Canals of Communication may be of such advantage in a commercial or agricultural country, that every attempt to render them more convenient, and less expensive in the construction, is entitled to public notice. In the Encyclopedia, an account, sufficiently perspicuous, is given of the common canals with locks; but in many cases it is very difficult to provide a sufficient quantity of water for the consumption of a canal where many boats are to pass. Different attempts have therefore been made, by ingenious men, to save water in the passing of boats or lighters from one lock of a canal to another; and, among these, perhaps none is more deserving of public favour than the following, by the late Mr James Playfair of Ruffell-Street, architect. We shall state his invention in his own words.

"The nature and principle of this manner of saving water consist in letting the water which has served to raise or fall a boat or barge from the lock, pass into reservoirs or cisterns, whose apertures of communication with the lock are upon different levels, and which may be placed or constructed at the side or sides of the lock with which they communicate, or in any other contiguous situation that circumstances may render eligible; which apertures may be opened or shut at pleasure, so that the water may pass from the lock to each reservoir of the canal, or from each reservoir to the lock, in the following manner: The water which fills the lock, when a boat is to ascend or descend, instead of being passed immediately into the lower part of the canal, is let pass into these cisterns or reservoirs, upon different levels; then, their communications with the lock being shut, they remain full until another vessel is wanted to pass; then, again, the cisterns are emptied into the lock, which is thereby nearly filled, so that only the remainder which is not filled is supplied from the higher part of the canal. Each of these cisterns must have a surface not less than that of the lock, and must contain half as much water as is meant to be expended for the passage of each vessel. The cisterns must be elevated twice its own depth (measuring by the aperture, or communicating opening of the cistern) under the level of the water in the higher part of the canal. The second cistern is placed once its own depth under the first, and so on are the others, to the lowest; which last is placed once its own depth above the level of the water in the lower part of the canal. The apertures of the intermediate cisterns, whatever their number may be, must all be equally divided into different levels; the surface of the water in the one being always on the level of the bottom of the aperture of the cistern which is immediately above. As an example of the manner and rule for constructing these cisterns, suppose that a lock is to be constructed twelve feet deep, that is, that the vessel may ascend or descend twelve feet in passing. Suppose the lock sixty feet long and six feet wide, the quantity of water required to fill the lock, and to pass a boat, is 4320 cubic feet; and suppose that, in calculating the quantity of water that can be procured for supplying the canal, after allowing for waste, it is found (according to the number of boats that may be expected to pass) that there will not be above 800 cubic feet for each; then it will be necessary to save five-sixths of the whole quantity that in the common case would be necessary: to do which ten cisterns must be made (the mode of placing which is expressed in the drawing, fig. I. Plate VII.), each of which must be one foot deep, or deeper at pleasure, and each must have a surface of 360 feet square, equal to the surface of the lock. The bottom of the aperture of the lowest cistern must be placed one foot above the level of the water in the lower part of the canal, or eleven feet under the level of the high water; the second cistern must be two feet above the level of the low water; the third three feet, and so on of the others; the bottom of the tenth, or uppermost cistern, being ten feet above the low water, and two feet lower than the high water; and, as each cistern must be twelve inches in depth, the surface of the water in the higher cistern will be one foot under the level of the water in the upper part of the canal. The cisterns being thus constructed, when the lock is full, and the boat to be let down, the communications between the lock and the cisterns, which until then have all been shut, are to be opened in the following manner: first, the communication with the higher cistern is opened, which, being at bottom two feet under the level of the water in the lock, is filled to the depth of one foot, the water in the lock descending one foot also at the same time; that communication is then shut, and the communication between the lock and the second cistern is opened; one foot more of the water then passes into that cistern from the lock, and fills it; the opening is then shut; the same is done with the third, fourth, fifth, sixth, seventh, eighth, ninth, and tenth, cisterns, one by one, until they they are all filled; and, when the tenth, or lowermost cistern, is filled, there remains but too feet depth of water in the lock. The communication between the lock and the lower part of the canal is then opened, and the last two feet depth of water is emptied into the lower part of the canal. By this means, it is evident, that instead of twelve feet depth of water being let descend into the lower part of the canal, there is only two feet depth that descends, or one-fifth of the whole; therefore, instead of 4320 cubic feet being used, there are only 720 cubic feet used: the remainder of the water in the cisterns being used as follows. When another boat is to mount, the sluices being then shut, and the boat in the lock, the tenth or lowermost cistern is emptied into the lock, which it fills one foot; the communication being then shut, the next lowest cistern, or the ninth, is emptied into the lock, which is thereby filled another foot; and so, in like manner, all the other cisterns are emptied one after another, until the higher cistern being emptied, which fills the tenth foot of water in the lock, there remains but two feet of water to fill, which is done from the upper part of the canal, by opening the higher sluice to pass the boat; by that means, the same quantity of water descends from the upper part of the canal into the lock, that in the other case descended from the lock into the lower part of the canal; so that, in both cases, the same quantity of water is saved, that is, five-sixths of what would be necessary were there no cisterns. Suppose again that, upon the same canal, and immediately after the twelve feet lock, it would be advantageous to construct one of eighteen feet; then, in order not to use any greater quantity of water, it will be necessary to have sixteen cisterns upon different levels, communicating with the lock in the same manner. Should, again, a lock of only six feet be wanted, after that of eighteen, then it will only be necessary to have four cisterns on different levels, and so of any other height of lock. The rule is this: for finding the number and size of the cisterns, each cistern being the same in supercicies with the lock, its depth must be such as to contain one half the quantity of water meant to be used in the passing of one boat. The depth of the lock, divided by the depth necessary for such a cistern, will give, in all cases, the whole number of cisterns, and two more: deduct the number two, therefore, from the number which you find by dividing the depth of the lock by the depth of one cistern, and you have always the number of cisterns required; which are to be placed upon different levels, according to the rule already given. The above is the principle and manner of using the lock, for saving water in canals, and for enabling engineers to construct locks of different depths upon the same canal, without using more water for the deep locks than for the shallow ones. With regard to the manner of disposing the cisterns, the circumstances of the ground, the declivity, &c. will be the best guide for the engineer.

But supposing a sufficiency of water, or admitting that this method of Mr Playfair's of saving it, where defective, is adequate to his fondest expectations—still, in passing numerous locks, where the rise is considerable, the interruption is so great, that it has often been wished that an eligible method of lowering and elevating boats could be devised, without the affluence of water-locks. Though this is evidently at first view practicable; and several different modes of doing it have been suggested, some of which have actually been carried into effect, yet all of them have been found to be attended with such inconvenience as to render an improvement in this respect still necessary.

In China, where water-carriage is more generally practised than in any kingdom of Europe, boats are raised and lowered from one canal into another by sliding them along an inclined plane; but the contrivances for effecting that purpose are so awkward, and such a number of hands are required, that it has in general been deemed inexpedient to resort to that mode of practice in Europe. Several devices, that discover considerable ingenuity, however, have been published, with a view to facilitate this operation: either by rendering the motion up the inclined plane more equable, or producing a power sufficient to move these great weights. But none of them have yet been so simple in their construction as could be wished, nor have they afforded satisfaction in practice. For the greater part of them, likewise, patents have been granted; so that whatever be their value, no engineer could avail himself of them without previously purchasing a licence from the patentee.

The following contrivance for this purpose is the invention of James Anderson, L.L.D., whose knowledge of economics is well known, and of whose public spirit there cannot be a doubt. Instead of applying for a patent, to secure to himself the fruits of his ingenuity, he published, for the good of his countrymen in general, his device, in the View of the Agriculture of the County of Aberdeen, which he drew up for the consideration of the board of agriculture. He introduces it to public notice with justly observing, that it possesses at least the merit of simplicity, in as high a degree, perhaps, as could be wished; and, "in the opinion (says he) of very good judges of matters of this sort, to whom the plan has been shewn, it has been deemed fully adequate to the purpose of raising and lowering boats of a moderate size, that is, of 20 tons, or downwards; and it is the opinion of most men with whom I have conversed, who are best acquainted with the inland navigations, that a boat of from 10 to 15 tons is better than those of a larger size. When several are wanted to be sent at once, they may be affixed to one another, as many as the towing-horse can conveniently draw. Were boats of this size adopted, and were all the boats on one canal to be of the same dimensions, it would prove a great convenience to a country in a state of beginning improvements; because the expense of such a boat would be trifling, that every farmer could have one for himself, and might of course make use of it when he pleased, by the aid of his own horse, without being obliged to have any dependence on the time that might suit the convenience of his neighbour; and if two or more boats were going from the same neighbourhood, one horse could serve the whole.

"You are to suppose that fig. 2. (Plate VII.) represents a bird's-eye view of this simple apparatus, as seen from above. A is supposed to be the upper reach of the canal, and B the lower reach, with the apparatus between the two. This consists of three divisions: the middle one, extending from C to D, is a solid piece of masonry, raised from a firm foundation below the level of..." of the bottom of the second reach; this is again divided into five parts, viz. d d d, where the wall rises only to the height of the water in the upper reach, and e e, two pillars raised high enough to support the pivots of a wheel or pulley g, placed in the position there marked.

"The second division b consists of a wooden coffer, of the same depth nearly as the water in the upper reach, and of a size exactly fitted to contain one of the boats. This communicates directly with the upper reach, and being upon the same plane with it, and so connected with it as to be water-tight, it is evident, from inspection, that nothing can be more easy than to float a boat into this coffer from the upper reach; the part of the wheel that projects over it being at a sufficient height above it, so as to occasion no loss of interruption.

"Third division. At i is represented another coffer, precisely of the same dimensions with the first. But here two sluices, which were open in the former, and only represented by dotted lines, are supposed to be shut, so as to cut off all communication between the water in the canal and that in the coffer. As it was impossible to represent this part of the apparatus on so small a scale, for the sake of illustration it is represented more at large in fig. 5, where A, as before, represents the upper reach of the canal, and b one of the coffers. The sluice k goes into two cheeks of wood, joined to the masonry of the dam of the canal, so as to fit perfectly close; and the sluice f fits, equally close, into cheeks made in the side of the coffer for that purpose; between these two sluices is a small space o. The coffer, and this division o, are to be supposed full of water, and it will be easy to see that these sluices may be let down, or drawn up at pleasure, with much facility.

"Fig. 6. represents a perpendicular section of these parts in the same direction as in fig. 5, and in which the same letters represent the same parts.

"Things being thus arranged, you are to suppose the coffer b to be suspended, by means of a chain passed over the pulley, and balanced by a weight that is sufficient to counterpoise it, suspended at the opposite end of the chain. Suppose, then, that the counterpoise be made somewhat lighter than the coffer with its contents, and that the line m n (fig. 6.) represents a division between the solid sides of the dam of separation, which terminates the upper reach, and the wooden coffer, which had been closed only by the pressure of its own weight (being pulled a very little from A towards B, beyond its precise perpendicular swing), and that the joining all round is covered with laths of cloth put upon it for that purpose; it is evident that, so long as the coffer is suspended to this height, the joining must be watertight; but no sooner is it lowered down a little than this joining opens, the water in the small division o is allowed to run out, and an entire separation is made between the fixed dam and this moveable coffer, which may be lowered down at pleasure without losing any part of the water it contained.

"Suppose the coffer now perfectly detached, turn to fig. 3., which represents a perpendicular section of this apparatus, in the direction of the dotted line p p (fig. 2.). In fig. 3. b represents an end view of the coffer, indicated by the same letter as in fig. 2. suspend- For example: suppose a hundred boats are following each other, in such a rapid succession as to be only half a minute behind each other: By the apparatus here proposed, they would all be elevated precisely as they came; in the other, let it be supposed that the lock is so well constructed as that it takes no more than five minutes to close and open it; that is, ten minutes in the whole to each boat (for the lock, being once filled, must be again emptied before it can receive another in the same direction): at this rate, six boats only could be passed in an hour; and of course it would take fifteen hours and forty minutes to pass the whole hundred; and as the last boat would reach the lock in the space of fifty minutes after the first, it would be detained fifteen hours and fifty minutes before its turn would come to be raised. This is an immense detention; but if a succession of boats, at the same rate, were to follow continually, they never could pass at all. In short, in a canal constructed with water-locks, not more than six boats, on an average, can be passed in an hour, so that beyond that extent all commerce must be stopped; but, of the plan here proposed, sixty, or six hundred, might be passed in an hour if necessary, so as to occasion no sort of interruption whatever. These are advantages of a very important nature, and ought not to be overlooked in a commercial country.

This apparatus might be employed for innumerable other uses as a moving power, which it would be foreign to our present purpose here to specify. Nor does its power admit of any limitation, but that of the strength of the chain, and of the coffers which are to support the weights. All the other parts admit of being made so immovably firm as to be capable of supporting almost any assignable weight.

I will not enlarge on the benefits that may be derived from this very simple apparatus: its cheapness, when compared with any other mode of raising and lowering vessels that has ever yet been practised, is very obvious; the waste of water it would occasion is next to nothing; and when it is considered that a boat might be raised or lowered fifty feet nearly with the same ease as five, it is evident that the interruptions which arise from frequent locks would be avoided, and an immense saving be made in the original expense of the canal, and in the annual repairs.

It is also evident, that an apparatus, on the same principle, might be easily applied for raising coals or metals from a great depth in mines, wherever a very small stream of water could be commanded, and where the mine was level-free.