208 METEORS AND METEORIC STONES. 



As we descend downwards from that limit of ex- 

 treme atmospheric rarity and gravitation, and cohe- 

 sion becomes more and more sensible, the motion 

 produced by the same variation of temperature 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 spectrum in 

 a glimmer of dawn to us, may, in the red and more 

 energetic part, give to that light air a degree of mo- 

 tion which shall send it completely round the atmo- 

 sphere, before the other part of the ray can reach 

 us from probably not the thousandth part of the dis- 

 tance. 



But though, in those upper parts of the atmo- 

 sphere, there is the least matter in the same space, 

 we must not on that account suppose that nature is 

 there least active. We have noticed, again and 

 again, that matter is the clog of motion ; and as the 

 most active substances that mingle with the atmo- 

 sphere have the greatest tendency to ascend in it, 

 we may properly suppose that they occupy the 

 upper parts of it ; and that their motions and oppo- 

 sitions are not only perfectly adequate to the pro- 

 duction of all the luminous meteors that appear 

 there, but also of forming out of the scattered mate- 

 rials which float at that airy height, the meteoric 

 stones of which so many are recorded as having 

 fallen to the ground. 



Lower than that, but still in air so fine that it 

 will float nothing that can be visible to our sight as 



