Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/868

786 CANAL The preservation of the banks at the water-line is also a of banks, matter of importance. "Pitching" with stones and " facing" with brushwood are employed, and in the author s experience the latter, if well executed, forms an economical and effectual protection. Flo. 3. Showing Drainage of Tow-path. Puddling, j n forming the alveus or bed of the canal care must be taken, especially on embankments, and even in cuttings where the soil is porous, to provide against leakage by using puddle, as shown as fig 2. An all-important matter, as affecting the construction of the works, is the possibility of getting clay in the district, or such other soil as may be worked into puddle, on the good quality of which the sta bility of the reservoir embankments and the imperviousness of the beds and banks of the canal mainly depend. These are the only points of general application, in the construction of canals, to which reference can here be made; and in applying them to each case the engineer must be guided, first, by theoretical knowledge, to bo acquired by a careful study of his profession; and, secondly, by that knoAvledge which can be gained only by expe rience. Mode of ISTot a little has been written on the best mode of con- conducting d uc ting traffic on canals, and the reader who wishes to canals. 1 stu( ty the subject fully is referred to the observations made by Mr Walker and Mr George Rerinie in the Transactions of the Royal Society and of the Institution of Civil Engi neers, and especially to the valuable researches on hydro dynamics by Mr J. Scott Russell in the Transactions of the Royal Society of Edinburgh. Mr Russell while experi menting on propelling boats at high speeds found that the primary wave of displacement produced by the motion of a boat moves with a velocity due to the depth of water in the canal, being the velocity that is due to gravity acting through a height equal to the depth of the centre of gravity of the cross-section of the channel below the surface of the fluid. The velocity is in no degree dependent on the form or velocity of the body which generates it, or on the breadth of the canal. A wave that had a velocity of 8 miles an hour was traced to a point where the channel became deeper, and its velocity was suddenly accelerated; the channel became alternately narrower and wider without producing any sensible effect, but when the wave once more reached that part of the channel which was of the original depth it resumed its original velocity. A fact of great practical value was established, that a boat, if raised by a sudden effort to the top of a primary wave, could be drawn along at 10 miles an hour with less fatigue to the horses than if drawn at the rate of 6 miles, while the waste was less severe on the banks of the canal. These investigations were made before the general establishment of railways, when swift canal travelling seemed a desirable attainment. But though boats propelled at high speed on canals have given place to railway carriages, yet the canal traffic at slow speeds must be conducted, and the cheapest means of effecting the " haulage" with the least danger to the banks is still an important inquiry, and has within the last few years afforded matter for some highly interesting papers and statements in the Proceedings of the Institution of Civil Engineers. These are communications on the employment of steam-power on the Gloucester and Berkeley Canal, by G. W. B. Clegram; l on the Grand Canal, Ireland, by Mr Healy; 2 on the Forth and Clyde, by Mr J. Milne; 3 and on the Aire and Calder, by Mr W. II. Bartholomew, 4 , to all which reference is made. One great objection to high speeds on canals is the Wasting wasting of the banks by the displacement produced in the banks propelling the vessel through the water. The wasting, indeed, takes place even with very low speeds, and as a matter of canal engineering it is necessary to notice it. To give an instance of the effect on the large scale: Mr lire says that the river steamers on the Clyde, going at a speed of 8 to 9 miles per hour, produce a swell which commences to rise when the vessel is " 2 or 3 miles off," a circumstance which was first noticed by Mr J. Scott Russell in 1837. The swell gradually increases as the steamer approaches, and at last becoming a wave of translation, it breaks on the river walls nearly abreast of the vessel, following her on her course along the river, as a violent breaking wave, measuring 8 or 10 feet from the hollow in the channel to the crest on the wall. A coating of heavy whinstone rock, from 2 to 3 feet thick, extending from low to high water mark is found necessary to enable the banks to withstand it. Mr Ure also found that the action of passing steamers, though very destructive to the banks, was useful in stirring up the mud from the bottom, which was carried off by the currents to an extent which he estimates to be from 20 to 25 per cent, of the whole quantity dredged from one particular part of the river where he carefully measured it. It will at once be apparent, that however inconvenient these wasting waves may be in a river, the waves in a canal, though smaller, are nevertheless a source of greater anxiety, acting as they do in a narrow artificial channel, formed at some places on high embankments, the failure of which would be attended with serious consequences. The wasting on canals where the traffic is conducted at a moderate speed is found to extend not more than 18 inches to 2 feet, that is 1 foot above and below the water-line, and Mr Clegram states that he has found on the Gloucester Canal that a facing of stone filled into a recess cut in the banks formed a complete protection. Brushwood, as already noticed, is also an effectual remedy. What has recently led to the consideration of the best Steam- means of protecting the banks of canals is the substitution towing. of steam for horse power in working the traffic, which has been entirely successful. The first attempt at using steam- power on canals was made on the Forth and Clyde Canal with Symington s boat, in 1789. Various experiments were made to introduce tugs, but these were ultimately abandoned in favour of steam-lighters, which now in great numbers navigate the canal, and make passages to Leith, Greenock, and other trading ports on the Firths of Forth and Clyde. This system, however, would not suit the trade of the steam- Gloucester Canal, which is chiefly frequented by sea-borne towing o vessels, and steam-towing has been introduced on that Gloucest navigation. The following extracts from Mr Clegram s Canalt paper 5 seem generally applicable to all navigations where towing is to be adopted. He says the ship canal leads from the Severn at Gloucester to the Severn at Sharpness Point. It is 16^ miles in length, and has a depth varying from 18 to 18 feet 6 inches, navigable by vessels of 700 tons register. Prior to the year 1860 all sea-going vessels passing through were towed by horses, the number of horses being regulated by a scale varying from 1 horse for a vessel of 40 tons to 9 horses for a vessel of 420 tons. The cost of this amounted generally to about one farthing per ton per mile on the 1 Minutes of Proceedings of Institution of Civil Engineers, vol. xxvi. p. 1. a }iid^ p. o. 3 ibM^ p. 10.

4 Ibid., p. 25. 6 Ibid., p. 1.