MODIFICATION OF THE GREAT LAKES 



241 



SUPERIOR 



MICHIGAN' 

 HURON 



ERIE 



ONTARIO 



JAN 



FEB 



MAR 



-y 



/ 



APL 



/ 



MAY 



^ 



JUNEJULY 



AUG. SEPT. 



\ 



'\ 



^V 



\ 



OCT. 



^. 



NOV. 



V 



DLC. 



\ 



FIG. 5— OSCILL.\TIONS OF THE SURFACE OF LAKE MICHIGAN, DUE TO CHANCES IN 

 THE VOLUME OF THE LAKE 



Compiled under the direction of the Chief of Kiigineer.s, U. S. A., from gage read- 

 ings at Milwaukee, Wisconsin, from August, 1S59, to June, 1897. Each horizontal 

 space represents a calendar year ; each vertical space one foot 



In the diagram, Fig. 6, A C B is the profile of a lake ba.sin, 

 A and B are fixt objects on opposite shores, and we will sup- 

 pose the water surface to have the position X X'. Assuming 

 the water in equilibrium, all parts of this surface have the same 

 height. If the height of A above the water at X l)e accurately 

 measured by the- surveyor's level, and the height of B above the 

 water at X' be similarly measured, then the difTerence between 

 the.>-;e two measurements gives the ditVerence in height between 

 A and B. After an interval of some years or decades the work 



^77777777777777777777777777/ 



FIG. 6 -DIAGRAM ILLUSTRATING THE METHOD OF USING A LAKE SURFACE FOR THK 

 DISCOVERY AND MEASURE.MENT OF EARTH MOVEMENTS 



