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stair, or a building, twenty or thirty feet high, we might jump from such a position, while we hold it fully stretched, and gradually descend to the ground without violence or injury. It is on this principle that the instrument called a parachute is constructed, by means of which an aëronaut, while pursuing his aërial excursions, has left his balloon, when elevated nearly a mile above the surface of the earth, and descended in a few minutes to the ground, without shock or accident. Perhaps some contrivance of this kind might be useful to prevent accidents in the case of fires in large towns—when persons have atteinpted to jump from the windows of a third story to preserve themselves from being involved and destroyed in the burning mass. The circumstances now stated prove, that there is a certain material substance, though invisible, around us, which offers a sensible resistance to any body having a large surface when it is pushed rapidly through it.
3. That air is a material substance, appears from its excluding all other bodies from the place it occupies. Thus, if we take a glass jar, and plunge it with its mouth downwards into a vessel of water, only a very small quantity of water will get into the jar, because the air, of which the jar is full, keeps the water out; otherwise, if it were empty of every material substance, the water would rush in and completely fill the jar. Hence, we may learn why a vessel cannot be filled with water by plunging its orifice downward, and why a funnel, if its
pipe fit closely to the neck of a bottle, is not convenient for pouring off liquors; for, in order to put water or wine into a bottle, the air must pass between the neck of the bottle and the funnel to let the air out as the water rushes in. And hence, the practice in such cases, suggested by necessity, of pulling up the funnel a little when the liquor stops, in order to let the air rush out between the pipe and the neck of the bottle. It is on the principle now stated, that the diving bell is constructed, by which a person may descend to a considerable depth into the sea, and yet not be immersed in water, nor deprived of air for breathing.
4. If we take a smooth cylindrical tube shut at one end, and fit a plug or cork exactly to its open end, so as to slide along it, if the plug be so tight and soaked with grease, as to prevent all passage of any fluid by its sides, we shall find that no force whatever can push it to the bottom of the tube. There is, therefore, something within the tube, though invisible, which prevents the entry of the plug, and, therefore, possessing the characteristic of matter, and this something is air.
5. Let us take a pair of common bellows, and, after having opened them, if we shut up the nozzle and valve-hole, and try to bring the boards together, we shall find it impossible. There is something included that prevents this, in the same manner as if the bellows were filled with flax or wool; but, on opening the nozzle, we can easily shut them by expelling this
something that is within, which will issue with considerable force, and impel anything that lies in its way. This something can be nothing else than the air of the atmosphere.
6. The air, though for the most part invisible, may, in certain cases, be rendered an object of sight. If we take a telescope of high magnifying power, and, in the forenoon of a hot summer day, when the sun is shining, look through it to distant objects, we shall perceive the air undulating about the objects somewhat like the waves of the sea, and rendering them undefined and obscure. This is the principal reason why very high magnifying powers cannot be used, with effect, on telescopes for land objects, in the day time, when the sun produces undulations in the atmosphere ; and the same cause frequently prevents distinct vision of celestial objects.
The above are clear proofs that the air, though not generally an object of sight, is, in reality, a material substance, as much so as water, wood, stones, or iron. This substance, in a state of rest, we call air; in a state of motion, we call it wind; and, in this state, its force is sometimes so great as to drive our wind-mills, impel our ships across the ocean, and even to overturn buildings, to tear up from their roots the largest trees, and to dash whole fleets to pieces of wreck.
The weight and pressure of the atmosphere, and the quantity
of matter it contains.
As air is demonstrated to be a body, like all other material substances, it must have weight, and the proportion its weight bears to other known substances is determined by experiment. If a bottle which contains about a quart be emptied of its air by means of an air-pump, or in any other way, and then accurately weighed in a nice balance, it will be found to be about sixteen grains lighter than it was before it was emptied of its air, which shows that a quart of air weighs sixteen grains. A quart of water weighs about 14,620 grains, or nearly two pounds. If this last number be divided by sixteen, the quotient will be nine hundred and thirteen, which shows that air is nine hundred and thirteen times lighter than water ; or, in other words, that it would require above nine hundred quart-bottles of air to weigh one quart-bottle of water. Other experiments which have been made to determine this point, lead to the result that, for every cubic foot of air, five hundred and twenty-three grains, or, one and one-fifth
ounce avoirdupois, are to be allowed ; and, since a cubic foot of water weighs 1,000 ounces, the one divided by the other gives a result of eight hundred and thirty-three, the number of times that water is heavier than air. It is impossible to arrive at very great nicety in such estimates ; but the general results of all the experiments which have been made on this point, lead to the conclusion that air is somewhere between eight hundred and nine hundred times lighter than water. These results, however, must be understood solely to apply to the air near the surface of the earth; for, as we ascend into the higher regions of the atmosphere, the air becomes gradually thinner and lighter, being less pressed with the air that is above.
We may now attend to the pressure which the atmosphere exerts upon the surface of the earth, and upon all bodies connected with it.
It has been proved by a variety of accurate experiments, that the atmosphere presses on every part of the earth's surface with a force, at an average, equal to about fifteen pounds on every square inch.
This has been ascertained by what is called the Torricellian experiment. Take a glass tube about three feet long, open at one end, and hermetically sealed at the other: fill it with quicksilver, putting the finger upon
open end, turn that end downwards, and immerse it in a small vessel of quicksilver, without admitting any air, then take away the finger, and the quicksilver will remain suspended