206 



Mr Cookson, The Oscillations of a Fluid 



Period for 



= 5 '8 x -£r where h 

 = depth in inches. 



Form of Free Surface. 



Section at CC 



Section at BE . 



Fig. 1. Annular Trough. 

 n = l: gravest mode. 



The two longest periods for the symmetrical class are therefore 

 3 s -35 and l s> 83, 



and for the unsymmetrical class 



6 s -97 and 2 S> 40. 



The oscillations in an annular trough are then much quicker 

 than those in a circular trough. Returning to the case of the 

 annular basin, it will be noticed that an additional nodal circle is 

 introduced every time we proceed to a higher harmonic and that 

 the nodal diameters are n in number. The contour lines between 

 two nodal circles must be closed curves of the shape of those in 

 Fig. 2, and the curves between a nodal circle and the boundary 



