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elevation above the surface of the
earth. VI. To illustrate its composition; or, the che
mical principles of which common
atmospherical air is composed. VII. To illustrate its beneficial effects in the
system of nature. VIII. To exhibit the evidences which its con
stitution affords of the wisdom and benevolence of the Creator.
Air is a material substance.
The first inquiry, then, is, What is that air, of the importance of which we hear so much asserted? We see nothing, it may be saidwe feel nothing. We feel ourselves at liberty to move about without any let or hindrance. Whence, then, the assertion that we are surrounded by a substance called air ? A few facts and illustrations only will be sufficient to elucidate this position.
1. If we take a rod, and make it pass rapidly through what appears empty space, we shall hear a sound and feel a slight resistance, as if something had intervened to prevent the motion of the rod.
2. If we take a large fan, or an umbrella, when fully stretched, and push it forcibly from us, we shall feel a very considerable resistance, and a person opposite will feel a certain impression made upon his face, as if some substance had come in contact with it. Were we to take a very large umbrella-say from twelve to fifteen feet diameter--and stand on the top of a high 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 attempted 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. 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.
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
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.