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Show 206 EXTENSIVE Ul!E UF HEAT. and every change of appearance tf1at we ?an ima rrine-for we can rationally imagme nothmg_ but a ~wdification of something we, by the opera~1~n of mind, have seen altered in its relation ; and It IS no more violation of propriety to suppos~ tha~ the sa!De heat which keeps the body warm. 111 hfe, which labours in the furnace and cooks m the fire, and which brings us the beauty of summer and the abu!l dance of autumn, can sport in the aur?ra bor~ahs, guide mariners in the needle, or blaze 111 the hg~ltninO" than it is to suppose that the never-thawmg ice 5~f Mont Blanc could be a river or part of a t!ee, or a human body, or that its component parts mig~t become the fuel of the most in!ense fla~1e t?at IS known, and that they are the chief matenals 1n the flames of our common fires. . At whatever place of the atmosphere water remams in a state of rest, the heat is always sue~ as. to balance to the utmost nicety both the gravitatiOn and the cohesion; and it is only the air which at the same point admits of variation_. The ot~ers are, compared with it, dull and passive properties; and they act only when heat is suspended, t_hough when once begun, they increa~e at t!'w _rate which has bee~ mentioned; and, as theu actiOn goes on, and watei has no solid cohesion at a temperate heat, the result is most conspicuous as gravitati_on. . The extreme mobility of the au. favours the actwn of all these principles; and that IS the reason why the aerial state is to be regarded !-ls th~ elementa_ry state in the formation of all matenal thmgs. Easily a'3 the air is moveable at the surface of the earth, It must become more and more so as. we ascend above that surface, till at its upper Jimit we can hardly imagine that it offers any res1stanc~ ~t !-lll. We, indeed, know nothing about absolute ln~nts m n~ture; but where there ceases to be any resistance IS the limit to our observation, and therefore ~here can be uo know1edge, and need be no speculatiOn beyond. 1 I MOBILITY OF AIR. 207 As. from its utmost density, at the bottom of the deepest pit or crevice in the eart~ to _which It can reach to its utmost degree of ranty m those elevated' reD"ions where, if we could ascend to it, it would el~de the observation even of our muscular feelinO" of resistance, which is our primary as well as ou~ ultimate test of the existence of matter, the atmosphere in all the compounds of which it is made up stands in the same perfect equipoise between he~t and those other principles which are the antagonists of heat, it follows that its susceptibility of change must be everywhere in the inverse ratio of its density; and that a difference of temperature will produce, in the upper or rare and delicate regions of the atmosphere, very great degrees of motion and disturbance, although it would produce no sensible effect in the deriser portions near the surface. Those upper parts of the atmosphere may be re .. garded as being sensibility itself, just on account of the inconceivably small portion of matter which there is in any assignable space. If we could sup .. pose that the last space of the atmosphere, taken even to a mile in thickness, could weigh a grain, or even the millionth of a grain, we should still be on the ground of observation, and not have arrived at the limit. At the limit both gravitation and cohesion are in the very article of entirely losing their do .. minion, and heat is beginning to be all-powerful. At that boundary, therefore, there is really nothing measurable, or even moveable, that can retard motion : and so it is perfectly consistent to suppose that the air moves, or, which is the same thing, the wind blows there with a rapidity equal to that of light itself, if not greater, and yet that, though we were exposed to its current, we should be no more • · sensible of the impact of that current than we are of the impact of light, which comes to us without anv difference of temperature from that of the body, and falls not on the eyes |