UNDERGROUND WATER 129 



when the water level is near the surface, a great number of 

 lesser variations. King, who has made an exhaustive study 

 of the movements of water in shallow wells by means of a 

 very exact self-registering apparatus ( Wisconsin gth Rep., 129), 

 tells us that the water level in such wells is in summer time 

 never still, but always moving in one direction or the other. 

 These movements in the water level are equally seen in the 

 varying discharge of water by springs or by drain-pipes. 

 They appear to be mainly occasioned by the expansion or 

 contraction of the air imprisoned within the soil between the 

 surface and the water-level. To a less extent, they are due 

 to the alteration in the viscosity and surface tension of the 

 water, brought about by changes in temperature. A rise in 

 temperature starts a fresh percolation by diminishing the 

 viscosity and surface tension of the water coating the soil 

 particles, while a fall in temperature causes water to rise 

 from the saturated soil into the unsaturated. The action due 

 to the influence of temperature on the physical properties of 

 water is thus in precisely the same direction as that due to 

 the expansion or contraction of included air. 



King found the discharge from a spring to be 8 per cent, 

 greater with a falling than with a rising barometer, and the 

 discharge from a drain-pipe diminished 15 per cent, for a rise 

 of 01 inch in the barometer. The height of water recorded 

 for the wells showed a regular daily fluctuation, the fall in 

 level during the day being more or less made up by a rise 

 in level during the night. The drain-pipes exhibited the same 

 changes, the discharge reaching its maximum about 7 a.m. 

 This diurnal variation is due to alterations in temperature ; 

 but the effect of the maximum daily temperature was not felt 

 till early in the next morning, owing to the slow progress of 



K 



