414 Prof. De Volson Wood on 



only a few miles, for although the atmosphere in the upper 

 regions is extremely rare, yet the actual number of molecules 

 in a cubic foot is large. Thus, according to our analysis, for 

 statical conditions, the topmost cubic foot of the 104-mile 

 column would contain about 1,000,000 molecules ; and at the 

 height of 95 miles it would contain about 1,000,000,000,000,000 

 molecules ; so that if the relative velocities of the meteor and 

 air be 20 miles per second, the meteor would encounter an 

 enormous number in the twentieth or even the hundredth part 

 of a second, after first entering the atmosphere. 



The height of the auroral arch — supposed to be within our 

 atmosphere — has been computed to be from 33 to 1000 miles 

 (see article "Aurora,''' Encyc. Brit.). But it has been shown 

 by experiment, that a vacuum may be produced through 

 which an electrical discharge cannot be passed, and yet the 

 atmosphere at the height of 150 miles under the most favour- 

 able condition, that of uniform temperature, is vastly more 

 rare than the most perfect vacuum ever produced by the most 

 perfect Sprengel pump ; and at the height of 200 miles under 

 the same conditions the vacuum would be some 10,000,000 

 times as great as the most perfect vacuum yet made ; while, 

 according to the probable law of the decrease of temperature 

 with the elevation, and in accordance with the probable mass 

 of a molecule of air, the extreme height falls far short of 150 

 miles. It is evident, therefore, that the assumed determination 

 of the height of the atmosphere by means of the auroral arch 

 is, to say the least, unreliable*. 



We have pursued this digression in regard to the atmo- 

 sphere partly for its own sake and partly to show, by way of 

 contrast and accumulative evidence, that the aether is a sub- 

 stance entirely distinct from that of the atmosphere, — that the 

 former cannot be considered as the latter greatly rarefied, as 

 some have supposed. Admitting the validity of the prece- 

 ding discussion, some of the distinctive properties are : — 



1. The different modes of the movements of the molecules 

 in the two substances in the propagation of a wave ; in one 

 the motion being a to-and-fro movement and in the other a 

 transverse movement. These are distinctions recognized by 

 the best writers upon the subject, and are especially noticed 

 by Maxwell in an article on Ether in the Encyclopaedia Bri~ 

 tannica. 



2. It is impossible for a wave to be transmitted in air with 

 the known velocity of light, unless its temperature be increased 



* Some writers incline to the view that the aurora is due to a cosmic 

 rather than a terrestrial origin. ' Science/ 1885, p. 395. 



