3. The supplying of water is one of the most important problems connected with canals. In the old canals, which were generally small and ran beside natural streams, the water require ments could be readily met by damming the stream and leading a short feeder to the canal. Under more difficult conditions, lakes or arti ficial reservoirs and sometimes long feeders have to be resorted to. It is almost essential that the supply be such that it can reach the canal by gravity flow, since in a canal of any magnitude the question of pumping is seldom to be considered.
Water has to be supplied to canals for three main purposes, namely, to fill the canal prism and to replenish loss in the levels between locks; to furnish water sufficient for lockages and kr unavoidable leakage at locks; and to furnish power for operating the machinery at locks and other structures, and for electric lighting along certain portions of the canal, particularly at the locks.
An independent water-supply for filling the canal prism is required only in land-lines and then only at the opening of navigation or after water has been drawn out of the levels for repairs or other purposes. In river-lines the natural flow of the stream will, of course, fill the canal prism. The losses in land-lines in clude seepage through embankments, waste over spillways, evaporation from water-sur faces and transpiration through aquatic plants. For river-lines there is less waste through seepage and none over spillways.
The amount of water required for a lock varies not only with the height of the lift, but also with the volume of traffic. The same quantity of water is used in lockage, whether the boats are large or small and whether the lockage is up or down. Therefore the effi ciency, or economy, of the lifting operation is increased with the larger boats. For a given amount of traffic the water-supply varies ac cording as the boats are large or small and according to the manner in which they pass the lock— whether singly or in groups and whether lockages in the same direction are made in sequence or alternate with lockages in the opposite direction. A canal is essentially an inland transportation line in which the grades are overcome by water-power. That the power is applied directly from water to boat does not alter the case. All things considered, the canal lock is a fairly efficient water-driven machine. In addition, the lock has the ad vantage of simplicity, quick operation and avoidance of strain on the boat.
The critical points of supplying water to canals are usually the summit levels. Although lower levels may require more water, it is generally more readily obtainable for such portions, especially under the modern method of stream utilization. Proceeding downward in either direction from a summit, the water supply required at any lock is equal to the sum of the losses from the source of supply down to the lock, plus the water required at the lock, minus the natural inflow, if any, between the source of supply and the lock. From this it appears that the water-supply required may vary greatly from point to point. In providing water for a summit level of a canal it is neces sary, therefore, to obtain a supply adequate for the points of greatest demand on both sides of the summit.
Thus it will be seen that the problem of estimating the water-supply required by a canal, like the problems already discussed, is also complicated. Many items can be deter mined only approximately. It is necessary to allow a liberal excess, or reserve supply, as a factor of safety.
Historical.— The earliest artificial water channels were for irrigation and drainage; not to reclaim swamp land, other soil being too plentiful, but to regulate the overflow of rivers. These date from an immemorial past, certainly 3500 B.C. in Babylonia and Egypt, more prob ably 7000 at least. Very early also the larger ones must have been used for boat navigation, to transport agricultural and building materials; these combined drains and canals still exist in England, called and the workers on them and have given to the language the word for construction laborers. At what period the first ones were dug primarily for navigation, and incidentally for irrigation, cannot be told. There is a tradition that the Suez Canal was dug under the old kingdom of Egypt before 2000 ac.; it was certainly opened or reopened for small boats by Nechq, about 600 B.C. About this time also Nebuchadrezzar of Babylon opened the Royal Canal between the Tigris and Euphrates, but Mesopotamia had been well canalized be fore. These two countries, indeed, invited canals, with their flat surface and long levels, and easy digging in sand or clay. It is probable that China also had canals before the Christian era, but evidence is wanting. The first canals were of course on one level; but with the light boats and great engineering skill of the ancients the step was not long to damming the water at different levels and hauling the boat over. The first system, not yet disused, was to pull the boat up an inclined plane and let it down by gravity; and this remained the only available method till modern times. Under Alexander and his successors int and the Seleucid empire canals were much used an important one was from Alexandria to the Nile, whose mouths were shut off by sandbars. Marius had one constructed 102 a.c from the lower Rhone to the Mediterranean. Under Claudius there was one from the Tiber to the sea; and in Great Britain there are two which date from the Roman time, the Foss Dyke and the Caer Dyke, in Lincolnshire, of 40 and 11 miles respectively. In the 4th century Lombardy was canalized,— a very favorable spot from its great plain and many rivers; and near the end of the 5th cen tury Odoacer carried one from the Adriatic to Mentone above Ravenna. The downfall of Roman civilization stopped their development for a while; but under Charlemagne a fresh extension began, that monarch building canals to connect the Danube both with the Rhine and the Black Sea. In the Netherland bogs the system is that of nature itself, and began very early; here the canal is not so much an artifi cial channel as a remnant of the original sea, around which the land is built. In Britain as early as 1121 Henry I deepened and made navigable the old Foss Dyke. The Grand Canal of China, about 1,000 miles long, a large part of it made up of canalized rivers, was completed in 1289. That country has many other great systems connecting its internal waterways.