662 Professor George Ckrystal [May 17, 



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. 



3. Fluctuating Wind and other Denivellations. — Due to the propa- 

 gation of trains of waves on the surface of the lake by the passage of 

 wind squalls, and associated with the rapid variations of wind 

 pressure shown by the self -registering anemograph. Such wave 

 trains may also be started by passing steamers or other accidental 

 causes. 



4. Su'eU 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 

 given point. The general effect as seen at any one place is, however, 

 a fairly regular pattern of small progressive waves of apparently 

 constant length, usually diversified by wave maxima at approximately 

 equal intervals. This system persists for some time after the wind 

 falls ; and in this stage is usually spoken of as " swell." 



5. Seiche Denivellations. — These are stationary oscillations of the 

 whole lake, having nodes (i.e. places of no vertical motion), ventral 

 points (i.e. places of no horizontal motion), and periods depending 

 only on the configuration of the lake basin. 



The three last forms of denivellation — which for shortness we 

 may call solitary tcave, swell, and seiche — all make themselves felt on 

 the limnogram ; and it may be worth while to show you a few ex- 

 periments to make clear the distinction between them, which I find 

 are often imperfectly understood. 



Experiment Shoiving the Solitary Wave. — You will see that the 

 sudden sweep given to the water at the end of this shallow trough 

 has raised a hump on its surface, which travels along the trough* 

 without very rapid alteration of form, is reflected at the end, and travels 

 backwards, and so on. Observation shows that the particles of water 

 are affected by this wave only while the hump is immediately over 



* The trough used was rectangular, 8^ in. broad, 6 ft. 8 in. long, the depth 

 of the water about 3 inches. The experiment will not succeed if the depth of 

 the water be great compared with the dimensions of the wave, or if the depth 

 vary rapidly. It would fail, for example, in the parabolic troughs used in the 

 two succeeding experiments. 



