( 445 ) 

 — a-\-h = c (5) 



«=|-^ + -+l)r^'i (G) 



^ *'o '"o ' 



The velocity witli which the aether slides along the earth is 

 found to be 



„3 — — 



V — . e ''" h sin , (7) 



4V 



where d is the angle between the radius of the point considered 

 and the direction of the velocity c. Now, Prof. Planck remarks 



that, by (6), if only — be large enough, a will be very small rela- 



''o 

 tively to 6, so that, as (5) shows, b is nearly equal to c. But then, 

 the value of v given by (7) will be a very small fraction of c itself. 

 If the quotient of the pressure and the density had the same 

 value for aether as for air of 0°, and if the force of gravity acted 

 with the same intensity on the aether as on ponderable matter, we 

 should have 



— = 800 , approximately. 



The sliding would then be absolutely imperceptible, but it should 

 be noticed that this would be due to an enormous condensation, 

 the ratio » between the densities for ?■ = r^ and r :^ cc being by (3) 



In order that the aether may follow the earth in its motion in 

 so far as is necessary for the explication of the phenomena, wc 

 need not require that the condensation should have such a high 



k 

 value. Of course, it would be less, if either — or g were smaller 



P 

 than for air. 



We can easily determine what degree of condensation must 

 necessarily be admitted. Indeed, the constant of aberration may be 

 reckoned to correspond to within Vs pCt. to the value given by 

 the elementary theory of the phenomenon ; consequently, in the theory 

 of Stokes, the velocity of sliding should be no more than about 



u 

 Vü pCt. of the earth's velocity. Now, putting — = 10 , 1 find for 



a 



the maximum value of the velocity of sliding 0,011c. If — ^ 11 , 



