SEICHES AND OTHER OSCILLATIONS 
39 
1. Volume Denivellations. — Caused by precipitation or evapora- 
tion. These are usually of slow variation, easily traced to their 
causes, and evidently not directly concerned with seiche phenomena. 
2. Persistent Wind Denivellations. — ^Due to the heaping up of 
the water at one end of a lake, or in shallow places, where the bottom 
friction prevents the development of an under return current to 
counteract the surface wind current. These denivellations are slow 
and irregular in their variation, and again easily traced to their 
cause. 
a. Fluctuating Wind and other Denivellations. — Due to the 
propagation of trains of waves on the surface of the lake by the 
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passage of wind-squalls, and associated with the rapid variations of 
wind pressure shown by the self-registering anemograph. Such w^ave 
trains may also be started by passing steamers or other accidental 
causes. 
4. Swell Denivellations. — After a persistent wind has blown for 
some time over a stretch of water of a certain length, a kind of 
dynamical equilibrium is established between the wind and the water, 
and the surface becomes covered with more or less regular trains of 
progressive waves. Owing to reflection at banks and retardation at 
shores and shallows, and also to unsteadiness of the wind, there is an 
interference of superposed trains, which spoils the wave pattern, and 
prevents absolutely regular periodicity in the denivellation at any 
