PERCOLATION, AND EVAPOEATION. 



41 



Ouly a small area in the county is likely to have sucli a porous 

 subsoil as is here represented by a gauge containing broken pieces 

 of chalk. 



The results given in Table XI 1 1 are of the greatest value in 

 this inquiry, for here ■^e have a soil in as nearly as possible the 

 usual condition of the soil over the Chalk area of Hertfordshire. 

 The table gives the percolation of rain through 3 feet of soil 

 ■svith grass growing on the surface, at Nash Mills, for the forty-two 

 years ending 31st March, 1884, and the difference from the rainfall, 

 considered as evaporation, and the following general results may 

 be deduced from it : — 



Table XIV gives the percolation of rain through 3 feet of soil 

 such as usually covers the Chalk in Hertfordshire, with grass 

 growing on the surface, at Lea Bridge, for the twenty-one years 

 ending 31st March, 1873, and the difference from the rainfall, 

 considered as evaporation, and the following general results may 

 be deduced from it : — 



That two independent series of experiments extending over 

 different periods of time, and carried on at a considerable distance 

 apart, should give a mean annual percolation differing only 2^ per 

 cent, from each other, is in itself a testimony to their general 

 accuracy. Of the two series, the experiments at Xash ]Mills seem 

 to be more likely to give accurate results than those at Lea Bridge, 

 in the fii'st place because they have extended over a much, longer 



