128 RELATIONS OF SOIL TO WATER 



various times, according to the preceding rainfall, and may 

 occur in October, November, December, or, more rarely, in 

 January. The greatest height is reached occasionally in 

 February, but more generally in March or April, after which 

 a decline sets in. The decline is slower than the rise. We 

 have already seen that the commencement of large percolation 

 through the soil is generally in October or November, and 

 continues till the end of February ; the rise and fall of the 

 water in the wells thus follows, at a somewhat later date, the 

 order of the percolation through the soil. 



In Wisconsin, with a severe winter climate and a water 

 level only a few feet below the surface, a great rise in the well 

 water does not commence till April, after the spring thaw. 



The alterations in the height of the underground water are 

 considerable, extending to several feet ; from three to five feet 

 is the ordinary rise in the deep wells at Harpenden. It is at 

 first sight puzzling that a few inches of autumn drainage 

 should produce an increase of several feet in the height of the 

 underground water. The explanation is however simple. 

 The water in a soil, as we have already seen, merely occupies 

 the interspaces between the particles. Supposing then the 

 interspaces in a perfectly dry soil to amount to 40 per cent, 

 of its volume, it is evident that four inches of rain would 

 saturate ten inches of soil, and raise the level of the under- 

 ground water to that extent. The effect produced in this 

 direction is however really far greater, for the subsoil is not 

 dry, but already holds a considerable amount of water; a 

 small addition of drainage water thus suffices to complete the 

 saturation of a considerable depth of subsoil. 



Besides the considerable variations in the level of under- 

 ground water determined by the season of the year, there are, 



