum it tho samo mo mont and thoy will reach tho giound togothoi There is another beautiful experiment, illustrât
ing tho resistance of tho on Wo hovo a toll glass tube fitting closel) the fountain plate, and into tho pump pluto is screwed a bliss tube with a stop cock Tho au 13 now exhausted fiom the receiver, and when tho exhau«tion is completo the stop cock is closed, thus piovont mg any accoss of nu 011 unscrewing tho tubo from tho pump plate Now I immerse tht lowei end of the tubo into a basin of watet, open the 6top cock, and the jircssuro of tho air on the water in the basin will foice it upwards through tho tube into the recen ei and there being uo air to resist its ascent, it will form it self into a beautiful fountain, which will cou tmuo to play until the equilibrium is lestored lho pressure of the an is proved iu this experi
mont, but wo may fut thor exemplify its pressure by tho Magdeburg heim«phore Iheso aio two hollow half circles of bia»s, mode to fit one anothei closely lho) aro furnished with handles ouo of which moy be unscrewed and a tubo with a stop cock screw ed on 111 its place This tube, with the hemisphere attached is now scienod on to the pump plate with tho stop cock open, soaa to have freo communication liotween the interior of the sphero and tho oxhausting sy 1 mgc of the air pump The air 13 now to bo exhausted from tho intorioi of the cups When this is done, the stop cock is again turned, to piereut tho air gottmg m agnm Tho cup is then taken off tho pluto The piossuro of 011 13 now so great all round tho sphere as to prevent its two hemispheres being separated They are pie«sed together by a force amounting to lo lbs on every square inch If the diameter of tho spheres bo 6 inches, its section will bo about 28 squaio inches, which multiplied by 15, will givo 420 lbs, the amount of force necessary to sopa rate the two hemisphere«,and if 400lbs bo sus ponded from the spheres the weight will be sup ported by the pressure of the atmosphere This experiment was mado by Otto Guoncke on a largo «colo His sphere w as of a largo size and when exhausted the hemispheres could not be drawn asunder by twenty horses
It is to the pressure of air that wo owe the action of the common pump of tho uro engine, and of the syphon The pump is not a pneu matic but a hydrauho machine, although it ow es its power to the pressure of air It wasimented by Etcsebes, a mathematician of Alexandria, about 120 y ears before Christ, and it was very much nnpi oved when tho pressure of air became known There aie threo kinds of pumpa-viz , the sucking, the lifting and the foicmg pump I shall only notico tho first It oousists of a pipe open at both ends, m which is a movable cyhnacr or piston ns big as the bore of the pipo in that part wherein it works, and contrived to fit the pipe «o exactly as not to allow any air to pass between it and tho «ides of the pipe where it acts In the piston there is a valve opening upwaids, like a trap dooi, to allow tho air and water to ascend readily through it, but to pio vent cither of thein descending This piston ia called the bucket It is moved up and down by o handle, to which is attached a connecting rod The pipe consists of two ports, of which the first or wider part is called thebairol, tho other the suction pipe, which ia of smaller di
ameter At the joining of the working barrel with tho suction pipe theie is a fixed valve, also opening upwards Lastly, the lower end of the suction pipe is immeracd in water, which is ad mitted by small holes to prevent the entrance of dirt, and at tho top of the working barrel is a pipe for the dein cry of the wator which ia raised Tho mode of raising watoi by means of a pump is tina When the handle ia raised, the piston rod descends ond brings the piston valve to the fixed valve When tho handle is drawn down, the piston ia taised and as it is au tight, a vacuum is produced between tho two valves Tho air between the low or valvo and tho w atcr forces open the lower valve, and by tho pi essui o of the atmosphere on the surfaco of tho water, the air being íondoicd less dense in the pump, tho woter is forced 0 short way up tho ban c1 Wheu the piston again descends to the lowoi valve, the air between them is again forced out by forcing opou the upper valve, nnd when tho jiiston is îaisod n vacuum ia again formed tho mr below the lower valve rushes up, and the water in cou3equenco is afain raised a httlo fuithei This oporation continues until tho watei raises ahovo tho lower valvo At eveiy stioko afterwards tho water passes through tho valve of the descending piston, and 13 raised by it on its ascent until it issues out at tho spout AriBtotlc, a Gicok philosopher, assorted that
tue water rose into tho vacuum because mature abhoicd a vacuum All naturo, ho asserted, was full of being and a Bpace void of all niuttor could not be produced His followeis said that if water did not follow tho piston, there would be a void But nature abhors a void, thoro foronatei follows the piston But one obser vation will overturn this wretched reasoning Suppose the pipo shut at the bottom the piston eau be drawn up, and thus a void produced Ai istotlo'a friends say " Ko," and talk of cer
tain thinga which occupy tho spaco But if, ob tho) say, the space is occupied, why need tho
water rise ? How can it ri»e ? Then this ia
ovidently not tho causo Wo must seek it olso whoro, and in the pressuie of tho air we find it This discovery is due to Galileo
Tho syphon is not a pump, but a bent tube, so contrived ns to convey liquids fiom ono vessel to anothoi at a lower lovel, by raising
thom first above their natural Ici el in the first vessel Hie syphon has one leg shorter than tho other, the Bhorter leg is immersed in tho liquid which is to bo transforied from one vesael to tho other At the commencement of the operntion the shoitcr leg ia ltnmcrBodin the first ves6c!, and the oir removed in any way mo«t convenient, by bucI ing it out of tho tube, 01 filling the tubo with liquid whilo its ends aro held up But in any case, as Boon as tho sy
phon is filled and nivmcrsod, tho ntmosplienc pressure on tho surfoce of the wator in tho full vc'solfoiccB tho liquid up tho tube towoi ds tho highest point and if this point bo not more than 32 feet high in lho case of water, and 30 inches in coso of mercury, tho fluid will pass this point and fiU tho wholo of the lubo to the mouth of tho long leg Another vosscl ib now placed under tho open end, and the liquid 111 the one vosbcI will be tianBferrcd to tho othei You
will understand whv ono leg of tho syphon must bo longer than tbo other by considering thal when the instrument 13 left to itself the atmos phone pressure ib acting as much nt 0110 ex tromity of the syphon as at tho other If you raise the liquid to the ovtrcnio highest point and withdraw tho mouth the liquid falls back at ouco It will do tho same if wo get the liquid no further than half woy down tho long leg bo cause at that point the upward pressure pro vails against the downwnid pressure , but bo yond that point the downward pressure prevails against the upwaid, and tho liquid will flow out lho experiment wo mado a short timo ago to prove tho pressure and rosistanco of the air, is colled the ' Byphon fountain Hie bi phon is used m somo places to carry water from one ploco to another wheie tho intervening ground ia higher than tho fountain head Hero a pipo is laid, and by nicons of a forcing pump tho water ib driven to the end of the pipe, and onco running thero is no need of the force pump ony longei, as bv tho jiressuro of tho air it will con tinue running Now, wera it poBeiblo to pro vent the admission of air with the water, tho stream would continue to run until tho ¿bun tain was exhausted, but the wator in tho upper part of the tube is in tho samo stnto as wator undci tho receiver of an air pump-it gives off tho an which is chemically combined with it lins an accumulates and finally chokes tho rnpe bo much that the w ator refuses to run, and the force pump must bo ogam applied Still, in cases of irrigation this mothod, whero it could bo applied, would ho a last saving of labor and timo lhis combination of air with water is very distinctly seen by means of tho air pump If a glass of cold spring wator he placed under the receiver of an air pump, and the air rarefied, small bubbles will bo observca adhering to tho sides of tho glass, and griidu ally approaching tbo surfaco of the water If wo boiled tho water in lacuo, that is, under tho receiver while tho au is exhausted, we should
thi
th extract all the air from the water Tho air pump givos us a, great variety of experiments ii lustrativo of tho air's elasticity and expausi
bihty Tho veiy operation of exhausting, as it is called-that is, withdrawing tho mr from any- thing-is an mstnnco of its groat expansibility, but tina we cannot at present exhibit to now
Aftor sovoial othoi interesting experiments had bion made, tho lecturei went on to speak of thi baiomiter lo explain in a ftw words »hat tho bnronietei ib, I must recall to youl recollée turn what I said about tho weight of tho atmos
phcro as computd with lueiciuy and watei, vi/ A column of air out inch square, and reaching fiom tho surfaco of tho cm th to tho greatest height of the atmosphere, would be equul in weight to a column of mcieiir) one îuch s quin o and SO indus high, or a column of water ont inch square mid 31 ftet high It is tho pi casino of the ntmosphtio wluth mol es tho mu cur) keep up in a barometu Tho aimplo mode of moiling a biiometer is to tako o w eil mado tubo of glass 33 or 34. inches long, and, filling this with moieuri, invert its open end into a oup of tho somo metal Now this ap- peals vory easy, but thoro are a good niiny practical difliculties which render the construe tion of o barometer a work of gieot nitety Mercury will absorb impurities, and air will persist m getting botweon the inercui) and the gla«s, and so lender the insti mnont lmporftct aud a variety of impediments occur which re- quire scioutiuc application to oiercomo Hav- ing thus prepared tho tube, we must lind a moons of asceitamiug the amount of use and foil in our instrument Foi this purposo wo mark tho sides of tho lustiument (supposing that wo have fixed tho tube in u woolen ii ame) with numbers corresponding to tho length, oi height rather, of tho column of mci eui), tom mencing fi om tho lovel of the mercurv in tho cup or cistorn below We shall find this amount to precisely 30 inches But it 13 not necessary to rommoncn Uio ooiilo /mm tho lovU of tho nioreuty in tho elstern, or tho neutral point, as it 13 called, but from about tho 27th mch, and to continue to tho 31st Should tho insti union t be required for measuring moun- tains, then the scale must commencent thol2th inch Lach inch is divided into ten pints, mid each of theso again into huiiotnths, by means of a sliding stale, called the nonius or vernie)
This inca ures oxactl) ouo inch and one tenth in length, and is numbered from the top dow n wards lhi« is used foi ncciuato leadings of tho
mstrumont Hiero oro several forms of the barometer, but it is not necessary to describe them Suflice it to explain how tho height of a mountain mav bo osceitaincd by means of the barouietei It ia an ascertained fact that in as cendmg lofty mountains tho mercury in the barometer will fall one tenth of nu inch foi ovoiy 103 feet of OBcent (in perpendicular height), so that if you oscond a mountain with a barometer in your hand, and find that, ni
though nt starting thomeicurv stood at 28, yet now, on the trouutam, it stands at 27, you may concludo that you have attained a height of 1000 feet It is thus thut baromctois aro
pressed into tho sónico of »'.onouts, who uso them to ascertain tho height to which they havo
ascended in a balloon
Having thus glanced at tho vanous propel
ties of air, wo will return foi a moment to tho property of pressure I supposo )ouhtvonll heard of the air gun This instrument ow cs its power to the expunaivo forco of condensed on It resembles a common gun, with tho addition of a round boll undei it lins bull is hollow, oud the air is forced into it by means of a con dousing syringo A portion 13 nllo-vcd to escopo each time the tuggtr is diown, so that it piesses against the bullet, and acts in tho «nine way as gunpowder Into the ball enough an can geno rally bo condensed to servo foi fifteen 01 twenty chargos , but tho force with which the bullets aropiopollcd will decrease at each dischnrgi bulee the invention of tho air pump, it has beon piovcd that au ia tho entire volntlo of sound Nothing is moro wondirful than tho vibratory motions by which sound 13 piojiagntod When wo sie a man felling a tieeut somo distance from us, we may soo the Btroko of tho nxo full on tho trco, but wo do not hoai tho t ni iel of tho stioko (if wo montar enough to lum it) until somo time afteiwnrda Kow, why do we not Iltur tho sound at tho instant of its production '
Tor tho obvious looBon that the sound must trurol to oui car thiough the an But now the question aiiBcs-How does it travel ? lins mu) bo shown by comparing air with water If wo drop a pebblo into the watei, we seo a aenea of lings aiising from the spot where the pobblo ontoied tho water, und giodually enlaigmg thonisclves as thoy recedo fuithoi fiom the centre, until, finally, the conti 0 of agitation is nt lest, and tho diverging circles become moro und moio gentle until at last thoy ic-ise to bo apparent Wo may apply tho suino oxamplo to air Tho sound of tho axe, like I j dropping of the stono, onuses tho pal tides of air to bo forcibly imptllcd against othti piutitlos, which, hating icconcd tho momentum, 111 thou turn piesa against otheis These waves will do crease in stiength us tho distance 11111 easts, but with a velotity noiulv uinfoini, and winch 111 air is about 1112 feet per second
Hie principal paita of airarotwogusos, called o\)gcn and nitiogen Owgon, 11 its pure state, 13 lcmaikable for its cncigy 111 promoting combustion, îcspiiation, and other chemical changes If you place in animal in pine oxigtn it will d10 from excess of vital ictiou When wo blotf the fue with a pim of billows or with oui hot, wo causo it to bum up more biilliantlj by tho oxtia supply of oxygtn lho propoi ties of nitrogen, on tho contrniy, appeal to be of a negative chai ictei It suppoits neither combustion no1 respnotioii It has no poisonous qualities like some gasea, y ot au am mill cannot hie m it, simply fi oin tho ihstnco of oxygen, ond not fiom tho pi 1 judicial elliot of nitiogen Its use 111 the utmosphorois to di lutt it-to modify and subdue the activ lty of ox) gen, andtlioicfoioit exists in greater ubundanco Every atom of oxygon 13 accomponied by four of nitrogei and it is a Biugulni fuct that al
though nitiogen is lighter than oxygen, these two gasea an always prisent 111 the samo pro portion, no matter if the mr bo taken fiom lho ¡over hospital from the Antic Octun, fiom tho dc=ci t of Sall ira, or from tin top of tho Andes This fact will appear the more woneltrful if jou take into consideration tho innumciable som tes
of vitiation which ttnd to lmparo the n«cful jiroporticB of oxygen, by dopimng it of its power to support life ond combustion Indeed, these two vtry actions aro tho chief means of vitiating it, for during the combustion of om candles, lamps, and fires, and during tho re spiration of animals, n quantity of carbon is hbiratid, every 0 parts of which unitt with 10 parts of oxygen to form a compound gas tailed carbonic acid, which resembles nitiogen m not supporting animal hfo or combustion lins rany be shown by confining a quantity of mr m 0 gloss tube, and placing therein 0 lighted taper If we do not admit any moro air, tho cundió will tonsumo tho oxygen, jnoduco carbonic and, and go out for tho want of a furthe- supply of the gua ntce«Biir) to its existence I havo al lind., suid that this oxgyen is noces ary to sup poit combustion If we placo a lighted co dlo undii the receiver of au air pump and exhuuat the air, out goes oui candle Why 9 Becauso the gas necessary to ita continuing 111 a state of combustion is withdrawn But, furtheimoro, as oxygen is consumed b) our tespirution 01 breathing, so ib carbonic acid produced by tho samo action, and consequent!) it is reasonable to supposo that tho brooth of our lungs will not support combustion If we take a glasB tubo and exhaust the air from it, and admit mr from our lungs b) breathing it m through a tube, lho light is at onco extinguished Lvdontly no oxygen there, or the light would ou'muc to
burn
Kow docs it not appear strange that tho quantity of corbonio acid in tho atmosjiheio should consist only of 5 volumns in 10,000 of at
uiosphciicair, considering tho enormous amount of oxygon consumed by tho innumeiubic land and water animals, which all depend moro or less on ox) gen for their existence, mid the various operations wo cairy on which require combustion? By a beautiful arrangement, however, the carbonic acid thus made ia turned into footi foi plants foi tho vcgctablo world The green parts of plants, under tho lnlluciico of light, absorb carbonic ncid, retain tho carbon, and restore pure oxygen to tho air Duung tho night, howover, plants absorb oxygen, and give
ki
out carbonic acid, but in the course of tho 24 hours they give out considerably moro oxygen than thoy consume, and in this wa) a compen- sation is made foi tho loss of oxygou occasioned by combustion and respiration. I havo not tuno now to outer upon tho subject of air in motion, whioh would oocupy a long time , neithor can I say anything at present conctrmiig icriol navi
gatton Pethops nt bohío luture timo, when wo are hard up foi a subject or a speakoi, I may take np tho-Biibject ogam In tho limited tuno afforded mo it baa beon nnpossiblo to discuss tho subject fully, but I trust I have been suffi cundy plain to have made it interesting, and that I havo not tned you too much I am much indebted to Mr Gray foi his ablo assist anco in conducting tho various experiments, and I om sine you will join with mo in thanking him
At tho conclusion of the lecture, which was hcai lily applauded, tho first verso of tho Nation Anthem waa sung by oil, and the audi enco dispersed
Hine 13 an improiemciit which wo would suggest as tending to relievo tho necessarily tedious part of a sciintific lecture, viz , nitro
ducmg music onco or twice Tho attention of tho poople could bo onhv ened by this means, whon otherwise three hours of a lecture would naturally tiro them