s a term used to express an important operation in the practice of the engineer, that of removing mud and other deposited matters from the bed of rivers, canals, harbours, and basins.

In describing the several methods by which dredging has been successfully employed for preserving the necessary depth of water in our harbours and tracks of inland navigation, it is not our intention to enter minutely into geological discussions regarding the deposition of silt or mud; but we cannot allow the process of silting to pass without at least hinting at the cause of such depositions. If we consider the tendency to waste and decay in the higher lands from the agency of moisture, heat, and frost, we shall find that every rill of water carries along with it a portion of separated matter; and these rills being so many tributary streams to the great rivers forming the drainage of vast tracts of country, we need not be surprised to find the beds of such rivers as the Thames, the Humber, the Tay, and the Clyde, &c. much incumbered in the central parts by numerous sand-banks, while their margins are skirted with the finer or more minute particles called silt and mud. Nor can we avoid noticing this marked difference in the separation of the deposited matters at the mouths of rivers which flow with a very gentle current towards the sea. From the greater specific gravity of the salt water, it is found to preserve its course up the respective rivers, according to the rise of the tide, in a distinct stratum under the fresh water. A considerable proportion of the heavier matters, as gravel and sand, may thus be conceived to be arrested in their progress, whereas the lighter particles, floating at or near the surface, are either borne along with the stream into the ocean, or, by getting into the eddy waters, formed by the projecting obstacles along the banks of the respective rivers, are, in this manner, allowed to be deposited in the form of silt, sleek, or mud.

The more ponderable matters, accordingly, accumulate in the form of sand-banks and small islets in the central parts of the stream, while all the creeks and sinuosities on the margin are 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 on our shores, as for example at Sandwich in Kent, that that ancient sea-port is left almost in the state of an inland town. Great importance is therefore justly attached to such means as may be instrumental in preventing so great an evil; and we shall endeavour to direct the attention of our readers to the different modes of dredging, by treating the subject under the following heads, viz. the Scouring Basin, the Harrow, &c. the Spoon Apparatus, and the Bucket dredging machine, worked by the power of men, by horses, and by steam.

The Scouring Basin is a water-tight compartment of a harbour, furnished with sluices, and set apart for containing a quantity of tidal or river water, to be run off at low tides for scouring or floating away the stuff which may have been loosened in the process of dredging; or in situations where the command of head-water is considerable. It is used for cleansing the bottom and bar of harbours, without the assistance of any dredging apparatus; indeed, it is only in conjunction with such basins, natural or artificial, that the several modes of dredging, simply by loosing the stuff, can be rendered effectually or permanently useful. All harbours left in a state of dryness every tide, at low water, ought, if possible, to be furnished with a scouring basin, especially those situated at the embouchure or upon the banks of rivers, where the deposition of mud is most apt to take place. We cannot, perhaps, illustrate this better than by referring to the harbour of Montrose in Forfarshire, where the great natural basin connected with that harbour is flooded every tide by the waters of the ocean, to the extent of about five square miles, and has been estimated to contain about fifty-five millions of cubic yards of backwater, which produces so great a current, that the shifting sand-bank called the Annet, is prevented from being thrown across the mouth or entrance of that harbour in gales of wind from the eastward. By this means a great body of water is kept passing through the comparatively small entrance of the harbour, which not only keeps the navigation open, but is sufficient to preserve it of considerable depth, even at the lowest ebb of the tide. The same observation is strictly applicable to the entrance of all great rivers or estuaries, where the navigation can only be preserved by a strong current of water. We accordingly find that 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 basin of this kind at Ramsgate, where the silt of the outer harbour is dredged or loosened, and raked into the tracks or courses of the water issuing from the sluices in the scouring basin; by which a considerable portion of the stuff is carried out of the harbour into deep water. It is, however, to be regretted that the very circumscribed position of this harbour prevented that eminent engineer from enlarging this basin sufficiently; so that the good effects of this design have never, it is believed, been fully experienced. In extensive chains or plans of wet docks, much use may be made of this mode of scouring or floating away mud, by opening numerous sluices from one dock into another; and also by taking the advantage of such a command of head water for clearing the outer harbour, or receiving basin, by a judicious and well-directed system of sluices.

The Harrow, and even the common plough, and Dredge also a kind of dredging-frame, made of timber and plate iron, like a box without bottom or top, is chiefly used for loosening or dragging loose stuff, and bringing it within reach of removal, by the waters of the scouring basin, the spade, and barrow. These, and other implements of less note, have been used with good effect in navigable rivers and harbours. Such modes are much practised in Holland, upon the extensive flats at the entrance of the great navigable rivers of that country, in connection with the operation of the sluices and strong currents issuing from their extensive basins and canals. In Great Britain, dredging is now almost exclusively confined to the spoon and bucket apparatus, for lifting and removing the stuff. But, in the improvement of the navigation of the river Clyde, even these have now, in a good measure, been laid aside by the enlightened trustees; having given way to the natural and more successful operations of narrowing the channel and confining the current, which has at length produced the depth of about nine feet, instead of five feet, as formerly; from which the trade and commerce of the city of Glasgow derive the most decided advantages. The spoon dredging-boat has been long, and is, indeed, still much used by the Dutch, with whom it, in all probability, originated. It is also at this moment more extensively employed on the Thames than the bucket-machine, or any other contrivance for lifting ballast, and improving the navigation of that river. Referring to Plate LXXIII, and to the figure on it marked Spoon Dredging Boat, the reader will at once comprehend this simple apparatus. The boat varies in size according to the situation in which it is to be used; but is generally from twenty to sixty tons burden. These boats are built so as to float upon an easy draught of water; they are sometimes flush-decked, and carry their cargo upon deck, but the greater part, and especially the larger sizes, are in the state of open boats, or have a kind of inner sole or floor. When the excavated matters are not to be employed in banking or for ballast, a convenient mode of getting quit of them is to have an aperture or open in the bottom, by which the stuff is afterwards dropped into deep water, in the manner represented in one of the compartments of the boat in the diagram.

The spoon apparatus consists of a strong ring or hoop of malleable iron, about six or seven feet in circumference, properly formed for making an impression upon the soft and muddy ground. To this ring a large bag is strongly attached with thongs, which are sometimes made of bullock's hide, but more generally of tanned leather. 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 long pole or handle is attached to the spoon, and a rope to the bottom of the bag for directing their position at the commencement of each operation. The pole or handle varies in length and thickness according to the depth of water, from fifteen to thirty feet. This apparatus is generally worked with a wheel and pinion, or winch, and the chain or rope is brought from the spoon to the winch through a block suspended from a small crane for bearing the spoon and its contents to the side of the boat, and bringing it over the gunwale to be emptied into the boat. The purchase rope is led upon deck by a snatch block in the proper direction for the barrel of the winch. These dredging boats, when placed for work, are moored at head and stern with guy ropes for shifting their birth at pleasure. They are managed and wrought with from two to four men, who with this simple apparatus can lift from twenty to sixty tons in one tide, at the depth of about two and a half or three fathoms, when the ground is somewhat loose, and favourable for the operation.

In Holland and Flanders, the spoon apparatus is much used in deepening the extensive tracts of canal, where the labour is more easily performed than on the Thames, from the absence of tide or current, and from the more manageable nature of the stuff at the bottom. There the excavated matters are very generally of a mossy description, which, after being compressed in moulds, is used as turf-fuel. On the Thames, this operation is conducted upon a great scale, and in the most systematic manner, under the immediate direction of the Trinity Board of London; and the stuff dredged and brought up from the bottom, consisting chiefly of mud and gravel, is not only useful for deepening and improving the navigation, but is sold to good advantage as ballast to the shipping of the port of London, particularly to the colliers; and to such an extent is this process carried on, that the Ballast Hills of Shields and Newcastle, which form no small curiosity from their vast extent, have been chiefly brought from the Thames.

In proportion as the commerce of a country extends, its shipping increase in dimensions, and a greater depth of water is consequently required to float them; and greater difficulty and expense are accordingly experienced in constructing the necessary harbours along the coast, and in preserving a sufficient depth of water. To effect the objects in view, recourse has been had to various means, such as extending piers and breakwaters, together with the apparatus above described. These have been succeeded by a still more powerful apparatus, termed the Bucket Dredging Machine.

This powerful engine is said to have been first worked by men only; when the principles on which it acts were more fully ascertained, horses were employed, but it is now generally worked by the power of steam. Different situations are found to require peculiar modes of application. Thus we find, that at the port of Greenock, on the Clyde, after using one of these machines with horses working in a covered gin-tract, or circular path in the boat, it is now found to be more suitable and expedient to resort to the use of manual labour, applied by crane-work, with a wheel and pinion. Perhaps in all situations where fuel is very expensive, and where the work is not of sufficient extent for the full and ample employment of a steam-engine, it will be found better to have recourse to manual labour than to the power of horses, which is applied under many disadvantages on board of a boat of this kind, as experience has shown in the Clyde. Indeed, the only question with us is, whether, in cases of this kind, it would not be much better to use the spoon apparatus.

The bucket dredging-machine is the same in principle, whether the power applied be that of men, horses, or steam. We shall, however, in our description, refer to a vessel with a steam-engine for working the buckets, Plate LXXIII. We hope that this apparatus, with the connecting diagrams, will appear sufficiently obvious, without the necessity of having recourse to numerous alphabetical references, which often tend to distract the general reader, without being useful or necessary to those professionally instructed, who must, in their operations, proceed upon experience, and a more precise knowledge of the subject than can be conveyed in an article of this limited extent. We, therefore, deem it unnecessary to give references to the parts of the vessel or dredging machinery, consisting of the moveable frame and revolving buckets for lifting the deposited matters from the bottom, and the mode of discharging the stuff into the receiving boats; as this will be easily understood upon a simple inspection of the plate.

In describing the operation of dredging with the bucket-machine, we may suppose the vessel brought to the position in which she is intended to work, where she is moored by the head and stern, and provided with the necessary crane-work and ropes for shifting her birth at the discretion of the boatmen. The bucket-frame, which consists of two beams of timber, is supported on a rod of iron with shores of wood; on these beams the full buckets move upon iron rollers fixed to the timber, while the empty buckets and endless chain form a curve in descending to the bottom, which is essential to the manner in which the buckets are respectively intended to touch the ground. The bucket-frame, so constructed, is then lowered with proper precaution till the empty buckets come in contact with the mud, and, according to the tenacity of the stuff, a greater or less impression is made by the buckets on the ground. The operation of lowering and raising the frame is usually performed by a piece of crane-work, distinct from the machinery of the steam-engine; but a considerable improvement has been made in the dredging-machine lately constructed for deepening the harbour of Dundee, in which the bucket-frame is lowered and raised with great regularity by a power taken from the steam-engine; as will be seen by tracing the line of the purchase-chain from the lower part of the bucket-frame to the machinery of the engine. Things being in this state of preparation, the buckets are ranged on the frame and put in motion by the power of men, horses, or steam.

The reader may next attend to the direction of the buckets, as delineated on the plate, from the lower point, where they are shown touching the ground, to the upper extremity of the frame, where the stuff is tilted into a kind of trough. This operation will readily be understood by directing our attention to the crown or upper-wheel of the train of machinery, connected with the steam-engine; which is placed upon the same axle with the upper-tumbler of the bucket-frame with which the train of buckets is made to revolve by means of an endless chain. The buckets at the lower end of the frame are continually filling, while those at the upper extremity are, at the same time, emptied. The tumbler, buckets, and endless chain, form very important parts of this apparatus, and we have, therefore, given separate views of these, upon an enlarged scale, in the connecting diagrams on the plate, from which the general dimensions may be ascertained. Instead of a cylindrical barrel, the tumbler is necessarily formed into a polygonal figure by bars of iron, to which the links of the endless chain are exactly fitted; and in revolving upon its axis, it takes round the chain and buckets, and so produces the complete effect of filling and discharging the dredging-buckets into the receiving barge, or boat, moored alongside for this purpose, as represented in the Plate.

If these excavated matters are not required as ballast for shipping, as on the Thames, or for embanking and filling up behind the piers, and other purposes connected with the works at the place, the receiving boat is generally made with two holds sloping towards the keel or bottom, for the purpose of lessening the width of the discharging apertures, which are shut with hatches, or rather with hinged doors, as will be observed by the dotted lines delineated on the elevation of Dredging, the receiving boat in the plate. These hatches or doors open outwards, and therefore, as the boat receives its cargo, the increasing pressure of the water, acting in a contrary direction, prevents the doors from being forced open by the weight of the stuff, till it be transported to the place of its destination; when the chains, fixed to ring-bolts on these doors, are loosened by turning a windlass or crane work upon deck, when the whole contents instantly drop into the water. On the Continent much use is made of this description of boat in dredging operations.

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, it was first used at the port of Hull in the Humber. We know that the application of the steam-engine was proposed as part of the establishment at the royal dock-yards, for towing or dragging ships, by an ingenious person of the name of Jonathan Hull, so far back as the year 1736; who, at that early period, obtained a patent for his invention. About the year 1787, or nearly forty years afterwards, the late Mr Miller of Dalswinton, distinguished for his curious nautical experiments, introduced the steam-engine into one of his boats, with three keels, for plying upon the open sea. The late Earl of Stanhope, well known for his many ingenious contrivances, also laboured in the same important branch of mechanics. These circumstances alone are sufficient to prove the steam-boat to be of British origin; but it is probable that steam was not applied to the dredging-apparatus prior to the year 1800, nor brought into general use in Great Britain sooner than 1812, by Mr Bell of Helensburgh, on the Clyde.

Since the first rude and simple attempts at this machine, very considerable improvements have been made both in its construction and application. The figure of the dredging-machine, delineated in the plate, is simple in its form, and contains the latest improvements, both in the position and arrangement of the machinery and of the dredging-frame. The form of the vessels on which this apparatus is mounted is very different; but the great object to be attended to in framing vessels for this purpose, is to obtain such a sufficient degree of strength as not only to withstand the tremulous motion of the engine and machinery, which is apt to shake the timbers of the vessel, but to resist the strains to which a vessel so employed must be continually liable.

These vessels are very different in their tonnage and dimensions, including a range from about 50 to upwards of 220 tons. The smallest machine on this principle is, perhaps, that already noticed at Greenock, which, exclusively of receiving boats, is worked by about eight men. Perhaps the largest vessel in which a steam-engine is employed, is at the works of the Caledonian Canal; where about the same number of men are occupied in attending the vessel and steam-engine. At Greenock, from 70 to 90 tons of silt and mud are lifted per day; but on the Caledonian Canal, about 400 tons have been removed in the same period, in deepening Loch Doughfour, where the bottom is of rough gravel. Here, independently of the value of the apparatus, the expense has been found to be about ninepence per cubic yard, including the expense of removal in barges; a rate which, compared with the usual tedious operation of digging and carting in tide harbours that are quite accessible, will be found to be extremely moderate. When we consider the situations in which the process of dredging is usually performed, this mode of excavating under water cannot fail to strike every one as an immense improvement in the hands of the engineer.

On the Thames, the bucket dredging-machine is also in use, though the work there is chiefly done with the spoon apparatus. One of the dredging machines on the river at London has a bucket frame working at each side of a vessel of about 100 tons burden, furnished with an engine of the power of sixteen horses, which excavates about 300 tons per day.

It is found that gravel and small stones are more easily excavated than silt or mud. In a general way, we may estimate that a bucket dredging machine, in good order, fitted up with one of Watt and Bolton's steam-engines, of the power of about twelve or fifteen horses, on a vessel of 100 or 120 tons, with two or more receiving boats, according to the distance of removal, and working in a situation sheltered from the lift or swell of the sea, and in a tide way, not exceeding one or two miles per hour in velocity, nor more than two fathoms in depth, will lift from 120 to 160 tons, or at the rate of about 30 or 40 tons per hour; and according to the situation of the work the expense will be from fourpence to one shilling per cubic yard. The cost of a dredging apparatus, complete, with an engine of the power of twelve or fifteen horses, will be about L3500. The expenditure of coals for such an engine may be taken at the rate of about 2½ cwt. per hour; and the daily expense of the whole, including the value of the apparatus, and men for every purpose, will amount to from three to five pounds, according to the circumstances of the place where it is employed.

The strength of the vessel and fitness of the machinery, and also 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 consideration of the engineer; whose regulation of all the parts of this apparatus will be much guided by attention to the actual operation, as more or less suitable to the peculiar situation of the works in which it is to be employed. What we have been able to bring under the reader's notice in this article, we trust will be found sufficient to give him an idea of the construction of the apparatus, and the principles upon which it acts. To those who may not take much interest in the details of complicated machinery, we presume it will 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. (II. II.)