116 MOTION OF A LIQUID IN TWO DIMENSIONS. [CHAP. IV 



In any case of transformation by orthomorphic projection, 

 whether the motion be irrotational or not, the velocity (d\jrjdn) is 

 transformed in the inverse ratio of a linear element, and therefore 

 the kinetic energies of the portions of the fluid occupying corre- 

 sponding areas are equal (provided, of course, the density and 

 the thickness be the same). In the same way the circulation 

 (fdty/dn.ds) in any circuit is unaltered by projection. 



