SECT. 5] 



MICKOSEISMS 



703 



transferring a mass of fluid proportional to a^ from a trough to a crest of the 

 first wave, i.e. through a distance proportional to a^. The vertical displacement 

 of the centre of gravity is, therefore, increased by an amount proportional to 

 «ia2, and, if the water is assumed to be incompressible, it can be shown that the 

 pressure variation at the sea bottom is given by 



(^2)00 = 2paia2o-2 cos lot. 



Fig. 4. Experimental verification of the pressure effect of progressive and stationary 

 waves. (After Cooper and Longuet-Higgins, 1951.) 



R. I. B. Cooper and Longuet-Higgins (1951) confirmed the theoretical work 

 by producing standing and interfering waves in a tank and measuring the 

 pressure variations. Fig. 4 shows some of their results ; (a) shows the effect of 

 a progressive wave at a depth of 8.8 cm ; (b) shows the effect at a depth of 



