301 



The energy at any point in the wave front of the wavelets must be small 

 compared to the energy at any point in the main wave, for two reasons. In 

 the first place only a fraction of the energy of the original wave passes through 

 the apertures. In the second place, what does get through spreads out to 

 form the wavelets and thus greatly reduces the energy propagated in a partic- 

 ular direction. If the speed of propagation decreases with the energy of the 

 sound wave, and, therefore, with the intensity, it would seem that our photo- 

 graphs should show two results: the velocity of a wavelet should be less than 

 that of the main wave, and the wave front of a wavelet should not be cir- 

 cular, because the energy at a point in the wavelet falls off rapidly as the dis- 

 tance from the pole of the wave increases. One need not cite Stokes's law, 

 for the pictures clearly indicate a variation in intensity along the front of the 

 wavelets. Yet, taking into consideration the breadth of the apertures the 

 wavelets are circular, showing that the velocity of the pole of the wave is not 

 greater than the velocity tangent to the grating surface. Nor does the breadth 

 of the aperture, and, therefore, the energy passing through, appear to make 

 any difference in the velocity. It will be noted that the photographs show 

 apertures of four different sizes. 



The photographs show that the main wave and the poles of all the wave- 

 lets are tangent to one another, and since the wavelets are circular, that the 

 velocity of the attenuated wavelet propagated tangent to the grating surface 

 is not less than the velocity of the main wave of much greater intensity. 



Physics Laboratory, Indiana University, December, 1915. 



