44 



J. HOPKINSON" — HEETFOKDSHIEE EAINFALL, 



mean rainfall and percolation in so many of the drier years as have 

 together a mean rainfall about equal to the county mean. Out 

 of the twenty-one years 1 find that the ten driest years have 

 a mean rainfall of 25*87 ins., and a mean percolation of 10"28ius., 

 and that the eleven driest years have a mean rainfall of 2 6' 2 3 ins , 

 and a mean percolation of 10'44ins. Averaging these results, the 

 mean rainfall is 26-05 ins., almost the same as our county mean, 

 and the mean percolation 10-36 ins. Proceeding in the same way 

 with the summer and winter rainfall of these years, the general 

 results are as follows ; — 



If the accuracy of these conclusions, and also of the experiments 

 on -which they are founded, be admitted, it follows that with the 

 mean county rainfall the percolation at Rothamsted would exceed 

 the mean of that at Nash Mills and Lea Bridge by 1-84 in., or 

 13 per cent, of the rainfall in the summer, 1-70 in., or 14 per cent, 

 of the rainfall in the winter, and 3-54 ins., or 135^ per cent, of the 

 rainfall in the year, the excess representing the decreased amount 

 of evaporation due to the absence of vegetation at Rothamsted. 



In the course of a discussion at a meeting of the Institution of 

 Civil Engineers in March, 1891, Dr. J. H. Gilbert [now Sir Henry 

 Gilbert] said that he and Sir John Lawes considered that the 

 deduction from the amount of percolation through soil destitute 

 of vegetation which should be made for the presence of vegetation 

 ■would vary from 2 to 7 inches or more per annum from downs 

 or waste lands to heavily cropped land; and that "taking the 

 average of a large area around London, partly covered with vege- 

 tation and partly bare, over a number of seasons, they thought 

 that between 3 and 4 inches should be deducted," a conclusion 

 which is confirmed, or at least very strongly supported, by the 

 above investigation. 



In order to test the difference in percolation and evaporation in 

 wet and dry years, where there is vegetation, I have selected from 

 the forty- two years' record of percolation at Nash Mills, the six 

 wettest summers, winters, and years, and the six driest summers, 

 winters, and years, and have computed the averages of rainfall, 

 percolation, and evaporation, and the ratio of each to the mean 

 rainfall, percolation, and evaporation of the whole period. 



