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} |
+-----------+---------------+-------------+
From this Table it appears that the Air in proceeding upwards is
rarefied in such manner, that a sphere of that Air which is nearest the
Earth but of one inch diameter, if dilated to an equal rarefaction with
that of the Air at the height of ten semi-diameters of the Earth, would
fill up more space than is contained in the whole Heavens on this side
the fixed Stars, according to the preceding computation of their
distance[41].” And it likewise appears that the Moon does not move in a
perfectly free and un-resisting medium; although the air at a height
equal to her distance, is at least 34000^{190} times thinner than at the
Earth’s surface; and therefore cannot resist her motion so as to be
sensible in many ages.
[Sidenote: It’s weight how found.