HAVING briefly described the earth and the sea, let us now consider. that fluid which surrounds them both; and which, in some measure, supports and supplies all animated nature. As upon viewing the bottom of the ocean from its surface, we see an infinity of animals moving therein, and seeking food; so were some superior being to regard the earth at a proper distance, he might consider us in the same light. He would, from his superior station, behold a number of busy animals, immersed in the ærial fluid, that every were surrounds them, and sedulously employed in procuring the means of subsistence. This fluid, tho' too fine for the perception of its inhabitants, might, to the nicer organs of sight, in a superior being, be very visible; and, while he at once saw its operations, he might smile at the varieties of human conjecture concerning it: he might readily discern, perhaps, the height above the earth to which this fluid atmosphere reaches; he might exactly determine that peculiar form of its parts, which gives it the spring or elasticity with which it is endued: he might distinguish which of its parts were pure incorruptible air, and which only made for a little time to assume the appearance, so as to be quickly returned back to the element from whence it came.

Such where the reflections of Dr. Goldsmith, in the beginning of his dissertation upon air, in his Natural History: and he concludes them by lamenting the little knowledge which mankind had attained concerning this element. As for us, says he, who are immersed at the bottom of this gulph, we must be contented with a more confined knowledge; and, wanting a proper point of prospect, remain satisfied with a combination VOL. IV. 3 A

of its effects. But since the time of Goldsmith, our experimental philosophers have discovered much more of the properties of air: they have demonstrated that water may be changed into real and pure air, and that air may be turned into water, but after all we know very little of this element, although we daily live, breathe, and move in it.

One of the first things that our senses inform us of, is, though the air is too fine for our sight, yet it is very obvious to our touch. Although we cannot see the wind contained in a bladder, we can very readily feel its resistance; and though the storm may want colour, we often fatally experience that it does not want force. We have equal experience of the elasticity of the air: the bladder when pressed, returns again, when the pressure is taken away; and a bottle when filled, often bursts from the spring of the air which is included.

-Thus far the slightest experience reaches; but by carrying experiment a little farther we learn that air is also heavy: a round glass vessel being emptied of its air, and accurately weighed, has been found lighter than when it was weighed with the air in it. Upon computing the superior weight of the full vessel, a cubic foot of common air is found to weigh something more than an ounce.

From this experiment, therefore, we learn, that the earth, and all things upon its surface, are every where covered with a ponderous fluid, which rising very high over our heads, must be proportionably heavy.

For instance, as in the sea, a man at the depth of twenty feet sustains a greater weight of water, than a man at the depth of ten feet; so will a man at the bottom of a valley have a greater weight of air over him, than a man on the top of a mountain.

From hence we may conclude, that we sustain a very great weight of air: it is true, we do not feel the pressure, but this is owing to its being equal on every side. As in morals, we seldom know the blessings that surround us till we are deprived of them, so here, we do not feel the weight of the ambient fluid till a part of it is taken away. If by any means we contrive to remove the pressure of the air from one part of our bodies, we are soon made sensible of the weight upon the other parts. Thus if we put our hand upon the mouth of a vessel from whence the air has been taken away, there will then be air on one side and none on the other; upon which, we shall instantly find the hand violently sucked inwards; which is nothing more than the weight of the air upon the back of the hand, that presses it into the space which is empty below.

Thus it is evident, that the air presses with great weight upon every thing on the surface of the earth; so, by other experiments, we learn the exact weight with which it presses. If the air be exhausted out of any vessel, a drinking glass for instance, and this vessel be set with the mouth downwards in the water, the water will then rise up into the empty space, and fill the inverted glass; for the external air will, in this case, press up the water where there is no weight to resist; as one part of a bed being pressed, makes the other parts, that have no weight upon them, rise. In this case, therefore, as before observed, the water, being pressed without, will rise in the glass; and would continue to rise, if

the glass were tall enough, thirty two feet high. In fact, there have been pipes made purposely for this experiment, of above thirty two feet high; in which, upon being exhausted, the water has always risen to the height of thirty two feet: there it always rested, and never ascended higher. From this, therefore, we learn, that the weight of the air which presses up the water, is equal to a pillar or colum of water, which is thirty two feet high; as it is just able to raise such a column, and no more. In other words, the surface of the earth is every were covered with a weight of air, which is equivalent to a covering of thirty two feet deep of water; or to a weight of twenty nine inches and a half of quicksilver, which is known to be just as heavy as the former.

It is easily found by computation, that to raise water thirty two feet high, will require a weight of fifteen pounds upon every square inch Now, if any person be fond of computations, he has only to calculate how many square inches are in the surface of an ordinary human body, and allowing every inch to sustain fifteen pounds, we may amaze ourselves at the weight of air we sustain. It has been computed, that our common load of air amounts to nearly forty thousand pounds weight: this is wonderful! but wondering is not the way to grow wise.

Notwithstanding this be our ordinary load, there are at different times very great variations. The air is not, like water, equally heavy at all seasons; but sometimes is lighter, and sometimes more heavy. It is sometimes more comprest, and sometimes more elastic, which produces the same effect as an increase and diminution of its weight. The air, which at one time raises water thirty two feet in the tube, and quicksilver twenty nine inches, will not, at another time, raise the air to thirty feet, or the other to twenty six inches. This makes, therefore, a very great difference in the weight we sustain; and we are actually known, by computation, to carry at one time four thousand pounds of air more than at another.

The reason of this surprising difference in the weight of air, is either owing to its pressure from above, or an increase of vapour floating in it. Its increased pressure is the consequence of its spring or elasticity, which cold and heat sensibly affect, and are continually changing.

This elasticity of the air is one of its most surprising properties; and, it is said, that the possible expansion of it is unknown. A body of air that may be contained in a nutshell, may, by heat, be dilated immeasurably. And, on the contrary, philosophers tell us that, all the air contained in a large house, may be compressed into a cavity not larger than the eye of a needle. In short, no bounds can be set to its compression or expansion; but, perhaps, it ought only to be said, that the experiments hitherto made have been indefinite. In every situation, it retains its elasticity; and the more closely we compress it, the more strongly does it resist the pressure. If to the increasing the elasticity on the one hand by pressure, we increase it on the other by heat, the force of both soon becomes irresistible; and Monsier Amontons, a French philosopher, supposed, that air thus confined, and expanding, was sufficient for the explosion of a world.

Various attempts have been made to ascertain the height to which the atmosphere is extended all round the earth. These attempts commenced soon after it was discovered by Torricelli, that air is a gravitating substance. By the glass tube, before mentioned, which was invented by this philosopher, the weight of the air, through its whole height, in any square measure of it was demonstrated: and from hence its height above the surface of the earth was estimated to be a little more than five miles.

But this calculation cannot be depended on; for as the air is an elastic fluid, the upper parts must expand to an immense bulk, and thus render the above calculation extremely precarious. By experiments made in different countries, it has been found, that the spaces which any portion of air takes up, are reciprocally proportioned to the weights with which it is compressed. Allowances must therefore be made in calculating the height of the atmosphere.

Philosophers, therefore had recourse to another method for determining the altitude of the atmosphere, namely, by a calculation of the height from which the light of the sun is refracted, so as to become visible to us before he himself is really seen in the heavens. By this method it has been supposed to be demonstrated, that at the height of forty five miles, the atmosphere had no power of refraction; and consequently, beyond that distance was either a meer vacuum, or next to it, and not to be regarded. This theory became very generally received, and the height of the atmosphere was spoken of as familiarly as the height of a mountain, and reckoned to be as well ascertained, if not more so, than the height of most lofty mountains are. Very great objections, however, which have never been removed, arise from the appearances of some meteors, like large globes of fire, not unfrequently to be seen at vast heights above the earth. A very remarkable one of this kind was observed by Dr. Halley, in the month of March, 1719, whose altitude he computed to be betwixt sixty nine and seventy three miles and a half; its diameter two thousand eight hundered yards, or upwards of a mile and a half; and its velocity about three hundred and fifty miles in a minute. Others have been seen apparently of the same kind, but whose altitude and velocity were still greater particularly that very remarkable one, August, the 18th, 1783, whose distance from the earth could not be less than ninety miles; aud its diameter not less than the former one; at the same time, that its velocity was not less than a thousand miles in a minute.

Fire balls in appearance similar to these, though vastly inferior in size, have been sometimes observed at the surface of the earth. Of this kind Dr. Priestly mentions one seen on board the Montague, on the 4th of November, 1749, which appeared as big as a large millstone, and broke with a violent explosion.

From analogical reasoning, it seems very probable, that the meteors which appear at such great heights in the air, are not essentially different from those which, like the fire ball just mentioned, are met with on the surface of the earth. The perplexing circumstances with regard to the former, are, that at the great heights above mentioned, the atmosphere ought not to have any density sufficient to support the flame

or to propagate sound; yet these meteors are commonly suceeded by one or more explosions, nay, are sometimes said to be accompanied with a hissing noise as they pass over our heads.

The meteor of 1719, was not only very bright, insomuch that for a short space it turned the night into day, but was attended with an explosion which was heard all over Great Britain, occasioning a violent concussion in the atmosphere, and seeming to shake the earth itself.

That of 1783, also, though much higher than the former, was succeeded by explosions, and, according to the testimony of several persons, a hissing noise was heard as it passed. Dr. Halley, acknowledged that he was unable to reconcile these circumstances with the received theory of the height of the atmosphere; as, in the regions in which this meteor moved the air ought to be three huudred thousand times more rare than what we breathe.

In the meteor of 1783, the difficulty is still greator, as it appears to have been twenty miles farther up in the air. Dr. Halley, offers a conjecture, indeed, that the vast magnitude of such bodies may compensate for the thinness of the medium in which they move,

Whether or not this is the case, cannot be assertained, as we have so few data to go upon; but the greatest difficulty is to account for the brightness of the light. Appearances of this kind are with great probability attributed to electricity, but the difficulty is not thus removed: if the electrical fire pervades the vacuum of a common are pump, yet it does not in that case appear in bright well defined sparks, as in the open air, but rather in long streams, resembling the aurora borealis.

From some late experiments indeed, it has been concluded that the electrical fluid cannot penetrate a perfect vacuum. If this be the case, it shews that the regions which we speak of are not such a perfect vacuum as can be made artificially by the air pump; but whether it be so or not, the extreme brightness of the light shews that a fluid was present in those regions, capable of confining and condensing the electric matter as much as the air does at the surface of the earth; for the brightness of these meteors, considering their distance, cannot be supposed inferior to that of the brightest flashes of lightning.

It should seem therefore that the height of the atmosphere is not yet known. The beginning and ending of twilight, indeed, shew that the atmosphere begins to reflect the sun's rays at about forty five miles high; but probably this may be only the height to which the aqueous vapours are carried for it cannot be thought an unreasonable supposition, that light is refracted only by means of the aqueous vapour contained in the atmosphere; and that where this ceases, it is still capable of supporting the electric fire at least, as bright and strong as at the surface. That it does extend much higher, is evident from the meteors already mentioned for all these are undoubtedly carried along with the atmosphere; otherwise that of 1783, which was seen for about a minute, must have been left a thousand miles to the westward, by the earth flying out below it in its annual course round the sun.

We have before mentioned the vast pressure of the atmosphere, and poticed the weight which a human body of common size is supposed to