and the Storage of Water. 
17 
selves act as collectors of water, by condensing the vapour in the 
atmosphere, which is caught by the leaves, whence it drops on 
the soil below. The appearance of a tree on a foggy evening, 
with drops depending from the end of every twig, and the ground 
beneath saturated with water, must be familiar to all. The fact 
must be equally well known that, in a drier condition of the 
atmosphere, trees collect the water from the earth by the spong- 
ioles of their roots, and return it to the atmosphere from the 
leaves ; this process being most active in spring and summer. 
Thus the winter-rains conduce to the humidity of the atmo- 
sphere during the droughts of summer. 
The capillary action which takes place in the soil materially 
assists the process of evaporation. As the surface is dried up 
in summer the water gradually rises from the soil beneath to 
feed the roots of the vegetation and to form the dew which 
so refreshes the plants after a hot summer's day. A contrary 
process takes place in the wet season of winter, when sur- 
face-water is too plentiful, percolation being then assisted by 
the opening of the pores of the soil by the thaw which follows 
a sharp frost. A rapid thaw after a fall of snow produces a 
much greater flood than is due to the mere quantity of water 
produced by the melted snow. The break-up of a frost is often 
accompanied by a heavy fall of rain, which, added to the melted 
snow and rapid percolation of the water, causes unusually high 
floods in the rivers. It is stated that all the great floods on the 
Thames valley during the past 120 years have been due to this 
cause. 
The depth to which the rain percolates through the soil 
varies considerably at different periods of the year. Messrs. 
Lawes and Gilbert, as the result of their experiments, found 
that out of an average rainfall of 28 inches, l0|^ inches perco- 
lated through 20 inches of soil, 10 inches through 40 inches, 
and 8 inches through 60 inches ; and that after the warm and 
comparatively dry weather of the autumn there was less water 
going through 40 inches than through 20 inches ; but that 
when the winter-rains accumulated, the reverse happened, and 
there was sometimes more passing through sixty inches than 
twenty.* 
Taking all circumstances into consideration, Mr. Hawkesley 
gives it as his experience, which is confirmed by Mr. Glaisher, 
that the evaporation and absorption of the rainfall vary through- 
out England generally between 10 and 18 inches ; the former 
applying to steep precipitous mountains of non-absorbent rock, 
* " Eainfall and Evaporation," ' Trnns. Instit. Civil Engineers,' vol. xlv. 
VOL. XIV. — S. S. C 
