130 EFFECTS OF WINDS AND OF 



(6) in the probable errors shown in table No. 28, page 110, and (c) in the 

 discussion of these probable errors on pages 111-113. It appears that from 

 observations at Mackinaw alone the mean elevation of the whole of Lake 

 Michigan-Huron on any day may possibly be determined with a probable 

 error less than 0.010 foot. From the three stations Milwaukee, Harbor 

 Beach, and Mackinaw together, this probable error may be reduced still 

 more. It appears that the change in elevation of the mean surface of the 

 whole of Lake Michigan-Huron in one day may possibly be determined 

 with a probable error less than 0.007 foot an accuracy hitherto un- 

 attainable. 



The evaporation from Lake Michigan-Huron in a single day, measured in 

 depth of water taken off the whole surface of the lake, probably varies 

 between limits which are approximately 0.000 foot and 0.021 foot. The new 

 accuracy now attainable in determining the fluctuation in the elevation of 

 the mean surface of the lake from day to day evidently makes the proposed 

 method of measuring the small variations in evaporation much more power- 

 ful than it otherwise would be. Heretofore the method seemed to be barely 

 within the range of possibility. Now it seems to be certain that the method 

 will succeed. It certainly will succeed so far as success hinges on the 

 evaluation from day to day of the total content of the lake. 



APPLICATION TO REGULATION OF THE GREAT LAKES. 



The vision is gradually taking form that the elevation of the water surface 

 of each of the Great Lakes must be regulated by movable dams, or the 

 equivalent, for the benefit of navigation, for the benefit to be derived in 

 connection with the development of power in hydro-electric stations on the 

 connecting streams, and to provide for the increased use of the water from 

 Lake Michigan-Huron for sanitary purposes at Chicago. It is now clear 

 that the return to the people of the United States from such regulation 

 would be many times its cost. 



When that regulation becomes a reality, as it certainly will in due time, 

 it will then become important to detect each fluctuation in elevation of each 

 lake surface as soon as possible after it occurs and as accurately as possible. 

 Each fluctuation must be detected soon after it occurs in order that the 

 desirable change in regulation may be made. It must be detected accurately, 

 because the total range within which the regulation must operate is small, 

 and the means of regulation, the movable dams or the equivalent, will 

 produce but very slow alterations in lake elevations. The quantities to be 

 dealt with, for the desired valuable regulation, are hundredths of feet of ele- 

 vation rather than tenths. The promptness and accuracy with which all 

 fluctuations may be recognized is greatly increased by the outcome of this 

 investigation. This is indicated in part by the comments on accuracy made 

 in connection with the proposed application to the study of evaporation. 

 It may be emphasized from another point of view by noting that there are 



