Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/867

Rh CANAL 785 elevations in cumulo by groups of locks at places where it can be most advantageously done. This leads to a saving of attendance and expense in working the canal, and causes fewer stoppages to the traffic. But to prevent waste of water the locks must be placed sufficiently far apart, say 100 yards, or an intervening pond or increased width of canal must be formed, so that a descending boat does not let off more water than the area below will receive without raising its surface so much as to lose the surplus water over the waste weirs. The mode of overcoming the difference of level between the various level reaches is, with few exceptions, by locks, which generally have a lift of 8 or 10 feet, though in some cases it is somewhat greater. The dimensions of the locks ought to be regulated by the traffic ; but they should, in order to save water, be as nearly as possible the size of the craft to be passed through them, allowing from 6 inches to a foot of extra breadth and draught of water. The barge-canals in England have locks about 8 feet in breadth, and from 70 to 80 feet long, and their use in raising or lowering boats from the different reaches is so well known as not to require explanation ; and for details as to the construction of the masonry of the chamber and walls, and the timber and iron work of the gates and sluices, reference is made to &quot;Rankine s Engineer ing.&quot; The water is generally admitted into and flows from each lock by sluices formed in the gates, and the passage of a boat occupies from three to six minutes, depending on the lift. Sir William Cubitt, on the Severn navigation, introduced the water through a culvert parallel to the side Avail of the lock, and opening in the centre by means of a tunnel, which admits of 16,000 cubic feet of water flowing into or out of the lock in li minute ; and in little more than that time loaded vessels can be passed through. 1 Inclined planes and perpendicular lifts, which have the advantage of saving water, were adopted so long ago as 1789 on the Ketling Canal in Shropshire, and afterwards on the duke of Bridgewater s canal. Mr Douglas of New York constructed the Morris Canal in the United States with 23 inclined planes, having gradients of about 1 in 10, with an average lift of 58 feet. The boats weighed, when loaded, 50 tons, and after being grounded on a carriage, were raised by water-power up the inclines with great ease and expedition. The length of the Morris Canal, between the rivers Hudson and Delaware, is 101 miles, and the whole rise and fall is 1557 feet, of which 223 were overcome by locks, and the remaining 1334 by inclined planes. 2 When first describing this work the author stated that the principal objection to the inclined planes for moving boats was the injury they were apt to sustain in supporting great weights while resting on the cradle. A slimly-built canal boat, 80 feet long, and loaded with 30 tons, could not be grounded on a smooth surface without straining her timbers, but this objection has to some extent been overcome on an inclined plane constructed by Mr Leslie and Mr Bateman on the Monkland Canal, where the boats are not wholly grounded on the carriage, but are transported in a caisson of boiler-plate containing 2 feet of water, and are thus water-borne. This inclined plane is wrought by two high- pressure steam-engines of 25 horse-power each. The height is 96 feet, and the gradient 1 in 10. The maximum weight raised is 80 tons, and the transit takes about ten minutes. The average number of boats passing over the incline is about 7500 per annum. Mr Green introduced on the Great Western Canal a perpendicular lift of 46 feet. Sir W. Cubitt also introduced three inclined planes, having gradients of 1 in 8, on the Chard Canal, Somersetshire. 1 Minutes of Proceedings of Institution of Civil Engineers, vol. v. r. 310. 2 Stevenson s Sketch of Civil Engineering in North America, Lori lon, John &quot;Weale. One of these inclines overcomes a rise of 86 feet ; and they are said to act very satisfactorily. 3 An essential adjunct to a canal is a sufficient number of Waste waste-weirs to discharge surplus water accumulating during weirs - floods, which, if not provided with an exit, may overflow the tow-path, and cause a breach in the banks, stoppage of the traffic, and damage to adjoining lands. The number and positions of these waste-weirs must depend on the nature of the country through which the canal passes. Wherever the canal crosses a stream a waste-weir should bo formed in the aqueduct ; but independently of this the engineer must consider at what points large influxes of water may be apprehended, and must at such places not only form wa-ste- weirs of sufficient size to carry off the surplus, but form artificial courses for its discharge into the nearest streams. These waste-weirs are placed at- the top Avater-level of the canal, so that when a flood occurs the water flows over them and thus relieves the banks. The want of these has occasioned overflows of canal banks, attended with very serious injury to the works, and lengthened suspension of the traffic ; and attention to this particular part of canal construction is of essential importance. Stop-gates are necessary at short intervals of a few miles Stop-gates, for the purpose of dividing the canal into isolated reaches, so that in the event of a breach the gates may be shut, and the discharge of water confined to the small reach inter cepted between two of them, instead of extending through out the whole line of canal. In broad canals these stop- gates may be formed like the gates of locks, two pairs of gates being .made to shut in opposite directions. In small works they may be made of thick planks slipped into grooves formed at the narrow points of the canal under road bridges, or at contractions made at intermediate points to receive them. Self-acting stop-gates hav.e been tried, but their success has not been such as to lead to their general introduction. When repairs have to be made stop-gates allow of the water being run off from a short reach, and afterwards restored with comparatively little interruption to the traffic. Their value in obviating serious accidents has been well exemplified in the author s own experience. The water during a flood flowed over the towing-path of the Union Canal connecting Edinburgh and Glasgow, and the uncontrolled current carried away the embankment and the soil on which it rested to the depth of 80 feet, as measured from the top water-level. The stop-gates were promptly applied, and the discharge confined to a short reach of a few miles, otherwise the injury (which was, even in its modified form, very considerable) would have been enor mous, not only to the canal works but to the adjoining lands. For the purpose of draining off the water to admit of Offlets. repairs after the stop-gates have been closed, it is proper to introduce, at convenient situations, a series of exits called &quot; offlets,&quot; which are pipes placed at the level of the bottom of the canal, and fitted with valves which can be opened when required. These offlets are generally formed at aqueducts or bridges crossing rivers, where the contents of the canal can be run off into the bed of the stream, the stop-gates on both sides being closed so as to isolate the part of the canal from which the water is withdrawn. In executing the work, provision must be made for the Drainage of proper drainage of the tow-path, which should be made tow T aUl - highest at the side next the canal, and sloped with a gentle inclination towards the outside. The drainage of the tow- path should be carried to a sky drain, and at intervals passed below it into the canal, as shown in fig 3. 3 Minutes of Proceedings of Institution of Civil Engineers, vol. x rii. . 205. TV. - 99