PERCOLATION, AND EVAPORATION. 



37 



being a more episode in a period of declining rainfall, if measnred 

 by centuries instead of decades, so that we should look forward to a 

 considerably reduced rainfall in the distant future, and to a lowering 

 of the plane of permanent saturation in the Chalk of the Thames 

 Easin from natural causes. The destruction of forests and the 

 lowering of the surface of the ground by subaerial denudation must 

 tend to reduce the rainfall, while this reduction, and the loss of 

 water which must take place from its gradual percolation to a lower 

 level owing to the secular cooling of the earth, must in time lower 

 the surface and lessen the contents of the underground reservoir on 

 which we now depend for a considerable portion of our water-supply. 



As examples of old records in the Thames Valley may be 

 mentioned Sunbury in Middlesex, with a mean annual rainfall 

 of 23-72 ins. for the forty-two calendar years 1797 to 1838; 

 Sandhurst in Kent, with a mean of 23-89 ins. for the forty 

 calendar years 1820 to 1859; and Cobham in Surrey, with a 

 mean of 23-75 ins. for the thirty-five calendar years 1825 to 1859. 



At the commencement of this statement I showed that the mean 

 rainfall derived directly from the records of our ten stations, would 

 most probably be about 1 per cent, in excess of the true mean for 

 the county. If we make this deduction our mean annual rainfall will 

 be 26*07 inches. It will now be shown that this probable excess 

 of 1 per cent, is more than accounted for by the smaller of our two 

 principal river-basins, that of the Colne, having a greater rainfall 

 than the larger one, that of the Lea, and each of these unequal 

 areas being represented by the same number of rainfall stations. 



Tables II to VI give the rainfall at five stations in the catch- 

 ment-basin of the Iliver Colne. For the twenty years 1872-92, 

 they show a mean annual rainfall of 29-56 ins., the mean summer 

 rainfall being 14-69 ins., and the mean winter rainfall, 14-87 ins. 



Tables VII to XI give the rainfall at five stations in the catch- 

 ment-basin of the River Lea, (Two of these stations, Hitchin and 

 lloyston, are just over the watershed on the north.) For the 

 twenty years 1872-92, these tables show a mean annual rainfall 

 of 25-87 ins., the mean summer rainfall being 13-16 ins., and the 

 mean winter rainfall, 12*71 ins. 



To reduce these values to the mean for the half-centiiry, a 

 deduction of 5 per cent, is required (see p. 36). The corrected 

 values are as follows : — 



The area of Hertfoi'dshire is 633 square miles. It will be 

 convenient, and sufficiently precise for our purpose, to consider 



