Models of the Aether. 331 



Comparing this equation with that which determines the value 

 of Q, we have 



' d^ 



or substituting for CT, 



The isotropy with respect to x and z gives the equation 



, 8 a\ 8 _ 



-+ ^0- h" V ^ 



But by integration by parts we obtain the equation 



' U. v -^ o= _ 



and by the condition of incompressibility the second member 

 may be written 



A . (tojty) . (d/ty) . V- 2 Vo, or - A . v . (d/ z df) . V-^o ; 

 so we have 



On account of the isotropy, we may write J for 



and, therefore, 



The deviation from isotropy shown by this equation is very 

 small, because of the smallness of df(y, t)/dy. The equation is 

 therefore not restricted to the initial values of the two members, 



