CANAL, an aqueduct made for the purposcs'of inland navigation. This great improvement in the conveyance of com modities has arrived at a high degree of perfection, and enables us to transport them even overmoxntains,whereitwould appear impossible to preserve a commu nication, or rather a continuity of water carriage with the subjacent plains. This is effected by the means of locks built of masonry, each of which serves as the con junction of two different levels. The locks are made only large enough to ad mit the vessels employed in the business, and have two gates, one at each end. When a vessel should ascend to a supe rior level, the upper gate is shut, and the vessel being brought within the lock,the lower gate is also closed, and the upper one opened. By this means the water flowsm, and the vessel is raised to the in tended height. The upperzate is clos ed as soon as the vessel has passed, but the water in the lock is preserved for the purpose of letting a vessel down, which is done by shutting the upper gate after she is in the lock, and opening the lower one ; so that she is lowered gradually to the next level. The water in all cases is let in or out by means of a small hatch, making its rise and fall very gradual; else the gates would be torn from their hinges by the rush of so large a body, and the vessel would be endan gered. We have instances of about twen ty locks all in half a mile's distance ; but they require very powerful springs to supply a due quantity of water. Some times canals are raised above the level of the country ; and we have instances where one canal passes over another.
The particular operations necessary for making artificial navigations depend upon a numher of circumstances. The situation of the ground ; its vicinity or connection with rivers ; the ease or dif ficulty with which a proper quantity of water can be obtained : these, and many. other circumstances, necessarily produce great variety in the structure of artificial navigations, and augment or diminish the labour and expense of executing them. When the ground is naturally level, and unconnected with rivers, the execution is easy, and the navigation is not liable to be disturbed by floods ; but when the ground rises and falls, and cannot be re duced to a level, artificial methods of rais ing and lowering vessels must be employ ed, which likewise vary according to cir cumstances.
In Mr. Donaldson's " View of the Pre sent state of Husbandry," it is observed, that the canals already completed or forming have had wonderful effects upon the agriculture, as well as upon the manufactures and general state of ma ny parts of the kingdom ; these, and the navigable rivers, render the carriage of bulky articles more easy and less expen sive. The conveyance of manure, fuel, &c. into districts, whither, without that medium, they could scarcely have been transmitted, has tended materially to the improvement ofthese particular districts; and the ease with which the inhabitants can export the produce of the country to otherwise, almost inaccessible markets, while it tends to the same end, has also considerable effects on the general mar kets of the kingdom, and lessens the number of horses that would be requisite for transporting these articles from one place to another.
Owing to some cause or other, inland navigations in many parts of the island have proved ruinous to the adjoining lands ; while in many others the injury done to the soil in the districts through which these inland navigations are car ried, by obstructing the free passage of the rivers to the sea, and by their fre quently overflowing their banks, and de stroying the crops in the low grounds, is infinitely greater than any commercial advantages that can possibly be derived from them, except by those who'are more immediately interested. To render ca nals, or inland navigations of any sort, of general utility, says he,much circumspec tion is necessary in framing the acts of Parliament : so that, while the commerce of the country is increased, its agricul ture may not be injured. It might, he thinks, be a wise regulation, that in eve ry instance, without exception, all sorts of manure should be carried at one half or one third of made payable for articles of any other description. Were this point attended to, and minute investigation made as to the probable consequences that were likely to result from granting leave to form canals, and deepen the beds of rivers, for the pur pose of inland navigations, these means of lessening the expense of carriage would not so often prove injurious to the best interest of the country,----its agricul tural improvement.