narrows at its lower end, forming a gorge which can be obstructed by a
comparatively short dam. The accompanying condition is that the nature of
the soil, i.e., the character, strata, and lie of the rock, clay, etc.,
as the case may be, is favorable to assuring a good foundation. In Great
Britain, as a rule, dams for reservoirs have been constructed of
earthwork with a puddle core, deemed by the majority of English engineers
as more suitable for this purpose than masonry.

Earthwork, in some instances combined with masonry, was also a form usual
in the ancient works of the East, already referred to; but it would
appear from the experience of recent years that masonry dams are likely
to become as common as those of earthwork, especially in districts
favorable to the construction of the former, where the natural ground is
of a rocky character, and good stone easily obtained.

As to the stability of structures of masonry for this purpose, as
compared with earthwork, experience would seem to leave the question an
open one. Either method is liable to failure, and there certainly are as
many cases on record of the destruction of masonry dams as there are of
those constructed of earthwork, as instanced in Algeria within the past
few years. As regards masonry dams, the question of success does not seem
so much to depend upon their design, as far as the mere determination of
the suitable profile or cross section is concerned, as that has been very
exhaustively investigated, and fairly agreed upon, from a mathematical
point of view, but to be principally due to the correctness of the
estimate of the floods to be dealt with, and a sufficient provision of
by-wash allowed for the most extreme cases; and, lastly, perhaps the most
important of all, the securing a thoroughly good foundation, and a
careful execution of the work throughout.