CANAL. An artificial channel filled with water, formed for the purpose of draining, irrigation, supplying towns with water, or of inland navigation. The Cay uga Canal, which drains the marshes at the bead of the lake, and empties into the Ontario Lake, is an illustration of the first. Those of Ancient Egypt are the best instances of the second. The supplying stream from the Croton river to the re ceiving' basins in New York, illustrates the third. It is to the fourth kind, that for inland navigation, that the term is now almost wholly restricted.
There is no country in the world where the advantages of canals are more appre ciated than in China. From time imme morial the rivers that intersect that vast empire have been united by innumerable canals ; and the Grand Canal is said to be the most stupendous work of the kind that has ever been executed. Russia, too, exhibits a remarkable degree of en terprise in the construction of canals for the purpose of inland navigation ; and though innumerable difficulties peculiar to that country for a long period impeded the progress of works of this description, that empire is now traversed by an un broken line of water communication from St. Petersburg to the Caspian Sea.
The section of a canal is usually a tra pezium, of which two sides are parallel and horizontal, and the other two equally inclined to the horizon. The inclination depends on the nature of the soil. It is least in tenacious earth, and greatest in loose soil ; but no soil will maintain itself unless the base of the slope exceeds its height at least in the ratio of four to three. In loose soils the base requires to be twice as great as the height.
A canal is usually confined between a bank on one side, and a towing path on the other, the breadth of whose upper surface must be sufficient for a road on which the animals employed in draught may easily pass. This requires the breadth o? the upper surface to be at least 9 feet. The usual mode for the other bank is to make the breadth at top equal to the height, measured from the bottom of the canal: but in this case there should be a berm of a foot, or a foot and a half, at the level of the water, which increases the thickness of the bank at bottom, and prevents the wash of the banks from falling into the canal. To prevent the entrance of rain-water, a counter-ditch is formed on the outside of each of the banks. The form of a well constructed canal will therefore present the following figure :— The dimensions of navigable canals must depend on the size of the vessels in tended to navigate them. In order that they may enable two vessels to pass each other with freedom, the breadth at bot tom is usually made twice as great as the breadth of the beam of the vessels ; the depth requires to be at least one foot more than the vessel's draught of water.
The bed of a canal must be absolutely level, or have no more slope than is ne cessary to convey water to replace that which hasbeen wasted. Hence, when a canal intersects a sloping country in a se ries of channels at different levels, means must be provided to enable vessels to pass from one level to another. This is commonly effected by means of a lock.
The invention of locks as a means of carrying a canal through an undulating country has given an entirely new fea ture to the inland navigation of Europe. Various nations have claimed the honor of this invention ; but it would appear that the controversy which has arisen on the subject is not yet settled. A lock
is a chamber, formed of masonry, occu pying the whole bed of the canal where the difference of level is to be overcome. This chamber is so contrived that the level of the water which it contains may be made to coincide with either the upper or lower level of the canal. This is effected by two pairs of gates, One of which pairs is placed at each end of the chamber of the lock. By this means, while the gates at the lower end of the chamber are opened, and those at the upper end are closed, the wa ter in the chamber will stand at the lower level of the canal; and on the con trary, when the lower gates are closed, and the upper ones are opened, the level of the water in the lock will coincide with the level of the water in the upper part of the canal. In the first case, a boat may be floated into the lock from the lower part of the canal; and if then the gates be closed, and water is admitted into the lock from the upper level unite the surfiice of the lock is in a line with the water above, the boat will be floated up, and on the opening of the upper gates may be passed onward. By revers ing the course of procedure, boats may be as readily conveyed from the upper to the lower level. (See Lem.) The supply of water required for main taining a canal depends on the lockage or quantity wasted in passing a vessel through the locks, on the evaporation from the surface, and on the leakage. It has been found by experiment that the annualquantity of evaporation from the canal orLanguedoe is 32 inches; that is to say, the body of water required to supply this waste is equal to a parallelo piped whose base is the whole surface of water in the canal, and whose altitude is 32 inches : in most calculations it has been customary to take this altitude at 86 inches. With respect to the leakage, when the soil is porous the inner surface of the banks may be lined with an earth retentive of water, or a portion of the middle of each bank may be built up with earth of this character. The opera tion of lining a bank with clay, or earth retentive of moisture, is called puddling.
The advantages derived from canals are now so generally known and acknow ledged, as to render it almost superfluous to allude to the question. The beneficial effects of canals are felt in a greater or less degree by all classes of society : by their means the manufacturer is enabled to collect his materials and his fuel with less labor and expense ; the farmer ob tains a supply of manure at a cheap rate, and a ready conveyance of his produce to the most profitable market; and the merchant is enabled to extend his com merce by exporting greater quantities and varieties of goods from places remote from sea, and by more easily supply ing a wider extent of inland country with articles of foreign produce. In short, general arguments in favor of ca nals are superseded by the rapidly im proving and thriving state of all the ci ties, towns, and villages in their neigh borhood; while the great works of every kind to which they have been conducted, and to which a large portion of them owe their rise, are their best recommenda tion. Experience has shown that the formation of railroads does not yet su persede the necessity for canals, as where cheapness, and not expedition, is re quired, the canal will be preferred. The general introduction of steam propulsion on our canals would be of great service. It has been tried on the Erie, and Chesa peake, and Ohio Canals.