174. The Earth is surrounded by a thin fluid mass of matter, called the _Air_, or _Atmosphere_, which gravitates to the Earth, revolves with it in it’s diurnal motion, and goes round the Sun with it every year. This fluid is of an elastic or springy nature, and it’s lowermost parts being pressed by the weight of all the Air above them, are squeezed the closer together; and are therefore densest of all at the Earth’s surface, and gradually rarer the higher up. “It is well known[39] that the Air near the surface of our Earth possesses a space about 1200 times greater than water of the same weight. And therefore, a cylindric column of Air 1200 foot high is of equal weight with a cylinder of water of the same breadth and but one foot high. But a cylinder of Air reaching to the top of the Atmosphere is of equal weight with a cylinder of water about 33 foot high[40]; and therefore if from the whole cylinder of Air, the lower part of 1200 foot high is taken away, the remaining upper part will be of equal weight with a cylinder of water 32 foot high; wherefore, at the height of 1200 feet or two furlongs, the weight of the incumbent Air is less, and consequently the rarity of the compressed Air is greater than near the Earth’s surface in the ratio of 33 to 32. And having this ratio we may compute the rarity of the Air at all heights whatsoever, supposing the expansion thereof to be reciprocally proportional to its compression; and this proportion has been proved by the experiments of Dr. _Hooke_ and others. The result of the computation I have set down in the annexed Table, in the first column of which you have the height of the Air in miles, whereof 4000 make a semi-diameter of the Earth; in the second the compression of the Air or the incumbent weight; in the third it’s rarity or expansion, supposing gravity to decrease in the duplicate ratio of the distances from the Earth’s center. And the small numeral figures are here used to shew what number of cyphers must be joined to the numbers expressed by the larger figures, as 0.^{17}1224 for 0.000000000000000001224, and 26956^{15} for 26956000000000000000. +-----------------------------------------+ | AIR’s | | _________________/\ _________________ | | / \ | | Height. Compression. Expansion. | +-----------+---------------+-------------+ | 0 | 33 | 1 | | 5 | 17.8515 | 1.8486 | | 10 | 9.6717 | 3.4151 | | 20 | 2.852 | 11.571 | | 40 | 0.2525 | 136.83 | | 400 | 0.^{17}1224 | 26956^{15} | | 4000 | 0.^{105}4465 | 73907^{102} | | 40000 | 0.^{192}1628 | 26263^{189} | | 400000 | 0.^{210}7895 | 41798^{207} | | 4000000 | 0.^{212}9878 | 33414^{209} | | Infinite. | 0.^{212}6041 | 54622^{209} | +-----------+---------------+-------------+