Unavoidable changes of level are usually overcome by locks, the invention of which has beep disputed by the Utiteh and the Vene tians. Leonardo da Vinci is said to have ap plied them in 1497 to the Milanese canals. A lock is a chamber of masonry constituting the bed of the canal between ,the ppper and lower levels, at the point where it is desired to trans fer boats from one to the other, and is fur nished with gates at each end, and with sluices communicating with both the levels. When a boat is to be passed from the lower to the upper level, the water is suffered to escape from the lock until its surface coincides with the lower level. The gates at the lower end are then opened, and the boat is floated into the lock. The sluices which communicate with the upper level being then opened, the level of the water in the lock is raised until it coincides with the upper level of the canal. The upper gates are then opened, and the boat is floated out of the lock. The operation of lowering a boat is precisely the reverse. Every time the operation is performed, a quantity of water, equal to the contents of the lock, is lost from the upper level. To make this loss as small as possible, locks are made only just wide enough to admit the wiclestboats used 9n the canal; and in POMO cases OP are formed side by side, with a communication between them, so that, whenever one has to be emptied, one-half of its contents may be transferred to the adjoining one, and saved for a future occasion. This arrangement also saves time. Inclined planes, up and down which the boats may be conveyed on trucks or sledges, have in a few cases been used as substitutes for locks.
The usual mode of moving boats upon a canal is to tow or draw them by means of a long rope, by horses driven along a raised towing-path formed along one bank of the canal. To save expense, the older canal bridges were made so small that it was neces sary to detach the horses on coming to them, and to get the boats through by manual labour, or by mere impetus ; but on the best modern canals the arches are made large enough to include a towing-path as well as a water-way. The same remark applies to tunnels. In several of the older tunnels the boats were forced through by the laborious and dangerous process of legging, which was performed by men lying upon their backs on the boat, and thrusting their feet against the sides or roof of the tunnel. In some cases, ropes or chains worked by steam - engines have been used for hauling boats through. The attempts made to propel canal-boats by steam power, have been of questionable suc cess. Paddle-wheels of every kind disturb the water so much as to injure the banks. In 1814 screw propellers were for the first time used on canals in Scotland, on the Union Canal. The steamer was a tug-boat, capable of drawing eight or ten heavily laden barges : it had two Archimeclean screws on either side of the bow. A steam tug was tried on the
Grand Junction Canal in 1815, provided with a submerged propeller invented by Captain W. H. Taylor, differing somewhat from the screw-propeller. It produced scarcely any wave when going four miles through the water, and gave promise of being a useful means of traction for goods traffic. On the Glasgow, Paisley, and Androssan Canal fly boats for passengers, drawn by horses at a considerable speed, have been run success fully: but, amidst conflicting statements, it is difficult to decide whether in any case the or dinary walking pace of a horse can be ex ceeded on a canal without a greater loss of power than would attend the like increase of speed upon a railway or perfect road. From experiments made while railways were yet in their infancy, it would appear that, while at a very slow pace heavy goods may be conveyed much more economically on a canal than. in any other way, the economy turns in favour of a railway where the velocity exceeds four miles per hour, while at high velocities the economy of the canal disappears even as compared with an ordinary road.
Mr. Watson patented in 1839 a form of canal boat which could be lengthened or shortened according to the length of the lock which it had to pass through. It was divided into two or more separate and independent water-tight portions, connected together by means of hinges or coupling links and bolts ; so that they might either be separated and placed side by side, or the ends turned round and doubled back without being unhinged or uncoupled.
A singular mode of ascending and descend ing canals was proposed by the late Mr. Smith of Deanston, as a means of saving expense in locks and gates at changes of level. He pro posed to divide the canal into a series of basins, the water levels of which should be from 12 to 18 inches above each other. The extremity of each basin is so contracted as to permit only the free passage of a boat ; and at this spot is placed a single gate, hinged to a sill across the bottom; the head pointing at a given angle across the stream, and the late ral faces pressing against rabbits in the ma sonry. The gate is constructed of buoyant materials, or made hollow so as to float and be held up by the pressure of the water in the higher level. On the top of the gate is a roller to precipitate the passage of boats. When a boat is required to pass from a higher to a lower level, the bow end, which must be armed with an inclined projection, depresses the gate to as great a degree as the depth of the immersion of the boat, and as much water escapes as can pass between its sides and the walls of the contracted part of the basin. The same action takes place in ascending, except that a certain additional amount of power must be expended to enable the boat to surmount the difference of level between the basins.