CANAL of Communication, an artificial cut in the ground, supplied with water from rivers, springs, &c. in order to make a navigable communication betwixt one place and another.

The particular operations necessary for making artificial navigations depend upon a number of circumstances. The situation of the ground; the vicinity or connection with rivers; the ease or difficulty 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 reduced to a level, artificial methods of raising and lowering vessels must be employed; which likewise vary according to circumstances.

A kind of temporary sluices are sometimes employed for raising boats over falls or shoals in rivers by a very simple operation. Two posts or pillars of masonry-work, with grooves, are fixed, one on each bank of the river, at some distance below the shoal. The boat having passed these posts, planks are let down across the river by pulleys into the grooves, by which the water is dammed up to a proper height for allowing the boat to pass up the river over the shoal.

The Dutch and Flemings at this day sometimes, when obstructed by cascades, form an inclined plane or rolling-bridge upon dry land, along which their vessels are drawn from the river below the cascade into the river above it. This, it is said, was the only method employed by the ancients, and is still used by the Chinese, who are said to be entirely ignorant of the nature and utility of locks. These rolling-bridges consist of a number of cylindrical rollers which turn easily on pivots, and a mill is commonly built near by, so that the same machinery may serve the double purpose of working the mill and drawing up vessels.

A Lock is a basin placed lengthwise in a river or canal, lined with walls of masonry on each side, and terminated by two gates, placed where there is a cascade or natural fall of the country; and so constructed, that the basin being filled with water by an upper sluice to the level of the waters above, a vessel may ascend through the upper gate; or the water in the lock being reduced to the level of the water at the bottom of the cascade, the vessel may descend through the lower gate; for when the waters are brought to a level on either side, the gate on that side may be easily opened. But as the lower gate is strained in proportion to the depth of water it supports, when the perpendicular height of the water exceeds 12 or 13 feet, more locks than one become necessary. Thus, if the fall be 17 feet, two locks are required, each having 8½ feet fall; and if the fall be 26 feet, three locks are necessary, each having 8 feet 8 inches fall. The side-walls of a lock ought to be very strong. Where the natural foundation is bad, they should be founded on piles and platforms of wood; they should likewise slope outwards, in order to resist the pressure of the earth from behind.

Plate CXIV. fig. 1. A perspective view of part of a canal: the vessel L, within the lock A.C.—Fig. 2. Section of an open lock: the vessel L about to enter.—Fig. 3. Section of a lock full of water; the vessel L raised to a level with the water in the superior canal.—Fig. 4. Ground section of a lock. L., a vessel in the inferior canal. C, the under gate. A, the upper gate. G.H, a subterraneous passage for letting water from the superior canal run into the lock. K.F, a subterraneous passage for water from the lock to the inferior canal.

X and Y (fig. 1.) are the two flood-gates, each of which consists of two leaves, resting upon one another, so as to form an obtuse angle, in order the better to resist the pressure of the water. The first (X) prevents the water of the superior canal from falling into the lock; and the second (Y) dams up and confines the water in the lock. These flood-gates ought to be very strong, and to turn freely upon their hinges. In order to make them open and shut with ease, each leaf is furnished with a long lever A.b, A.b; C.b, C.b. They should be made very tight and close, that as little water as possible may be lost.

By the subterraneous passage G.H (fig. 2, 3, &c.) which descends obliquely, by opening the sluice G, the water is let down from the superior canal D into the lock, where it is flopped and retained by the gate C when shut, till the water on the lock comes to be on a level with the water in the superior canal D; as represented, fig. 3. When, on the other hand, the water contained by the lock is to be let out, the passage G.H must be shut by letting down the sluice G, the gate A must be also shut, and the passage K.F opened by raising the sluice K: a free passage being thus given to the water, it descends through K.F, into the inferior canal, until the water in the lock is on a level with the water in the inferior canal B; as represented, fig. 2.

Now, let it be required to raise the vessel L (fig. 2) from the inferior canal B to the superior one D; if the lock happens to be full of water, the sluice G must be shut, and also the gate A, and the sluice K opened. so that the water in the lock may run out till it is on a level with the water in the inferior canal B. When the water in the lock comes to be on a level with the water at B, the leaves of the gate C are opened by the levers C b, which is easily performed, the water on each side of the gate being in equilibrium; the vessel then falls into the lock. After this the gate C and the sluice K are shut, and the sluice G opened, in order to fill the lock, till the water in the lock, and consequently the vessel, be upon a level with the water in the superior canal D; as is represented in fig. 3. The gate A is then opened, and the vessel passes into the canal D.

Again, let it be required to make a vessel descend from the canal D into the inferior canal B. If the lock is empty, as in fig. 2, the gate C and sluice K must be shut; and the upper sluice G opened, so that the water in the lock may rise to a level with the water in the upper canal D. Then open the gate A, and let the vessel pass through into the lock. Shut the gate A and the sluice G; then open the sluice K, till the water in the lock be on a level with the water in the inferior canal; then the gate C is opened, and the vessel passes along into the canal B, as was required.

It is almost needless to spend time in enumerating the many advantages which necessarily result from artificial navigations. Their utility is now so apparent, that most nations in Europe give the highest encouragement to undertakings of this kind wherever they are practicable. The advantages of navigable canals did not escape the observation of the ancients. From the most early accounts of society we read of attempts to cut through large isthmuses, in order to make a communication by water, either between different nations, or distant parts of the same nation, where land-carriage was long and expensive. Herodotus relates, that the Cnidians, a people of Caria in Asia Minor, designed to cut the isthmus which joins that peninsula to the continent; but were superstitious enough to give up the undertaking, because they were interdicted by an oracle. Several kings of Egypt attempted to join the Red-Sea to the Mediterranean. Cleopatra was exceedingly fond of this project. Soliman II., emperor of the Turks, employed 50,000 men in this great work. This canal was completed under the caliphate of Omar, but was afterwards allowed to fall into disrepair; so that it is now difficult to discover any traces of it. Both the Greeks and Romans intended to make a canal across the Isthmus of Corinth, which joins the Morea and Achaea, in order to make a navigable passage by the Ionian sea into the Archipelago. Demetrius, Julius Caesar, Caligula, and Nero, made several unsuccessful efforts to open this passage. But, as the ancients were entirely ignorant of the use of water-locks, their whole attention was employed in making level cuts, which is probably the principal reason why they so often failed in their attempts. Charlemagne formed a design of joining the Rhine and the Danube, in order to make a communication between the ocean and the Black Sea, by a canal from the river Almutz which discharges itself into the Danube, to the Reditz, which falls into the Maine, and this last falls into the Rhine near Mayence; for this purpose he employed a prodigious number of workmen; but he met with so many obstacles from different quarters, that he was obliged to give up the attempt.

The French at present have many fine canals: that of Briare was begun under Henry IV. and finished under the direction of cardinal Richelieu in the reign of Louis XIII. This canal makes a communication betwixt the Loire and the Seine by the river Loing. It extends 11 French leagues from Briare to Montargis. It enters the Loire a little above Briare, and terminates in the Loing at Cepoi. There are 42 locks on this canal.

The canal of Orleans, for making another communication between the Seine and the Loire, was begun in 1675, and finished by Philip of Orleans, regent of France, during the minority of Louis XV. and is furnished with 20 locks. It goes by the name of the canal of Orleans; but it begins at the village of Combleux, which is a short French league from the town of Orleans.

But the greatest and most useful work of this kind is the junction of the ocean with the Mediterranean by the canal of Languedoc. It was proposed in the reigns of Francis I. and Henry IV. and was undertaken and finished under Louis XIV. It begins with a large reservoir 4000 paces in circumference, and 24 feet deep, which receives many springs from the mountain Noire. This canal is about 64 leagues in length, is supplied by a number of rivulets, and is furnished with 104 locks, of about eight feet rise each. In some places it passes over bridges of vast height; and in others it cuts through solid rocks for 1000 paces. At one end it joins the river Garonne near Thoulouse, and terminates at the other in the lake Tau, which extends to the port of Cette. It was planned by Francis Riquet in the 1666, and finished before his death, which happened in the 1680.

In the Dutch, Austrian, and French Netherlands, there is a very great number of canals; that from Bruges to Ostend carries vessels of 200 tons.

The Chinese have also a great number of canals; that which runs from Canton to Pekin extends about 825 miles in length, and was executed about 800 years ago.

It would be an endless task to describe the numberless canals in Holland, Russia, Germany, &c. We shall therefore confine ourselves to those that are either already finished, or at present executing, in our own country.

As the promoting of commerce is the principal intention of making canals, it is natural to expect that their frequency in any nation should bear some proportion to the trade carried on in it, providing the situation of the country will admit of them. The present state of England and Scotland confirms this observation. Though the Romans made a canal between the Nene, a little below Peterborough, and the Witham, three miles below Lincoln, which is now almost entirely filled up, yet it is not long since canals were revived in England. They are now however become very numerous, particularly in the counties of York, Lincoln, and Cheshire. Most of the counties betwixt the mouth of the Thames and the Bristol channel are connected together either by natural or artificial navigations; those upon the Thames and its reaching within about Canal.

20 miles of those upon the Severn. The duke of Bridgewater's canal in Cheshire runs 27 miles on a perfect level; but at Barton it is carried by a very high aqueduct bridge over the Irwell, a navigable river; so that it is common for vessels to be passing at the same time both under and above the bridge. It is likewise cut some miles into the hills, where the Duke's coal-mines are wrought.

A navigable canal between the Forth and Clyde in Scotland, and which divides the kingdom in two parts, was first thought of by Charles II. for transports and small ships of war; the expense of which was to have been £500,000, a sum far beyond the abilities of his reign. It was again projected in the year 1722, and a survey made; but nothing more done till 1761, when then Lord Napier, at his own expense, caused make a survey, plan, and estimate on a small scale. In 1764, the trustees for fisheries, &c., in Scotland caused make another survey, plan, and estimate of a canal five feet deep, which was to cost £79,000. In 1766, a subscription was obtained by a number of the most respectable merchants in Glasgow, for making a canal four feet deep and twenty-four feet in breadth; but when the bill was nearly obtained in Parliament, it was given up on account of the smallness of the scale, and a new subscription set on foot for a canal seven feet deep, estimated at £150,000. This obtained the sanction of Parliament; and the work was begun in 1768 by Mr Smeaton the engineer. The extreme length of the canal from the Forth to the Clyde is 35 miles, beginning at the mouth of the Carron, and ending at Dalmuir Burnfoot on the Clyde, six miles below Glasgow, rising and falling 160 feet by means of 39 locks, 20 on the east side of the summit, and 19 on the west, as the tide does not ebb so low in Clyde as in the Forth by nine feet. Vessels drawing eight feet water, and not exceeding nineteen feet beam and seventy-three feet in length, pass with ease, the canal having afterwards been deepened to upwards of eight feet. The whole enterprise displays the art of man in a high degree. The carrying the canal through moss, quicksand, gravel, and rocks, up precipices and over valleys, was attended with inconceivable difficulties. There are eighteen draw-bridges and fifteen aqueduct bridges of note, besides small ones and tunnels. In the first three miles there are only five locks; but in the fourth mile there are no less than ten locks, and a very fine aqueduct bridge over the great road to the west of Falkirk. In the next five miles there are only four locks, which carry you to the summit. The canal then runs eighteen miles on a level, and terminates about a mile from Glasgow. In this course, for a considerable way the ground is banked about twenty feet high, and the water is fifteen feet deep, and two miles of it is made through a deep moss. At Kirkintilloch, the canal is carried over the water of Logie on an aqueduct arch of ninety feet broad. This arch was thrown over in three stretches, having only a centre of thirty feet, which was shifted on small rollers from one stretch to another; a thing new, and never attempted before with an arch of this size; yet the joinings are as fairly equal as any other part, and admired as a very fine piece of masonry. On each side there is a very considerable banking over the valley. The work was carried on till it came within five miles of its junction with the Clyde; when the subscription and a subsequent loan being exhausted, the work was stopped in 1775. The city of Glasgow, however, by means of a collateral branch, opened a communication with the Forth, which has produced a revenue of about £600 annually; and, in order to finish the remaining five miles, the government in 1784 gave £50,000 out of the forfeited estates, the dividends arising from this sum to be applied to making and repairing roads in the Highlands of Scotland. Accordingly the work has been resumed; and by contract, under a high penalty, must be entirely completed in November 1789. The aqueduct bridge over the Kilvin (now finished, and supposed the greatest of the kind in the world) consists of four arches, and carries the canal over a valley 65 feet high and 420 in length, exhibiting a very singular effort of human ingenuity and labour. To supply the canal with water was of itself a very great work. There is one reservoir of 50 acres 24 feet deep, and another of 70 acres 22 feet deep, into which many rivers and springs terminate, which it is thought will afford sufficient supply of water at all times. This whole undertaking when finished will cost about £200,000. It is the greatest of the kind in Britain, and without doubt will be of great national utility; though it is to be regretted that it had not been executed on a still larger scale, the locks being too short for transporting large rafts.