or aqueducts, two
percussion drilling machines are usually mounted on a carriage, the
motive power being derived from compressed air sent up the tunnel in
pipes. The holes when driven are charged with explosives and fired. In
the Thirlmere tunnels, driven through very hard Lower Silurian strata,
the progress was about 13 yds. a week at each face, work being carried
on continuously day and night for six days a week. Where the character
of the country through which the aqueduct passes is much the same as
that from which the supply is derived, the tunnels need not be lined
with concrete, &c., more than is absolutely necessary for retaining the
water and supporting weak places in the rock; the floor, however, is
nearly always so treated. The lining, whether in tunnel or
cut-and-cover, may be either of concrete, or brickwork, or of concrete
faced with brickwork. To ensure the impermeability of work constructed
with these materials is in practice somewhat difficult, and no matter
how much care is taken by those supervising the workmen, and even by the
workmen themselves, it is impossible to guarantee entire freedom from
trouble in this respect. With a wall only about 15 in. thick, any
neglect is certain to make the work permeable; frequently the labourers
do not distribute the broken stone and fine material of the concrete
uniformly, and no matter how excellent the design, the quality of
materials, &c., a leak is sure to occur at such places (unless, indeed,
the pressure of the outside water is superior and an inflow occurs). A
further cause of trouble lies in the water which flows from the strata
on to the concrete, and washes away some of the cement upon which the
work depends for its watertightness, before it has time to set. For this
reason it is advisable to put in the floor before, and not after, the
sidewalls and arch have been built, otherwise the only outlet for the
water in the strata is through the ground on which the floor has to be
laid. Each length of about 20 ft. should be completely constructed
before the next is begun, the water then having an easy exit at the
leading end. Manholes, by which the aqueduct can be entered, are usually
placed in the roof at convenient intervals; thus, in the case of the
Thirlmere aqueduct, they occur at every quarter of a mile.

Timber aqueducts.

In some parts of America aqueducts are frequently constructed of wood,
being then termed flumes. These are probably more extensiv