356 MODIFICATION OF GREAT LAKES BY EARTH MOVEMENT 



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The main principles of such a system are, however, simple, and may 

 readily be stated. The most important is that the direct measurement 

 of the heights of individual points should not 

 be attempted, but comparison should always be 

 made between two points, their relative height 

 being measured by means of the water surface 

 used as a leveling instrument. 



In the diagram, figure G, A C B is the profile 

 of a lake basin, A and B are fixed objects on 

 opposite shores, and we will suppose 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 be accurately measured by the 

 surveyor's level, and the height of B above the 

 water at X' be similarly measured, then the dif- 

 ference between these two measurements gives 

 the difference in height between A and B. After 

 an interval of some years or decades the work is 

 repeated. The water surface then has some dif- 

 ferent position, Y Y', and the heights measured 

 are of A above Y and of B above Y 7 . The differ- 

 ence between the two heights gives, again, the 

 relative height of A and B; and if earth move- 

 ment has tilted the basin toward A or B, the 

 change in their relative height may be shown 

 by the difference in the two results of measure- 

 ment. 



As the water is in fact not still, but in contin- 

 ual motion, the mere running of lines of level 

 from A and B to the water does not suffice, and 

 it is necessary to determine from observations 

 on the oscillating water surface what would be 

 its position if still. Such observations are made 

 by means of gages. These are of various forms, 

 but each consists essentially of a fixed point, or 

 zero, close by the water, and a graduated scale, 

 by means of which the vertical distance of the 

 water surface from the zero is measured. 



Changes in the volume of the lake influence 

 all parts of its surface equally and at the same 

 time. To eliminate their effects from the meas- 

 urements it is only necessary that the gage 

 observations at the two stations be simultaneous. The effects of wind 

 waves can be prevented by breakwaters. Disturbances due to cur- 

 rents propelled by strong winds can be avoided by choosing times 



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