Dr. Hales, by means of a press, condensed the air 33 times; and, afterwards, by forcing water in an iron globe, into 1,551 times less space than it naturally occupies. The dilation of the air, by virtue of its elastic force, is found to be very surprising. In experiments made by Mr. Boyle, it dilated to 10,000, and even, at last, in 13,679 times its space; and this altogether by its own expansive force, without the help of fire. In fact, it appears that the air we breathe is compressed by its own weight into at least the 13,679th part of the space it would occupy in _vacuo_. But if the same air be condensed by art, the space it would take up when most dilated, will be, according to the same author's experiments, as 550,000 to 1.
It is only by means of the experiments made with pumps,[62] and the barometrical tube, by Galileo and Torricelli, that we came to the proof, not only that the atmosphere is endued with _weight_ and _pressure_, but also of the measure and quantity of that pressure. The rise of water in a pump was formerly attributed to the horror that nature had of a vacuum. This absurd notion was refuted about the middle of the seventeenth century, by the following occurrence. The Duke of Florence, having occasion to raise water to the height of 50 or 60 feet, ordered a common pump to be made for that purpose; but when it was completed, the workmen were astonished to find that it would not work. The matter was referred to Galileo, but he was unable to account for it in any way. All they were able to determine was, that water would not rise in a common pump more than 32 or 35 feet. The fact remained inexplicable till philosophers caught the idea of atmospheric pressure; since when, the suspension of mercury in the barometer, and water in a pump, have been well understood.[63]
That the air is a heavy body, has been demonstrated by a variety of experiments. The air next the earth is more dense than that at a distance, because, as it is of an elastic or springy nature, it is pressed down by the whole weight of the superincumbent air. Its general force of gravity appears, from its surrounding the earth, and always accompanying it in its orbit round the sun. As the matter of which the air is composed is always variable, so likewise will its weight or gravity be, as barometers of various kinds and structure evince. The weight of the air at the earth's surface, is found by the quantity of mercury that the atmosphere balances in the barometer; in which, at a mean state, the mercury stands 29½ inches high. And if the tube were a square inch wide, it would at that height contain 29½ cubic inches of mercury, which is just 15 pounds weight; and so much weight of air every square inch of the earth's surface sustains; and every square foot, as containing 144 inches, must sustain a pressure of 2,160. At this rate, a middle-sized man, whose surface is about 15 square feet, must sustain a weight of 32,400 pounds, or 16 tons; for the air, like other fluids, presses equally upwards, downwards, and sideways, in every direction. But because this enormous weight bears equally on all sides, and is counterbalanced by the spring of air diffused through all parts of the body, it is not in the least felt by us.[64]