112 
THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
This supply for 60,000 inhabitants, it will be readily observed, 
was equal to 7 gallons per day per head; whereas not less 
than 20 gallons per head should be available. For North¬ 
ampton alone, therefore, a daily supply of 1,200,000 gallons 
at least is required; and since the town is rapidly increasing, 
and a number of villages may also claim to be supplied by 
the town, owing to the character of an Act of Parliament 
applied for and obtained by the Water Company, certainly 
no less than 2,000,000 gallons per day ought to be aimed at. 
The Kiver Nen and its tributaries has a drainage area of 
1,077 square miles, made up of 150 square miles of Lias, 
and 927 of oolite, much covered with alluvium. The drain¬ 
age area above Higham Ferrers Bridge is 888 square miles.* 
Higham Ferrers Bridge is situated fourteen miles N.E. of 
Northampton, a distance not exceeding that of some portions 
of the Middle Lias which have in all probability acted as 
collecting grounds for the water supplied to Northampton in 
the past; for as I shall be able to show in a later section, the 
Marlstone area nearest Northampton has in no way con¬ 
tributed to the town water supply. It appears, therefore, 
that so far as the average porosity of the Marlstone is con¬ 
cerned, the whole of the catchment area, viz., 383 square 
miles, might be utilised for feeding the Middle Lias. As, 
however, many things might modify the available amount of 
water, and with any amount of care only a rough idea of the 
quantity could be obtained, I have tried to err on the side of 
caution by basing the following calculations on a supposed 
catchment area of 150 square miles. 
The rainfall of Northamptonshire, taking an average of a 
§-ood many years, is 23T inches per annum, and since 1 inch 
of rain = 22,687 gallons per acre, or 14,519,680 gallons per 
square mile, it follows that a rainfall of 23-1 inches over 
150 square miles would give about 50,310 millions of gallons. 
Of this water a good proportion is lost by evaporation 
from exposed surfaces, and from the leaves of plants. The 
amount of water evaporated in different seasons would vary 
considerably, and be greatest when the rainfall was most 
evenly distributed throughout the year; it would also be 
influenced by the character of the country as regards struc¬ 
ture, hills and valleys, and the geological character of its 
surface. Taking these latter into account for the district 
under consideration, and by comparing with estimates given 
by more competent authorities for other districts, I think an 
allowance of 10 inches per annum on account of evaporation 
would be sufficient. 
* “ The Water Supply of England and Wales,” by Chas. E. de 
Ranee, F.G.S., &c. 
