elevation, are the least affected by the power of gravity, are the least heavy, and are those which are kept at the greatest distance from each other. Close to the surface of the earth, the reverse is the case, and thence density and rarity. The weight and the elastic force being always equal, an universal equilibrium is always preserved in the atmosphere.

The agency of the air is wonderful.* It is in the first instance the vehicle of all kinds of odours, by the transportation of which we are acquainted with the good or with the bad qualities of food, and of innumerable other things. It is equally faithful and diligent in conveying to us knowledge, through the ministration of sounds, at the rate of 1142 feet per second, which may be looked upon as so many couriers, who run backwards and forwards to bring us news of transactions which occur at a considerable distance. It is likewise the channel of light, which is not one of its least phænomena; a substance possessing a surface sufficient to reflect the light, and yet not dense enough to be obvious to sight. And happy for us it is so; for had we but a glimpse of the air, we should

Spect, de la Nat.

never

never have any distinct view of any other object. It would indeed be a misfortune to us of the most serious kind. The exhalations of the earth would then be discovered; the least smoke would disfigure the rich landscape of the universe; we should discern all those gross humours, which an incessant perspiration drives from the bodies of all animals; and all the filth that arises from vile receptacles. Life, in short, would be disagreeable and irksome: we should be miserable, and should fly from the annoyances which would be the unavoidable result of such a faculty.

But if our obligations to the great Author of all, for the advantages we particularly derive from the element of air, are thus great, what should not our gratitude to him be, when we further contemplate its vivifying energy through the other parts of the creation? The universe abounds with an infinite variety of living creatures. Some are inhabitants of the air, others range round the fields, or crawl upon the ground. Thousands reside in the utmost recesses of the thickest woods, in the hearts of leaves, and under the bark of trees; some in the chinks and N 2 crevices

crevices of walls, and others in the most gloomy caverns. The very bowels of the earth are hollowed, and filled with inhabitants. All these animals, how different soever from each other, with respect to their nature and way of life, enjoy the same benefits in common. They all of them partake and breathe the same fluid, air.

The subject of air, which we are thus about to consider, like that of every other branch of true philosophy, though known to the ancients, and in some degree investigated by our forefathers, by Friar Bacon in particular, may yet very fairly be supposed to date its origin from the beginning of the last century. Drebbel, by the invention of the thermometer, and Toricelli, by that of the barometer, undoubtedly gave it birth.* As a science, it must be confessed to be of the greatest importance. Our very mental powers are affected by it. We are gay or sad, according as the heavens are cloudy and Is the air thick and disturbed, we feel a sadness at the heart, which vanishes as soon as it is restored to its wonted serenity. And hence it has with justice been remarked,

serene.

that

* Kirwan.

that the improvements that atmospheric air, and the other permanently elastic fluids, have received, have been of more advantage to mankind, and have reflected more honour upon the philosophers who have made them, than those of any other branch of natural philosophy what

soever.

N 3

LET

LETTER XVII.

AS the part most familiar to us, let us first turn our consideration to that most stupendous body of air, the atmosphere. The atmosphere which every where surrounds our globe, consists of a pellucid, elastic, and apparently homogeneous fluid, which we denote by the name of common, or atmospheric air. When more closely examined, it is found to contain, besides vapours, which vary wonderfully, according to the diversity of situations and winds, three fluids mixed together, and widely differing in their nature. The largest part, which certainly exceeds the others three times or more in bulk, is neither fit for supporting fire, nor for respiration; it probably derives its origin from the vital part, in consequence of some changes not yet perfectly known, perhaps proceeding from the addition or subtraction of phlogiston, and may therefore be called corrupted air. The next is that which is called dephlogisticated, but which Bergman, with the historian of the French Academy, thinks should be distinguished by the appellation of vital,

* Bergman.