DREDGING,

Is a term used to express the important operation in the practice of the engineer, of removing deposited matters from the beds of navigable rivers, harbours, canals, and basins.

In describing the several methods by which dredging has been successfully employed, it is not our intention to enter into geological discussions regarding the ultimate tendency of the process of deposition; but we cannot allow it to pass without at least hinting at its original cause, a knowledge of which may lead the inexperienced practitioner more readily to the proper means of removing an evil so generally complained of in our most secure and sheltered harbours. If the universal tendency to waste and decay in the higher lands, from the agency of moisture, heat, and frost, be considered, we shall find that every rill of water must carry along with it a portion of separated matter. These rivulets being so many tributary streams to the great rivers which form the drainage of vast tracts of country, we need not be surprised to find that the beds and embouchures of the Scheldt, the Meuse, the Rhine, and the Elbe, or of the Thames, the Humber, the Tay, and other great tributaries to the German Ocean, should be variously silted up; and that even this great basin itself should be much encumbered by numerous banks of deposited matters. To the agency of these, combined with the effects of cross-running tides, we ascribe the existence of the Dogger Banks, the Yarmouth Sands, the Flemish Banks, and even the great platforms of Holland, and the opposite planes of the Fens of Lincoln.

There is also a marked difference to be noticed in the separation and distribution of these matters of deposition. In those rivers which flow with a very gentle current toward the sea, fine silt, or what is sometimes termed ouse, is produced; while rivers of greater fall, and consequently of more velocity, carry forward the grosser particles proportionally farther from their embouchures. Another circumstance which deserves our notice is the greater specific gravity of salt water than fresh, which (as has been ascertained by the writer of this article, on the Dee at Aberdeen, and other rivers where he has made observations) preserve their course in distinct films, the salt water under the fresh. The salt water thus flows up the courses of the respective rivers to an extent corresponding to the fall of their beds and the rise of the tide. A considerable portion of the heavier matters, as gravel and sand, are arrested in their progress sea-ward, where the current is languid; while the lighter particles floating at or near the surface are either borne along with the stream into the expanse of the ocean, or settle in the eddy-waters. In this way the projecting obstacles along the margin are formed, and thus accumulate in the form of sand banks and small islets; the creeks and sinuosities are also silted up, and too often render the connecting harbours and shipping places so shallow, as to be unfit for the purposes of floating ships of burden. To such a degree has this been experienced in some situations, as, for example, Sandwich in Kent, that this ancient sea-port is left almost in the state of an inland town ; while other ports have been more or less deteriorated. Importance is therefore justly attached to such means as may be instrumental in counteracting or preventing the tendency of this process of silting. We shall accordingly direct the attention of our readers to some of those means, both natural and artificial.

Where recourse can be had to natural means in keeping a navigable channel clear, they will always be found preferable to those which are artificial. The great agent laid to our hand by nature for this purpose is the judicious use and application of the drainage waters of the connecting district, and the preservation of the full and ample flow of the tide waters. But in many instances these means are tampered with, and rendered ineffectual, by the reckless thirst of acquiring land at the expense or by the exclusion of the back-waters arising from the flow of the tide. Notwithstanding all that can be said by professional men upon the subject, this practice prevails in many parts of the united kingdom to a frightful extent. With a view to put a stop to this, an act (in which the late eminent engineer Mr Rennie, and the writer of this article, had some connection) which originated with the admiralty, was passed in 1806, entitled “An act for the preservation of public harbours of the united kingdom,” Geo. III. cap. 153. But this act wants amendment, and has been rarely acted upon, especially to the extent contemplated by Earl Grey, then Lord Harwich, and first lord of the admiralty.

In noticing the natural means of cleaning harbours, we cannot perhaps better illustrate the subject than by reference to Montrose, in Forfarshire, where some of the conterminous proprietors have from time to time proposed to make firm ground of part of the great natural basin connected with the harbour of that port. This basin is flooded every tide to the extent of about five square miles, and is estimated, especially in spring-tides, to contain about fifty-five millions of cubic yards of the back-waters of the tide, which passing four times in the twenty-four hours through this harbour, produces so powerful a current that the shifting sand-bank at the entrance, called the Annet, is prevented from being thrown across the mouth of it in gales of easterly wind. In this state of the weather the Annet-bank has a continual tendency westward, while the back-waters of this great natural basin check its progress, and not only keep the navigation open, but are sufficient to preserve a considerable depth in it during the lowest tides.

In the course of forming the bold design of opening a harbour near Lowestoffe, and carrying an inland navigation to the mercantile city of Norwich, Mr Cubit the engineer projected an entrance to this harbour, with a pair of gates measuring fifty feet in width, and of a depth so considerable that their ground-sill is sunk about twelve feet under low water of spring-tides, the rise of which upon this coast is very limited: the gates are laid at this depth, so as to be capable of receiving the largest class of mercantile ships. They are also ingeniously contrived for letting off the head-waters of Lake Lothing. This extensive sheet of water, which pervades a great portion of the lower district of Suffolk, is to be used as back-water to scour and keep the entrance of the harbour open against the effects of storms from the cast sea acting upon the sand banks which so much encumber this part of the coast. This great work is only in the progress of being executed, but very sanguine expectations are entertained of its success.

In the improvement of the navigation of the river Clyde,’ the means resorted to have been those of narrowing the’ channel and confining the current; and these have been so successful that a depth of about ten feet has been obtained, instead of only five feet as formerly, by which the trade [7:8:194]and commerce of the city of Glasgow derive the most decided advantages. Formerly only barges came up to the city, now sea-borne ships from all quarters of the globe are seen at its quays. But in forming a design upon the principle of narrowing the channel of a river, and thereby shutting out a portion of the tide waters, several elements connected with the local circumstances of the place require to be carefully weighed before attempting to lessen the capacity or water-way of a river. Much depends on the level or rise of the bed of the river, the perpendicular rise of the tide on the coast, the situation of its embouchure, and other circumstances which favourably and fortunately present themselves upon the Clyde. The application of the steam tug-boat, in towing vessels through narrow channels, has removed objections of the most serious nature upon this navigation.

Many other circumstances might be adduced to show the advantages of supplying the natural means in our power to the scouring of navigable tracks. The writer of this article has now (1833) under consideration the improvement of the harbour of Ballyshannon, in the county of Donegal, which possesses the natural means to an immense extent of clearing its embouchure, having almost the entire drainage waters of the county of Fermanagh collected in Lough Erne, which extend to about fifty miles in length, and in some places three miles in breadth, the entire overflow of which passes over the bar of Ballyshannon harbour.

The most eminent engineers, both of our own country and of France, have introduced scouring basins into their designs of tide harbours. Mr Smeaton constructed a tidebasin of this kind at Ramsgate in Kent, where the silt of the outer harbour is loosened by artificial means, and dredged into the tracks or courses of water issuing from the sluices of an artificial tide-basin. By this means a considerable portion of the deposited stuff is carried out of the harbour into deep water. It is, however, to be regretted that there is not a more extensive collection of back-waters here, as, from its very circumscribed position, that eminent engineer was prevented from enlarging his scouring basin to a sufficient extent to ensure the best effects of his design.

At Dover there is a good example of artificial scouring upon a small scale. This harbour is often choked up in a single tide, with the debris of the heights of Dover, and with the flinty gravel which surrounds this coast. Upon these occasions, the back-waters of an adjoining basin are conducted in great cast-iron pipes of about three feet in diameter. The water issuing from these pipes is made to act upon a system of temporary weirs, consisting of dealboards set on edge, which are shifted about at pleasure in different directions, so as to bring the water from the sluices to bear upon the gravel; and it is astonishing with what facility the entrance “to this harbour is by these means cleaned and rendered accessible to the packets, from a state of being quite shut up.

In extensive plans of docks for floating ships much use is or may be made of a system of soughs or sluices for scouring or floating away mud. By opening these sluices from one dock into another, as is done by Mr Hartley, engineer for the Liverpool docks, the advantages of such a command of head-water is attended with the most beneficial effects, both for clearing the several docks, and also the outer harbours or receiving basins of this great port.

The common plough and harrow, differing very little from their ordinary construction, and also a kind of frame of timber shod with plate iron, and provided with stilts or handles somewhat like a great shovel, are often used for loosening and dredging stuff within reach of removal by a stream of back-water, with which it is afterwards floated out of the respective harbours or navigable tracks. These and various other simple implements, suited to local circumstances, are sometimes employed by manual labour, and also with horse power, where the bottom is sufficiently firm to admit of the necessary trackage. Nor must we omit to mention the use of the common wheelbarrow and spade so much used in operations of this kind.

The spoon dredging apparatus, with its boat, as represented in Plate CCV., fig. 13, is of long standing, and was probably first used by the Dutch, who still apply it extensively. It is also much employed upon the river Thames, for lifting ballast and deepening the navigation of that greatest of all commercial rivers. Referring to fig. 13, the reader will at once comprehend the construction and application of this simple apparatus. The boats for this purpose vary in size according to the situation in which they are to be worked, say from twenty to sixty tons burden. They are built to float upon an easy draught of water, and are sometimes flush-decked, carrying their cargo wholly upon deck; but for the greater part, and especially those of the larger size, they are in the state of open boats, with a kind of inner sole or floor. When the excavated matters are not to be employed in banking on the sides of the river, or ballasting ships, the boat is formed with a kind of hopper-hold, somewhat upon the principle of the fishing smacks with wells, being convenient for getting quit of the stuff in deep water. In this case a hold of two compartments, one fore and one aft, are formed, as represented by the dark shading and dotted lines in fig. 13. Each of these hoppers has an aperture or opening in the bottom, through which the stuff is dropped by the flap-door, marked letter A, as represented in the figure. In some circumstances, also, the stuff is received into a system of sheet-iron boxes, and lifted out of the boat by a tackle, to be emptied. The spoon or shovel, marked letter B, which accompanies these boats, consists of a strong ring or hoop of malleable iron, the cutting part of which is of steel, and is about six or seven feet in circumference, properly formed for dredging upon soft mud or gravelly ground. To this ring a large bag made of bullocks’ hide, but more generally of tanned leather, is strongly attached with thongs. The bag is perforated with a number of small holes for allowing the water to drain off, and its capacity may be about four or five cubic feet. A pole of from thirty to forty feet in length is fixed to the spoon, or rather two poles are so laid together with hoops and rings as to be lengthened or shortened at pleasure, to suit the depth of water in which the apparatus is required to work. A rope is also attached to the bottom of the bag, for directing its position at the commencement of each operation. This apparatus is generally worked with a chain or rope, brought from the spoon to a winch, worked with wheel and pinion, through a block suspended from a small crane used for hauling the bag and its contents along with the progress of the boat, and in lifting the spoon over the gunwale to be emptied into the hopper of the boat. The purchase-rope is led along the deck to the winch, by a snatch-block placed in a proper direction for this purpose. The boat is moored at the head and stern, and its berth may be shifted at pleasure. These boats are generally managed and worked with from two to four men, who with this simple apparatus can lift in a tide from twenty to sixty tons of stuff from the bottom, at the depth of two and a half or three fathoms, when the ground is somewhat loose, and favourable for the operation. In Holland this apparatus, and all these simple modes of dredging, are much practised upon the extensive flats at the entrance of their great navigable rivers, in connection with the sluices and natural currents issuing from their extensive basins and canals. On the British coast, dredging, when carried on to any extent, [7:8:195]is now confined chiefly to the spoon and bucket machines; and here steam, wherever it can be applied, is the great moving power. In Holland the excavated matter is very generally of a mossy description, which, after being strongly compressed in moulds by that industrious people, is in a state to be speedily used as turf fuel. On the Thames the spoon dredging machine is conducted upon a large scale, and in the most systematic manner, under the immediate direction of the Trinity Board. The stuff brought from the bottom consists chiefly of mud and gravel. This is not only a useful operation for deepening and preserving the navigation, but the stuff’ itself is sold to good advantage as ballast for shipping. To such an extent is this carried, that the colliers, or shipping from London to Newcastle, have raised ballast hills in the neighbourhood of Shields, which from their vast extent have become objects of no small curiosity.

In proportion as the commerce of a country extends, its ships increase in their dimensions, and a greater depth of water is consequently required to float them; and hence greater difficulty and expense attends the construction and preservation of harbours for their accommodation. To effect these objects, and especially to obtain a greater depth of water, recourse has been had to various means, such as the extension of piers, the formation of breakwaters, and deepening by means of the process of dredging now described. These simple modes have, however, been succeeded by a still more powerful engine termed the bucket dredging machine.

Since the date of the last edition of this work, very considerable improvements have been effected in the system of dredging, both by the application of the power of steam, and the form and construction of the apparatus itself, as well as that of the vessel containing it. The machine is said to have been first worked by men only: when the principles upon which it acts were more fully ascertained, horses were employed, and worked round a covered gin-track or circular path within the boat. A machine of this description, worked by horse power, was used for some years at the port of Greenock, though it was there ultimately found more suitable and expedient to resort to manual labour, applied by crane-work with wheel and pinion. Perhaps, in all situations where fuel is very expensive, and where the work is not of sufficient extent for the full employment of steam, it will be found better, in a process of this kind, to employ manual labour than horses, which, from the circumscribed and hampered nature of the track on board of the vessel, must be very disadvantageous, as experience has shown on the Clyde. Indeed, the only question seems to be, in such cases where steam cannot be profitably employed, whether it were not better to use the spoon apparatus, as upon the Thames, where manual labour is at the highest rate.

In this article we shall give a minute detail of one of the best and latest constructed dredging machines, employed by Messrs Gibb upon the harbour works of Aberdeen. This machine was entirely constructed by Messrs John Duftus and Company of that port, both as regards the vessel and the machinery. In describing it we refer ‘to Plates CCIV. and CCV. Fig. 1, Plate CCIV., represents a longitudinal section of the vessel, exposing to view an elevation of the steam-engine and bucket apparatus, with its framework. The ground to be dredged is also shown, with the buckets in contact, and the attendant barge astern. Fig. 2 is a plan of the vessel and machinery. Her extreme length as taken by measurement is ninety feet, and the extreme breadth twenty-two feet; the length of the ark or well which contains the bucket-frame is fifty-four feet, and its width four feet one inch in the clear. The after-part of the vessel being thus formed into two compartments, these are connected at the stern by four strong timbers, placed across the well, and immediately above the keel. Upon these, timbers are placed in sloping directions for supporting the planking. This vessel draws only four feet of water, but the bucket frame can be lowered so as to dredge in a depth of fifteen feet. The steam engine is of the usual form of marine condensing engines; the diameter of the cylinder is twenty-four inches, and the length of the stroke thirty-one inches, being equal to the power of about sixteen horses. From the nature of the process of dredging, the resistance is extremely irregular, causing violent shocks; and therefore malleable instead of cast iron is used as much as possible in the construction of the moving parts. For the same reason a heavy fly-wheel becomes necessary, to regulate the motion. As a vessel of this description, in a tide harbour, must frequently get aground in places where it is unequal, the whole machinery is liable to be strained. On this account it is necessary that all pipes connected with the engine should be of copper, and for the same reason every facility is given for disengaging the parts of the machinery, to prevent fracture or derangement. From the corrosive nature of the stagnant water in harbours, which must be used for condensing, it is necessary to line or case the air-pump with copper or brass, and also to make the buckets, air-pump, valves, and rod, of the same metal. It has been observed that cast iron wastes more rapidly when exposed to the action of this harbour-water than in the open sea.

Subjoined are the dimensions of some of the principal parts of the' engine. These, to the practical engineer, may hardly seem to be necessary; but as the Aberdeen machine has been found to answer every expectation, and as it appears to the writer to be similar to those so successfully employed on the river Clyde, it may‘be of consequence to the general reader to know, from such data, the proper strength and size for its different parts. This engine is set upon a cast-iron cistern, measuring three feet in width, and prolonged as far as to contain the air-pump, foot-valve, and condenser. The cistern and cylinder, set on the top, are strongly bolted down through the bottom of the vessel. On the top of the cistern the columns of support for the fly-wheel shaft are set at a sufficient height to connect with the dredge-gearing, which is stayed by a circular entablature, as represented in the plan of the engine at letter a. It has also a diagonal stay, securely bolted to a bracket, cast on the side of the cylinder. The side levers of the engine are seven feet eight and a fourth inches long; between centres one and three-fourth inches thick of plate, and three and three-fourth inches thick on the back; fifteen inches broad in the middle, with forked ends one and three-fourth inches thick, for laying hold of the cross-rail and cylinder siderods. The cylinder cross-head is seven and a half inches deep at the middle, tapering to four inches where the side-rods are attached, and five and a fourth inches in diameter at the eye, and two inches in thickness. The cross-head of the air-pump is four and a half inches diameter at the eye, five and a half inches deep, and one and three-fourth inches thick. The cylinder side-rods are two and one-eighth inches diameter at each end, and two and two-eighth inches at the middle. The side-rods of the air-pump are finished with forked ends, for embracing the side-lever centre on each side, and fitted with straps and braces. The main centres are four and five-eighth inches diameter, keyed into the side levers. The parallel motion is constructed with radius and parallel boxes, as is usual in this description of engine. The connecting-rod is of malleable iron, three inches diameter at both ends, and three and three-fourth inches at the centre, fitted into the eye of the cross-tail with a gib and cutter. The fly-wheel [7:8:196] is ten feet three inches in diameter, and eight inches deep by four inches broad in the rim, as represented in dotted lines upon the cross section of the vessel, fig. 3, Plate CCV., and at fig. 1, Plate CCIV. The shaft is of cast iron, seven and three-fourth inches diameter. The engine is also provided with a governor and its necessary connections, and is driven from the fly-wheel shaft by means of a pulley and belt.

The boiler, marked c, fig. 1, Plate CCIV., is of iron, measuring ten feet four inches across the end, eight feet four inches long, and four feet eight inches in height in the middle, with two furnaces two fleet three inches wide. The water-ways between the flues are four inches wide; and the flues fifteen and a half inches, making one and a half turns in the length of the boiler. The sides and tops of the furnaces, the bottoms and sides of the flues, are three eighths of an inch thick, and the top half an inch in thickness. The copper steam-pipes are of the thickness No. 10 on the wire-guage, the funnel a of the engine is two feet in diameter and twenty-five feet in height, made of plate-iron an eighth of an inch in thickness.

Fig. 3, Plate CCV., represents a cross section of the vessel accurately drawn to the scale, showing its outline, and the manner in which it is constructed. The machinery is supported upon the three keels ABC of the vessel by their respective keelsons, or beams placed immediately over them. The bottom of the vessel is further supported by the like means, DD. The central keel extends only to the fore part of the ark or well. This cross section also exhibits an elevation of the train of wheels for raising and lowering the bucket-frame. From the main spur-wheel on the lying shaft, marked E, fig. 3, Plate CCV., down to the lowest wheel on the same shaft, with the chain-barrel, marked L, the connection is in the following order. The main spur-wheel is of that description called a mortisewheel: it is eight feet in diameter, constructed in a very ingenious manner, to revolve upon a friction nave as follows: The nave is three feet seven inches in diameter, smoothly turned on the circumference, and fixed to the lying shaft with keys. The wheel is also particularly turned in the eye to coincide with the nave, and is furnished with eight pincing plates and screws marked f, for tightening it at pleasure, and made to pass with sufficient force upon the nave, in order to carry the spur-wheel round in its fair work along with it; but if overstrained, it immediately slips, and thereby any injurious consequences to the apparatus are prevented. The cogs or teeth of this wheel are made of hard wood; all the teeth of the other wheels are made of cast iron. The spur-pinion g, the half of which only is shown, is on the same shaft with the fly, as is also the small spur-wheel h. The pinion g works in the main spur-wheel, and is three feet six inches in diameter. The small spur-wheel h, the half of which only is shown, connects the engine and the other wheels for working the chain-barrel: it is three feet seven and a half inches in diameter, and works in the wheel i, measuring four feet two and three fourth inches in diameter. On the shaft of the last-mentioned wheel, which is four and an eighth inches square, there is fixed a pinion j, of one foot three inches diameter, working in the wheel K, which is three feet eleven inches in diameter; and on the same shaft there is fixed a wheel l, two feet six inches in diameter, working in another wheel L, which is four feet ten inches in diameter: it is fixed upon the shaft that carries the purchase chain-barrel, and is the last wheel of the train. The barrel of cast iron, Plate CCIV., fig. 1 and 2, measures five feet ten inches in length within the flanges, two feet in diameter, and two feet eight inches over the flanges. It makes six and one tenth revolutions in one minute, being equal to thirty-eight strokes of the piston. This barrel is provided with an offset-clutch, for engaging and disengaging it from the engine, for the purpose of raising and lowering the bucket-frame; and as a precaution against accident, this clutch is provided with a friction-nave similar to that already described for the main spur-wheel. This becomes indispensably necessary in situations like that of Aberdeen, where tree-roots, stones, or the like, obstruct the buckets. The machinery would otherwise run great risk of being torn in pieces and destroyed. This barrel is also provided with a brake-wheel and friction-hoop, for lowering or fixing the bucket-frame to any required depth. This is effected by means of a weight constantly acting over a pulley at the extremity of the lever s, Plate CCIV. fig. 1, for pressing the friction-hoop upon the circumference of the brake-wheel with force sufficient to hold the bucket-frame in any position. But when this weight is partially removed, which is done by the hand, the bucket-frame is lowered to the intended depth at the discretion of the master of the vessel, or person who has charge of the work. The upper tumbler is square, as shown in fig. 1, Plate CCV. and in the vertical section, fig. 2, showing the flange and part of the body of the tumbler. In the original form it is cast in one piece, the body being an octagonal prism, and is afterwards brought to the square form by bolting the triangular bars a on the alternate sides. Fig. 12 represents the upper end of the bucket-frame, the shaft M which carries the tumbler N, and the great bevelled-wheel 0, which is shown in section; also a view of one of the buckets P attached to the links QQof the bucket-chain. RR are the plumber-blocks, and SS the brackets for supporting the shaft. Fig. 4 is a cross section of the under tumbler of the bucket-frame, which is five sided: a is the flange, b the strong studs to which the tumbler bars arc bolted, c the tumbler bars, and d the bolts by which they are secured. Fig. 5 is a vertical section of the flange, with the same letters of reference, showing the method of fixing the tumbler bars. The lying shaft is of cast iron, in five lengths, as represented in the longitudinal section, Plate CCIV. fig. 1. It is six and a half inches in diameter; the second length of the shaft is furnished with an offset clutch at m, for disengaging the bucket-frame without stopping the engine. This clutch is put into and out of gear by the lever n; but the clutch is more particularly shown in Plate CCV. fig. 6 and 7. The coupling-boxes for the lying shaft are fifteen inches long by one and three fourth inches thick, fastened together by four screwed bolts of one and a fourth inch square.

The bucket-frame, with its train of buckets, as represented in Plate CCIV. fig. 1, is fifty-two feet in length Between the centres of the lower and upper tumbler shafts. The frame is of the best oak timber, each side being of one entire piece. The buckets, one of which is represented in Plate CCV., fig. 8, are perforated in the back and bottom with small holes for draining off the water. They measure one foot nine inches in depth, one foot two inches in width from back to breast at the bottom, and one foot five inches -at the mouth; the breadth from side to side is two feet two inches. The mouth-piece or cutting edge a is of tempered steel; b is a side view of the double link connected with the bucket. Fig. 9 is an edge view of this link, fig. 10 a side view of the single link, and fig. 11 an edge view of it. These links are twenty-one inches long from centre to centre, three inches in breadth by two in thickness, with a ring of steel one fourth of an inch in thickness welded into the eye of each. The bolts for the chain are two inches in diameter, coated with steel. The weight of each double link is about 84 lbs. and of each single link about 44 lbs. The trussing rod for the bucket-frame, marked tt, Plate CCIV. fig 1, is three inches in breadth [7:8:197]by one and a fourth inch in thickness, meeting in the centre, and forming a breadth of seven inches. The friction rollers along the upper side of the bucket-frame are ten in number, as shown in the longitudinal section; they are seven inches in diameter, with a flange measuring twelve inches in diameter. The bucket-frame is lowered and regulated upon the ground to be dredged by a tackle formed by two three-sheaved iron blocks, with 185 feet of three-fourth inch chain run through them for a tacklefold. This tackle is suspended from a frame of cast iron, as shown at 0, Plate CCIV., fig. 1, and is represented in dotted lines at P. It is attached to the bucket-frame by a malleable iron cross-bar and two side rods, the other end being attached to the cast-iron barrel already described. The iron ties marked xx, Plate CCIV., fig. 1, of which there are four, are used, along with the vertical beam, for supporting the extremity of the bucket-frame: they are of malleable iron, measuring five eighths of an inch in diameter.

The dredging vessel is moored with anchors of two and a half cwt. each, and is moved in any required direction when dredging, by means of four winches of cast iron, viz. one of double power on each bow of the vessel, having a five-eighth inch chain of 300 feet in length, with couplings, and one single-power crab on each quarter, with two iron leading-locks, having also 300 feet of half-inch chain. This vessel and its apparatus requires nine men to work it; when dredging in clean gravel it will discharge into the receiving-barges at the rate of 120 tons per hour; the period for working is regulated by the tides, but generally about four hours is obtained each tide, or eight hours per day, which is equal to 960 tons. But when the machine works upon a clayey bottom, about sixty tons per hour may be taken as the average quantity lifted; and when the stuff is mixed with gravel and stones, the quantity is still less. It is not uncommon with the buckets to take up stones of upwards of one hundredweight, and occasionally the roots of large trees.

In some instances the receiving barges are built with a hold for carrying their load, especially where the mud or sleek is thin. It is received into plate-iron tanks, fitted so as to fill the hold. (Tanks of this, description, which contain about one ton each, are used at the harbour works of Dundee.) Theyare fitted with ears or handles, and are lifted out of the boat by a tackle, and emptied into ships as ballast, or they are landed on the quay to be otherwise disposed of. But where these excavated matters are not found necessary for the purposes of the port, they are thrown into deep water, for accomplishing which the barges are furnished with a copper-hold and flap-doors in the bottom, for letting off the stuff, as shown in Plate CCIV., fig. 13.

Though we have not been able to trace the invention of the bucket-dredging machine to any particular person, yet we believe it is strictly of British origin, and, so far as our information goes, was first used at the port of Hull in the Humber. It may here be curious to remark, that Jonathan Hulls was of that place; he obtained a British patent for working vessels by steam in 1735, although, strange to say, it was not introduced into practice till 1812, when the late Mr Henry Bell fitted out the canal steamboat upon the Clyde. Since that period the steam dredging machine has come into very general use. On the Clyde there are now no fewer than three such vessels as we have described employed, besides a diving bell for the removal of large boulder stones, which in various parts obstruct that navigation. The engines and apparatus used on the Clyde are chiefly of the manufacture of Messrs Girdwood and Berry of Glasgow.

The dredging machine delineated in Plates CCIV. and CCV. was constructed by Messrs John Duffus and Company, Aberdeen. As before noticed, it is simple in its form, and contains the latest improvements both in the build of the vessel and the position and arrangement of the machinery. The great object to be attended to in framing these vessels is, to obtain such a degree of strength as not only to withstand the tremulous motion of the engine and dredging buckets, but also to be capable of resisting the strains to which they are continually liable in taking the ground, or in the fair way of shipping.

The cost of the vessel, engine, and bucket apparatus, complete, with her twelve lighters, may be taken atL.5000. The expenditure of coal is at the rate of four cwt. per hour, and the daily expense of working her at L.3. 3s. But these items of course vary in amount, according to local circumstances and the situation of the port.

The strength of the vessel, the power of the engine, fitness of the machinery, and the security of the whole against accident by fire, are circumstances connected with the application of the dredging-machine which will always meet with the attentive consideration of the engineer, whose reg'ulation in all the parts of this apparatus will be guided by the actual operation to be performed, as more or less suitable to the peculiar situation of the works in which this apparatus is to be employed. What we have been able to bring under the professional reader’s notice in this article, we trust will be sufficient to give him an idea, not only of the construction of the apparatus, and the principles upon which it acts, but also to afford such details as cannot fail to be highly useful in the construction of such an apparatus. To the general reader, who may not take much interest in the details of complicated machinery, we presume our section, elevation, and plan of the bucket dredging-machine will be sufficiently obvious. To him it will also have been interesting to know how operations of this kind are performed, the quantity of work that may be done, and the rate of its expense. We shall also be happy if our observations upon the baneful consequences of shutting out tide water by embanking shall happen to come under the eye of those possessed of legislative power, and be the means of rendering more effective the act of 1800, for the preservation of the navigable rivers of the kingdom, as noticed at page 193. (τ. τ.)