784 Water supply. feet above the river, the canal being carried across in a cast-iron trough. 1 It must be obvious, that to construct a navigable channel through a country varying in level, and affording, perhaps, no great facilities for obtaining a supply of water, infers high engineering skill. Vast reservoirs must in some cases be formed for storing the water necessary to supply, during dry seasons, the loss by lockage, leakage, and evaporation. Feeders must be made to lead this water to the canal, hills must be pierced by tunnels, valleys must be crossed on lofty embankments, or spanned by spacious aqueducts, and, above all, the whole must be conceived and laid out with scrupulous regard to the all-inipcitant object of securing the works against injury from an overflow of water during floods, and a consequent inundation of the surrounding country. Moreover, the necessity of laying out the canal in level stretches, and surmounting elevations by means of locks or inclined planes, occurring at intervals, often occa sions much difficulty and greatly restricts the resources of the engineer. Taking, then, all these circumstances into consideration, and bearing in mind that canals were the pioneers of railways, we think it may safely be affirmed that the canal engineers of former days had more serious physical difficulties to contend with than are experienced in carrying out the railways of modern times, if we except such works as the Britannia Bridge, the high-level bridge of Newcastle, the Boxhill tunnel, and some other kindred works. But, indeed, their mechanical difficulties were also greater, for the introduction of steam, and its wide-spread application to all engineering operations, afford facilities to the engineers of the present day which Smeaton at the Eddystone, Stevenson at the Bell Rock, and Rennie and Telford in their early navigation works, did not enjoy. The distinguished merits of the engineers who practised in the former and at the commencement of the present century, cannot indeed be over-estimated, and had it been within the scope of this article it would have been profitable and instructive to have described in detail some of the grand aqueducts and other works on the lines of our canals. For this reference is made to the articles AQUEDUCT, BRIDGE, TUNNEL, and RESERVOIR, all of which are more or less applicable to the formation of canals. We shall only therefore offer to the student the following sum mary of engineering principles generally applicable to all cases. A canal cannot be properly worked without a supply of water calculated to last over the driest season of the year, and in that respect, except as to the quality of the water, demands all the care requisite in investigating the sources of water for supplying towns. If there be no natural lake in the district, available for supply and storage, the engineer must select situations suitable for artificial reservoirs, and the conditions to be attended to in selecting their positions are the same as those for water-works. They must com mand a sufficient area of drainage to supply the loss by leakage, evaporation, and lockage, due to the length of canal, number and size of the locks, and probable amount of traffic. The capability of the district to afford this supply will depend on the area of the basin drained and the annual amount of rainfall. The offlets from the reservoirs must be at such an elevation as to convey water to the summit-level of the canal. The embankments for retaining the water must be erected on sites affording a favourable foundation, and, if possible, in situations where an embank ment of small height and length may dam up a large amount of water. It is further necessary to consider whether the subsoil of the valley forming the reservoirs is throughout of so retentive a nature as to prevent leakage, 1 Life of Telford, London, 1838. and it is essential to provide, by means of waste weirs, for the discharge of floods. The Caledonian Canal, to be afterwards noticed, is in this respect very favourably situated, the whole supply being obtained from natural lochs. In other cases, such as the Union, Forth and Clyde, Crinan, Birmingham, and other canals, it was necessary to construct large reservoirs in which the water is stored in winter and led in feeders to points convenient for supplying the canal in summer. Where the canal communicates with the sea or a tidal river, and where the natural supply is small, as at the Foss Dyke already referred to, the water is raised by pumping engines. It will readily be seen, therefore, how important it is to reduce to a minimum the loss of water due to leakage from deficient workmanship, as well as to lockage of the traffic through the canal, and (while on this subject) it may be stated that the up con sumes a greater amount of water than the down traffic, for an ascending boat on entering a lock displaces a volume of water equal to its submerged capacity ; the water so displaced flows into the lower reach of the canal and the lower gates are closed, the boat is then raised, and on passing into the higher reach of the canal its displacement lost on entering is supplied by water withdrawn from the higher reach. A descending boat, on the other hand, on entering a lock likewise displaces a volume of water equal to its submerged capacity, but the water in this case flows back into the higher reach of the canal, where it is retained when the gates are closed. Mr Fulton gives the consump tion of 25-ton boats through locks of 8 feet lift as about 163 tons of water in ascending, and 103 in descending. 2 Several proposals have been made for reducing the loss of water by side ponds to receive part of the water, but all such plans delay the traffic and have not come into general use. The barge-canals constructed in this country are between 4 and 5 feet in depth. When the soil in which they were made was retentive, they were formed as shown in the cross-section, fig. 1. But when the soil was porous, clay 1 Sections areas of barge canals. t ! <- FIG. 1. Section in retentive soil. puddle was introduced, as shown in fig. 2. Professor Rankine says the depth of water and sectional area of water-way should be such as not to cause any material FIG. 2. Section in porous soil. increase of the resistance to the motion of the boats beyond what it would encounter in open water, and gives the following rules as fulfilling these conditions : Least breadth at bottom = 2 x greatest breadth of boat. Least depth of water H ^ oot + greatest draught of boat. Least area of water-way = 6 x greatest midship section of boat. In laying out a line of canal the engineer is more restricted than in forming the route of a road or railway, where gradients can be introduced to suit the undulating surface of the country. A canal, on the contrary, must follow rigidly the bases of hills and windings of valleys, to preserve a uniform level, accommodation being made for the road traffic by erecting suitable " fixed " and " movable " bridges. It is important, as already stated, to lay out the work in long level reaches, and to overcome
f Fulton s Canal Navigation.