it is the true philosophical explanation. Particles of snuff are carried up into the nose, in opposition to gravity, by the pressure of the air.

19. Ella. As the air at different heights is of different degrees of density, does it not make a difference in the weight of bodies?

Mr. M. Certainly it does; it makes a very great difference. The more dense the fluid in which the body is weighed, the less it weighs as you have already seen that a body weighs less in water than in the air (see p. 329).

George. I would like to ask Miss Ida which is the heaviest' -a pound of feathers' or a pound of lead`?

Ida. They are both of the same weight, to be sure!

20. Mr. M. Do not be too certain of that; for I think, after a little reflection, you will change your opinion. A pound of feathers, cork, or any other bulky substance which just balances a dense body, as lead or gold, in the air, is really heavier than the lead or gold. If a lump of iron will balance a stone when both are suspended in a tub of water, they will not balance when the water is withdrawn; but the stone, which is the more bulky substance, will be found to weigh the most. So, also, if a bag of feathers balance a pound of lead in the air, if the scales are then placed in the exhausted receiver of an air-pump, the feathers will be found to weigh the most.

21. Ida. I understand the principle now. I perceive, also, that if the bag of feathers were lighter than the atmosphere, it would not weigh any thing at all in the open air, while it would weigh something in a vacuum.

Mr. M. The feathers, in this case, would rise as smoke does, until they became of the same specific gravity as the surrounding air. This is the principle on which the balloon rises, as it is filled with hydrogen gas, which is only one sixteenth of the weight of the atmosphere. Although hydrogen gas weighs nothing in the atmosphere near the earth, yet it has weight.

Ella. How high have persons ascended in balloons?

22. Mr. M. Gay-Lussac reached a height of more than four miles and a quarter, and brought down samples of air, which he analyzed, and found to consist of the same proportions of oxygen and hydrogen gases as air near the surface of the earth.

Frank. I have read that Napoleon III. sent up men in a balloon to reconnoiter the position of the Austrians before the battle of Solferino.

John. It must be a very rapid way of traveling "on the wings of the wind;" for on the first of July, 1859, Mr. Wise and three other persons ascended from St. Louis, Mo., and nineteen hours later landed in Jefferson county, N. Y., having passed over a distance of 1150 miles. This was traveling about a mile in a minute.

23. Ella. I noticed in some of the experiments that the outside of the glass receiver became covered with mist when the air was exhausted. I suppose there must be a cause for it, but I can not imagine why it should happen.

Mr. M. By rarefaction a greater degree of cold is produced in the receiver, and the vapor of the surrounding air is condensed thereby. The writer of the "Botanic Garden" has thus described it:

"Now in brazen pumps the pistons move,
The membrane valve sustains the weight above;
Stroke follows stroke, the gelid vapor falls,
And misty dewdrops dim the crystal walls;
Rare and more rare expands the fluid thin,
And silence dwells with vacancy within."

You will understand the last reference to "silence" when you study the subject of Acoustics, or Sound.

24. Ida. As high mountains, even in the torrid zone, are covered with snow, is not the air much the coldest in the upper regions of the atmosphere?

Mr. M. It has been found that it is two degrees colder at the dome of St. Peter's Church than on the ground; and if we were to continue to ascend, the temperature would diminish about one degree for every hundred yards. In latitude 36°, the mean height at which water congeals is only two miles.

25. John. Then, if we send a bucket of water up two miles in a balloon, it will come down ice.

Frank. Is it possible that water will always freeze, even in the bright sunshine, within two or three miles of us?

Mr. M. It is possible and probable; for the same reason that a thermometer will show a diminution of heat in an exhausted receiver, it will indicate cold as it is carried up to an atmosphere less dense.

26. Ella. I should think that cold would make the air more dense, and that heat would expand it.

Mr. M. That is the case with most substances; but air has a greater tendency to expand from diminished pressure than to be contracted by the cold consequent on such expansion. 27. George. It must be very difficult for the inhabitants of

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. BOTH the balloon and the wind-mill are atmospheric 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 ?

B

7. John. I suppose the column of water in c con- Fig. 27. tracts 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, to enable you to see its mode of action. You can examine its principle at your leisure.*

"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