680 CANAL upon the nature of the soil, and in many situations a covering of stonework is em- ployed. The breadth of the canal at the bottom should be more than twice that of FIG. 1. Cross Section of a Canal, a. Part of the water- way. 6. Tow path. e. Puddling of clay or cement to prevent percolation, d. Benne bank. e. Side drain. the boats upon deck, thus securing sufficient room for passing, and the depth should be at least one foot greater than the draught of water of the loaded boats. The tow path, built upon one side, is from 2 to 3 ft. above the level of the water, and from 12 to 14 ft. wide to allow the horses to pass each other with ease. The water that falls upon this should drain out- ward, and not into the canal ; and for receiving this a ditch called a side drain is sometimes constructed outside of the towing path, and another outside of the opposite bank. Where the soil is not retentive, the bottom and sides require to be puddled with clay tempered and well mixed with sand and gravel. This is put on in successive layers of 2 or 3 in. each, as the under layers set. Each layer, however, should be made to unite with the one it is laid upon, by working this up to roughen its surface. The puddling often requires repairs, and these in the colder parts of the country are convenient- ly made when the water is let out of the canal for the winter. Puddling serves to prevent the burrowing of animals under the canal, by which much mischief is often produced, a small hole rapidly becoming a large one by the flow of the water through it. Even in retentive soils puddle ditches are sometimes sunk in the banks, and filled with cement or clay, to pre- vent the percolation of water, as well as to in- tercept the progress of burrowing animals. Be- tween the tow path and the slope there is often a bench from 3 to 5 ft. wide, called a berme. The level of this is a few inches above the surface, and its purpose is to favor the growth of grass and aquatic plants to prevent the washing away of earth, and also to catch earth and stones that may roll from the tow path. Bennes may be constructed on either side of the canal ; the bank opposite the tow path is also called the berme bank. The bed of a cana' is made so nearly horizontal that the water wil flow with a gentle current from one end of a level to replace the water drawn off at the lower end. The levels are the spaces between two locks, each level being at a greater or less elevation than the one adjoining. These changes of level succeed each other rapidly in hilly dis- tricts, and they are often so great that severa locks are required, one immediately following another like a flight of stairs, in order to over 30me the difference of elevation. As ordinarily constructed, a lock is a chamber of timber or masonry, long and wide enough to receive the argest boats that navigate the canal. Indeed, he size of the boats is limited by that of the .ocks. Its bottom reaches to that of the lower Jevel, or pound, as it is called by the English, at the termination of which it is placed, and its
- op is a little above the surface of the water in
- he upper pound. Each end is closed by heavy
swinging doors, which open in the middle FIG. 2. Horizontal Section of a Canal Lock, a, a. Lock chamber, ft, 6. Gates, with balance beam left off to show the quoins, c, c. Mitre sills, d, d. Recesses for the gates to swing into. /, /. Hollow quoins to receive the quoin posts of the gates, g, g. Wing walls. against the direction of the current. The width of the two doors being a little greater than that of the lock, they meet before they form a straight line, and consequently brace against each other, making a close fit when the water presses against them. The upper gates, reaching only to the bottom of the upper pound, are as much shorter than the lower gates as the difference of elevation of the upper and lower level. The gates near their lower end are furnished with sliding valves, which
