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 
down 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 w r ell had been sunk, 
the bore pipe was cut off, and the permanent plant fixed, pump¬ 
ing tests w T ere again made, viien the bore hole w T as found to 
yield practically the amounts calculated at particular levels, or in 
other w r ords the yield u r as in proportion to the square root of 
the head, thereby roughly obeying the law of the discharge of 
w r ater 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 w r ell 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 4S 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 
