From this theorem it follows, that, at the height of 3i miles, the density of the atmosphere is nearly 2 times rarer than it is at the surface of the earth ; at the height of 7 miles, 4 times rarer ; and so on, according to the following table : Height in miles. Number of times rarer.
31 2 7 4 14 16 21 64 28 256 35 1024 42 4096 49 16384 56 65536 63 262144 70 1048576 And, by pursuing the calculations in this table, it might be easily shewn, that a cubic inch of the air we breathe would be so much rarefied at the height of 500 miles, that it would fill a sphere equal in diameter to the orbit of Saturn.
It has been observed above, that the atmosphere has a refractive power, by which the rays of light are bent from the right lined direction, as in the case of the twilight ; and many other experiments manifest the same virtue, which is the cause of many phenomena. Alhazen, the Arabian, who lived about the year 1100, it seems, was more inquisitive into the nature of refraction than former writers. But neither Alhazen, nor his follower, Vitello, knew any thing of its just quan tity, which was not known, to any tolera ble degree of exactness, till Tycho Brahe, with great diligence, settled it. But neither did Tycho nor Kepler discover in what manner the rays of light were refracted by the atmosphere. Tycho thought the refraction was chiefly caused by dense vapours, very near the earth's surface : while Kepler placed the cause wholly at the top of the atmosphere, which he thought was dense ; and thence he determined its altitude to be little more than that of the highest mountains. But the true constitution of the density of the atmosphere, deduced afterwards from the Torricellian experi ment, afforded a juster idea of these re fractions, especially after it was found that the refractive power of the air is pro portional to its density. By this variation in the density and retractive power of the air, a ray of light, in passing through the atmosphere, is continually refracted at every point, and thereby made to describe a curve, and not a straight line, as it would have done, were there no atmosphere, or were its density uniform.
The atmosphere, or air, has also a re flective power ; and this power is the means by which objects are enlightened so uniformly on all sides. The want of this power would occasion a strange al teration in the appearance of things, the shadows of which would be so very dark, and their sides enlightened by the sun so very bright, that prohably we could see no more of them than their bright halves ; so that for a view of the other halves, we must turn them half round, or, if immove able, must wait till the sun could come round upon them. Such a pellucid un
reflective atmosphere would indeed have been very commodious for astronomical observations on the course of the sun and planets among the fixed stars, visible by day as well as by night ; but then such a sudden transition from darkness to light, and from light to darkness, immediately upon the rising and setting of the sun, without any twilight, and even upon turn ing to or from the sun at noon day, would have been very inconvenient and offen sive to our eyes. However, though the atmosphere be greatly assistant in the il lumination of objects, yet it must also be observed that it stops a great deal of light.
The knowledge of the component parts of the atmosphere is among the discove ries of the moderns. The opinions of the earlier chemists were too vague to merit any particular notice. Boyle, however, and his contemporaries, put it beyond doubt, that the atmosphere contained two distinct substances, viz. an elastic fluid, distinguished by the name of air, and water in the state of vapour. Besides these two bodies, it was supposed that the atmosphere contained a great variety of other substances, which were continu ally mixing with it from the earth, and which often altered its properties, and rendered it noxious or fatal. Since the discovery of carbonic acid gas by Dr. Black, it has been ascertained that this elastic fluid always constitutes a part of the atmosphere. The constituent parts of the atmosphere are, according to Mr: Murray, By measure. By weight.
Nitrogen gas 77.5 2;) 55 Oxygen gas 21.0 23.32 Aqueous vapour 1.42 1 03 Carbonic acid gas .08 .10 100.00 It f..).J0It has been imagined, that a portion of hydrogen may exist in the atmospheric air. But in the usual analysis of it oxy gen is abstracted, and the residual air is found to be nitrogen. The nitrogen is probably not perfectly pure, and it is possible a small portion of hydrogen is mixed with it, which, from the quantity being very trifling, is difficult to be de tected.