of which may only go to prove the unsuitability of the site under
consideration as regards area, etc. The loss due to evaporation,
according to Mr. Hawksley, in this country amounts to a mean of about 15
in.; this and the absorption must vary with the geological conditions,
and therefore to arrive at a satisfactory conclusion regarding the amount
of rainfall actually available for storage, careful gaugings have to be
made of the stream affected, and these should extend over a lengthened
period, and be compounded with the rainfall. A certain loss of water, in
times of excessive floods, must, in designing a dam, be ever expected,
and under favorable conditions may be estimated at 10 per cent. of the
total amount impounded.

As regards the choice of position for the dam of a reservoir, supposing
that it is intended to impound the water by throwing an obstruction
across a valley, it may be premised that to impound the largest quantity
of water with the minimum outlay, the most favorable conditions are
present where a more or less broad valley flanked by steep hills suddenly
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 det