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CHAPTER II.

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

a

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

the

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

a

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in the tube about twenty-nine and a half inches above its surface in the vessel, sometimes more and sometimes less, according to the state of the atmosphere. It is evident, then, that the quicksilver is kept up in the tube to this elevation by the pressure of the atmosphere upon the surface of the mercury in the bason ; for, if the bason and tube are put under a glass, and the air extracted, all the quicksilver in the tube will fall down into the bason; and, if the air be re-admitted, it will rise to the same height as before; or, if an opening be made in the top of the tube and the air admitted, the quicksilver will sink into the bason. The pressure, therefore, by the atmosphere on the earth, is the same as if a coating of quicksilver twenty-nine and a half inches thick were spread over every part of the earth's surface.

Now, it is proved that a square column of quicksilver twenty-nine and a half inches in height, and one inch thick, weighs just fifteen pounds, which counterpoise a column of air of the same thickness, extending to the top of the atmosphere; and, consequently, that air presses with this force upon every square inch of the earth's surface; and, of course, 2,160 pounds on every square foot, and 19,440 on every square yard. The experiment now described is, in fact, nothing else than the common barometer. The tube of the barometer is filled with quicksilver, or mercury; it then stands in a bason of quicksilver, is connected with a ball containing quicksilver, on the surface of which the atmosphere presses, and, in most cases, stands at an elevation of about twenty-nine and a half inches, but subject to certain variations, according to the state of the atmosphere. When the weather is steady and serene, it rises to above thirty inches; when it is stormy and rainy, it frequently sinks to twenty-eight inches, or under, thus indicating the changes that take place in the weight of the air; and hence, it has obtained the name of the weather-glass.

Were the same experiment made with water, instead of mercury, a tube must be provided of about thirty-six feet long; and then it would be found, that the water in the tube would be supported by the atmospheric pressure to the height of thirty-two or thirty-three feet. This costly experiment, which has been seldom repeated, was first performed by the celebrated Pascal, at Rouen, in Normandy, in 1647. He exhibited the experiment both with water and with wine, in order to show the different. heights to which these fluids would rise, according to their respective densities. He procured, at a glass-house, tubes of crystal glass forty feet long, which were fixed to the mast of a ship, that was contrived to be raised or depressed, as occasion required. He appointed a day for performing this experiment, and invited all the philosophers and others who doubted of the pressure of the atmosphere to attend, and to be witnesses of the wonderful nature of his experiment. The result was, according to the calculations he had previously made, that the altitude of water in the tube was thirty-one and one-ninth Paris feet, equal to thirty-two feet two and a half inches English; and the altitude of the wine was somewhat greater, namely, thirty-one and two-thirds Paris feet, or thirty-two feet ten inches English; the wine, on account of its superior levity, rising about seven and a half inches higher than the water. He performed this experiment to convince the Aristotelian philosophers of those times of the folly of a notion which then prevailed, that the rise of the mercury in the Torricellian experiment and the rise of water in pumps were produced, not by the pressure of the atmosphere, but by an occult quality, which they denominated “ Nature's abhorrence of a vacuum." They asserted that, in the upper part of the tube, deserted by the quicksilver, there were contained some spirits, evaporated from the quicksilver; which, being rarefied, filled up that space, thus assisting Nature, in a great emergency, against her mortal enemy, a vacuum. “Well, then, gentlemen,” says Pascal, “ take your own way. Here are two tubes, the one I am to fill with water, and the other with wine. You will all readily admit that there is a greater quantity of spirits in wine than in water; and, consequ ntly, that, if the empty space between the upper surface of the fiuids and the top of the tube be filled with spirits, there will be a greater quantity of spirits in the upper part of the tube containing the wine, than in the tube containing the water; and, of course, the wine

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