128 EFFECTS OF WINDS AND OF 



This last statement is cautiously made, because the limits of this investi- 

 gation have made it impossible to study the seiches in Lake Michigan-Huron 

 with the same care that was given to Lake Erie seiches. 



GENERALIZATION AS TO PREVAILING CONDITIONS ON THE 



GREAT LAKES. 



Apparently, if one is to appreciate fully the meaning of the continuous 

 record from a gage recording elevations of the water surface at any place 

 in the Great Lakes, he must have the general conception stated in the 

 following long paragraph, and must appreciate that the gage is in one of the 

 several seiche areas of that lake. 



At the beginning of any hour on any one of the Great Lakes there are one 

 or more seiches in progress in each of the several seiche areas of that lake, 

 and all oscillations (seiches) are taking place about a certain equilibrium 

 surface which is not horizontal but which has a certain position and slopes 

 fixed by the winds and barometric gradients operating on the lake at that 

 instant. During that hour changes occur in the direction and velocity of 

 the winds and in the barometric gradients. These changes produce cor- 

 responding changes in the position and slopes of the equilibrium surface 

 about which all oscillations due to inertia .(seiches) take place. The changes 

 in the equilibrium surface produce new forced oscillations in every seiche 

 area. The forced oscillations develop new seiches with the natural period 

 of each seiche area, which new seiches, being superposed on the seiches 

 already there, modify them, sometimes mainly in phase, sometimes to par- 

 tially stop them, but as a rule to increase their range. From each seiche 

 area forced oscillations are sent continuously into adjacent seiche areas, 

 tending to develop in these areas new seiches, which are superposed on the 

 old, as indicated in the preceding sentence. In general, whenever a change 

 occurs in the equilibrium surface when there is a new set of wind and 

 barometric impulses over a lake there will be a considerably larger seiche 

 produced in some one of the various seiche areas than in the others. This 

 may come about because (a) the new impulse happens to be timed to cor- 

 respond in the period of its change to the natural period of that seiche area, 

 or (6) the new impulse may happen to reinforce a particular wave of a seiche 

 in progress in that area and so to decidedly increase its range, or (c) the 

 forced oscillations sent in from the adjacent areas may happen to coincide 

 with each other and with a seiche in progress and so to decidedly increase 

 its range. The exchange of energy between different seiche areas will tend 

 to produce a very much greater apparent damping of this particular large 

 seiche than of the other seiches in progress and tend to bring about a nor- 

 mal distribution of seiche range to the different areas. As time progresses 

 there will be continual modification of seiches by new wind and baromet- 

 ric impulses, usually tending toward an increase in range of the seiches. 

 The true damping by friction, on the other hand, tends continually to 

 reduce the range of the seiches. The net effect of these two counter in- 



