required to be flushed out, laborers passed
through the gallery and broke down the timber barrier, the silt forming a
wall sufficiently thick to resist the pressure of the water for the time
being, and allow of the retreat of the Forlorn Hope--if the latter had
luck--before giving way.

One method adopted in Algeria, which has the advantage of permitting the
sediment to be utilized together with the irrigation, this sediment being
very fertilizing, is to pump air down through hose extending to the
bottom of the reservoir, the pumps being actuated by steam power or
turbine, and the sediment thus stirred up and run off with the water
through the irrigation pipes. As an example of one of the early types of
masonry dams in France, reference may be made to Fig. 13, on which is
shown an elevation and cross section of the Lampy dam, forming a large
reservoir for feeding the Languedoc canal.

I will now refer to some of the most notable masonry dams in existence,
commencing with France, where perhaps the finest is that known as the
Furens, in connection with the St. Etienne Water Works, constructed
between the years 1859-66, and designed by the engineers Graiff and
Grandchamps. It is curved in plan, struck with a radius of 828 ft. from a
center on the down stream side, and founded upon compact granite, the
footings being carried down to a depth of 3 ft. 3 in. below the surface
of the rock. It is of rubble masonry, in hydraulic mortar, carried up in
courses of 5 ft. in depth.

The height is 170 ft. on the up stream side and 184 ft. high on the lower
side, with a breadth of 9 ft. 8 in. at the crest and 110 ft. at the base,
and the cross section is so designed that the pressure is nearly constant
in all parts, and nowhere exceeds 93 lb. to the square inch--13,392 lb.
to the square foot. The contents is equal to 52,000 cubic yards of
masonry, and the cost of erection was L36,080. The capacity of the
reservoir is equal to 352,000,000 gallons.

The reservoir discharges into two tunnels (see Fig. 11), driven one above
the other through a hill into an adjacent valley. The lower tunnel
contains three cast iron pipes, with a masonry stopping of 36 ft. long.
Two of these pipes are 16 in. diameter, with regulating valves, and
discharge into a well, from whence the water can be directed for the town
supply or into the river. The third pipe, of 81/2 in. diameter, is always
open, and serves to remove any deposit in the reservoir, and to f