and the Storage of Water. 
13 
often filled with flood-water, while the former were never full.* 
The water-way of the arches in the clay districts varied from 
8 to 17 superficial feet for each square mile of drainage area, 
while in the chalk districts the water-way varied from one-third 
of a foot to 2 superficial feet per mile of drainage area. 
The water absorbed during the wet season of autumn and 
winter in these underground reservoirs slowly gravitates to the 
fissures. Through these natural channels it continues to flow 
in the direction of the dip of the strata until stopped by some 
impervious material, when, having risen high enough to sur- 
mount the barrier or dam, it is discharged in the form of springs. 
These springs feed and maintain the water-courses with a regu- 
larity only to be found under such circumstances, rendering 
them extremely valuable for purposes of water supply and for 
driving machinery, some of the best mills in England being 
found on streams fed from the chalk formation. When the 
rainfall is so exceptionally heavy that the chalk becomes sur- 
charged, and incapable of absorbing and holding water, it finds 
fresh vents, and, bursting out, forms those remarkable streams 
known as " bournes," the flow from which only occurs at inter- 
vals separated by long periods. 
Many of the deep-seated springs in the chalk and limestone 
are not affected by the rain until several months after its fall, 
the water taking some considerable time to saturate the rock 
and travel along the underground channels to the point of dis- 
charge. The heavy rainfall of a winter and autumn affords 
a supply for the following summer. The maximum of these 
springs is generally between May and July, and the minimum 
between October and December. The state of the stream may 
thus be predicted beforehand ; a wet winter may be expected 
to afford plentiful supplies for the following summer, but a dry 
autumn and winter must inevitably be followed by a deficiency 
in the following summer and an unusual depression in the 
water level. 
The perennial character of the chalk springs is affected by 
their altitude above the level of the sea ; those placed about 
100 feet above the sea being scarcely or ever dry, while those at 
a greater elevation are more liable to fail in a dry season. 
The quantity of rain which finds its way to the water-courses, 
is affected not only by the geological condition of the soil on 
which it falls, but also by the vegetation with which that soil is 
covered, and the meteorological condition of the atmosphere at 
the time. 
* " Chalk Water Supply," ' Trans. Instit. Civil Engineers,' vol. xlvii. ; " Fresh 
Water Floods of Rivers," idem, vol. xxvii. 
