1905 - 6 .] The Hydroclynamical Theory of Seiches. 143 
If the end of the trough is carefully raised and then lowered 
to its original position, the water moves in such a way as if the 
surface plane slid through a tranverse line across the centre. A 
seiche, or oscillatory motion of the entire body of water, is thus 
generated. The water rises and falls everywhere except at the 
central transverse line, which, from analogy with vibrating strings 
and air-columns in organ-pipes, is called a nodal-line. As in the 
case of vibrating strings, a node is a position of minimum transla- 
tional motion but of maximum rotational motion, so in a seiche 
there is minimum vertical motion hut maximum horizontal motion 
at a node. The liquid particles move backwards and forwards 
through a comparatively wide range across the central transverse 
line, in the direction of the seiche motion. 
In this present example of a uninodal seiche there is a nodal 
/7a. /• 
line in the middle of the vessel. The ends are positions of 
maximum vertical displacement, i.e. loops or ventral segments, and 
it is evident that exactly as in the case of open organ-pipes, the 
ends of a lake will always he ventral segments. If we compare the 
oscillating liquid with a stretched string, however, we find the 
circumstances reversed, since in the latter case the ends are 
always nodes ; and when the fundamental vibration is in existence, 
the centre is a loop. 
Taking advantage of the fact that at a node in a seiche, 
horizontal motion is a maximum, horizontal motion may he 
excited at other parts of the water-surface with the object of 
producing a seiche of a different period. Evidently, in the case 
referred to above, any such seiche will always have its nodes 
symmetrically situated about the central transverse axis. 
The problem, then, was to determine for any particular curve 
