The atmosphere, or body of air encompassing the earth on all sides, is generally divided into _three_ regions. The lowest region extends from the earth to the place where the air is no longer heated by the rays which the earth reflects: this region is the wannest. The _middle_ region begins where the preceding one ends, and goes to the summit of the highest mountains, or even the highest clouds; this is the space where rain, hail, and snow are engendered: this region is much colder than the preceding one. The _third_ region extends from the middle one to the utmost height of the atmosphere; whose limits have not been ascertained.[60] If the air were of an equal density throughout, the height of the atmosphere might be determined: but since the density of the air decreases with the pressure, it will be more rarefied and expanded the higher we go; and by this means the altitude of the atmosphere becomes indefinite, and terminates in pure ether. But though we cannot assign its real height, it is certain, from observations and experiments, that a distance of 45 or 50 miles is the utmost limit where the density is sufficient to refract the rays of light. For the beginning and ending of twilight show, that the height at which the atmosphere begins to refract the sun’s light is about 45 English miles; and therefore that may be reckoned the altitude of the air to the least degree of density. The air is justly reckoned among the number of _fluids_, because it has all the properties by which a fluid is distinguished. It requires but little attention to be convinced of this. The air yields to the smallest force impressed on it; its parts are easily moved among themselves; it presses according to its perpendicular height, and its pressure is every where equal. That the air is a fluid consisting of such particles as have no cohesion among themselves, but easily glide over one another, and yield to the smallest impression, appears from the ease and freedom with which animals breathe in it, and move through it without any difficulty or sensible resistance. The ease with which it is penetrated, and driven about in every direction, and the motion of it in pipes and channels, however crooked and intricate, demonstrate its fluidity. It is also known to be a fluid, by the easy conveyance which it affords to sound.