88 RELATIONS OF SOIL TO WATER 



at the lower end (see Table XI). A fall in temperature will 

 cause this new percolation to cease and will tend to bring- 

 about a redistribution of water towards the surface. A sudden 

 fall in the barometer may act on the air contained in the 

 column in the same manner as a rise of temperature. These 

 changes all occur in a natural soil, though the results from 

 them are not generally large ; we shall have to refer to them 

 again when we speak of the movements of underground water 

 (p. 129). 



Percolation, without a constant supply of water above, is 

 only possible if air can enter the soil to take the place of 

 the water leaving it. Percolation may be stopped for a time 

 by a slight rain falling on the surface and closing the air 

 passages. This has been observed in the case of the Rotham- 

 sted drain-gauges. 



King has made some experiments upon the influence of 

 temperature upon the rate of percolation through sand, a 

 constant water supply being provided. He found that a rise 

 of temperature from 9 to 24 C. increased the rate of per- 

 colation 50 per cent. This result he thinks may possibly 

 be above the truth. According, however, to Briggs' calcula- 

 tion, the experimental results obtained by King are precisely 

 those which might have been predicted from the known 

 diminution of the viscosity of water with a rise in temperature. 

 It is obvious therefore that soils will drain much more freely 

 in summer than in winter. 



The resistance of clay to the percolation of water is a fact 

 with which all are familiar ; the resistance is absolute when 

 the clay is in a puddled condition, that is when it has been 

 reduced to a mass of single particles. This resistance may 

 be greatly modified by the coagulation of the clay with lime 



