ee HYDRODYNAMICAL RELATIONS 



gible viscosity effects. In practical cases of a source very near a free 

 surface the incident wave is spherical and of finite duration, and the 

 nature of an explosive source may furthermore be significantly modified 

 by venting of the explosion products. Very little experimental evi- 

 dence is available on these questions and the whole problem of obhque 

 incidence at a free surface is one on which much further work is needed. 



