HISTORY OF COLCHESTER CORPORATION WATER WORKS. 25
increase the supply from existing wells. After a number of
pumping experiments, it was decided that the present new well
should be increased to 13 feet in diameter, and carried well
clown into the chalk; but, unfortunately, it was only carried
down through the London-clay into the mottled clay of the
Woolwich and Reading Bed series to a total depth of 79 feet
from the engine house floor. The present works (except the
well which was brought into use in 1891) were then constructed
and completed in 1893, and continues to be the chief source
of supply to the town; indeed up to 1906 it was the only supply.
After fourteen years more or less continuous pumping, and
towards the end of the dry summer season of 1901, when nearly
1,000,000 gallons per day were taken out of the well for weeks at
a stretch, the rest level began to show signs of lowering. It was
feared that at this rate of extraction, combined with more or less
continuous pumping, the rest level might become permanently
lowered, and the yield from the bore hole decreased in proportion;
hence it was decided to obtain a supplemental supply, which was
brought into use in 1905. Since that date the rest level has
partially recovered, principally owing to using Lexden Springs,
which yield about 40% of the average supply to the town.
Before deciding upon sinking and enlarging the new well in 1889,
an elaborate series of pumping tests were made, extending over a
considerable time, to find out by calculation what would be the
probable yield at different depths. After the well had been sunk,
the bore pipe was cut off, and the permanent plant fixed, pump-
ing tests were again made, when the bore hole was found to
yield practically the amounts calculated at particular levels, or in
other words the yield was in proportion to the square root of
the head, thereby roughly obeying the law of the discharge of
water from pipes, and showing that somewhere in the bore hole
there was a large fissure having a very free discharge. This
bore hole may be looked upon as a remarkable one, so far as the
quantity of water it yields. The only regret is that the well was
not carried down into the chalk formation so that headings could
have been driven therefrom into the chalk at any time. So far
as the well and boring are concerned (geologically), there is
nothing abnormal, except that at a level of 48 feet from the
surface there is a bed of Septaria in the London-clay, which
yielded a considerable quantity of water, but is plugged off