matter floated into them through the weir or syphon. Some basins are released from overflowing by a waste gate; this is the case with the basin to the Oxford canal at Oxford. This basin is so near the river Isis as to admit of an easy communication with it; and, availing itself of this circumstance, the bank between them has Been pierced through, and the embankment at the piercing right and left has been faced by masonry. See plate I. fig. 7. From the waste gate, VV, the walls right and left are splayed off at the embankments of the canal approaching the basin, and turned circular at their opposite ends, a rebate is left in the masonry for the gates to hang in at W W. The tops of the walls are coped with strong freestone, and the gates X are of oak, but without paddles. Returning again, to the basin, it will be necessary to observe that guard-rails are found necessary to be fixed against their walls to prevent the boats from striking against them. The guard-rails consist of a series of strong oaken piles driven down into the bed or bottom of the basin, slanting to the side of it at intervals of about ten feet; and on their facing near their tops are broad planks or rails of oak strongly bolted to them with iron bolts. These generally form a continued chain, traversing_the whole facing or embankment of the basin. Iron rings are fixed into the rails or piles at convenient intervals, for the purpose of allowing the bargemen to lash their boats to, while taking out their cargoes. In some cases large stones are worked in the walls for this purpose, and called bumpingstones, to which rings may be fixed for the lashing to of the boats.

It will be necessary, in this place, to notice the several contrivances had recourse to for supporting the water in a canal, against accidents to its embankments, and other unforeseen events arising out of the imperfection of the means employed for such purpose. Safety-gates are among the expedients made use of in such a dilemma; they are a contrivance for stopping the water in a long line of a canal when there is danger of the embankments giving way. Plate I., fig. 8, shows the plan of a safety gate or gates: P, P, the walls of masonry or brickwork built in each opposite bank of the canal; R, R, R, R, piers of masonry to strengthen their extreme ends; E, E, small projections worked up to stop and hinge the gate to; K sinkings in the wall to admit of the gate laying in flush with the wall; D the safety-gate. The plan is shown with two gates, one for the purpose of supporting the water in the upper, and one also for the lower reach of the canal; and these can be shut as required by the repairs to be done, whether up or down the canal. These gates move upon the same principles as lockgates, and require a similar contrivance, excepting that they are commonly in a single gate only. The walls of the safety-gate should be built on piled foundations of adequate substance at bottom, racking back outside, and battering inside, with a slight curvature: they should also be coped at their tops. The gates should be of good sound oaken wood, and prepared in a simiLar manner to lock-gates. See plate I., fig. 6. Advantage is sometimes taken, for the purpose of economy, of forming the safety-gates in the

pier walls about the bridges, if bridges of masonry happen in eligible places for the purpose. They are of the greatest utility on a canal, and claim particular attention in arranging the most eligible site for their erection; and their mode of building should be of the most substantial kind. They are indispensable in long levels, to protract dilapidation, where the cuttings are much embanked

There is also a contrivance of a similar nature called stop-gates, the construction of which does not differ materially from the safety-gate, except in its being made to lie flat at the bottom of the canal, instead of being balanced above, as is the case with safety-gates. The mode of raising the stop-gate is by a chain, which is fixed to the gate under water, and, when it is required to be raised, the chain is used for that purpose. Stopplanks are also often employed on canals for stopping water; they are a very simple contrivance, and consist in previously working up walls in the two opposite banks, formed with a groove or chase, into which the stop-planks are forced, and pressed down to the bottom of the canal. These answer the purpose effectually on narrow canals. Stop-bars are another contrivance, similar in manner to stop-planks, the walls for which are grooved to receive the bar. These are used as toll-bars, at the toll-offices on the canal line, and are to be so contrived as to be opened and shut by the overseers attending at the toll-houses. They may be esteemed the turnpike gates of a canal.

Aqueducts are frequently employed on a canal, for the purpose of extending it over rivers, and between two opposite ridges of high land. For this latter purpose they were often erected by the Romans, to convey water for their baths and fountains, as the ruins of many of which, still in existence, fully demonstrate. But the Roman aqueducts were never intended for any other purpose than to convey water for the people's use; hence they were confined in their dimensious, and were a little more than long narrow walls, with a void through them as a passage for the water. The aqueduct at Chapanost, near Lyons, is raised upon arches of masonry, on the tops of which is a narrow channel for the water, arched over at top, the size of which is six feet high and three feet wide, lined in the inside with a lining of strong cement about six inches in thickness, which is quite perfect even at this time. There is another at Montpelier, which passes the river De Baunon, and crosses the valley, of a similar construction. Louis le Grand ordered an aqueduct, which is built after the same manner, and which conveys the water to Versailles. They are also numerous in every part of Italy, and wherever else the Romans extended their power; but since the discovery of Galileo, which demonstrated the important effects of the weight and pressure of the atmosphere, aqueducts in the Roman manner have become useless; as his discoveries showed that water would not only elevate itself to the syphon line, but might be raised to about thirty-four feet above that line, by the means only of the atmospheric pressure.

Plate II. fig. 1, ENGINEERING, is the plan of a design of an aqueduct bridge to cross a navi

gable river, supposed of 300 feet wide. A, A, the piers; B, B, the wing piers and walls; CCC the water way; the longitudinal dotted lines show the course of the aqueduct in the superstructure. Fig. 2 is the elevation: it has been deemed proper to show a bridge of this description, with some reference to architectural design, which is too often neglected, when the expense would not be increased by attending to it, as whether a stone or brick is placed in one way or another, there can be no difference in its expense by such placing, but in this placing arises all the difference between the artist and the artisan. Fig. 3 is the cross section of the bridge; C the spring of the centre arch, showing the splaying sides and its intrados: H the solid masonry above the springing line; D, D, the embankments and towing-path to the aqueduct; EE the width of the water way at top; FF the width of the water way at bottom. The rounded parapets I,I, are proposed to be raised above the towing path right and left, as a protection to the passengers moving on the sides of the aqueduct. The construction of an aqueduct bridge requires all the talents necessary to be displayed in the erecting of other bridges, with the additional skill of giving to the road way water-tight qualities. The piers of a bridge of this description should stand on piled foundations, in order to give them the greatest firmness, and the usual process of erecting pieces of masonry in water should be followed. Caissons, or water-tight boxes, should be made to raise the piers in, until they are built above the top-water level; the abutment piers should be formed behind a coffer-dam, and the whole of these parts of the substructure should be raised to a similar height previously to raising the centering on which the arch is to be built. For a particular description of the caisson, coffer-dam, and other details connected with bridge-building, see the article MASONRY.

Stone is the eligible material for building aqueducts, inasmuch as a greater firmness will result to the parts, and without a firmness and equilibration in it, the aqueduct will be subject to leakage and dilapidation. The embankments at the two opposite ends or wings should turn somewhat outwards, to allow of more easy approach, as is common in most bridges. The sides or banks of the aqueduct, crossing the bridge, should be formed of solid masonry or brick-work, and of good substance, battered over as they approach their tops, and somewhat curved, which will add to their strength and solidity. The edges of the wall should be coped, and the banks or towing-paths paved. The lining of the aqueduct should be conducted in a similar manner to puddling, excepting only it should not be common lining of that nature. Parker's cement makes an excellent lining, and is perfectly watertight this might be laid on the masonry to a consistence of about an inch in thickness, smoothly spread, to the face of which a coating of common puddling might be applied, which together would be a secure protection against leakage.

The multiplication of canals in every part of the country, with the experience arising out of it, has produced the application of new buildings of various kinds in aid of them: and there has

been an aqueduct erected to the Shrewsbury Canal, composed almost wholly of cast-iron, and which may be considered as the first of the kind ever formed. In the Rev. Mr. Plimley's Report of the Agriculture of Shropshire, is the following account of it:-'The canal passes the valley of Tern at Long, for a distance of sixty, two yards, upon an aqueduct made of cast-iron, excepting only the nuts and screws, which are of wrought iron; and I believe this to be the first aqueduct for the purposes of a navigable canal which has ever been composed of this metal. It has completely answered the intention, although it was foretold that the different degrees of heat and cold would be such as to cause expansion and contraction,' from which it was concluded to be improper for this purpose. It is necessary only to observe, that the objections to iron were, founded in fact, as all metals are more or less influenced in their form by different degrees of temperature; and it was extremely probable, in the situation of an aqueduct, which is exposed so much to the vicissitudes of heat, cold, and oxidation, that it might turn out to be improper for that purpose. Time can only be the test of the propriety of establishing aqueducts of cast iron. Nevertheless, there are several at this time. erecting, but, in some of these, there is a greater combination of stone, and also of wood, than has been employed in the one at the valley of Tern. Mr. Fulton, in his treatise on canal navigation, proposes that the butments and piers should be raised of stone, after which it will be necessary to extend pieces of timber across the span, and each to be traced back and covered with planks, to form a stage or scaffolding; on this is fixed the iron work of the aqueduct which may be all cast in open sand, and of the following dimensions; for instance, the span 100 feet, and the versed sine of the arch one-sixth of the span; then three segments of a circle, each in three. pieces, about thirty-six feet long, eight inches by four inches diameter, and to be united; then three straight bars to extend from one pier to another, to be of the above diameter, may also be cast in three pieces, and which bars are to extend along the tops of the segments to the piers, and form a line parallel to the horizon. The bars and segments to be united by perpendicular stirrups, ten or fifteen feet distant from each other. The mortise in the lower end of the stirrup, being thirteen inches long, will be sufficient to secure the segment, and leave room for a hole two inches square, through which a cross brace is to pass and fasten the segment at proper distances. The brace to have a mortise cast on each side of the stirrup, in order to allow of tightening the work by wedges. The trough plates should be at least one inch thick; the side plates six feet wide, and as long as can conveniently be cast, which may be twelve feet, and perhaps more, the flange to be made outside of these plates. The bottom plates may be six feet wide and thirteen feet long, and, in order to support the horsepath, two of these plates laid across the stage and screwed together, with a flange under them, will compose a length equal to one of the side plates. The whole may in this manner be screwed together on packings of wool and tar, and the

seams pitched. On the plates composing one side of the trough, brackets about three feet from the top must be cast and fixed, in order to support the horse-path: perpendicular rails, eight feet long, being raised from the arms of the bottom plates will support the outside of the horsepath. It is added, that, by pursuing this method of forming an aqueduct of cast iron, very few patterns will be required; two will be sufficient for the trough-plates, and but few will be required for the springs, rails, and spurs.

Mr. Jessop undertook a most stupendous work of this kind on the Ellesmere Canal, for crossing the Dee River, in which are employed nineteen ponderous pillars of stone, at fifty-two feet distance from each other, the centre one of which is 126 feet high. On the tops of these pillars are supported a number of elliptical cast-iron ribs, which, by means of uprights and horizontal bars, support an aqueduct of cast iron 329 yards long, twenty feet wide, and six feet deep, formed of massy sheets of cast iron cemented and riveted together, having, on its southern side, an iron platform for the horse or towing path. It will be necessary to observe, that every aqueduct ought to be provided with stop-gates at the most convenient parts of its length, as well as syphons or other means to drain off the water if required, for repairs or accidents. The stop-gates will be necessary to effect this purpose, and provision should be made for their erection in the first beginning of the work. They are most usually placed at the approaches or wings of the aque duct.

Tunnels, or subterraneous passages, are now become familiar, as a means of conveying canals through ridges of high ground and mountains; they are also formed as roads through hills and under the beds of great rivers, to keep up easy communications between parts otherwise inaccessible, except by circuitous routes. Perforations for this purpose were not unknown to the ancients, for we find the Romans frequently making them in order to carry forward their aqueducts. For this purpose they were not required on a large scale, but, although small, they gave rise to the practicability which, to the ingenious, was a sufficient stimulus to create the means of performing greater undertakings; and which has now been acquired and effected through the multiplied local impediments which have presented themselves to inland navigation. The first subterraneous canal or tunnel ever made for the navigation of boats, was at Beziers, on the Languedoc Canal in France; and it is believed the first in England was at Worsley, by the late duke of Bridgewater, but this was to establish a communication to his coal mines only. However, there are now almost as many tunnels as there are canals, and the business of making them is so well understood, that they are set about with as few preliminaries as the deep cutting of a

canal.

Previously to beginning a tunnel, the hill through which it is to be formed should be bored in several parts, to ascertain the nature of the soil to be excavated. This being well ascertained, the needful tools and machinery can be collected, that the work may proceed without delays. In

setting out a tunnel, it is necessary to observe, that it must be quite straight from one end to the other. It tracing the line of it over the surface of the high ground, a number of small round poles are made use of, their tops painted white, to give them a more obvious direction to the sight: these are set up firmly with braces, at about 150 yards apart, and called, by the navigators, excavators, or miners' bench-marks. When the line of the tunnel is accurately traced out, another row of bench-marks should be fixed parallel and opposite to the first, at a small distance vertically from the greatest diameter of the tunnel. These bench-marks will be the places where the shafts or pits are to be sunk, for the purpose of assisting the excavators or miners, and also of allowing the soil to be raised through. In sinking the shafts, which should be at least seven feet in diameter, and as deep perpendicularly as the lowermost line of the bottom of the tunnel, a kirb should be made of wood, consisting of two circular ribs, and placed four or five feet from each other, and boarded over. With this kirb the shafts can be sunk to any proposed depth, without danger of the ground giving way; and the steining of the sides of the shafts with brick-work may or may not be had recourse to, in proportion to the strength of the soil, which, if good, may be supported by braces and planks only. If the places of the shafts are all marked out at the time of the tracing of the tunnel line, and sunk a small distance into the surface of the hill, they will, in the event of the bench-marks being broken down, remain as guides to the

excavator.

After the work has arrived so far, and every other needful arrangement is made, in collecting machinery for removing the soil, the work should be begun by cutting down the headings on each side of the hill intended to be perforated; which should be excavated, and the soil removed, to allow of getting quite up to the approaches of the tunnel. When the approaches are made, the form of the tunnel should be traced on the section of the hill, either by a mould or otherwise. The work having so far proceeded, the perforating is to commence; and this is done by opening a communication from the first or second shaft into the ground in the line of the tunnel, and working opposite to it right and left. Preparation should now be made at the top of the shaft for raising up the soil, and this is done in several ways, the most common of which is to station at the top four or more men, standing on a platform raised three or four feet from the ground; these men turn the handle which is at both ends of a roller, and to which a rope is fastened, with a square bucket or buckets appended to each end of it; they are so regulated in size, as that two can move in the shaft or pit at once: the men in working the roller, that is by turning it round, raise one bucket while another descends, and in this manner they keep raising the soil as fast as it is excavated. In some cases, a horse-gin or turn-beam is employed, similar to those used at coal mines. As the tunneling proceeds, the upper ground should be well propped with pianks, and every part secured; as the miners advance in the line, abundance of ribs also

should be prepared to turn the arch of the tunnel upon, and moulds and trammels made for the inverted arch forming the bottom.

The section of a tunnel consists in forming an inverted arch at its bottom, composed of masonry a: brickwork: this arch is generally the segment of a circle, and is laid on a good bed of puddling. On the two ends of this arch rise the soffit arch of the tunnel, which in figure is commonly a simple catenaria, a parabola, or ellipse, of sufficient elevation to admit of boats riding easily through it. Plate II. fig. 4, is the section of a tunnel: A the inverted arch at bottom; B, B, the foundations of the soffit arch D, D; E the shaft or pit for taking up the soil excavated from the tunnel; F, F, the guard-rails and chockblocks fixed to the sides of the tunnel to keep off the boats. The dotted line at C is the drain or sough to convey away the percolating waters; G the soil to be rammed in upon the soffit arch. Plate JI. fig. 5, is the plan of the inverted arch to a larger scale. This arch is commonly wrought with a trammel rod, which consists, if executed in brickwork, in setting up a vertical piece of wood, to which another piece is adjusted by a centre which allows of an easy motion right and left, and describes the figure of the segment by its motion. A is the trammel rod; B the vertical shaft to which it is fixed. The trammel can be shifted at pleasure, and is an easy contrivance for forming inverted arches. Plate II. fig. 6, is the section of the soffit arch of a tunnel to a larger scale, showing it with its centering. The beam A may be fixed upon the chock-blocks previously worked into the inverted arch, at F, fig. 4. The queens D, D, may be notched into the back of the beam A, or mortised through it and wedged with sliding wedges. The strutts B, B, may be of one and a half or two inch deal, bolted with nuts and screws to the ribs G, G, and to the queens D, D. The same bolt which fastens the stitts to the ribs, may be made also to fasten the lap-joints in the ribs; the straining beam O may be in two thicknesses put on to each side of the ribs and heads of the queens, with a shoulder to the latter, and one bolt in each will suffice to fasten the whole. The meeting of the centre or rib, at the apex E, should be lapped and bolted with a nut and screw. This may be braced if required. The void F will be found useful to the workmen, in allowing them an easy access into the tunnel to notice the state of the work, and also to see if any settlement is taking place in the centering, &c., which if found weak, cross-braces may easily be fixed in it.

It is not common to make a horse-path in a tunnel, and this is one of the greatest inconveniences attending on boats passing through them, as the bargemen are obliged to scramble through as they can, by placing their hands against the walls, and pushing on their boat in any way. It would be perfectly easy to remove this inconvenience, by making the diameter of the tunnel somewhat larger, and working into the walls, one foot above the top water line, or if it were below that line it would not much signify, a number of iron brackets, projecting from the brickwork about three feet; on these brackets could be laid cast iron plates sufficiently strong to ad

mit of horses passing over them. On the edge of this platform should be a small cast-iron rail; or there could be uprights at all the brackets, with a perforation in their tops, to admit of a rope or chain passing through them, which would act as a guard-chain or rope to the horse-path. The tops of the iron plates might be covered with a strong soil and gravel for the horses to walk upon, and thus would be removed one of the greatest defects in the navigation of a tunnel.

The centering ribs should be made in three or more parts, and they should be so adjusted together, that on the parts approaching one another they should produce the required arch, when they may be fixed with nuts and screws, taking care that the meetings be tight and close. The lower or inverted arch being turned a small way, the ribs for the soffit arch may be placed, three or four of which will be enough at a time; and these are fixed on a piece of timber called a sleeper, which is fastened at the base of the pit, and upon the chock-blocks; as the ribs are raised and adjusted to their places, their upper edges must be covered with laths to support the bricks of the tunnel arch. When the first yard of the arch is turned, the ground above the brick-work of the part formed must be well rammed and filled up, till it is as sound and firm as possible, and this must afterwards be repeated all through the work. After this is done and finished, more ribs must be placed and lathed, and the arching again begun. The workmen now look at their work with more firmness, as they see and comprehend it more completely: they begin now at each end of the already erected arch, one set of workmen working one way, and one set another, which they continue till another yard be done, which is filled up and rammed as before. It will be sometimes necessary, to avoid the effects of the percolating waters, to make at intervals, or wherever it becomes troublesome, cross-drains or soughs running quite under the tunnel, into which an opening can be made for the water to empty itself, and which may be carried off, or turned into the lower reach of the canal.

After three or more yards of the tunnel are completed, the work should be left to settle and get firm: this will be effected in a week or two, according to the state of the weather and the nature of the soil in which the tunnel has been made. If the above circumstances be at all favorable, a week will be sufficient; the nuts and screws may then be taken out of the ribs, and the first part of the tunnel centering taken down to be fixed in another part, and this may be continued all through the work, which will produce a great saving in ribs, &c. Stiff clays are the best for forming tunnels through, as they require less shoring and other supports than looser soils; in some siliceous and argillaceous soils the expences of shoring sand supporting the ground above, with the danger attending the work afterwards, is of more expense and diffculty, than at once beginning the work by open cutting, which in an instance or two has been obliged to be had recourse to, after many attempts at perforating such soils. The expense of canal tunneling, as well as others, must vary as the earth varies through which it is intended to be

made; in favorable circumstances, pretty accurate estimates might be formed; and as a reference, the following account, taken from Dr. Rees' Cyclopedia, article, Canal of the Tunnel at Blisworth, on the Grand Junction Canal, will suffice, as being one of the latest which has been finished. 'The internal width is sixteen feet and a half, the depth below the water-line to the inverted arch which forms the bottom is seven feet, and the soffit, or crown of the arch, is eleven feet above the same line. The side walls are the segments of circles of twenty feet radius, the top arch one of eight feet radius. The side, or top walls, are seventeen inches in thickness, or two bricks; and the bottom, or inverted arch, thirteen inches in thickness, or one brick and a half; every fifth course of the top arch, and every eleventh of the side walls, is composed of two heading bricks, or wedge-like, one inch thick on the inside, and three at the back; also every fifth and eleventh course as above (but between the courses of heading bricks), composed of bricks laid obliquely across the others, the front and back corners being cut off for that purpose in the making, for more effectually breaking the joints of the work obliged to be done in such short lengths. The mortar that was used was composed of one bushel of Southam lime (blue lias), and three of good sand; six inches under the water-line, on each side of the tunnel, slide-rails, of five inches square, were placed to keep the barges off the walls, and fixed by pieces of oak let into the wall below them, which rails project nine inches from the wall, and have at every nine feet a chock of wood upon the rails for the bargemen to set their pole against for shoving their barges along. It is added, that this tunnel was contracted for at £15 13s. per yard running. The soil, principally, through which it was made was a hard blue clay, with two or three thin rocks in it: sufficient headings had been executed several years before at the company's expense. The same contractors were paid 10d. per cubic yard for excavating the deep cutting at one end of the tunnel, and 11d. a yard for the other. The expenses of driving the headings were from 36s. to 42s. 6d. per yard running; nineteen shafts, or tunnel pits, some of them sixty feet deep, were sunk for the use of the above tunnel, which cost about 30s. per yard in depth, including the steining.' From such data very probable estimates may be calculated, taking into account the different prices of labor and materials where such a work is proposed to be effected. The success attending tunneling having been generally complete for canals, undertakings of the same nature have been proposed for various other improvements in our inland communications. Highgate was fixed upon as one for this purpose, which certainly presented a spot in every way eligible for the undertaking, and from which the public might have derived considerable accommodation. The heading of this tunnel was begun on the Holloway side of Highgate, and extended about 350 yards with about thirty yards of vertical cutting. The whole was marl and strong blue clay, with a compactness that required very little propping; every thing in nature was favorable to the success of this tunnel, and the work went on with adequate

effect till the arching commenced. It is too well known, that this arch has now given way, and it is for the engineers concerned in it to say, why.It could not have failed by reason of the soils in which it was formed, as their compactness rendered their bearing on the arch of little or no importance. The shape of the arch made it capable of supporting ten times what was ever upon it, if the bricks had received adequate attention in their bonding. If it failed for want of sufficient foundation, it is to be regretted: the inverted arch was not adequate for that purpose, as a very slight inspection will demonstrate. And if it fell down for want of support in this quarter, it must have been from the engineers' ignorance of the nature of the pressure in such arches. Arches formed of common bricks, when they are to assume a large diameter, require the utmost attention in their construction. The brick of the common size is too small to make any figure as a voissoir in such arches, as it will not allow of falling into the radii of their curvature; if such arches exist, it is by virtue of the cement more than by reason of the shape of the materials of which they are formed. In the tunnel now making across Hyde-park, for conveying the superfluous water from the neighbourhood of Paddington, the bricks which are to form the arch are moulded into the shape of wedges, and are called joggle-bricks; the form of the jogglebrick is previously determined by drawing its shape from the centre of curvature of the arch, of which it is to form a voissoir, and they may be varied, as the figure of the arch is composed of more than one centre of curvature; hence the parabolic arch will require joggle-bricks of one pattern for the top, and of another for the sides, such an arch being composed of different segments.

Bridges have continued to be erected through every succession of time, and are a leading point in developing the progress of science and the arts, as they appear generally well or ill contrived in proportion as they have advanced. Arcuation, within these last fifty years, has received deductions, arising from mathematical investigations, which has ranked it very high among the late discoveries made in science. It has now been shown that the voissoirs of an arch can be given that form, in relation to the whole matter surrounding them, as to produce an equilibrated figure: and this discovery has so far extended itself as to supply the principle by a table, formed by Dr. Hutton, which sets forth its quantities to every species or degree of curvature. bridges, to which the attention of engineers, however, is commonly called, are simple buildings, applied to the purposes of a canal; they consist, most frequently, of wood, and their beauty consists in making them light, but adequately strong.

The

Of the different species of canal bridges the draw-bridge is the most common; it consists of the following particulars: plate I., fig. 9, is the plan of a drawing-bridge; D the canal to be passed. The foundations, right and left of the canal, are supplied either by brick-walls, built battering, or by a row of four or more piles of oak, forced down into the embankment, with a