304 THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
50ft., there was a small rise at the Berry Wood Asylum well, 
at Duston, three and a quarter miles away (see page 253). 
Both wells were being used at the time my information was 
obtained; the exact height of the former is indefinite, and 
the minimum height of the latter only known to me, hence I 
do not wish to attach too much importance to any calcula¬ 
tions founded on them.* Take the rest-level of the water at 
the Billing Boad well as being 40ft. above the Marlstone, and 
the rest-level at Duston as 9ft. above the same bed, and the 
Artesian gradient as 7-^ft. per mile, then the 40ft. of water 
at Northampton would sustain 40ft.-f-3^x 7jft. = 64§ft. at 
Duston, subtract the 9ft. of water there and we get about 
55ft. as the increased height of the Rock-bed, and the dip 
would amount to a trifle over 17ft. per mile. 
If this dip were maintained, and all other conditions 
remained the same as supposed, then a sector of a circle 
having a radius of twelve miles could be filled before the 
water would stand at the height of 110ft. at Northampton, 
as may be ascertained by a simple calculation. It is quite 
certain, however, that the dip increases as we get further 
away to the west or north-west, and equally certain that it is 
less than this northwards, so that I have based the following 
calculation on an area bounded by two radii at an angle of 
125° with each other and an arc eight miles distant, in order 
to be well within the region of probability. 
Area capable of being filled =irr <2 ‘ X if^ = 70 square miles 
nearly. 
One square mile of rock, 4ft. thick, would contain, at 
§ gallon per cubic foot, 74,342,400 gallons, and therefore the 
whole 70 square miles nearly 5,204 millions of gallons. 
I do not say this is correct, but to show that it is reason¬ 
able, I would point out that for about forty years an average 
of something like 500,000 gallons per day has been pumped 
from the Marlstone, besides that lost by springs; this would 
amount to 7,300 millions of gallons, and reasoning on the 
assumed capacity of the Marlstone as given above, we will 
suppose that to get a “head” of 90ft., such as was obtained 
at first, 4,300 millions of gallons were already stored in the 
Marlstone, then 3,000 millions of gallons would have to be 
supplied by percolation during the period of forty years, this 
would amount to 75 millions of gallons per year, or 200,000 
gallons per day. This the Marlstone seemed capable of 
yielding, but not much more, for when the Billing Road well 
* Pumping was continued at Northampton for some time, after the 
rough estimate of 40 or 50 feet was given me, before I obtained the 
information about Duston well. 
