MOBILITY OF AIR. 207 



As. from its utmost density, at the bottom of the 

 deepest pit or crevice in the earth to which it can 

 reach, to its utmost degree of rarity in those ele- 

 vated regions where, if we could ascend to it, it 

 would elude the observation even of our muscular 

 feeling of resistance, which is our primary as well 

 as our 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 

 heat and those other principles which are the antag- 

 onists 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 denser 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 mo- 

 tion : 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 any 

 difference of temperature from that of the body, an<l 

 falls not on the eyes. 



