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equal to 4000 miles. Then, by the rules of trigonometry, Rg g

[graphic]

As the sine of the angle c D A .. 81° SO'.. 9.995203

Is to the side A c, 4000.. 3.602060

So is radius, sine of 90° 10.000000

To the side c D, 4044J miles 3.606857

From C D, equal to the semi-diameter of the earth and atmosphere, subtract c A, or the semi-diameter of the earth, and the remainder, E D, equal to 44£ miles, will be the height of the atmosphere. In this operation, the logarithms of the second and third terms of the proportion are added, and the logarithm of the first term subtracted from the sum.

Thus 3.602060 10.000000

13.602060 9.995203

3.606857

The sumo result is produced by the following proportion:—

As Radius 10.000000

Is to A c 4000 3.002060

So is the secant of A C D = SJD 10.004S00

To C D = 4044J 3.006800

It appears, then, that in ordinary cases, the air, at the height of forty-four miles and a half, is capable of reflecting to us the rays of light. But, as a sensible illumination has been perceived when the sun is much further below the horizon than what has been now stated, there is some reason to conclude, that the air is sufficiently dense for reflecting a sensible degree of light at the height of nearly two hundred miles.

Various considerations, founded on meteoric phenomena, serve to prove that the atmosphere extends to a much higher elevation than fortyfour or fifty miles. In the year 1719, a remarkable luminous meteor, or fire-ball, was seen, whose altitude was computed to be seventy-three miles above the surface of the earth. On the 18th of August, 1783, a brilliant fire-ball passed over Britain and the adjacent countries; and, from various circumstances which were particularly marked by different observers, it was calculated that its elevation above the earth could not be less than ninety or a hundred miles. In passing over certain parts of England, a loud report was heard and a hissing noise. The meteor of 1719 is said to have been attended with an explosion, which was heard over the whole island of Great Britain, occasioning a violent concussion of the atmosphere, and seeming to shake even the earth itself. Now, in these, and multitudes of similar phenomena, we have instances of fire and flame being supported, and sounds conveyed to the earth from a height of ninety or a hundred miles; and, consequently, even in these elevated regions, notwithstanding the great rarefaction of the air, it must still have the power of supporting flame and propagating sound. Even although the fire-balls alluded to be supposed to consist of electrical matter—which is the general opinion—yet the difficulty is not thereby removed; for, it is found, by some late experiments, that the electrical fire cannot penetrate a perfect vacuum. And, therefore, there is reason to conclude, that we are still ignorant of the precise extent of the atmosphere, and of the nature of the fluids which occupy its superior regions. That the meteors now referred to, however elevated, were not beyond the limits of the atmosphere, appears from this consideration, that the atmosphere revolves with the earth in its course round the sun, at the rate of 68,000 miles an hour. Now, as the meteor of 1783 moved from north to south, if it had been beyond the limits of the atmosphere, it would have been left, in the course of a minute, more than a thousand miles to the westward, by the earth flying out before it, both in its annual and diurnal course.

In short, it appears not altogether improbable to suppose, that the visible universe is filled with some fine elastic fluid or air, but of such a rarity as to be no sensible hindrance to the celestial orbs in their rapid motions through the regions of space; and that this fluid accumulates itself around every planetary body, in proportion to the quantity of matter it contains—the larger bodies attracting more of it, and the smaller bodies less; and thus forming an atmosphere around each, corresponding to its nature and destination. And, if this be the case, the atmosphere of the earth can have no definite boundary, but may be said to mingle with the atmospheres of all the other planets which belong to our system. There is a certain portion of atmospheric air, however, which must always be considered as attached to the earth, and which revolves with it in its diurnal rotation, and is carried along with it in its course round the sun. If the atmosphere did not revolve along with the earth, we should constantly experience an easterly wind, blowing with an immense velocity of more than a thousand miles an hour, which would produce a most tremendous hurricane, which would level with the ground houses, trees, forests, and every prominent object on the surface of the earth. But the particular region where the motion of the atmosphere terminates, it is impossible for us to ascertain.

CHAPTER VI.

The composition of the atmosphere.

For a long series of ages, air was considered by philosophers as one of the four elements of which all things are composed, the other three being fire, earth, and water. But the discoveries of modern chemistry have fully demonstrated that all these are compound bodies— that even the air itself, fine and invisible as it is, is not a simple substance, but compounded of different ingredients. This is one of the most curious and interesting discoveries of modern times; and little more than seventy years have elapsed since it was first surmised that the atmosphere is not a simple and homogeneous, but a compound fluid. The experiments which led to this discovery were first made by Dr. Priestley, on the first of August, 1774, on which day he obtained what was then termed dephlogisticated air, nowknown by the name of oxygen gas, and which forms one of the constituent principles of atmospheric air. It was also discovered in the year 1775, by M. Scheele, a Swedish chemist, without c

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