214 MOBILITY OF AIR. 



cohesion are in the very article of entirely losing 

 their dominion, 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 consis- 

 tent to suppose that the air moves, or which is 

 the same thing, the wind blows there with a ra- 

 pidity equal to that of light itself, if not greater, 

 and yet that, though we were exposed to its cur- 

 rent, we should be no more sensible of the im- 

 pact 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, and falls 

 not on the eyes. 



As we descend downwards from that limit of 

 extreme atmospheric rarity and gravitation, and 

 cohesion becomes more and more sensible, the mo- 

 tion produced by the same variation of tempera- 

 ture must gradually become less and less; but 

 the atmosphere is so rare even where densest, that 

 it is probably more sensible to changes of heat 

 than even our sense of muscular resistance ; and 

 therefore we cannot even feel it to any thing near 

 its boundary. 



Thus even at moderate elevations, elevations not 

 greater than the summits of our loftiest mountains, 

 the atmosphere may be thrown into very great 

 action by very slight causes ; and the very first 

 pencil of the morning light which streams upon an 

 atmosphere thick enough for dividing that light, 

 and sending down the extreme violet of the spec- 

 trum in a glimmer of dawn to us, may, in the red 

 and more energetic part, give to that light air a 

 degree of motion which shall send it completely 

 round the atmosphere, before the other part of the 



