46 



J. HOPKrNSO:^ — HEETFOEDSHIEE EAINFALL, 



between tbe rainfall and percolation proceeds at an uniform rate, 

 for every inch of rainfall there Avill be a difference of 0-48 inch in 

 percolation, and on this supposition the following will represent 

 approximately the mean annual percolation and evaporation in the 

 two divisions of our county, the mean for the county being added 

 for comparison : — 



The River Colne leaves our county at Harefield, and at about 

 that point, or a little higher, it has been proposed to withdraw 

 from the Chalk a large quantity of water for the supply of Loudon. 

 The River Lea is regularly gauged at Feilde's Weir, and above 

 that point is included all the area of chalk within its basin. It 

 will be convenient to estimate the areas of these two catchment- 

 basins, irrespective of the boundaries of Hertfordshire, above these 

 points. The method I have pursued is that of drawing the areas 

 on stout paper, cutting them out, and weighing them. The result 

 1 have arrived at is that the area of the Colne above Harefield 

 is approximately 235 square miles, and that the area of the Lea 

 above Feilde's Weir is approximately 410 square miles. In the 

 catchment-basin of the Colne about 195 square miles are on 

 the Chalk, and about 40 square miles are on Tertiaries (Reading 

 Reds and London Clay). In the catchment-basin of the Lea about 

 295 square miles are on the Chalk, and about 115 square miles are 

 on Tertiaries (Reading Beds and London Clay). The following 

 appear to be approximately the areas drained by the tributcary 

 streams, and by the main streams outside the basins of their 

 tributaries : — 



