such cities as Quito, and others situated on high mountains, to cook their food by boiling, as water boils at a lower temperature as the pressure of air on its surface is diminished. I have read that on Mt. Blanc, Saussure found the temperature of boiling water to be 180°, while it was 212° at the sea level. Ella. He could not boil potatoes soft in water of that temperature. But why could he not make the water hotter by more fire?

28. Mr. M. More fire can not raise water to a greater temperature than 212° at the ocean level, nor more than 180° on the summit of Mt. Blanc, unless it is confined as in a steam boiler.

Frank. That is the way they measure the altitude of mountains by boiling water. For every 520 feet in height, the boiling point is lowered one degree.

Ella. I have just calculated the height of Mt. Blanc to be 16,640 feet, or 32 times 520. But how did Saussure

"Breathe the difficult air

Of the iced mountain top ?"

29. Mr. M. He breathed, indeed, with difficulty; but the change was so gradual that he experienced no permanent injury. Persons in going up in balloons have ruptured bloodvessels, and have had the blood start from their "very fingers' ends" by the withdrawal of a portion of the atmospheric pressure to which they had been accustomed. Yet it has been noticed that the inhabitants of Quito, Mexico, and other elevated places do not suffer in this way, because they gradually become accustomed to the rarity of the atmosphere; and, moreover, they have larger chests than those living in lowlands, because a larger bulk of air is necessary to furnish the requisite amount of oxygen to sustain life.

30. George. It appears, then, that as we rise from the ocean level, the air becomes so rare that we breathe it with difficulty; and if we should descend a few miles into the earth, it would become so dense that we could not breathe it.

Mr. M. This shows the law of adaptation; that the Creator has adapted our bodies to that particular sphere of existence in which he designed us to move. Yet this is but one example, out of thousands, of a law which pervades all animated nature.

LESSON VII.-ATMOSPHERIC MACHINES.

1. Mr. M. Boтн the balloon and the wind-mill are atmos pheric machines; but I desire now to call your attention to others of a somewhat different character. You have seen that even the process of smoking a cigar is on the principle of atmospheric pressure. Can you think of any other illustration of this principle?

Ida. I have seen boys with straws for tubes, and their cheeks for air-pumps, allowing the atmosphere to force sweet cider into their mouths.

2. Ella. I believe that somewhere in South America the ladies take tea in that manner.

John. The negroes in some of the West Indies are said to steal rum from full casks by filling a bottle with water, and inverting it in the bung-hole of the cask, somewhat as Torricelli made his barometer. In this case, however, the water, being more dense than the rum, descends, while the rum rises into the bottle.

3. George. Liquids are often transferred from one cask to another by means of a bent tube.

Fig. 24, the

Siphon.

Mr. M. This is the siphon. It is first filled, and one end is immersed in the liquid to be discharged. It is always necessary that the end from which the liquid runs should be lower than the surface of the liquid in the vessel. Can either of you explain the action of the siphon?

4. John. The liquid in the long column will run out by the force of gravity, and a vacuum would be formed in the tube, did not the pressure of the atmosphere constantly force up a corresponding quantity out of the cup to supply its place.

Ida. There must be a siphon in that piece of apparatus called Tantalus's cup, which will never get full, although a small stream of water is poured in for hours. The water runs out through the siphon as fast as it is poured in. Would not this be a good way to discharge the water from a leaking ship?

5. Mr. M. The only trouble would be that, if Fig. 25, Tanta- the siphon acted at all, the water would run into the ship instead of out of it.

lus's Cup.

Fig. 26, Siphon
Fountain.

I will now show you a siphon fountain in the air. I have no doubt that the annexed figure will sufficiently explain its action.

Frank. It is perfectly plain. The water is discharged precisely as from any other siphon, and the long column in the tube causes the fountain by hydrostatic pressure.

6. Mr. M. I wish now to show you one of the effects of running water. I will take the long open tube B c, to which the branch a is attached, and hold it upright, with the pipe a reaching to the bottom of the open jar, which is filled with water. I now pour a pitcher of water into the funnel B. You see the jar is emptied; for the water, running up through a and down the pipe c, is discharged with the water poured in at the top. Who can explain it ?

Fig. 27.

B

7. John. I suppose the column of water in c contracts in its descent in the tube, just as a stream of molasses does in air, and consequently does not entirely fill the tube. The water, too, by its friction, tends to draw in the air of the tube a, and the external air forces the water of the jar up into the partial vacuum so formed. It is very curious, but is it of any practical use?

8. Mr. M. It has been made of great use, for marshes have been drained on this principle; and in the circulating system of animals there are arrangements of blood-vessels by which a current of blood passing along one vein may assist in emptying a lateral branch. It is by no means necessary for the stream of water to descend vertically, as it may run at any angle, or even horizontally.

Ida. Does Hiero's fountain depend upon atmospheric pressure?

Mr. M. It depends on the pressure of a column of water and the elasticity of air. The one I have here is mainly constructed of glass, Fig. 28, Hiero's Foun- to enable you to see its mode of action. You can examine its principle at your leisure.*

tain.

"Hiero's fountain" explained. Water is poured into the glass vessel B until it is nearly full, while the glass vessel C contains only air. Into the vessel A is now poured a little water, which flows through the pipe F, and displaces some of the air in C by

George. Will not the explanation of the common pump belong to this lesson?

9. Mr. M. Its principle has already been illustrated in the account which was given of the discovery of the weight of the atmosphere. It is evident that we have only to exhaust the air in a pipe, the open end of which is placed in water, and the water will be pressed up to fill the vacuum. Here are illustrations of two different kinds of pumps, one of which is the forcing-pump, which illustrates the principle of the fireengine.*

Frank. Darwin very prettily explains the action of the common pump in the following lines:

"Thus does the sliding piston bear

The viewless columns of incumbent air;
Press'd by the incumbent air, the floods below,
Through opening valves, in foaming torrents flow,
Foot after foot with lessened impulse move,
And, rising, seek the vacancy above."

forcing it up through D. There is now a pressure of air on the water in B equivalent to the pressure exerted (on the principle of the hydrostatic paradox) by the column of water in F, and this pressure is exerted to throw the water up through E and cause the play of the fountain. Thus the contents of B are actually transferred to C, and the air that was in C passes into B. When C thus becomes filled with water and B with air, the fountain must stop.

*The common pump, represented by Fig. 29, consists of three parts, the suction-pipe, the barrel, and the piston. The suction-pipe, f e, is of sufficient length to reach down to the water in the well. The barrel, C B, is a perfect cylindrical cavity, in which the piston G moves, air-tight, up and down by the rod d. It is commonly moved by a lever, but in the figure a rod and handle, D E, are represented. On one side is the spout F. At the top of the suction-pipe at H there is a valve, b, opening upward, and also one at a. When the piston is raised from the bottom of the barrel, a vacuum is produced in the barrel, the valve b opens, and if the pipe, fe, be full of water, the water rushes into the barrel, being pressed up by the atmosphere resting on the water in the well. On depressing the piston the water rushes up through the valve a, and after a few movements the water is poured out at the spout F.

In the forcing-pump, Fig. 30, the piston, g, has no valve. On the box at H is a valve, b, opening upward, and when the piston is elevated the water rises into the barrel, B C. During the

Fig. 29.

Fig. 30.
E

downward movement of the piston the valve b shuts, and the water passes by a channel around m, through the lateral pipe M O M N into the air-chamber, K K. The entrance to this air-chamber is closed by a valve at a, and from the chamber proceeds a tube, H G, open at both ends. After a few movements of the piston the lower end of this tube becomes covered with water, the air is compressed into the space G H, and thereby the water is thrown out in a continuous jet, S.

10. Mr. M. Do you know of any instrument besides the forcing-pump and the fire-engine that acts upon the principle of condensed air?

John. I have seen an air-gun charged with air instead of powder.

Mr. M. That is a good illustration of the principle that the density and elasticity of air are directly as the force of compression. Here is a drawing of the air-gun. Air is con

Fig. 31, the Air-gun,

densed into the ball A, which is attached to the

gun. A bullet is then put into the

barrel, and by a peculiar lock a portion of the condensed air is let in behind the bullet, which is thrown out with almost the force of gunpowder.

George. Can another bullet be thrown out without refilling the ball?

11. Mr. M. Yes, a dozen or more; but each one with less force than the one before it, as the air in the ball gradually loses its density, and consequently its elasticity. Sometimes the air-chamber is in the stock of the gun, which makes it more convenient.

John. I have made little fountains by inserting through the cork of a bottle half filled with water a common pipe-stem, and after blowing through the tube the water would spout up to a considerable height. Was not this owing to condensed air pressing on the surface of the water?

Mr. M. It was caused by condensed air; but you had to be careful to let the tube reach below Fig. 32, Bottle the surface of the water?

Fountain. Ella. Is not the kite an atmospheric machine? 12. Mr. M. Yes; and although it is a playing machine, it is elevated on strictly scientific principles. It is really pulled up an inclined plane of air by the tension and weight of the string.*

W

T

B

*Fig. 33. The kite here appears in the act of rising from the ground. As the wind, coming from the direction of W, falls upon the oblique surface of the kite, it is resolved into two forces, one parallel to that surface (B O), and the other perpendicular to it (Y O), of which the latter only will produce any effect, carrying the kite along the line O A. But the kite is also pulled in the direction ST. It is therefore under the influence of the two forces O A

Y