t be Exercised in
order that the supply upon which such large populations depend may not
be interrupted, and that the country through which such large volumes of
water are conveyed may not be flooded in consequence of the failure of
any of the works.

Construction.

Practically only two types of aqueduct are used in England. The one is
built of concrete, brickwork, &c., the other of cast-iron (or, in
special circumstances, steel) pipes. In the former type the water
surface coincides with the hydraulic gradient, and the conditions are
those of an artificial river; the aqueduct must therefore be carefully
graded throughout, so that the fall available between source and
termination may be economically distributed. This condition requires
that the ground in which the work is built shall be at the proper
elevation; if at any point this is not the case, the aqueduct must be
carried on a substructure built up to the required level. Such large
structures are, however, extremely expensive, and require elaborate
devices for maintaining water-tightness against the expansion and
contraction of the masonry due to changes of temperature. They are now
only used where their length is very short, as in cases where mountain
streams have to be crossed, and even these short lengths are avoided by
some engineers, who arrange that the aqueduct shall pass, wherever
practicable, under the streams. Where wide valleys interrupt the course
of the built aqueduct, or where the absence of high ground prevents the
adoption of that type at any part of the route, the cast-iron pipes
hereafter referred to are used.

Masonry aqueducts.

The built aqueduct may be either in tunnel, or cut-and-cover, the latter
term denoting the process of cutting the trench, building the floor,
side-walls, and roof, and covering with earth, the surface of the ground
being restored as before. For works conveying water for domestic supply,
the aqueduct is in these days, in England, always covered. Where, as is
usually the case, the water is derived from a tract of mountainous
country, the tunnel work is sometimes very heavy. In the case of the
Thirlmere aqueduct, out of the first 13 m. the length of the tunnelled
portions is 8 m., the longest tunnel being 3 m. in length. Conditions of
time, and the character of the rock, usually require the use of
machinery for driving, at any rate in the case of the longer tunnels.
For the comparatively small tunnels required f