s built at the end of the plugging, projecting 25 ft.
beyond the sides of the culvert and 8 ft. above, the space between them
being filled up with cement concrete tied into the rock, and on this the
puddle wall rests. This bank, like almost all others pierced by outlet
pipes or culverts, was not destined to be perfect. In 1867, four years
after the completion, spurts of water showed themselves in the culvert
in front of the puddle wall, which began to settle, and the water had to
be drawn off to admit of repairs. Diagram No. 10 shows a structure of a
different character to any of these already described. This character of
work is adopted on the North Poudre Irrigation Canal, in N.E. Colorado.
Timber is there plentiful, and a dam of this character can be rapidly
constructed, although probably not very durable, owing to liability to
decay of timber. That represented is about 25 ft. high.

The author has now concluded the consideration of earthwork dams, and
proposes making a few remarks upon those of masonry or concrete, with
reference to some of the most important, as shown on the diagrams. Their
stability, unlike those of earthwork, may be considerably increased where
the contour and nature of the ground is favorable by being curved in
plan, convex toward the water, and with a suitable radius. They are
especially suitable for blocking narrow rocky valleys, and as such
situations must, from the character of the ground, be liable to sudden
and high floods, great care is necessary to make sufficient provision for
overflow.

When of masonry, the stones should be bonded, not merely as they would be
in an ordinary vertical wall, where the direction of the stress is
perpendicular, but each course should be knit in with that above and
below it in a somewhat similar manner to what is termed "random" work.
And lastly, if hydraulic mortar be used, a sufficient time should elapse
after construction before being subjected to strain, or in other words,
before water is allowed to rise in the reservoir. For this latter reason,
and also the liability to damage by sudden floods during the progress of
the works, dams of Portland cement concrete, on account of their quick
consolidation, possess advantages over those of hydraulic masonry apart
from the necessity in the latter instance of constant supervision to
prevent "scamping" by leaving chinks and spaces vacant, especially where
large masses of stone or Cyclopean rubble are used.

Aga