216 DE RANCE: UNDERGROUND AVATER-SUPPLY AND RIVER FLOODS. 
pure, but is kept at a uniform temperature of about 10° C (50° F.), 
rendering it cool in summer, and keeping it far from freezing in 
winter. Every million gallons of water drawn from the chalk carries 
with it in solution, on an average, li tons of the chalk througli 
which it has percolated, causing an additional storage room for 110 
gallons of water, so that the yield of a well draining a given area^ 
other things being equal, ought to gradually increase in yield until 
the maximum limit of permeability is reached. 
The Chalk of Yorkshire is drained over 364 square miles by the 
river Hull, 90 miles by the Derwent and Foulness, while 88 miles 
drain northwards and eastwards to Flamborough Head and Bridlington 
Bay, and 206 miles southwards into the mouth of the Humber. 
Much of it is heavily covered with the thick Glacial Drift of Holder- 
ness, chiefly consisting of impermeable clay, from oO to 200 feet in 
thickness, their surface rising to 109 feet above the sea, their base 
sinking to 130 feet below it. The chalk, according to Mr. J. R. 
Mortimer,''^ is SOO feet thick, and rests on impermeable neocomian 
or Kimeridge Clays. It is subject to the curious Gypsey Races," 
local temporary autumnal rivers, corresponding to the ''Bourns" 
of the chalk of the South of England. 
In my work on " The Water Supply of England and Wales," 
pubhshed in 1882, I have attempted to show what is the probable 
supply of water available in all the river basins of South Britain, and 
what is the amount required to satisf)" the demands upon that supply, 
with the result that it appears to be amply demonstrated that the 
rainfall of this country is more than sufficient to meet all the require- 
ments of human consumption, manufacturing processes, and the pur- 
poses of canalization, and yet with these resources large districts still 
suffer from all the ills of polluted water supply, whilst other areas 
are devastated by floods, representing unproductive rainfall passing 
to the sea. In no county would this unproductive rainfall be more 
valuable than in that of Yorkshire, where water is not only wanted 
for human consumption, but for the varied processes of trade, and 
nowhere could it be more largely increased by dumb-wells in the 
manner suggested. 
* Mill. Proc. Inst. C.E., vol. iv. 
