LIMITS OF THE AIR. 171 



point is supposed to be the true limit of the air. 

 The air has consequently a true surface or level, 

 like that of a fluid. 



Could we take our stand upon the surface of 

 our fair satellite the moon, as she " walks in 

 brightness," and look toward the earth, sup- 

 posing that the atmosphere enveloping it were 

 coloured throughout, so as to be visible to our 

 eyes, it would be seen to be of the form of an 

 oblate spheroid, the lesser axis of which would 

 pass through the poles of the earth. In short, 

 its outline must correspond pretty nearly to 

 that of the globe which it envelopes, supposing 

 it to be of equal thickness in every region. 

 This, however, is not the case, since it is more 

 dense near the poles, and more rare near the 

 equator. But the effect of this would be sim- 

 ply to exaggerate the oblately spheroidal out- 

 line of the air. It has been conjectured by 

 M. Biot, and others, that beyond the limits of our 

 own air there exists a ' sort of ether, which per- 

 vades all space. In confirmation of this hypo- 

 thesis, it has been supposed by some astrono- 

 mers that it is only on such grounds that the 

 retardation of the motion of certain comets is to 

 be explained, the cause lying in the resistance 

 to their motion offered by such a medium. If 

 such a fluid exist at all, it must possess elas- 

 ticity and tenuity in a degree, probably, as far 



