FIG 2 



three-dimensional motion. Thus we have here, I believe for the first time, a genuine 

 physical interpretation of hydrodynamic mass as a mass of liquid entrained by the body. 



The corresponding problem of interpretation with a free surface still awaits 

 investigation, although some computations of the hydrodynamic mass itself have been 

 made by Bloh [13] for spheres and ellipsoids half immersed and totally immersed. 



The problem of rotation also offers opportunities for investigation. If we con- 

 sider a rotating plate, in two-dimensional motion, it is found that two regions A and 

 B exist in which liquid is trapped and moves round with the plate, not as a rigid body 

 but consistently with irrotational flow, Fig. 3. This points to a method of interpreting 

 hydrodynamic mass due to rotation. 



Problems concerning free streamlines have been intensively studied in recent 

 years in relation to the cavities formed behind bodies moving at high speed and the 

 water entry of missiles. 



The two-dimensional problem has been greatly simplified by Max Shiftman's 

 [14] method of reflection across free streamlines whereby an image of the actual flow 



