CANAL

CANALS or 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 intitled to public notice. In the Encyclopædia, 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 Ruffel-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 passing of each vessel. The cistern the most elevated is placed 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. 1. 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 two 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-sixth 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 superficies 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 assistance 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, LL. 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 expence of such a boat would be so 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 sort 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 pushed a very little from A towards B, beyond its precise perpendicular swing), and that the joining all round is covered with lifts 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 water-tight; 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. suspended by its chain, and now perfectly detached from all other objects, and balanced by a counterpoise i, which is another coffer exactly of the same size, as low down as the level of the lower reach. From inspection only it is evident, that, in proportion as the one of these weights rises, the other must descend. For the present, then, suppose that the coffer b is by some means rendered more weighty than i; it is plain it will descend while the other rises; and they will thus continue till b comes down to the level of the lower reach, and i rises to the level of the higher one. "Fig. 4. represents a section in the direction A B (fig. 2.), in which the coffer i (seen in both situations) is supposed to have been gradually raised from the level of the lower reach B, to that of the higher A, where it now remains stationary; while the coffer b (which is concealed behind the masonry) has descended in the mean time to the level of the lower reach, where it closes by means of the juncture r s, fig. 6. (which juncture is covered with lifts of cloth, as before explained at m n, and is of course become water-tight) when, by lifting the sluice t, and the corresponding sluice at the end of the canal, a perfect communication by water is established between them. If, then, instead of water only, this coffer had contained a boat, floated into it from the upper reach, and then lowered down, it is very plain, that when these sluices were removed, after it had reached the level of the lower reach, that boat might have been floated out of the coffer with as much facility as it was let into it above. Here then we have a boat taken from the higher into the lower canal; and, by reversing this movement, it is very obvious that it might be, with equal ease, raised from the lower into the higher one. It now only remains that I should explain by what means the equilibrium between these counter-balancing weights can be destroyed at pleasure, and the motion of course produced. "It is very evident, that if the two corresponding coffers be precisely of the same dimensions, their weight will be exactly the same when they are both filled to the same depth of water. It is equally plain, that should a boat be floated into either or both of them, whatever its dimensions or weight may be, so that it can be contained afloat in the coffer, the weight of the coffer and its contents will continue precisely the same as when it was filled with water only: hence, then, supposing one boat is to be lowered, or one to be raised at a time, or supposing one to be raised and another lowered at the same time—they remain perfectly in equilibrium in either place, till it is your pleasure to destroy that equilibrium. Suppose, then, for the present, that both coffers are loaded with a boat in each, the double sluices both above and below closed; and suppose also that a stop-cock n, in the under edge of the side of the lower coffer (fig. 4. and 6.), is opened, some of the water which served to float the boat in the coffer will flow out of it, and consequently that coffer will become lighter than the higher one; the upper coffer will of course descend, while the other mounts upwards. When a gentle motion has been thus communicated, it may be prevented from accelerating, merely by turning the stop-cock so as to prevent the loss of more water, and thus one coffer will continue to ascend, and the other to descend, till they have assumed their stations respectively; when, in consequence of a stop stop below, and another above, they are rendered stationary at the level of the respective canals (A). "Precisely the same effect will be produced when the coffers are filled entirely with water. "It is unnecessary to add more to this explanation, except to observe, that the space for the coffer to descend into must be deeper than the bottom of the lower canal, in order to allow a free descent for the coffer to the requisite depth; and of course it will be necessary to have a small conduit to allow the water to get out of it. Two or three inches free, below the bottom of the canal, is all that would be necessary. "Where the height is inconsiderable, there will be no occasion for providing any counterpoise for the chain, as that will give only a small addition to the weight of the undermost coffer, so as to make it preponderate, in circumstances where the two coffers would otherwise be in perfect equilibrium: but, where the height is considerable, there will be a necessity for providing such a counterpoise; as, without it, the chain, by becoming more weighty every foot it descended, would tend to destroy the equilibrium too much, and accelerate the motion to an inconvenient degree. To guard against this inconvenience, let a chain of the same weight, per foot, be appended at the bottom of each coffer, of such a length as to reach within a few yards of the ground where the coffer is at its greatest height (see fig. 3.); it will act with its whole weight upon the highest coffer while in this position; but, as that gradually descended, the chain would reach the ground, and, being there supported, its weight would be diminished in proportion to its descent; while the weight of the chain on the opposite side would be augmented in the same proportion, so as to counterpoise each other exactly, in every situation, until the uppermost chain was raised from the ground. After which it would increase its weight no more; and, of course, would then give the under coffer that preponderance which is necessary for preserving the machine steady. The under coffer, when it reached its lowest position, would touch the bottom on its edges, which would then support it, and keep every thing in the same position, till it was made lighter for the purpose of ascending. "What constitutes one particular excellence of the apparatus here proposed is, that it is not only unlimited as to the extent of the rise or depression of which it is susceptible (for it would not require the expenditure of one drop more water to lower it one hundred feet than one foot); but it would also be easy to augment the number of pulleys at any one place as to admit of two, three, four, or any greater number of boats being lowered or elevated at the same time; so that let the succession of boats on such a canal be nearly as rapid as that of carriages upon a highway, none of them need be delayed one moment to wait an opportunity of passing: a thing that is totally impracticable where water-locks are employed; for the intercourse, on every canal constructed with water-locks, is necessarily limited to a certain degree, beyond which it is impossible to force it.