CHAPTER IX.

PNEUMATICS.

146. =What Pneumatics Teaches.=--As Hydrostatics treats of the pressure and equilibrium of liquids, Pneumatics treats of the same in air and the gases, or aeriform substances. The name comes from the Greek word πνευμα, meaning air, breath, spirit.

147. =Air Material and has Weight.=--That air is a material substance has been already proved to you, for it was shown in § 46 that it has impenetrability, one of the essential properties of matter. It has extension also, for bodies of air can be obtained in various shapes confined in vessels, so that we can speak of cubes and spheres of air; and besides, the ultimate atoms (§ 15) of air must have shape or extension. That air has weight can be proved by weighing it as you would any other substance. Let a hollow globe, A, Fig. 92, having a neck with a stop-cock, B, be emptied of air and weighed. If now you open the stop-cock, and so let in the air, the other beam of the scale will rise, because the globe is heavier than it was before. The additional weight required to make the scales balance will indicate the weight of the air which the globe contains. It is one eight hundredth (1/800) of the weight of the same volume of water. How the globe can be emptied of the air will be shown in another part of this chapter.

148. =Air Attracted by the Earth.=--The weight of the air is simply the result of the attraction of the earth (§ 52). Air is attracted by the earth just as water is; and the water takes its place below air because it is attracted more strongly than the air. It is from the attraction of the earth that air descends into any hollow spot in the earth when water is removed from it. It takes the place of the removed water because from the influence of attraction it gets as near to the earth as possible. If you put into a vial mercury, water, and oil, the mercury will be at the bottom, because it is more strongly attracted by the earth than the other fluids. The water will be next, then the oil, and lastly, over all, there is air, that being less attracted than any of the other substances. It is this attraction of the air by the earth that gives us the chief phenomena of Pneumatics.

149. =Why Some Things Fall and Others Rise in Air.=--Most substances fall in air for the same reason that very heavy substances sink in water. They fall because the earth attracts them more strongly than it does the air. The reason that some substances rise in air is precisely the same as that given in § 136 for the rising of substances in water. The air being attracted more strongly than they are pushes them up to get below them, as cork or wood is pushed up by water. Thus a balloon filled with hydrogen gas rises in air for the same reason that a bladder filled with air rises in water. So, also, smoke rises in air, just as oil rises in water.

150. =Thickness of the Earth's Air-Covering.=--The air makes a covering for the earth about fifty miles deep. If the earth were represented by a globe a foot in diameter, the air might be represented by a covering a tenth of an inch in thickness. The line _a_, Fig. 93, gives us the curve of the surface of such a globe, and the space between _a_ and _b_ represents the comparative thickness of the covering of air. This is ascertained by calculation from the pressure of the air upon the earth. It is just as the depth of water may be calculated from the pressure which it makes. We do not take this mode of ascertaining the depth of water, because we can measure it from the surface by sounding. But we should be obliged to adopt it if we lived at the bottom of water, as we do at the bottom of the sea of air.

151. =How the Air-Covering Adheres to the Earth.=--The earth flies on in its yearly journey around the sun at the rate of 1100 miles per minute, and yet it holds on to this loose airy robe by its attractive force, so that not an atom of it escapes into the surrounding ether. Of itself it is disposed to escape; and it would do so, and be diffused through space, if the attraction of the earth for it were suspended. For, unlike liquids, the air has no disposition to keep together; that is, there is no attraction between its particles. On the other hand, there is a repulsion, so that they are disposed to keep far apart, and are kept together only by pressure. It is the pressure of the earth's attraction that keeps them together to the extent of fifty miles all around it.

152. =Compressibility of Air.=--In looking at the influence of gravitation upon air, it must be remembered that air is very compressible, while water is very nearly incompressible. While, therefore, in a body of water the particles are very little nearer together at the bottom than at the surface, the particles of the air are much nearer together close to the earth than they are far away from it. For as all the particles of the air are attracted or drawn toward the earth, those below are pressed together by the weight of those above. The air is therefore thinner as we go up from the surface of the earth, and in the outer regions of the sea of air it is too thin to support life. Even at the tops of very high mountains, or the heights sometimes reached by balloons, disagreeable effects are often experienced from the thinness of the air. The air has been compared, in regard to its varying density at different heights, to a heap of loose compressible substance; as, for example, cotton-wool, which is quite light at the top, but is pressed more and more together as you go toward the bottom. Hydrogen gas has only one fifteenth the weight of air at the surface of the earth; and therefore the hydrogen balloon rises till it reaches a height where the air is so thin that the balloon is of the same weight with an equal bulk of air, and there it stops.

153. =In what Aeriform Substances and Liquids are Alike.=--You have seen in § 36 and § 38 how the air and gases differ from liquids. But in one very important respect they are alike, viz., the movability of their particles. Hence pressure is in air, as well as in water, equal in all directions, so that in the experiment with the bladder, in § 126, it makes no difference in the result whether there be water or air in it. For the same reason pressure is as the depth in aeriform substances as in liquids, and the laws of specific gravity apply to the one as well as to the other.

You are now prepared to understand the results of _the action of gravitation upon air and the gases_; or, in other words, the principal phenomenon of Pneumatics.

154. =Pressure of the Atmosphere.=--The amount of the pressure of the atmosphere is very readily estimated, the mode of doing which I will speak of in another part of this chapter. It presses with a weight of fifteen pounds on every square inch. Suppose that you extend your outspread hand horizontally in the air. You feel no pressure upon it, but there is a pressure of some two or three hundred pounds of air upon it. If your hand be five inches long and three broad it presents a surface of fifteen square inches, on every one of which the atmosphere is pressing with the weight of fifteen pounds. That is, there is a pressure on the upper surface of your hand of a column of air weighing 225 pounds. So, also, on the lid of a box only thirty inches square, there is a pressure of 13,500 pounds. The whole pressure on the body of a man of common size is about fifteen tons. But why is it that the lid of the box is not broken in, your hand not borne down, and your body not crushed? It is simply from the fact, shown in the previous chapter in regard to liquids, and in this in regard to aeriform substances, that the pressure is equal in all directions. The lid and the outspread hand are therefore balanced by an upward pressure equal to the downward, and the body has the pressure on all sides the same. If the air could be removed from within the box the lid would be crushed in; if from under the hand, that would be borne down; and if from one side of the body, the body would be forced violently in that direction till it met with an opposing pressure.

But besides this equal pressure of the air on all sides, there is air within the pores and interstices of all bodies that are not very dense, and its particles are subject to the same laws as are those on the outside.

All this can be made clear to you by the air-pump.

155. =Air-pump.=--In Fig. 94 you have a representation of an air-pump as commonly arranged. At _a a_ are two pump-barrels, the pistons in which are worked by means of the handle, _b_. Those pumps are very nicely made, and the frame-work, _d e d e_, to which they are attached, is very strong and firm, so that the pumps may work evenly. There is a large, smooth, metallic plate, _f_. At _c_ is a bell-shaped glass vessel, close at the top, but open at the bottom, the edge of which is ground very true, so that it may fit exactly on the metallic plate. In the middle of the plate is an opening which leads to the pump-barrels, and it is through this that the air is pumped out of the glass receiver, _c_. If we wish to let the air in after we have pumped it out we loosen the screw at _g_, for from the opening here is a passage to the opening in the middle of the plate.

The operation of the air-pump can be made clear by the plan in Fig. 95. But one pump-barrel, _a_, is represented, with a piston, _c_, working in it. In the piston there is a valve, _i_, opening upward, and also one at _b_, in the beginning of the passage leading to the centre of the plate where is the receiver, _d_. The working of the instrument is thus: If the piston be forced down, the air under it, being compressed, will close the valve at _b_, and will rush upward through the valve _i_ in the piston. Let the piston now be raised; the resistance of the air above it will close the valve _i_, while the valve _b_ will be opened by the air rushing from the receiver, _d_, through the passage, _e_, to fill the space between the piston and _b_. You see, then, that every time that the piston is drawn up air passes out of the receiver through the valve _b_ into the space between this valve and the piston. None of this air which has passed out can go back again, for the moment that you press upon it by forcing downward the piston the valve _b_ is shut down, and the air escapes from the pressure by passing out through the valve _i_. Each time, therefore, that you work the piston up and down you pump out some of the air from the receiver; and if you pump for some time there will be exceedingly little air left in it, and that will of course be diffused throughout the receiver. It will be thin, like that in the upper regions of the atmosphere.

156. =Experiments.=--When the receiver is full of air it can be moved about on the plate easily, and can be lifted from it. But work the pumps a few strokes and you will find that the receiver is firmly fastened to the plate, for the air within, being made thin, presses with little force compared with the air outside. If the pumps be worked for some time no force could release the receiver from the pressure without breaking it. But loosen the screw, _g_, and thus let the air in, and the equality of the pressure on the outside and inside is at once restored. Take off now this large receiver, and place a small glass jar, open at both ends, on the plate, with the hand covering the upper opening, as represented in Fig. 96. On exhausting the air the hand is so firmly pressed into the glass that it requires considerable force to disengage it from the pressure. If we tie a piece of bladder or India rubber over this jar, as in Fig. 97, and then pump out the air, the bladder at first is pressed in as represented, and if we pump on it at length bursts with a loud report. It would make no difference in the result of the experiment if the jar were shaped as in Fig. 98, for the pressure is the same in all directions. The resemblance between air and liquids in this respect may be illustrated thus: Suppose that a flat fish covers with one of its sides the end of the tube of a pump. He feels no uncomfortable pressure, because the water in the pump and that below it press equally upon him. If, now, the pressure of the water in the pump could be suddenly taken off by the piston, the fish would be pressed upward into the tube, as the bladder is pressed upward in Fig. 98, or downward in Fig. 97, or as the hand is pressed downward in Fig. 96. The Magdeburg Hemispheres, Fig. 99, illustrate very impressively the pressure of the atmosphere. They consist of two hemispheres whose edges at A fit very accurately upon each other. The air is exhausted through the stem where you see the stop-cock, and then the handle B is screwed on. The force required to pull these hemispheres apart depends upon the extent of their surface. In the famous experiment at Magdeburg, in 1654, by Otto von Guericke, the inventor of the air-pump, two strong hemispheres of brass of a foot in diameter were employed, and it required the force of thirty horses to separate them. In Fig. 100 you see a receiver with an opening at the top. Cemented in this opening is a wooden cup, _a_, terminating in a cylindrical piece, _b_. If mercury be poured into the cup, on exhausting the air from the receiver the mercury will be forced through the pores of the wood by the external air, and will fall in a silver shower. A tall jar, _c_, is placed there to receive it, to prevent any of it from going down into the opening in the metallic plate.

157. =The Sucker.=--The boy's sucker illustrates the pressure of the air. It is simply a circular piece of leather with a string fastened to its centre, as seen in Fig. 101. When the leather is moistened and pressed upon a smooth stone, on pulling the string a vacuum is made between the middle of the leather and the stone, and the leather adheres by its edges to the stone, just as the receiver adheres to the plate of the air-pump when the air is pumped out. There are many animals that have contrivances of a similar character. The gecko and the cuttle-fish furnish interesting examples, as noticed in my Natural History, pages 198 and 320. Snails, limpets, etc., adhere to rocks by a like arrangement. Some fishes do the same. There is one fish called the remora, that attaches itself by suckers to the side of some large fish or a ship, and thus enjoys a fine ride through the water, without any exertion on his part. In all such cases it is water instead of air that makes the pressure, but the principle is the same. Flies and some other insects can walk up a smooth pane of glass, or along the ceiling over-head, because their feet have contrivances akin to the boy's sucker. The hind-feet of the walrus are constructed somewhat like the feet of the fly, enabling this huge animal to go up smooth walls of ice.

158. =Density of the Air Dependent upon Pressure.=--The fact that the degree of the density of the air is dependent on pressure has been already shown in § 152. The same thing can be shown in various ways with the air-pump. If a small bladder, partly filled with air, Fig. 102, and loaded with a weight so as to sink in water, be placed in a jar of water, and the whole be set under the receiver of the air-pump, on exhausting the air the bladder will swell out with the expanded air in it, and will rise as seen in the figure. The reason is, that the pressure being taken off the surface of the water, the bladder bears only the pressure of the water, and not that of the air with the water, and so the air in it expands and becomes less dense. If an India-rubber bag be partly filled with air, Fig. 103 (p. 119), and put under the receiver, on exhausting the air, the surrounding pressure being thus taken off from the bag, the air in it becomes expanded, that is, rarefied. For the same reason, if a vessel with soap-bubbles in it be placed under the receiver, on pumping out the air the bubbles will become much enlarged. A very pretty experiment illustrating the same thing may be tried in this way. Let an egg with a hole made in its small end be suspended in a receiver, as represented in Fig. 104, a wine-glass being beneath it. On exhausting the air the egg will all run out of the shell into the wine-glass, and then, on admitting the air, it will run back again into the shell. The explanation is this: There is air in the large end of the egg. As soon as the pressure of air is taken off from all about the egg the air in the egg expands, forcing out the contents; but when the air is admitted into the receiver the air in the egg is at once condensed into its former small bulk by the surrounding pressure.

159. =Hydrostatic Balloon.=--The philosophical toy represented in Fig. 105 illustrates very beautifully the influence of pressure upon the density of the air. The balloon in the jar of water is of glass, with a small orifice at its lower part. Care must be taken in putting water in the balloon to have just enough to make it of a little less specific gravity than water. In that case it will be at the top of the jar, with a very little of its top above the surface of the water. Now tie a piece of India-rubber cloth over the top of the jar, and the apparatus is complete. On pressing upon the India rubber the balloon will go down in the jar, and on taking off the pressure it will rise. The explanation is this: The pressure upon the India rubber is felt through the whole body of the water in the jar, and forces a little more water into the orifice of the balloon, condensing the air that is there. The balloon consequently becomes heavier, and has a greater specific gravity than water, and sinks in it. But when the pressure is taken off, the condensed air in the balloon, by its elasticity, returns to its former bulk, expelling the surplus water just introduced, and the balloon, becoming therefore as light as before, rises. Grotesque figures of glass may be managed in the same way. The Cartesian image, Fig. 106, is an example. This has air in its upper part, _a_, and water up to _c d_. When pressure is made on the India rubber more water is forced into the image through the tail, _b_, and it goes down like the balloon, to rise again when the pressure is taken off.

160. =Air in Substances.=--I have said that there is air in the pores and interstices of wood, flesh, and a great variety of substances. In all these cases the presence of the air can be made manifest by taking off the pressure of the surrounding air, and thus allowing the air in these substances to expand. If an egg be placed in a jar of water, Fig. 107, under the receiver of an air-pump, on exhaustion being made air-bubbles will constantly rise in the water from the egg. So, too, a glass of porter, Fig. 108, will have its surface covered with foam, the carbonic acid gas in it escaping freely when the pressure of the air upon it is taken off. The same thing may be seen to some extent even in water, for it always contains some air. For the same reason a shriveled apple, with the pressure of the air taken from it, will become plump and fair, but will shrink at once to its shriveled state when the air is admitted into the receiver.

161. =Elasticity of the Air.=--All the phenomena cited in § 158, § 159, and § 160 exhibit the elasticity of the air. It is from this property that it is always disposed to expand. It will do so whenever pressure is taken from it, or when it can overcome pressure to which it is subjected. This property is most strikingly exhibited when the air is much condensed by pressure. And the greater the condensation the stronger is the expansive or elastic force.

162. =The Condenser.=--In Fig. 109 you have the plan of an instrument called the Condenser. In A B, a cylinder, moves the piston, P. Air is admitted to the cylinder at F, and into the receiver, V, at G. The valve at F prevents any air from escaping from the cylinder, and the valve at G prevents it from escaping from the receiver. The operation of the instrument is this: If the piston be pressed downward, the pressed air in the cylinder shuts the valve F and opens G, and so enters the receiver V. If now the piston be raised, air rushes in at F to fill the space in the cylinder. It can not come from V, because the valve G is shut by its pressure. By working the piston for some time you can get a body of air into V of very great density. You see that this instrument is the very opposite of the air-pump. In the receiver, V, you have condensed air, while in the receiver of the air-pump you have rarefied air. If you compare the two instruments you will see that the opposite results are owing to different arrangement of the valves.

163. =The Gasometer.=--Gas is distributed in pipes from the gasometer at the gas factory by the agency of the elasticity occasioned by condensation under pressure. The apparatus, Fig. 110 (p. 122), consists of a large round vessel, G, open below, and sunk in a larger vessel of water, _w_. We will suppose the vessel, G, to be full of water. Gas is introduced into it through the pipe, _p r_, the gasometer rising as it fills with the gas. P is a weight balancing the gasometer, and so permitting it to rise as the gas enters. The gasometer being filled, the gas is to be distributed. For this purpose weights are put upon the gasometer, so that the gas may be compressed. Under this pressure it by its elasticity seeks for more room, and obtains it by escaping through the pipe _o b c_. As the pressure on the gas needs to be regulated, there is sometimes a gauge, _h i_, attached, which shows the amount of the pressure. It is a bent tube with water in the bend. You see at once that the greater the pressure upon the gas the higher will the water be in the branch, _h_, of the gauge.

164. =Air-Guns and Pop-Guns.=--These illustrate the elasticity of condensed air. The air-gun is constructed in this way: A receiver, like V, Fig. 109, is made so that you can screw it on and off from the instrument. After being charged with condensed air it is screwed upon the gun, its stem communicating with the barrel. In order to discharge the gun there is a contrivance connected with the trigger for raising the valve, G, so that some of the condensed air may enter the barrel. On doing so, it by its sudden expansion rapidly forces out the contents. The principle on which the common pop-gun operates is the same. There is air confined between the two corks, _a_ and _b_, Fig. 111 (p. 123). As the rod, R, is pushed quickly in, the cork _b_ is carried nearer to _a_, so that the air between them is condensed. With the condensation the expansive force is increased; and when it becomes so great that the cork _a_ can no longer resist it, it throws the cork out, and so quickly as to occasion the popping sound.

165. =Powder and Steam.=--The explosion of powder furnishes a good illustration of the expansive force of condensed air or gases. These gases are produced so suddenly from the powder that at the instant they are in a very condensed state, and therefore expand powerfully. So, also, steam has power in proportion to its condensation. When formed under the confinement of a boiler, on being allowed to escape it expands with great force. The application of the expansive power of steam will be treated of particularly in another part of this book.

166. =Retardation by Condensed Air in Gunnery.=--When a ball is fired it is constantly retarded in its flight by the resistance of the air, for it has to push the air away on every side in order to make its way through it. Of course, then, the more condensed the air is the greater is the resistance. Now it is condensed air that the ball is obliged to remove; for as it goes forward it, by its rapid pressure, condenses the air directly before it. And the more rapid is its flight the greater is the condensation, and therefore the greater the resistance. Besides, the retarding effect is increased by the tendency to a vacuum behind the ball. All this can be made clear by Fig. 112. Let B be a ball going very rapidly in the direction indicated by the arrow, the cloud representing the condensed air before it, and the space included in the two lines the vacuum behind it. It is obvious that the more rapidly the ball goes the less readily is the air pressed out of the way, and therefore the more it is condensed in front of the ball. At the same time the more rapid is the ball the less readily does the air close up behind it, and therefore the greater is the tendency to a vacuum there. For these reasons there is more retarding influence exerted by the air upon a ball in the first part of its course than in its latter part.

167. =Pressure of the Air on Liquids.=--If you plunge a tumbler into a vessel of water, and turning it over hold it so that its open part is just under the surface, it will remain full. The reason is that the weight of the air pressing upon the surface of the water in the vessel prevents the water in the tumbler from passing downward. Now if you introduce a bent tube under the tumbler, as in Fig. 113, and blow through it, the air that you force up into the tumbler presses the water down, taking its place. That is, the pressure of the air acts in opposition to the pressure of the air outside upon the surface of the water in the vessel. You take a jar, _a_, Fig. 114, and filling it with water, turn it over with its open end downward, the water will remain in the jar. You have here a representation of the pneumatic trough used by the chemist in collecting gases. To fill the jar _a_ with gas he puts the mouth of the retort from which the gas issues under the jar _a_, and the gas passing upward expels the water, as the water is expelled by the breath from the tumbler in Fig. 113. In Fig. 115 (p. 125) is represented an experiment which shows not only that the pressure of the air sustains the column of water in the cases cited above, but also that it makes no difference in what direction this pressure is exerted. Take a large tube, _a_, closed at one end and open at the other, and fill it even full with water. Place, now, a piece of writing-paper over its mouth, and carefully invert the tube, as seen in the figure. The paper will remain, and the water will not run out. It is the pressure of the air that sustains the water, and the paper only serves to maintain the surface of the water unbroken. If the paper were not there the particles of the air would insinuate themselves among those of the water, and pass upward in the tube. You can try this experiment with a wine-glass, and may even succeed with a tumbler. We see in these experiments the reason that a liquid will not run from a barrel when it is tapped, if there be no vent-hole above, unless there be so large an opening made as to let the air work its way in bubbles among portions of the liquid. It is this entrance of the air that causes the gurgling sound in pouring a liquid from a bottle.

168. =Amount of Atmospheric Pressure.=--If, instead of the jar _a_, in Fig. 114, you have a tube thirty-four feet high, and closed at the top, situated as the jar _a_ is, it will remain full of water. If the tube be longer the water will stand only at thirty-four feet, leaving a vacuum above it. It makes no difference what the size of the tube is; the result will be the same in all cases.[2] That is, a column of water thirty-four feet high can be sustained by the pressure of the atmosphere. It is easy, therefore, to estimate the weight or pressure of the air. The pressure of the column of water is found to be fifteen pounds to the square inch of its base, and this, of course, is the amount of pressure or weight of the atmosphere which it balances. Mercury is thirteen and a half times as heavy as water, and therefore the air will sustain a column of it only about thirty inches in height.

169. =Barometer.=--The weight of the atmosphere varies to some extent at different times, and the barometer is an instrument for measuring these variations. It is constructed on the principles developed in the previous paragraphs. In Fig. 116 is a representation of the instrument. A B is a glass tube about 34 or 35 inches long, closed at one end. It has been filled with mercury, and then inverted in a cup of the same liquid, C. The vacuum above the mercury is called the Torricellian vacuum, from Torricelli, an Italian, who first developed the principles of the instrument. The mercury generally, as stated in § 168, stands at about the height of thirty inches. But it varies from this with the weather. When the weather is bright and clear the air is heavier than this, and, pressing upon the mercury in the vessel, forces it up higher in the tube. But when a storm is coming the air is apt to be lighter, and therefore pressing less strongly on the mercury in the vessel, the mercury in the tube falls. The barometer is of great service, especially at sea, in affording the sailor warning of an approaching storm. An incident is related by Dr. Arnot which strikingly illustrates its value in this respect. He was at sea in a Southern latitude. As the sun set after a beautiful afternoon the captain foresaw danger, although the weather was perfectly calm, for the mercury in the barometer had suddenly fallen to a remarkable degree. He gave hurried orders to the wondering sailors to prepare the ship for a storm. Scarcely had the preparations been made when a tremendous hurricane burst upon the ship, tearing the furled sails to tatters, and disabling the masts and yards. If the barometer had not been observed the ship would have been wholly unprepared, and shipwreck, with the loss of all on board, would have been the result.

A water-barometer could be made, but it would be an unwieldy thing, for the tube must be over 34 feet long. Besides, it would not answer in very cold weather, as the water would freeze. So short a column of the heavy fluid, mercury, balances the weight of the atmosphere that a barometer made with this is of very convenient size; and then there is no danger of the mercury's freezing, except in the extreme cold of the Arctic regions.

170. =Barometer a Measurer of Heights.=--The atmosphere, as stated in § 152, diminishes regularly in density as we go upward. The rate of this diminution has been accurately ascertained, and therefore we can estimate heights by the amount of pressure on the mercury in the barometer. At a height of 500 feet the barometer will be half an inch lower than in the valley below. At the summit of Mont Blanc it stands but half as high as at its foot, indicating a height of 15,000 feet. Du Luc, in his famous balloon ascension from Paris, saw the barometer at one time standing at about twelve inches, showing an elevation of 21,000 feet.

171. =Relation of the Air's Pressure to the Boiling Point.=--Water heated to 212 degrees of Fahrenheit boils, that is, it becomes vapor. Now if water be heated on the summit of a high mountain it boils before it arrives at this degree of temperature. On the top of Mont Blanc it boils at 180 degrees, that is, 32 degrees below the boiling point of water at the foot of the mountain. This is because the pressure of the air acts in opposition to the change of water in to vapor, and the less the pressure is the less heat will be required to vaporize the water. We may illustrate this influence of the pressure of air upon boiling by the following experiment. Let a cup of ether (which boils at 98 degrees) be placed under the receiver of an air-pump. On rarefying the air by the pump the ether will boil. The general effect of pressure upon boiling may be prettily illustrated by another experiment. Boil some water in a thin flask over a spirit-lamp. Blow out the lamp, and, corking the flask tightly, let the boiling cease. If, now, you pour some cold water over the flask the boiling will commence again with considerable force. Why? Because you condense the steam which is over the water by the application of cold, and thus take off the pressure. Then, again, if, while the water is boiling, you pour hot water over the flask, the boiling ceases, because the heat favors the accumulation of steam, and therefore renews the pressure on the surface of the water.

You can see from what has been stated that most liquids have the liquid form because of the pressure of the atmosphere upon them. If there were no atmosphere, ether, alcohol, the volatile oils, and even water, would fly off in vapor; and the earth would be enveloped in a vaporous robe, for the particles of the vapors would be held to the earth by attraction, just as the particles of the air are now, § 151.

172. =Syphon.=--The pressure of air upon fluids is beautifully exemplified in the operation of the syphon. This instrument is simply a bent tube having one branch longer than the other. Its operation is shown in Fig. 117. The tube having been first filled with the liquid, has its shorter branch in the liquid of the vessel A, which is to be emptied, and the other in the vessel B, which is to receive the liquid. As you see it here, the opening of the long branch is below the surface of the liquid in B. It is manifest, therefore, that the air presses equally upon the surfaces in both vessels, tending to support the fluid in the tube, just as the fluid is supported in the jar in Fig. 114. But, notwithstanding these equal pressures, the liquid runs up the tube from A, and down its longer branch into B. Why is this? As the pressure of a column of fluid is as its height, there is greater pressure or weight in the longer branch than in the other; and it is this difference in weight that causes the flow from A into B through the syphon. The difference in the columns in the two branches is not the difference in length of these branches, but the distance between the levels of the fluid in A and B, that is, the distance from _a_ to _b_. The operation, then, of the instrument is this. There is a constant tendency to a vacuum at C, the bend of the tube, from the influence of gravitation on the excess of fluid in the long branch over that in the short one. This tendency is constantly counteracted by the rise of fluid in the short branch, it being forced up by the pressure of the air upon the surface of the fluid in A.

If the syphon were so placed that the surface of the liquid in A is precisely on a level with that in B, as represented in Fig. 118, the liquid would remain at rest, for as pressure is as the height, § 121, and the pressures on the two surfaces are equal, there would be an exact balance. But let the surface in B be in the least lower than in A and the flow will begin. And the greater the distance between the two levels the more rapid will be the flow, for the greater will be the influence of gravitation in the long branch.

Again, if the end of the long branch of the syphon be free, as in Fig. 119 (p. 130), the syphon will operate in the same way, for the air, pressing in all directions equally, tends to support the column of fluid in the long branch by a direct upward pressure, but is prevented from doing so by the excess of fluid in it above what is in the shorter one. The operation of the syphon is commonly represented in this way; but I have given first the arrangement in Fig. 117, in order that you might more clearly see the principle of the instrument.

173. =Uses of the Syphon.=--The syphon is used chiefly for discharging liquids from one barrel or vessel into another. For convenience, it is often constructed after the plan of Fig. 120. To the long branch, B C, is attached the tube ED. It is used in this way: The end of the short branch, A, being introduced into the liquid to be drawn off, you place your finger upon C, and after filling the syphon by suction at E, you remove the finger and let the liquid run. The syphon has sometimes been used to drain pits and mines. It of course can never be used where the elevation over which the tube is to bend is over 34 feet from the surface of the water to be discharged, for then the air would not press the water up to the bend of the syphon.

174. =Cup of Tantalus.=--This cup, Fig. 121, has a syphon in it, the short branch, _b_, opening into the cup, and the long branch, _d_, having its outlet in the bottom. As you pour water into the cup it will remain there until you pour enough in to cover the bend of the syphon. As soon as this is done, the syphon being filled, the water suddenly flows out from the outlet, _a_, of the long branch.

175. =Intermitting Springs.=--The operation of an intermitting spring is essentially the same with that of the cup of Tantalus. You have a representation of such a spring in Fig. 122. There is a cavity in a hill, supplied with water from a passage above. There is also a passage from it which takes a bend upward like a syphon. Now when the water in the cavity is low it will not run out from the syphon-like channel; but when the cavity becomes filled above the level of the bend the water will at once flow out, just as it does from the cup of Tantalus as soon as the bend of its syphon is covered.

176. =Pumps.=--In Fig. 123 you have a plan of a common pump. A tube, C D, extends down into the well, W. Above this is the barrel of the pump, A B, in which the piston works up and down. There is a valve, F, in the piston, and another, E, at the bottom of the barrel. Both of them open upward. We will suppose that the pump is entirely empty of water. If, now, the piston descend, the valve E shuts down, and F opens, letting the pressed air between the piston and E pass upward. See what will happen when the piston rises. The air above the piston can not get below, for its pressure will shut the valve F. But there will be a tendency to a vacuum below the piston as it rises, and the air will go up through the valve E to fill up the space. But why does the air rise? Because of the pressure of the air upon the surface of the water in the well. This forces up in the pump the water and the air above it, just in proportion as the downward pressure in the pump is lessened. If the pumping be continued all the air will soon be expelled, the water following it and flowing out at the opening, G. It is obvious that the pump will be useless if the valve E be over 34 feet above the surface of the water in the well, as the pressure of the atmosphere will not sustain a column of water higher than this.

177. =Suction.=--In common language, the operation of the pump is attributed to what is called a principle of suction, as if there was a drawing up of the water. But the water, you see, is not drawn, but forced, up. So it is with all operations of a similar character. In sucking up a fluid through a tube the fluid is forced up, because the pressure downward in the tube is removed. But how is it removed? It is done by a movement of the tongue downward from the roof of the mouth, thus causing a tendency to a vacuum, as the upward movement of the piston in the pump causes this tendency under it. To fill the space made by the movement of the tongue the air is forced up the tube, the liquid following; and, as in the case of the pump when the air is all expelled, the liquid will begin to discharge into the mouth.

178. =Forcing-Pump.=--The forcing-pump is constructed differently from the common pump. Its plan is given in Fig. 124. It has a pipe, C D, and a barrel, A B, like the common pump. It has also the valve E at the bottom of the barrel. But it has no valve in the piston. Connected with the barrel is another pipe, F G, from which the water issues. This has a valve, H, opening upward. The operation of the pump is obvious. As the piston is drawn up E opens and H shuts, and when it is forced down E shuts and H opens.

179. =Fire-Engine.=--The fire-engine has commonly two forcing-pumps, with a contrivance for making the water issue in a uniform stream. This contrivance can be explained in Fig. 125. The discharging pipe, L M, extends down into a large vessel, I K, which is filled with air. The uniformity of the stream depends upon the elastic force of compressed air, as you will see if I explain the operation of the machine. When the water is forced through the opening H, it rises to the level N O, compressing the air in I K, for the tube L M is too small to allow all the water to escape that comes from the larger tube, H E. Now the moment that the piston ceases to force the water through H, the elastic force of the compressed air operates, shutting down the valve H, and forcing the water up L M. The result, you see, is a continuous forcing up of the water through this tube, and therefore a uniform stream.