The word dam is very often used incorrectly in Scotland to indicate a reservoir or sheet of water, instead of the structure made use of to form the reservoir, which is its proper meaning. A reservoir requires a sufficient outlet at the bottom by means of a tunnel, culvert, or iron pipes provided with suitable sluices, and these ought properly to be so arranged as that access can be had to them even when the reservoir is full.
Most of the disasters from the bursting of reservoirs have arisen from the want of sufficient waste-weir, and from the embankments being overtopped in consequence by the water, and the outer slope being washed away, so as to deprive the puddle-wall of its support; but some accidents have occurred from the outlet being by a wooden box or trough through the embankment, and that being neglected and allowed to get rotten. The bursting of the Bilberry reservoir, above Holmfirth, which occurred in 1852, arose from the embankment having sunk to, and being allowed to remain at, a level actually below that of the waste-weir, so that it was overtopped; hut the Brad field reservoir embankment of the Sheffield water-works which burst in 1864, gave way before the water had risen to the level of the waste-weir; and much difference of opin ion exists as to the cause, some engineers contending that the disaster was caused by bad workmanship in the embankment itself, and others that it was owing to a landslip under the embankment.
Distributing reservoirs for towns, used chiefly for storing up the surplus water during the night, which otherwise might mostly go to waste, ought to hold at least half a day's supply, and ought to be placed high enough to command the highest parts of the town. They are generally built of masonry or brickwork, but are sometimes made of cast iron, and now occasionally of boiler-plate—in which last case they are best of circular form. There is one of that description on the highest part of Edinburgh castle. In India and in the south of Europe, where long droughts prevail, very large reservoirs have been constructed for supplying water for the purpose of irrigation.