110 THE OCEAN 



but opposite to the phase of the motion at the 

 centre of the lake, and so forth for any 

 number of nodes. Some basins are better 

 tuned for seiches than others, and in one lake 

 the commonest oscillation may have two or 

 more nodes, in another the commonest os- 

 cillation may be of the fundamental type with 

 one node. The period of the oscillation 

 depends on the shape of the basin. Roughly 

 speaking, it varies directly as the length of 

 the basin, and inversely as the square root 

 of the depth. One effect of a seiche is to 

 cause a to-and-fro motion of all the water 

 particles in the basin. At a node this to-and- 

 fro motion is purely horizontal, and at the 

 ends it is mainly vertical. Where the wave- 

 length of the oscillation is large compared 

 with the depth of the basin, as is the case in 

 all lakes of any size, then the component of 

 horizontal motion at any point at the bottom 

 of the lake is the same as at the surface verti- 

 cally above. At every part of the bottom 

 the motion is tangential to the slope of the 

 bottom. We can, therefore, by observing 

 the current produced at the surface by a seiche, 

 find out the current which will be caused at 

 the bottom of the lake. The velocity of these 

 currents may be calculated when the shape 

 of the lake is known. 



These oscillations in fresh-water lakes have 



