206 
Mr Oookson, The Oscillations of a Fluid 
Fig. 1. Annular Trough. 
w = l: gravest mode. 
The two longest periods for the symmetrical class are therefore 
3 S, 35 and 1»*83, 
and for the unsymmetrical class 
6 S, 97 and 2H0. 
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 
