PHYSICS. 409 



the velocity of a current with the slope of the water surface and the 

 dimensions of the channel in which the current is flowing. The values 

 of the exponent (about 2.4) and of C are both determined by observa- 

 tions in such wise that they are constants of general application any- 

 where in the world on any free water surface, on any lake, on the open 

 ocean, or on any gulf, bay, or river. The outcome of this part of the 

 investigation is a method of computation and the necessary funda- 

 mental constants for use in it, which will enable one to compute the 

 disturbance of elevation produced at any point on a free water surface, 

 provided one has a chart or charts showing the horizontal projection 

 of the whole of that free water surface and the depth of the water at 

 ever>' point. This method and these fundamental constants were 

 derived, originally, for use in the problem of determining the laws of 

 evaporation from the Great Lakes. The importance of the method 

 and constants will be much better appreciated, however, if one notes 

 some of their many other possible apphcations. 



WTien the dream of regulating the elevation of the Great Lakes by 

 movable dams at certain of the outlets (St. Lawrence, Niagara, and 

 St. Mary's Rivers) becomes a reaUty, as it certainly will in due time, it 

 will then be important to watch the fluctuations of elevation of each 

 lake with the highest attainable accuracy, in order to secure the best 

 possible control. It will be important to correct the observed elevation 

 at each recording gage, from day to day, for the effects of winds, in order 

 to obtain true values of the mean lake elevation. Such necessary 

 corrections are as great as 1 foot on rare occasions at Buffalo. 



From the evidence derived from gages operated over long periods, 

 or during widely separated years, at various points of Lake IMichigan- 

 Huron, the eminent geologist G. K. Gilbert determined that the whole 

 region covered by this lake is slowly tilting to the southwestward, and 

 secured a determination of the rate of tilting. This determination 

 would obviously be strengthened if the corrections for wind effects at 

 the gages were applied by using the method and constants now avail- 

 able. 



Elevations determined by precise leveling are referred to mean sea- 

 level by means of observations taken at tide gages. The mean sea- 

 level as fixed by the observations at a given gage is in error by an 

 amount dependent on the configuration of the shores and the bottom 

 in the surrounding region and upon the prevailing winds. The method 

 and constants now available will enable one to compute the necessary 

 correction to be applied to the observed mean sea-level to eliminate the 

 wind effect and so to obtain the true mean sea-level. Such a necessary 

 correction may be small at certain gages. It is important to prove it 

 to be small in such cases. The corrections are probably large enough 

 at some gages to predominate over the accumulated errors in the pre- 

 cise leveling for hundreds of miles from the gages. 



