TIDES, WAVES 



109 



staircase, the whole system turning in clock- 

 wise direction from top to bottom. 



Seiches — Long-period oscillations in the 

 ocean have now been recognised, but have 

 not yet been carefully observed. As in 

 lakes they may be of two kinds : the ordinary 

 seiche and the temperature seiche : — 



(1) A seiche is a standing oscillation of 



Fig. 5. — Diagrams of uninodai (upper) and binodal (lower) seiches 

 (the arrows indicate the direction and amplitude of the movements 

 of the water-particles, and the dotted and continuous lines indicate 

 the position of the surface at opposite phases). 



the whole body of water in a defined basin. 

 The simplest form of oscillation is where there 

 is no vertical motion at a central point in a 

 lake, and where the vertical motions at the 

 two ends of the lake are of opposite phase. 

 The point where there is no vertical motion is 

 called a node. The surface of the lake at the 

 extremes of the oscillation may be represented 

 by the plain and dotted lines in Fig. 5. We 

 may have two nodes, in which case the phase 

 of the motions at the two ends is the same. 



