126 EFFECTS OF WINDS AND OF 



the next seiche area, where it becomes a forced oscillation for the time being 

 and tends to set up a new seiche of the period natural to that area or to 

 modify such a seiche which may already be there. 



In each seiche area the seiche period is dependent upon the depth of the 

 water and the principal dimensions of the area. 



The apparent damping of seiches is dependent upon two things: (1) the 

 true damping, due to friction, and (2) the transmission from the seiche area 

 in question into other adjacent seiche areas. 



The true damping, due to friction, is of course dependent primarily upon 

 the depth of the water. In deep water the friction is smaller in proportion 

 to the total energy involved in the seiche, and hence the damping is small. 



The apparent damping by transmission from the seiche area under con- 

 sideration into adjacent seiche areas tends, of course, to be greater (1) if 

 the seiche area in question is bounded largely by other seiche areas rather 

 than by shores, (2) if the adjacent seiche, areas are large in comparison with 

 the seiche area under consideration, and (3) if the seiches in progress in the 

 adjacent areas are small relatively to those in the area under consideration. 

 If an adjacent seiche area has in it at a given time a large seiche, there is a 

 tendency for the net transmission of energy across the boundary to be such 

 as to increase the seiche in the area under consideration. In general, there 

 will be a transmission in progress in both directions across the boundary, one 

 tending to dissipate the seiche in the area under consideration and the other 

 to build it up at the expense of the adjacent seiche. In general, if an un- 

 usually large seiche is started in one seiche area of several in a lake, the ap- 

 parent damping in that area will be very rapid at first, until enough energy 

 has been transmitted across to adjacent seiche areas to build sufficiently 

 large seiches there to make the net exchange of energy across the seiche area 

 boundaries nearly zero. Thereafter the apparent damping will be mainly 

 true damping, due to friction, and therefore relatively slow. 



From the foregoing it is clear that if a lake has a compact area with 

 regular shores, has nearly uniform depths, and has these matters so related 

 that the whole lake is composed of only one or a few seiche areas, large and 

 persistent seiches will occur in that lake. On the other hand, if a lake has 

 a very irregular shore and a straggling area, if its depth varies greatly in 

 different parts, and if these matters are so related that the whole lake is 

 broken up into many seiche areas none of which predominates in size over the 

 others, the seiches in that lake will in general be small, and each new un- 

 usually large seiche will be rather promptly reduced to moderate size by 

 transfer of energy across seiche-area boundaries. 



Consider the contrast between Lake Erie and Lake Michigan-Huron 

 in the characteristics indicated in the preceding paragraph. Use plates 2, 

 5, and 6 in visualizing the contrast. 



As a whole, Lake Erie is comparatively compact, with regular shores. 

 Over more than one-half of its area the bottom is almost as flat as a floor, 

 with depths varying only from 11 to 14 fathoms for 120 miles along its axis. 



