DYNAMICAL THEORY OP SOUND 



where 6 is a parameter which varies from one line of motion 

 to another. The curves, which are identical in form with the 

 lines of force due to a small magnet, are shewn in Fig. 73. 



Fig. 73. 



To calculate the reaction on the sphere we divide the surface 

 into zones by planes perpendicular to Ox. The area of a zone 

 being 27ra 2 sin0S0, the resultant force on the sphere in the 

 direction of ^-positive is 



(7) 



The constant part of the pressure contributes nothing to the 

 resultant. The variable part is, if terms of the second order in 

 the velocities be neglected, 



.(8) 



