Page:The American Cyclopædia (1879) Volume III.djvu/692

682 CANAL they are so constructed 'that they may be opened at any level between the surface and bottom of the canal. The water is discharged through slide gates which move in grooves. Canals are supplied with water upon their upper levels, to replace what is consumed by evaporation, leakage, and the passage of the boats through the locks. The structures which thus supply water, whether they are built ex- pressly for this purpose, or are also used as canals for transportation of boats, are called feeders. In crossing rivers whose levels are below that of the canal, it is sometimes necessary to erect structures which require very skilful engineering. The aqueducts at Schenectady and at Cohoes, by which the Erie canal crosses the Mohawk river, are among the finest pieces of engineering architecture in the country. Fig. 5 is a view of the Seneca river aqueduct, by which that canal is carried FIG. 5. Seneca River Aqueduct. across the Cayuga marshes upon 80 arches of stone masonry resting upon piers 5 ft. in width. The arches are 22 ft. span, with a spring of about 10 ft. commencing 11 ft. above the sur- face of the water. The width of water in the aqueduct is 53 ft. at the surface. The total length of the structure is 894 ft. The work was commenced in January, 1849, and complet- ed in the spring of 1857, at a total cost of $200,000. The aqueduct by which the Ches- apeake and Ohio canal is carried over the Potomac river is 1,446 ft. long and 36 ft. high. The conduit has a 9-foot vent and discharges 68,000,000 gallons of water in 24 hours. It is supported by 8 piers of granite, which are im- bedded 17 ft. in the river bottom. It was con- structed from plans made by Major Turnbull of the U. S. topographical engineers, at a cost of about $2,000,000. The first aqueduct for canals in England was made by the duke of Bridgewater under the direction of James Brindley, across the Irwell. It consisted of three arches, the middle one of 63 ft. span. The aqueduct on the Lancaster canal, over the river Lune, designed by Rennie, is a bold and elegant structure, consisting of five arches, each of 70 ft. span, rising 70 ft. above the level of the river. The aqueduct at Slateford carries the Edinburgh and Glasgow Union canal across the valley of the Water of Leith. The arches and also the water channel are made of cast iron, the latter being built in with masonry. It is about 500 ft. long, and has 8 arches of 45 ft. span each, the canal being 70 ft. above the level of the river. The Pont-y-Cysyllte aque- duct, by Telford, which carries the Ellesmere canal across the Dee and the vale of Llangol- len, is celebrated for its magnitude, for the simplicity of its design, and the just proportion of its parts. The water channel is made of cast iron, supported on cast-iron arches which rest upon stone pillars. The aqueduct is about 1,000 ft. long, consisting of 19 arches, each of 45 ft. span. The pillars are 8 ft. broad at the top, and the height of the four middle ones is 115 ft. to the spring of the arch. The surface of the water in the canal is 127 ft. above the level of the river. In France there are some remarkable aqueducts of masonry. At D6goin and at Guetin, across affluents of the Loire, are structures of this kind respectively 810 and 1,298 ft. long. The aqueduct of the Ganges irrigation canal is probably the largest structure of this kind yet built, it having a waterway of 170 ft. in width by 10 ft. in depth. The motive power used on canals is various. In China the boats are towed by men, as they were in England on the Thames and Severn till near the close of the last century ; but on most European and American canals of small breadth and depth horses and mules are used. On the larger canals, especially in Europe, steam power is used to a great extent. The average rate of towage on the Erie canal is estimated to be less than two miles per hour, and the economical and satisfactory introduc- tion of steam power on this work has long oc- cupied the attention of the state authorities as well as the general public. Perhaps the sub- ject cannot be better set forth than by present- ing the views contained in a report recently made by a commission which was appointed by the legislature of New York " to practically test and examine inventions and any and all devices which may be submitted to them, by which steam, caloric, electricity, or any other motor than animal power may be practically and profitably used and applied in the propul- sion of boats upon canals." The act which appointed the commission provides that if the commissioners shall determine that one or more "inventions," not to exceed three in