BAROMETRIC PRESSURES ON THE GREAT LAKES 9 



(a) The problem of regulating the elevations of the water surface of each 

 of the Great Lakes and the rates of flow through the connecting streams, 

 so as to secure the greatest aggregate benefits to navigation, power, develop- 

 ment, and sanitation. 



(6) The problem of determining the laws of evaporation from large free- 

 water surfaces such as the surface of the Great Lakes. 



(c) The problem of correcting the observed elevations of the water- 

 surface at a tide-gage in such a manner as to remove the disturbances due 

 to winds and fluctuating barometric pressures and thereby to secure a more 

 accurate determination of mean sea-level than could otherwise be obtained 

 from said observations. 



(d) The problem of determining the direction and rate of the tilting, 

 which is believed to be in progress, of the land underlying and immediately 

 surrounding the Great Lakes. 



ORDER OF PRESENTATION. 



The order of presentation in this publication is briefly indicated in the 

 following paragraphs : 



(1) The manner in which the least-square solutions which serve to deter- 

 mine barometric effects were set up is first given, including the theoretical 

 basis of the observation equations. The principal facts in regard to the five 

 final barometric solutions are given, including the values computed from 

 them. The manner of using these values to compute hourly and daily 

 barometric effects is set forth. 



(2) Similarly, the manner of setting up the least-square solutions which 

 serve to determine the wind effects is given, together with the principal facts 

 in regard to the four final wind solutions and the values computed from them. 

 The formula, constants, and method for computing hourly wind effects are 

 set forth. The method used in computing daily wind effects is shown. 



(3) The accuracy of the computed barometric effects is discussed, using 

 both the internal evidence of the computations and external evidence. 



(4) Similarly, the accuracy of the computed wind effects is discussed. 



(5) The evidence as to the over-all accuracy attained in the attempt to 

 secure elevations of the mean surface of a whole lake by applying corrections 

 for wind effects and barometric effects at the gage stations is discussed. 



(6) The evidence obtained in regard to seiches, free oscillations under the 

 influence of inertia, in Lake Erie and Lake Michigan-Huron, is set forth and 

 discussed. 



(7) Certain generalizations are made as to wind effects, barometric 

 effects, and seiches. These generalizations are intended to help one, during 

 a first reconnaissance of a problem connected with any lake (or other large 

 body of water), to form a first approximate estimate as to the probable 

 magnitude and character of the wind effects, barometric effects, and 

 seiches on that lake and their probable bearing on the problem to be at- 

 tacked. 



