Composition of Waters of Land-Draiiiage and of Rain. 127 



Table I. — Pvain-fall, Evaporation, and Filtration in each of 8 years (Parkes). 



Yc'iirs. 



Evaporation. Rain per Acre. 



1836 

 1837 

 1838 

 1839 

 1840 

 1841 

 1842 

 1843 



IVr ci'ut. 



Tons. 

 3139 

 2137 

 2342 

 316S 

 2171 

 3251 

 2676 

 2680 



Mean 



20-61 



42-4 



57-6 



2095 



It will be seen that of the whole water falling in rain, 42'4 

 per cent., or, in round numbers, 2-5ths passes through to the 

 drains. This number is the average of 8 years, which vary 

 within rather wide limits, being in one case as low as 33, in 

 another as high as 57. 



At first sight it does not appear why this should be the case, 

 but a little consideration makes it evident. In heavy rains, 

 and when the soil is already saturated with moisture, all the 

 water which fulls, if it does not flow over the surface, will find 

 its way into the drains, there being, at such times, little oppor- 

 tunity for evaporation. When, however, the rains are frequent, 

 and comparatively light, with intervals of warm sunshine, the 

 quantity of water which would suffice to saturate the land is 

 carried off by evaporation before the next shower, and none at all 

 reaches the drains. Thus, as each year has its own peculiar distribu- 

 tion of heat and sunshine, so it will be with regard to percolation. 



This is well shown by the following Table, taken from Mr. 

 Parkes' s Essay be; fore quoted. 



Table II. — !Mean llain-fall, Evaporation, and I'iltratiou and Evaporation in 

 each month (8 years). — (Parkes.) 



