390 Prof. De Volson Wood on 



and is the value used by Sir William Thomson in determining 

 the probable density of the aether*. Later determinations of 

 the value of the solar constant by MM. Soret, Crova, and 

 Violle have made it as high as 2*2 to 2*5 calories. But the 

 most recent, as well as the most reliable, determination is by 

 Professor S. P. Langley, who brought to his service the most 

 refined apparatus yet used for this purpose, and secured his 

 data under favourable conditions; from which the value is 

 found to be 2*8+ calories f with some uncertainty still re- 

 maining in regard to the first figure of the decimal. We will 

 consider it as exactly 2*8 in this analysis, according to which, 

 there being 7000 grains in a pound and 15*432 grains in a 

 gramme, we have for the equivalent energy 



2-8x15-432 9 772x144 1QQ , . , 



ttt^k — x - x 7^r^ — 777: =133 foot-pounds 



7000 5 0-155 x 60 r 



per second for each square foot of surface normally exposed 

 to the sun's rays, which value we will use. Beyond these 

 facts, no progress can be made without an assumption. Com- 

 putations have been made of the density, and also of the 

 elasticity, of the aether founded on the most arbitrary, and in 

 some cases the most extravagant, hypotheses. Thus, Herschel 

 estimated the stress (elasticity) to exceed 



17 x 10 9 = (17,000,000,000) pounds per square inchj; 



and this high authority has doubtless caused it to be widely 

 accepted as approximately correct. But his analysis was 

 founded upon the assumption that the density of the aether 

 was the same as that of air at sea-level, which is not only 

 arbitrary, but so contrary to what we should expect from 

 its non-resisting qualities, as to leave his conclusion of no 

 value. That author also erred in assuming that the tensions 

 of gases were as the wave-velocities in each, instead of the 

 mean square of the velocity of the molecules of a self-agitated 

 gas; but this is unimportant, as it happens to be a matter of 

 quality rather than of quantity. Herschel adds, " Consi- 

 dered according to any hypothesis, it is impossible to escape 

 the conclusion that the aether is under great stress." We 

 hope to show that this conclusion is not warranted ; that a 

 great stress necessitates a great density; but that both may 

 be exceedingly small. A great density of the aether not only 

 presents great physical difficulties, but, as we hope to show, 



* Trans. Roy. Soc. Edinburgh, vol. xxi. part 1. 



t Am. Journ. of Arts and Science, March 1883, p. 195. Also Comptes 

 Rendus. 



\ i Familiar Lectures/ p. 282. 



