y be very short and its cost bear a relatively small
proportion to the total outlay upon a scheme of water supply, but where
distant sources have to be relied upon the cost of the aqueduct becomes
one of the most important features in the scheme, and the quantity of
water obtainable must be considerable to justify the outlay. Hence it is
that only very large towns can undertake the responsibility for this
expenditure. In Great Britain it has in all large schemes become a
condition that, when a town is permitted to go outside its own
watershed, it shall, subject to a priority of a certain number of
gallons per day per head of its own inhabitants, allow local
authorities, any part of whose district is within a certain number of
miles of the aqueduct, to take a supply on reasonable terms. The first
case in which this principle was adopted on a large scale was the
Thirlmere scheme sanctioned by parliament in 1879, for augmenting the
supply of Manchester. The previous supply was derived from a source only
about 15 m. distant, and the cost of the aqueduct, chiefly cast-iron
pipes, was insignificant compared with the cost of the impounding
reservoirs. But Thirlmere is 96 m. distant from the service reservoir
near Manchester, and the cost of the aqueduct was more than 90% of the
total cost. As a supply of about 50,000,000 gallons a day is available
the outlay was justifiable, and the water is in fact very cheaply
obtained. Liverpool derives a supply of about 40,000,000 gallons a day
from the river Vyrnwy in North Wales, 68 m. distant, and Birmingham has
constructed works for impounding water in Radnorshire, and conveying it
a distance of 74 m., the supply being about 75,000,000 gallons a day. In
the year 1899 an act of parliament was passed authorizing the towns of
Derby, Leicester, Sheffield and Nottingham, jointly to obtain a supply
of water from the head waters of the river Derwent in Derbyshire.
Leicester is 60 m. distant from this source, and its share of the supply
is about 10,000,000 gallons a day. For more than half the distance,
however, the aqueduct is common to Derby and Nottingham, which together
are entitled to about 16,000,000 gallons a day, and the expense to
Leicester is correspondingly reduced. These are the most important cases
of long aqueducts in England, and all are subsequent to 1879. It is
obvious, therefore, how greatly the design and construction of the
aqueduct have grown in importance, and what care mus