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 (dtyjdn) 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 

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



