PROM THE CHALK OF HERTFORDSHIRE. 
143 
with a humid atmosphere, will nearly all percolate beyond the 
reach of evaporation. At Nash Mills during the winter of 1858-59, 
9-64 inches of rain fell, of which only (H)9 inch, or less than 1 per 
cent., percolated through three feet of soil, while during the winter 
of 1879-80 only 5*84 inches of rain fell, of which 2*79 inches, or 
48 per cent., percolated. Again, very much more rain percolates 
during the six winter months than during the six summer months, 
the difference being chiefly due to the much larger amount absorbed 
by vegetation in summer than in winter. With an average annual 
rainfall at Nash Mills of about 28 inches, the percolation in winter 
averages 46 per cent, of the rainfall, and in summer about 4 per cent. 
With this rainfall and percentage of percolation, if 10 inches of 
rain fell in the winter and 18 in the summer, the percolation would 
he 5*32 inches, while if 18 inches fell in the winter and 10 in the 
summer, it would be 8-68 inches. The difference would really be 
much greater than this, for in a wet winter the percentage of 
percolation would on the average he much greater than in a dry 
one, and in a dry summer it would probably be nil. 
If we can rely upon the experiments at Strathfield Turgiss, 
Nash Mills, and Lea Bridge, of which I have given some of the 
chief results, as affording a fair average, it follows that out of a 
mean rainfall of 25j inches, the evaporation from a water-surface 
is 18 inches, or 70 per cent, of the rainfall; and out of a mean 
rainfall of 27£ inches there is evaporated from the surface of the 
ground or absorbed by vegetation, 20 inches, or 73^ per cent, of the 
rainfall, the percolation being 7\ inches, or 26^- per cent, of the 
rainfall. The evaporation from the ground thus appears to be 
little more than that from water, but this is owing to its frequent 
dryness. If the ground covered by vegetation were always moist, 
the evaporation would he doubled, but this is practically an im¬ 
possibility, as it would then greatly exceed the rainfall. It is 
probable that an appreciable portion of the water evaporated 
returns to the earth in the form of dew without being measured 
by the rain-gauge; but, as we have only to deal with the amount 
which percolates through the soil, this question need not be dis¬ 
cussed further. A small portion of the water which percolates 
into the Chalk, also, must be entirely lost to us every year, sinking 
through the comparatively retentive beds which form the base of 
this formation, and eventually arriving at the plane of temperature 
of the boiling-point of water, which, as already mentioned, is con¬ 
stantly approaching the centre of the earth. 
Assuming, as we are justified in doing, that a nearly constant 
quantity of water is evaporated from the surface of the ground or 
absorbed by vegetation, and taking the mean rainfall for Hertford¬ 
shire as 261 inches, being that of the last half-century, we may 
infer that 61 inches percolates through 3 feet of soil. It is then 
usually assumed to be out of the reach of evaporation and vegeta¬ 
tion, but this assumption is not fully warranted. Professor Ansted 
has expressed his opinion that there is no practical limit to the 
capillary action of chalk, and he has made experiments which have 
