MOBILITY OF AIR. 213 



face of the earth, it must become more and more 

 so as we ascend above that surface, till at its up- 

 per limit we can hardly imagine that it offers any 

 resistance at all. We, indeed, know nothing 

 about absolute limits in nature ; but where there 

 ceases to be any resistance is the limit to our ob- 

 servation, and therefore there can be no know- 

 ledge, and need be no speculation beyond. 



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 

 elevated regions where, if we could ascend to it, 

 it would elude the observation even of our mus- 

 cular 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 antagonists of heat, it follows that 

 its susceptibility of change must be every where 

 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 atmos- 

 phere, very great degrees of motion and disturb- 

 ance, although it would produce no sensible effect 

 in the denser portions near the surface. Those 

 upper parts of the atmosphere may be regarded as 

 being sensibility itself, just on account of the in- 

 conceivably small portion of matter which there 

 is in any assignable space. If we could suppose 

 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 ar- 

 rived at the limit. At the limit both gravitation and 



