294 Ground Water, Wells and Farm Drainage. 



the gradient of the ground water surface as it rises back 

 from the lake. In well 29, 150 feet from the lake, the 

 water stood on a certain date 7.214 feet above the level of 

 the water in the lake, thus showing a mean rise or gradient 

 of 1 foot in 20.79 feet. In the same locality, but outside the 

 area represented by the map, a well stands 1,250 feet back 

 from the lake and in this the water has a level 52 feet above 

 the lake or drainage outlet, which gives a mean gradient 

 or rise of 1 foot in 24.4. 



In Fig. 112 is represented an apparatus for demonstrat- 

 ing the position of the, surface of the ground water and the 

 difference of pressure at different distances away from and 

 above a drain tile, and Fig. 113 shows the observed differ- 

 ences of pressure under two sets of conditions. 



In^Fig. 114 is also represented the general slope of the 

 ground water surface and the modification of it by a line 



FIG. 113. Showing the changes in pressure at different distances from 

 the tile drain when the water is flowing. The lower curve shows the 

 pressure when the flow is from the stopcock a, Fig. 112, and the 

 upper set of curves represent changes which occurred during a period 

 of flow from the stopcock c. Fig. 112. 



of infiltration pipes, which is in effect a tile drain. The 

 rate of rise of the ground water back from a tile drain is 

 one of the chief factors in determining the distance apart 

 the drains should be placed in the field. 



