On the Constitution of the Luminiferous. Mther. 343 



This series is precisely that which Professor Challis has 

 found on page 281. And thus it appears that the solution in 

 which that series is concerned does in reality express the in- 

 terference of an indefinite number of plane waves, all travelling 

 with the velocity assigned by the ordinary theory, and all in- 

 definitely extended in the direction of their planes. These 

 waves cannot all originate from one source, and therefore this 

 solution does not relate to the problem of sound, in its ordi- 

 nary acceptation. , . , t^ c 



1 will only further allude to the difficulty to which Professor 

 Challis adverts in his last paragraph. The difficulty must 

 arise, in some way, from misunderstanding on words. The 

 most trifling examination of the process in the investigation of 

 the velocity of sound serves to show that the velocity does not 

 depend on the absolute pressure of the air in its normal state 

 of density, but upon the proportion of the change of pressure 

 to the change of density. This is increased by the suddenness of 

 condensation in one part, which when the elastic force is great 

 makes it still greater, and by the suddenness of rarefaction in 

 another part, which when the elastic force is small makes it 

 still smaller; thus in both ways increasing the change of 



pressure. 



I am, Gentlemen, 



Your obedient Servant, 

 Royal Observatory, Greenwicli, G. B. AlRY. 



April 10, 1848. 



XLVII. On the Constitution of the Liminifer am Mther. By 

 G. G. Stokes, M.A.^ Fellow of Pembroke College, Cam- 

 bridge '*. 

 THE phajnomenon of aberration may be reconciled with 

 the undulatory theory of light, as 1 have already shown 

 (Phil. Mag., vol. xxvii. p. 9), without making the violent sup- 

 position th"it the aether passes freely through the earth in its 

 motion round the sun, but supposing, on the contrary, that 

 the aether close to the surface of the earth is at rest relatively 

 to the earth. This explanation recjuires us to suppose the 

 motion of the a;ther to be such, that the expression usually 

 denoted by udx + vdy + ivdz is an exact differential. It becomes 

 an interesting question to inejuire on what physical properties 

 of tlie aether this sort of motion can be explained. Is it sufli- 

 cieiit to consider the jctlier as an ordinary fluid, or must we 

 have recourse to some property which does not exist in ordi- 

 nary fluids, or, to speak uiorc correctly, the existence of which 

 • Comiminlcatetl by the Autiior. 



