One of the most important accessory works of a reservoir is the bye wash, or waste weir ; a portion of the enclosure over which any water is allowed to pass after the reservoir itself is filled to its maxi mum water line. The length of the bye wash must be made sufficient to carry off in a thin stream, the surplus products of the most violent storms, or of the most continuous winter's rains ; and it must be made in such wise that the water flowing over it should not affect the stability of the dam, either by the cataract of the falling water, or by the velocity of the stream flowing from the foot of the bye wash. For the service of canals, or of town distribution, the water is drawn off by a series of sluice-gates, provided with the necessary contrivances to regulate the velocity of flow in the distributing channels; and it may be stated that, for a town distribution, the variable head upon the main pipes which is usually admitted, does not exceed 13 feet; and that in all cases it is desirable that the efflux of the water through the feeders, or the mains, should take place with as great regularity as it is possible to attain.
Some idea of the importance of the reservoirs formed for the supply of canals and town distributions may be derived from the following notes upon some of the most remarkable works of this description. Thus the Fountain Reservoir of the Croton aqueduct covers 400 acres, and has a depth at the dam of 38 feet : the Manchester Corporation Waterworks cover an area of above 408 acres, and have an average depth of 35 feet 8 inches, or they have a atoreage capacity of 584,866,716 cubic feet ; the Liverpool reservoirs have a storcage capacity of about 3,156,000,000 gallons ; the Yan Ycan reservoirs in Victoria, have an area of 1303 acres, and a storeage capacity of 6,400,C00,000 gallons. The Canal du Midi, of France, has at Gros Bois, a reservoir of 300,510,000 cubic feet capacitT, with a head of water at the dam of 89 feet 6 inches ; the reservoir of the Nantes to Brest canal has a capacity of about 262,395,000 cubic feet, and a head of 33 feet; the reservoir of St.. Feriol, on the canal du Midi, has a capacity of 223,090,000 cubic feet, and a head of 105 feet. The Birmingham canal has a reservoir of 80 acres „area, with a depth of 45 feet at the head ; the reservoir of the Union canal of Philadelphia, is not less than 730 acres in area, by an extreme depth of 40 feet, and it has a capacity of 572,000,000 cubic feet ; on the Rideau canal of Canada, there is a' darn 70 feet high ; the Turton and Entwystle reservoir, in Lancashire, is of 100,000,000 cubic feet in capacity, &c. In England, it mane that the usual ratio of the storeage capacity to the area of gather ing ground is from 30 to 40 thousand cubic feet per acre ; and it is found practically that the maximum quantity stored never exceeds 15 inches over the area of the gathering grounds, in the moat favour ably inclined and impermeable districts ; nor does it ever exceed 12 inches in flat retentive ones. The cost of forming large reservoirs
has rarely, if ever, fallen abort of 450/. to 500/. per million cubic feet of the atoreago capacity, leaving out of account the accessory works, or the purchase of the land. A very important element in the coat of all such works is to be found in the compensation to be given to the parties who were formerly entitled to the water privileges existing in the district ; but it is not possible to discover any general rule appli cable to the countless variety of cases of this description. Some remarks on the subject will be found under Warsat MILLS.
There is an important distinction to be made between the various kinds of reservoirs used upon town distributions, which may be mentioned here, though the discussion of the principles of their construction moat be referred to WATER SUPPLY. The distinction in question is the one between the open storeage reservoirs, and the covered reservoirs, from whence the waters are distributed, generally after filtration. The latter are extremely costly structures, and are, therefore, rarely made larger than would be required for a four or a seven days' supply at the maximum ; whilst the storeage reservoirs aro made large enough to hold a quantity equal, when the rain-fall is about 60 inches per annum, to about 120 days consumption ; when it is about 48 inches, to about 140 days' consumption ; and when it is about 22 inches (which is the case in the eastern parts of England), to about 200 days' consumption.
It may be interesting, archfeologically, to add that some of the most extraordinary mountain reservoirs in Europe are those which were constructed by the Moors of Spain ; that the Egyptians under the Pharaolia had executed some wonderful works for the purpose of storing the inundation waters of the Nile ; that thy native rulers and the Mohammedan conquerors of India vied with one another in the construction of the colossal tanks of that country ; and that even the Aztecs had adopted a peculiar system of aguadas, or underground reservoirs, for the purpose of storing the copious but partial rains of their country. The Moors and the Egyptians seem to have used the water they stored principally for the purpose of irrigation. The Greeks and the Romans supplied their towns with water from springs and rivers, and very rarely attempted to execute irrigation works, so that there are no authentic records of the formation of artificial storeage reservoirs by either of those nations. One of the most ancient works of this de scription whose history can be identified in modern times is the reservoir of St. F4riol, on the Canal du Midi; and nearly contem poraneously with it the system of catehwater ponds, which supply partially the fountains of Versailles, was executed by the orders of Louis XIV.