BAROMETRIC PRESSURES ON THE GREAT LAKES 129 



fluences on seiche range is to cause the seiche range in general to increase 

 during periods of increasing intensity of wind and barometric impulses, to 

 decrease rather rapidly during periods of decreasing intensity of wind and 

 barometric impulses, and still to decrease but slowly during periods when 

 such intensity is approximately constant. One must see all these things to 

 appreciate fully the meaning of a gage record, in addition to seeing in it the 

 fluctuations of elevation of the mean lake surface due to inflow from the 

 lake above, outflow to the lake below, run-off from the surrounding land, 

 rainfall on the lake surface, and evaporation from the lake surface. 



POSSIBLE APPLICATIONS OF RESULTS OF THIS 



INVESTIGATION. 



The outcome of this investigation is a more accurate knowledge than was 

 heretofore available of the wind effects and the barometric effects upon the 

 elevation of the water surface at any given point on the Great Lakes or on 

 any free water surface, and some increase in the available knowledge of 

 seiches and of their relation to accurate determinations of the mean elevation 

 of the water surface from a given gage record. Consider, now, very briefly, 

 some of the possible applications of the results of this investigation. 



APPLICATION TO A STUDY OF LAWS OF EVAPORATION. 



As stated on pages 1-4, the investigation here reported upon is a part of a 

 larger investigation having for its purpose determining the laws of evapo- 

 ration from large water surfaces, such as the surfaces of lakes and rivers. To 

 attain this end, it is proposed to consider each of the Great Lakes in turn as 

 an evaporation pan and to evaluate from day to day (1) the change of con- 

 tent, (2) the income, and (3) the outgo, including evaporation. To ac- 

 complish the purpose it is necessary to evaluate the change of content 

 from day to day with great accuracy. That change of content is measured 

 by the change in elevation from day to day of the mean surface of the whole 

 lake. Each recording gage measures the change in elevation of the surface 

 of the lake at the point at which the gage is located. Formerly the only 

 feasible way to fix the elevation of the mean surface of the lake was to take 

 it as equal to the mean of the elevations at the two or three points on the 

 lake at which first-class gages were operating. Now, as an outcome of this 

 investigation, the elevation of the mean surface of the whole lake may be 

 determined on any day by applying the known corrections for wind effects 

 and barometric effects at any gage to the observed elevation for that gage. 

 The proper weighted mean for the several gages may be taken and the few 

 abnormal values may be detected and rejected by a definite criterion. In 

 the place, then, of the former values of mean elevation of the whole lake 

 surface, of a certain degree of accuracy, one has now available values of a 

 much higher degree of accuracy. The increase in accuracy, which operates 

 to increase the possible accuracy of the evaporation investigation, may be 

 seen (a) by comparison of the two sets of residuals in tables Nos. 19 to 25, 



