154 
J. HOPKINSON —WATER AND WATER-SUPPLY 
The source of the Lea is in Beds; the source of the Colne, higher 
than the point where its waters are usually lost in swallow-holes 
in the Chalk, is in Middlesex, adding a little to the drainage-areas 
of these rivers. On the whole, the catchment-basin of the Lea, 
above the lowest point where the New River and the East London 
Companies are augmenting their supplies by deep borings, com¬ 
prises an area of about 430 square miles, and the catchment-basin 
of the Colne, above the point where it is now proposed to bore for 
an additional supply of water for London, comprises an area of 
about 235 square miles, or together 665 square miles, of which 
about half are occupied by permeable strata. Adding the area 
drained by a few streams which flow for some distance over the 
London Clay, but are frequently dry in summer, and when they run 
lose themselves in swallow-holes in the Chalk, we certainly cannot 
depend upon a percolable area of more than 400 square miles in the 
catchment-basins of the Colne and Lea, of which may be assigned 
about 150 square miles to the Colne and 250 to the Lea. 
Three inches of rain sinking into our underground reservoir 
throughout an area of 400 square miles gives a supply of 17,374£ 
millions of gallons of water per annum, or rather more than 471- 
millions of gallons per diem. The average supply to London is 
about 30 gallons per head of its population each day, but in very 
dry weather this is exceeded. In an exceptionally dry series of 
years the supply from the catchment-basins of the Colne and Lea 
would therefore suffice for about a million and a half of people, or 
between one-third and one-fourth the amount required by the five 
and a half millions or so comprising the population of London and 
its suburbs. In an average year, or in a sufficiently long series of 
years to ensure a rainfall closely approximating the average, the 
supply would be about 95 millions of gallons per day, and would 
suffice, at 30 gallons per head, for about three millions of people. 
In neither case, however, would there be any water to spare for the 
population in the areas of the Colne and Lea river-basins, nor would 
there be any overflow to feed our rivers. The area required to 
collect the water supplied to London from the Lea valley, about 
72£ million gallons per diem, at 6 inches of percolation per annum, 
is a little over 300 square miles, so that more water is being with¬ 
drawn from the Lea catchment-basin than it is collecting, inde¬ 
pendently of the amount required for its own resident population. 
Even if we suppose, as we have already seen is most probably the 
case, that a considerable portion of the water pumped from the 
Chalk in the Lea valley is derived from the Colne catchment-basin, 
the draught upon this extended area will still be greater than the 
supply, and the result must be that the Chalk will be gradually 
exhausted of the water it contains, and its plane of saturation 
lowered year by year. This is what is actually occurring, not only 
iu the valleys of the Colne and Lea, but also in the main valley of 
the Thames under London, and thus the conclusion at which we 
have arrived by calculation is confirmed by fact. 
Although the exceptional lowness of the underground water-level 
