PERCOLATION, AND EVAPORATION. 



49 



Lea Eridgo, and Eotliamstod, so far as either series of experiments 

 is available for the half-century ending 31st March, 1892, the 

 follo\ving' will be found to be the percentage to the mean in 

 each year : — 



The smallest ratio of percolation in any year was 21 per cent, 

 in 1849-50, the next, 23 per cent, in 1858-59; the smallest in 

 any two years in succession was 412- per cent, in 1873-74 and 

 1874-75, the next, 44^ per cent, in 1853-54 and 1854-55; the 

 smallest in any three years in succession was 60 per cent, in 

 1853-54 to 1855-56, the next, 64 per cent, in 1873-74 to 1875-76. 

 We may, therefore, expect to have three years in succession with 

 an average percolation of about 4 inches of rain, being 60 per cent, 

 of our county average of 6*76 inches. We may also have three 

 successive years with an average percolation of 9|- inches of rain, 

 being 44 per cent, above the average, as in 1878-79 to 1880-81. 



If, instead of taking the ratio to the mean of the three series 

 of experiments, we take the actual amount of percolation through 

 soil at Nash Mills and Lea Bridge, it will be found that for the 

 three years 1854-55 to 1856-57, the average annual percolation 

 at Lea Bridge was 3-63 ins., and that for the three years 1869-70 

 to 1871-72 at IS^ash Mills it was 3-75 ins. Even if we take such 

 a long period as six years, we get a mean annual percolation of 

 less than 4 J inches, for during the six years 1853-54 to 1858-59, 

 at Lea Bridge, the average was 4-33 ins., and during the six years 

 1866-67 to 1871-72, at Nash Mills, it was 4-30 ins. 



In three successive years, therefore, the average annual supply 

 from the catchment-basins of the Colne and Lea may be from about 

 35 to 40 per cent, less than the estimated average for half a centuiy, 

 and in six successive years it may be about 25 per cent, less than 

 this average. 



It may be noticed that the ratio of percolation during the 

 second, fourth, and fifth decade of the half-century corresponds 

 very closely with the ratio of winter rainfall during these decades, 



VOL. IX. 



-PART II. 



