CANALS.
Navigable canals are artificial water-ways. They are distinguished from the natural water-ways by having a very slight fall, in consequence of which the water has only a scarcely-perceptible current; so that boats require to be propelled by some means in both directions, while in natural waters they require propulsion only in going up stream. Nevertheless, nayig-ation by canals is decidedly more advantageous than by natural water-ways, for the reasons that it affords great safety, that the water-level is practically uniform at all times, and that rope-towage presents great conveniences. Furthermore, the course of the canals is much less wind ing than that of most rivers. Canals follow the elevations and depressions of the country through which they pass, being divided into a number of sections, or reaches, placed at different levels, the successive sections being connected with one another by means of locks. Canals may be built either to connect two rivers (or other bodies of water), or to afford a passage around dangerous places in otherwise navigable streams, or to shorten winding or irregular portions of their courses, or, finally, to confer the benefits of water-transportation upon regions naturally destitute of naviga ble water-ways.
Advantages of Canals.—Compared with transportation by roads, canal transportation presents decided advantages, as will become apparent from the statement that, while a horse is capable of drawing- twenty hundred weig-ht over a good road and about two hundred hundredweight on a level tramway, he can draw fifteen hundred hundredweight upon a broad canal. Many countries—Holland, for example—owe much of their commercial and industrial development to the admirable facilities for inland traffic which their systems of canals afford. Canal traffic, however, has lost much of its importance since the introduction of the railway, by which the de mands of commerce for the rapid and regular despatch of commodities are more satisfactorily met than by the canals, whose slower operations are liable to interruption in times of drought, and, in more northerly coun tries, by the winter's ice. In many countries, from this latter cause, canals are available for traffic during only six or eight months of the year. Their advantag,es, however, will appear most favorably where they constitute a series of connected waterways, and their construction is justified where they effect a notable saving in the time of passage between important points, or where they are made to connect large waterways and afford an outlet for the crude agricultural and mineral products of thinly-populated districts. The development of the railway, however, has reduced the sys
tem of canal transportation to a subordinate rank, and the day for planning and constructing large and independent canal systems has passed.
Canal-lozcage.—On a number of the more important European canals the transportation of freight requiring a prompt delivery is done by means of canal-boats propelled by steam; but these consume considerable time in the passage of the locks, and often occasion serious damage to the banks by the washing action of the waves they cause. In certain situations steam towing-vessels have been introduced, though with questionable advantage. A substantial improvement in canal transportation was effected by the introduction of the system of chain- or cable-towage, which has been suffi ciently treated of in what has preceded. (See pp. 3o2, 303.) Locating a Canal.—The preliminary work required in locating the line of a canal is substantially the same as that required in laying out a railway route. The width of the canal must be sufficient to permit two boats easily to pass each other, and its depth should be about one foot deeper than the draught of the heaviest-laden boats it is intended to serve. Where the canal intercepts the course of a brook which cannot be diverted into an other channel, the brook is carried beneath the canal through a culvert of masonry or through iron pipes. Deep valleys arc crossed by aqueducts, which are distinguished from viaducts only in that the highway will require to be impervious to water and to be given a special form adapted to its in tended service. \\There rising land is met, it is possible that a point may be reached where it will be found more advantageous to carry the canal through an underground channel than in a deep open cut. Figure 2 (fii. 52) exhibits a canal tunnel of this description. Such expedients, however, are objectionable, on account of the difficulties of passage, and are there fore resorted to only where they cannot be avoided.