Fluctuations of Ground Water. 



271 



recording the changes is also represented in the same figure. 

 The changes in the rate of discharge from the spring, which 

 are associated with changes in the pressure of the atmos- 

 phere, amount to as much as 8 per cent, of the total nor- 

 mal now. 



FIG. 101. Showing the barometric changes in the rate of seepage Into 

 tile drains. Lower curve, drain. 



337. Barometric Changes in the Discharge of Water from 

 Tile Drains. Using the same means for recording the rate 

 of discharge of water from tile drains it was shown that 

 changes occur here which are entirely analogous to those re- 

 corded from the spring, and Fig. 101 shows a week's record 

 of the changes both in atmospheric pressure and in the rate 

 of discharge from a system of tile drains. In this system 

 changes in the rate of flow as great as 15 per cent, of the 

 mean have been recorded, entirely independent of rainfall 

 and apparently due solely to changes in atmospheric pres- 

 sure. 



338. Diurnal Changes in the 

 Rate of Discharge from Tile 

 Drains. Besides the changes as- 

 sociated with changes of baro- 

 metric pressure referred to in 

 (337) there may also be diurnal 

 changes in the rate of discharge 

 which are due to the diurnal 

 changes which take place in the 

 soil air above the ground water. 

 As the air expands under the heat 

 absorbed it presses downward 

 "P the water, causing it to drain 

 away faster, which makes it 



temperature. 



