256 Dr. Pliny Earle Chase's Tests of 



they are no more than the observed phenomena of light and 

 heat force us to accept." 



Fiske (' The Unseen World,' p. 20) cites the numerical por- 

 tion of the above statement, and explains, in a footnote, that 

 " The figures, which in the English system of numeration read 

 as seventeen billions, would in the American system read as 



Prof. De Volson Wood (Phil. Mag. [5] xx. p. 390) says :— 

 " Computations 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 inch ; 



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, ' considered accord- 

 ing to any hypothesis, it is impossible to escape the conclusion 

 that the aether is under great stress/ " 



The mistakes which I. will endeavour to point out in the 

 foregoing paragraphs seem to have widely prevailed. They 

 are the more remarkable on account of the especial pains 

 which were taken by Herschel to guard against them, and on 

 account of some of the results, which Prof. Wood supposes to 

 be new, being identical with those of Herschel and having 

 been obtained by precisely the same methods. 



Herschel discussed the question of aetherial intensity, first, 

 on the corpuscular (' Familiar Lectures,' pp. 271-4), secondly 

 on the undulatory hypothesis (ibid. pp. 274-82) ; the second 

 portion of his discussion being made fuller and more satisfac- 

 tory, on account of the present general adoption of the theory 

 on which it rests. His procedure was substantially as 

 follows : — 



Let h = height of Earth's homogeneous atmosphere; 

 v s = velocity of sound ; 

 V\ = velocity of light ; 



g = gravitating acceleration at Earth's equatorial 

 surface ; 



