by its elastic quality, expands and contracts; and it being found, by repeated experiments in most nations of Europe, that the spaces it occupies, when compressed by different weights, are reciprocally proportional to the weights themselves; or, that the more the air is pressed, so much the less space it takes up; it follows that the air in the upper regions of the atmosphere must grow continually more and more rare, as it ascends higher; and indeed that, according to that law, it must necessarily be extended to an indefinite height. At the height of 3 miles the density of the atmosphere is nearly 2 times rarer than it is at the surface of the earth; at the height of seven miles, 4 times rarer; and so on, according to the following table. Height in miles.

3

7

Number of times rarer.

2

4

By pursuing these calculations, 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. Hence we may perceive how very soon the air becomes so extremely rare and light, as to be utterly imperceptible to all experience; and that hence, if all the planets have such atmospheres as our earth, they will, at the distances of the planets from one another, be so extremely attenuated, as to give no sensible resistance to the planets in their motion round the sun for many, perhaps hundreds or thousands of ages to come. Even at the height of about fifty miles, it is so rare as to have no sensible effect on the rays of light.

Mr. Boyle in his physico-mechanical experiments concerning the air, declares it probable that the atmosphere may be several hundred miles high; which is easy to be admitted, when we consider, what he proves in another part of the

same treatise, viz. that the air here about the surface of the earth, when the pressure is taken from it, dilates into 10,000, and even at last into 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 possess in vacuo. But, if the same air be condensed by art, the space it would take up when dilated, to that it possesses when condensed, will be, according to the same author's experiments, as 550,000 to 1.

Our direct experiments, however, not reaching to any great heights into the regions of the atmosphere, and not knowing how far air may be expanded, we are incapable of determining to what height the atmosphere is actually extended.

CHAP. VII.

Of the Meteors.

WE have seen that the atmosphere is a vast laboratory, in which nature operates immense analyses, solutions, precipitations, and combinations; it is a grand receiver, in which all the attenuated, volatilized productions of terrestrial bodies are received, mingled, agitated, combined and separated. Considered in this view, the atmospheric air is a chaos, an indeterminate mixture of mineral, vegetable, and animal effluvia, which the electric fluid is pervading and traversing continually. The grand changes it experiences, and of which we are sensible in extensive spaces by the appearance of water, light, or noise, are called meteors. As the state of the atmosphere is ever varying, the meteors assume different forms; some delighting us with their appearance, while others wear a terrifying

aspect. In this repository is collected the gentle dew and hoar-frost; here clouds are gathered and carried along by the wind, to refresh the earth in falling showers, give rise to rivers, spread vast inundations of water over the fields, or lay them under a covering of snow or hail; here mock-suns, mock-moons, haloes, and rainbows make their gaudy but transitory appearance; and here the water-spout, dreadful to the mariner; here. rolls the dreadful thunder, here lightnings dart their vivid flames, and sometimes, striking upon the earth, destroy its productions, fill its inhabitants with terror, and sometimes strike them dead; here the aurora, or streamers, the ignes fatui, or wandering fires, called also Jack with the Lantern; here falling stars, as they are ignorantly termed, or fiery balls of various sizes, appear with splendour during the gloom of night, and astonish mankind, who too often seem willing, with superstitious awe, to find portentous omens of dire calamities in these curious phenomena, rather than investigate their causes or discover their uses.