have the power of walking on a perpendicular pane of glass, or on the ceiling of a room, with their backs downward. This has been proved to arise from a power they possess of squeezing out the air between the inside of their feet and the surface on which they tread, and thus being supported against the outside of their feet by the pressure of the atmosphere.

4. It is owing, in a great measure, to the pressure of the atmosphere, that frosts occasion a scantiness of water in our fountains and wells. This is not caused, as is generally supposed, by the freezing of the water in the bowels of the earth. The most intense frost of a Siberian winter would not freeze the ground two feet deep, but a moderate frost will consolidate the whole surface of a country, and make it impervious to the air, especially if the frost has been preceded by rain, which has soaked the surface. When this happens, the water which was filtering through the ground is all arrested, and kept suspended in its capillary tubes by the pressure of the air, in the same manner as water is suspended in a tube which is closed at one end, as in the third experiment stated in the preceding chapter (p. 34.) A thaw melts the superficial ice, and allows the water to run out in the same manner as it does when the thumb, in that experiment, is removed from the top of the tube.

5. It is well known that a cask full of water, or spirits, will not run by the cock, unless a hole be opened in the top, or some other part

of the cask. The reason is, that the air presses upon the opening in the cock, and prevents the liquor from flowing; whereas, when an opening is made at the top, the pressure of the air from above forces it down. If, indeed, the hole in the cask is of large dimensions, it will run without any other hole, because air will get in at the upper side of the hole, while the liquor runs out by the lower part of it. For the same reason, a small hole is made in the lid of a teapot to insure its pouring out the tea, otherwise, when the tea-pot is quite full, it would be difficult to make the tea run out at the spout, the pressure of the air from below tending to prevent it.

A

The

On the same principle depends the performance of an instrument used by spirit dealers, for taking out a sample of their spirits. It. consists of a long tin-plate tube, AB, fig. 3, open at top at A, and ending in a small hole at B. end is dipped into the spirits, which rise into the tube; then the thumb is clapped on the mouth a, and the whole is lifted out of the cask. The spirit remains in it till the thumb be taken off; it is then allowed to run into a glass for examination. We are informed by certain travellers, that some of the rude tribes of Asia and Africa, with whom they have travelled, have a similar method of acquiring a draught of water, though they do not understand the

B

Fig. 3.

principle on which it depends. They provide themselves, in their journeys, with a long hollow cane, and, when they wish to drink, or to give their companions a draught of water from a stream or pool, they place the hollow cane in the water, and apply their mouth to the upper end, and draw out the air, when the water rushes in and fills the interior of the cane; they then apply their thumb to the upper end, take the cane out of the water, and apply it to the lips of their thirsty companion, when the water rushes into his mouth. This mode of taking a draught of water may, in some cases, be very convenient when we cannot easily apply our lips to the surface of a running

stream.

The action of the syphon depends on the same principle. A syphon is a bent tube, the one end of which is longer than the other, as AB, fig. 4. If the tube be filled with water, and the shorter leg be placed in a vessel of

E

Fig. 4. water, E, immediately upon withdrawing the finger from the longer leg, the water will flow out till all the liquid in the vessel is emptied. By means of this instrument, we can convey water from a cistern over a rising ground, provided its perpendicular elevation above the level of the fountain does not exceed thirty-two feet, and that the leg, from which the water runs

off, is below the level of the cistern; because the weight of a column of water, about thirty-two or thirty-three feet high, is equal to the weight of a column of air reaching from the surface of the earth to the top of the atmosphere. The pressure of the atmosphere upon the water of the vessel, or cistern, produces this effect.

It might be shown, that the common pump for raising water, the fire-engine, the steamengine, the forcing-pump, and many other pneumatic and hydraulic engines, derive their power and utility chiefly from that extensive and universal agent-the pressure of the atmosphere; without the assistance of which many of our most powerful engines would be arrested in their operations, and sink into feebleness and insignificancy. But this chapter shall be concluded by a few general remarks, suggested by this subject.

The discovery of the pressure of the atmosphere, and of its agency in the system of nature, formed a new era in the history of science. However common it is now to perform the Torricellian experiment, and to talk about the pressure of the atmosphere, it was a subject which, less than two centuries ago, struck with astonishment all the learned throughout Europe. So wonderful and incredible did it at first appear, that it was not till after the lapse of several years-till after opinions which had prevailed for ages had been overturned, and the most decisive experiments had been performed, in every possible way, that it was cor

dially received. And, indeed, when we consider the effects of this powerful agent, in the numerous operations both of nature and art, there is something which tends to excite our admiration more than all the fairy tales which the human fancy has created. We behold its operation in compressing the bodies of all animated beings-in counterpoising the internal pressure of the circulating fluids, and preventing the elastic force of the internal air from bursting the arteries and veins. We behold its operation in forcing-pumps and fire-engines

-in raising water from deep pits, and carrying it to the tops of the highest buildings-in giving motion to our spinning-machines, and in propelling large vessels along seas and rivers-in the action of Hiero's fountain, of syphons and barometers; and in many other cases where its agency could scarcely have been suspected. What can be more unlike than the working of a fire-engine, when spouting a column of water to the top of a building in flames, and the crawling of a fly upon a window-pane, or the ceiling of a room? Yet both these operations are performed by the same means, the pressure of the atmosphere.

But what appears no less striking than such operations, is, the pressure of the atmosphere upon our own bodies. It has already been stated, that this pressure amounts to above thirty thousand pounds. Were the half of this weight to fall on one side of our bodies, without being counteracted by any other power, it