perience to maintain, with Sir James Clark Ross, that "from however great a depth we may be enabled to bring up mud and stones of the bed of the ocean we shall find them teeming with animal life." Samples of the seabottom, procured with great difficulty and in small quantity from the first deep soundings in the Atlantic, chiefly by the use of Brooke's sounding machine, an instrument which by a neat contrivance disengaged its weights when it reached the bottom, and thus allowed a tube, so arranged as to get filled with a sample of the bottom, to be recovered by the sounding line, were eagerly examined by miscroscopists; and the singular fact was established that these samples consisted over a large part of the bed of the Atlantic of the entire or broken shells of certain Foraminifera. Dr Wallich, the naturalist to the "Bulldog" sounding expedition under Sir Leopold M'Clintock, reported that star-fishes, with their stomachs full of the deep-sea Foraminifera, had come up from a depth of 1200 fathoms on a sounding line; and doubts began to be entertained whether the bottom of the sea was in truth a desert, or whether it might not present a new zoological region open to investigation and discovery, and peopled by a peculiar fauna suited to its spécial conditions. In the year 1868, while the question was still undecided, two testing investigations were undertaken independently. In America Count L. F. Pourtales, one of the officers employed in the United States Coast Survey under Professor Pierce, commenced a series of deep dredgings across the Gulf Stream off the coast of Florida, which were continued in the following year, and were productive of most valuable results; and in Great Britain the Admiralty, on the representation of the Royal Society, placed the "Lightning," a small gun vessel, at the disposal of a small committee to sound and dredge in the North Atlantic between Shetland and the Faröe Islands. In the "Lightning," with the help of a "donkeyengine" for winding in, dredging was carried on with comparative ease at a depth of 600 fathoms, and at that depth animal life was found to be still abundant. The results of the "Lightning's" dredgings were regarded of so great importance to science that the Royal Society pressed upon the Admiralty the advantage of continuing the researches, and accordingly, during the years 1869 and 1870, the gun-boat "Porcupine" was put under the orders of a committee consisting of Dr Carpenter, F.R.S., Dr Gwyn Jeffreys, F.R.S., and Professor Wyville Thomson, F.R.S., one or other of whom superintended the scientific work of a series of dredging trips in the North Atlantic to the north and west of the British Islands, which occupied two summers. In the "Porcupine," in the summer of 1869, dredging was carried down successfully to a depth of 2435 fathoms, upwards of two miles and a half, in the Bay of Biscay, and the dredge brought up well-developed representatives of all the classes of marine invertebrates. During the cruises of the "Porcupine" the fauna of the deep water off the western coasts of Great Britain and of Spain and Portugal was tolerably well ascertained, and it was found to differ greatly from the fauna of shallow water in the same region, to possess very special characters, and to show a very marked relation to the fauna of the earlier Tertiary and the later Cretaceous periods. In the winter of 1872, as a sequel to the preliminary cruises of the "Lightning" and "Porcupine," by far the most considerable expedition in which systematic dredging had ever been made a special object left Great Britain. H.M.S. "Challenger," a corvette of 2306 tons, with auxiliary steam working to 1234 horse-power, was despatched to investigate the physical and biological conditions of the great ocean basins,. The "Challenger" was provided with a most complete and liberal organization for the purpose; she had powerful deck engines for hauling in the dredge, workrooms, laboratories, and libraries for investigating the results on the spot, and a staff of competent naturalists to undertake such investigations and to superintend the packing and preservation of the specimens reserved for future study. In these deep-sea dredgings it was frequently found that, while few objects of interest were brought up within the dredge, many echinoderms, corals, and sponges came to the surface sticking to the outside of the dredge, and even to the first few fathoms of the dredge-line. This suggested many expedients, and finally a long transverse iron bar was attached to the bottom of the dredge-bag, and large bunches of teazed-out hemp were fastened to the free ends of the bar (fig. 4). The "hempen-tangles" are now regarded as an essential part of the dredge, nearly as important as the dredgebag, and often much more conspicuous in its results. This addition to Ball's dredge is not, however, gererally available in dredging from a boat or in shallow water; the tangles are apt to catch on rocks or coral, and a turn of the drum of the donkey-engine is required to free them. Ball's dredge was still employed, with some slight modifications, the result of further experience. Fig. 4 represents the form of dredge which was found most suitable for great depths. The dredgeframe of hammered iron is 4 feet 6 inches long and 1 foot 3 inches broad; the scrapers are 3 inches wide, and are connected at the ends by bars of 1 inches round FIG. 4.-Deep-sea Dredge, iron. The arms are of inch with Tangle-bar. round iron, and slightly curved; they are bolted together to a stout iron bar which ends above in a swivel and ring. Two bars of square iron of some strength are attached by eyes to the round cross-bars at the ends of the dredge-frame, and have the other ends lashed to the iron bar which bears the tangles. These rods keep the dredge-bag at its full length, and prevent it or the tangles from folding over the mouth of the dredge. The dredge-bag is 4 feet 6 inches in length; the lower half is of twine netting so close as to retain everything except the finest mud, which indeed only partially washes through, and the upper half is of twine netting with the meshes an inch to the side. The bag is guarded by three loops of bolt-rope attached to the frame of the dredge, to the bottom of the bag, and finally to the tangle-bar. The canvas pads represented in the figure on the dredge-frame are only to protect the seizings of the loops. The dredge is suspended by an iron chain, which forms the first few fathoms of the dredge-line. The chain is not, however, directly fastened to the ring at the end of the arms, but is made fast to one of the end bars of the dredge-frame, and it is stopped to the ring by a single strand of bolt-rope. If the dredge get caught the stop carries away, the direction of the strain on the dredge is altered, and it probably relieves itself and comes up end upwards. In deep water a 28 b deep-sea lead is usually hung from the centre of the tangle-bar with four tangles on each side. Dredging was carried on in the "Challenger" from the main yard-arm. A strong pendant was attached by a hook to the cap of the main-mast, and by a tackle to the yardarm a compound arrangement of 55 to 70 of Hodge's patent accumulators was hung to the pendant, and beneath it a block through which the dredge-rope passed. The donkey-engines for hauling in the dredging and sounding gear were placed at the foot of the main-mast on the port side. They consisted of a pair of direct-acting, highpressure, horizontal engines, in combination of 18 horsepower nominal. Instead of a connecting rod to each, a guide was fixed to the end of the piston-rod with a brass block working up and down the slot of the guide. The crank-axles ran through the centre of the blocks, and the movable block, obtaining a backward and forward motion from the piston-rod, acted on the crank as a connecting-rod would do. This style of engine is commonly used for pumping, the pump-rods being attached to the guide on the opposite side from the piston-rod. At one end of the crank a small toothed wheel was attached, which drove one thrice the multiple on a horizontal shaft extending nearly across the deck, and about 3 feet 6 inches above it. At each end of this shaft a large and a small drum were fixed, the larger having three sheaves cast upon it of different sizes, the lesser being a common barrel only. To these drums the line was led, two or three turns being taken round the drum selected. In hauling in, the dredge-rope was taken to a gin-block secured to a spar on the forecastle, then aft to the drum of the donkey-engines on the port side, then to a leading-block on the port side of the quarter-deck, and across the deck to a leading-block on the starboard side corresponding in diameter with the drum used on the port side, and from this it was finally taken by the hands and coiled. The strain is of course greatest at the yard arm and the first leading block, and by this arrangement it is gradually diminished as the line passes round the series of blocks and sheaves. A change made latterly in the handling of the dredge had certain advantages. Instead of attaching the weights directly to the dredge-rope, and sending them down with the dredge, a "toggle," a small spindle-shaped piece of hard wood, was attached transversely to the rope at the required distance, 200 to 300 fathoms in advance of the dredge. A "messenger," consisting of a figure of eight of rope, with two large thimbles in the loops, had one of the thimbles slipped over the chain before the dredge was hung, and the other thimble made fast to a lizard. When the dredge was well down and had taken its direction from the drift of the ship, the weights, usually six 28-b deepsca leads in three canvas covers, were attached to the other thimble of the traveller, which was then cut adrift from the lizard and allowed to spin down the line until it was brought up by the toggle. By this plan the dredge took a somewhat longer time to go down; but after it was adopted not a single case occurred of the fouling of the dredge in the dredge-rope, a misadventure which had occurred more than once before, and which was attributed to the weights getting ahead of the dredge in going down, and pulling it down upon them entangled in the double part of the line. The great risk in dredging in very deep water is that of the dredge running down nearly vertically and sinking at once into the soft mud, and remaining imbedded until hauling in commences. During the earlier part of the voyage of the "Challenger "this accident seemed too often to defeat, at all events partially, the object of the operation; and, after various suggestions for modifying the dredge, it was proposed to try some form of the trawl in order to insure, so far as possible, the capture of any of the larger marine animals which might be present, and thus to gain a better general idea of the nature of the fauna. A 15-feet beam-trawl was sent down off Cape St Vincent to a depth of 600 fathoms; the experiment looked hazardous, but the trawl came up in due time all right, and contained, along with many of the larger Invertebrata, several fishes, The trawl seemed to answer so well that it was tried again a little farther south in 1090 fathoms, and again it was perfectly successful, and during the remainder of the voyage it was employed almost as frequently, and in nearly as deep water, as Ball's dredge. The deepest successful haul of the traw! was in the Pacific in 3125 fathoms, and the deepest haul of Ball's dredge was in the Atlantic at 3150 fathoms. During the voyage of the "Challenger" a course of about 70,000 nautical miles was traversed in three years and a half, and 362 observing stations were established at intervals as nearly uniform as circumstances would permit ; and at the greater number of these dredging or some modification of the process was successfully performed-52 times at depths greater than 2000 fathoms, and thrice at depths beyond 3000 fathoms. So fully convinced were the "Challenger" officers that they could dredge at any depths, that it was only want of time and daylight which prevented their doing so at their deepest sounding, 4575 fathoms. The Atlantic was crossed five times, and an erratic route through the Pacific gave a good idea of the conditions of the abysses of that ocean, while in the South Indian Ocean dredging and trawling were carried down close to the Antarctic ice-barrier. The expedition was successful, and the results were of the most interesting nature. Animal life was found to exist at all depths, although probably in diminishing abundance as the depth becomes extreme; and in all parts of the world at depths beyond 400 or 500 fathoms the fauna had much the same general character. The species usually differed in widely separated areas, but the great majority of forms, if not identical, were so nearly allied that they might be regarded as representative and genetically related. Although all marine invertebrate classes were represented, echinoderms in their different orders, sponges, and Crustacea preponderated, while corals and Mollusca were comparatively scarce. In the first two groups named many forms occurred allied to families which had been previously regarded as extinct or nearly so; thus among the echinoderms, stalked crinoids were by no means rare, and many species of regular Echinidea related to the Chalk genus Echinothuria, and many irregular species allied to Ananchytes and Dysaster occurred. The sporges were mainly represented by the Hexactinellider, the beautiful order to which the glass-rope sponge of Japan and the marvellous "Venus's Flower Basket" of the Philippines belong, the order to which the Ventriculites of the Chalk must also be referred. Dredging at these great depths is a difficult and critical operation, and, although by its means some idea of the nature and distribution of the abyssal fauna of the ocean has already been attained, it will be long before the blanks are filled up; for of the area of 140,000,000 square miles forming the "abyssal province" the actual amount hitherto traversed by the naturalist's dredge may still be readily reckoned by the square yard. (c. W. T.) DREDGING. Dredging is the name given by engineers to the process of excavating materials under water, raising them to the surface, and depositing them in barges. It is a process which has been useful from very early times in works of marine and hydraulic engineering, and it has of late years, by improved appliances, been brought to high perfection. Bag and Spoon Dredge.-The first employment of machinery to effect this object is, like the discovery of the canal lock, claimed alike for Holland and Italy, in both of which countries dredging is believed to have been practised before it was introduced into Britain. Dutch at a very early period used what is termed the "bag and spoon" dredge for cleaning their canals. It was The simply a ring of iron, about 2 feet in diameter, flattened and steeled for about one-third of its circumference, having a bag of strong leather attached to it by leather thongs. The ring and bag were fixed to a pole, which, on being used, was lowered to the bottom from the side of a barge moored in the canal or river. A rope made fast to the iron ring was then wound up by a windlass placed at the other end of the barge, and the spoon was thus dragged along the bottom, and was guided in its progress by a man who held the pole. When the spoon reached the end of the barge where the windlass was placed, the winding was still continued, and the suspending rope being nearly perpendicular, the bag was raised to the surface, bringing with it the stuff excavated while it was being drawn along the bottom. The windlass being still wrought, the whole was raised to the gunwale of the barge, and the bag, being emptied, was again hauled back to the opposite end of the barge, and lowered for another supply. This system is slow, and only adapted to a limited depth of water and a soft bottom. But it has been generally employed in canals, and is much used in the Thames. The writer had occasion to use it at the Fossdyke Canal, in Lincolnshire, where 135,000 tons were raised in the manner described. Dredging by Bucket between two Lighters.—Another plan, practised at an early period in rivers of considerable breadth, was to, moor two large barges, one on each side; between them was slung an iron dredging bucket, which was attached to both barges by chains wound round the barrels of a crab wincn worked by six men in one barge, and round a simple windlass, worked by two men in the other. The bucket, being lowered at the side of the barge carrying the windlass, was drawn across the bottom of the river by the crab winch on the other barge; and, having been raised and emptied, it was hauled across by the opposite windlass for a repetition of the process. This plan was in use in the Tay till 1833. Steam Dredges. In all large operations these and other primitive appliances have now, as is well known, been superseded by the steam dredge, which was first employed, it is believed, in deepening the Wear at Sunderland about the year 1796. The Sunderland machine was made for Mr Grimshaw by Boulton and Watt. Receiving improvements from Mr Hughes, Mr Rennie, Mr Jessop, and others, the steam dredge, as now generally constructed, is a most powerful machine in skilful hands, excavating and raising materials from depths of 15 to upwards of 30 feet of water according to the size of the machinery, at a cost not very different from, and in some cases even less than, that at which the same work could be performed on dry land. As to the kind of work that may be accomplished by dredging, it may be stated that almost all materials, excepting solid rock or very large boulders, may now be dredged with ease. Loose gravel is probably the most favourable material to work in ; but a powerful dredge will readily break up and raise indurated beds of gravel, clay, and boulders, and even find its way through the surface of soft rock, though it will not penetrate very far into it. In such cases it is usual to alternate on the bucket-frame a bucket for raising the stuff, with a rake or pronged instrument for disturbing the bottom. The writer in his own experience has raised boulders weighing upwards of a ton with a powerful dredge of the ordinary construction, and removed disintegrated or rotten rock at least to a limited depth, and he believes that in many cases the surfaces of submerged rocks may, by means of such appliances, be to some extent broken up and removed, so as to obtain in certain situations a considerable increase of depth, without recourse to cofferdams, which involve great expense. The construction of large river steam dredges is now carried on by many engineering firms. The main feature of the machine is the bucket-ladder, which is lowered through an ark formed in the vessel till it reaches the bottom. Along this ladder a series of buckets traverse which cut into the bottom at the lower extremity of the ladder and return loaded with the excavated material, which is discharged at the top of the bucket-ladder into a lighter or barge prepared for its reception. The machines are sometimes made with single and sometimes with double ladders, sometimes discharging at the stern of the vessel and sometimes at both sides, but it is obviously impossible to give illustrative drawings of the different forms of dredgers in sufficient detail to be practically useful. It may be stated that a first-class dredging machine to work in 30 feet water, and discharge over either side, of 60 horse-power complete, costs at present prices about £16,000 to £18,000. The steam hoppers employed to receive and remove the dredgings carry about 500 tons of excavations; they are 70 horsepower, and steam at about 9 miles per hour. The hopper harges are made with opening hinged bottoms, which can be opened when the place of deposit is reached, and the dredgings easily and quickly discharged. These steam barges cost about £8000. Large dredges, such as those constructed by Messrs Wingate of Glasgow for the Tyne and other places, will excavate at the rate of 460 tons per hour when working on favourable ground. Hopper Dredge.-Some improvements that have been suggested on the dredging plant hitherto used deserve notice. Among these may be mentioned that of Messrs Simons & Company, Renfrew, who have patented and constructed what they have called a hopper-dredge, combining in itself the advantages of a dredge for raising the material and a screw hopper vessel for conveying it to the place of discharge, both which services are performed by the same engines and the same crew. Messrs Simons have constructed seven hopper dredges on this plan, varying from 200 to 1000 tons of "hopper capacity." Silt Dredge.-Another of the recently suggested improvements is that by Mr C. Randolph, who, in 1879, proposed that, instead of the ordinary dredging buckets, pipes should be lowered until they come into contact with the sand or mud at the bottom. The tops of these pipes were to be in communication with powerful centrifugal pumps, so that the velocity of the in-flowing water through the pipes could be made so great as to carry with it a large percentage of the sand or mud from the bottom; and when the solid matter, and the water in which it is suspended, were raised to the desired height, they would flow freely to any required place for deposit of the suspended material. It is not known that this plan has been carried into practical operation. Dredging at Amsterdam and Suez Canals.—Another arrangement is that of raising the material by buckets in the ordinary way, and thereafter receiving it in a vessel and floating it off by pipes to the place of deposit. This, of course, can only be done where the place of deposit is close to the spot whence the material is dredged. Two plans have been proposed for effecting this. One of these has been used in the Amsterdam Canal, where the stuff is discharged from the buckets into a vertical cylinder, and is there mingled with water by a revolving Woodford-pump and sent off under pressure to the place of deposit in a semi-fluid state. At the Amsterdam Canal this was done by pipes made of timber, and hooped with iron like barrels. These wooden cylinders were made in lengths of about 15 feet, connected with leather joints, and floated on the surface of the water, conveying the stuff to the requisite distance, like the hose of a fire engine, under a head of pressure, it is believed, of 4 or 5 feet, and depositing it over the banks of the canal. A somewhat similar process was employed on the Suez Canal,-not. however, by using pumps, but simply by running the stuff to the banks on steeply inclined shoots, which were supplied with water when the material raised did not contain sufficient water to cause it to run freely. It is obvious, however, that these arrangements can only be applied in situations where the material to be excavated is of a very soft nature, and where the place of deposit is close at hand. In keeping clear the Suez Canal such appliances may be very useful, as the soft deposit of the canal has only to be raised and projected over the banks on either side. American Dredges.—Dredging in Canada and the United States is done by what are called Dipper and Clam-shell dredges, the bucket dredge being seldom used. The dipper dredge consists of a barge, with a derrickcrane reaching over the stern, suspending a large wroughtiron bucket which brings up the dredged material. To the bucket is attached a pole 6 inches by 4 inches in cross section, by which means it is guided while being drawn along the bottom; it is then raised, and its bottom being made to drop open, the contents fall into the barge moored alongside of the dredge. The bottom of the bucket is kept closed by a catch, which, by means of a rope, can be withdrawn at the proper moment. The clamshell is a box made of two similar pieces of wrought iron hinged together at one end; by a simple arrangement of the gearing the clam, mouth open, drops down and sinks into the bottom, and the first effect of heaving up is to close it, thus imprisoning a quantity of material which is raised and deposited as in the case of the dipper. Both kinds of dredges are worked by a steam-engine, and rough as they appear to be, they are extensively employed in deepening and widening river channels, making or deepening canals, and other such works. This is not the place to discuss the merits of different apparatus, which perhaps can only be settled by the actual performance of different arrangements when fully tested by practice. Having thus briefly noticed them, a few practical observations on dredging, as more immediately applicable to British rivers, have still to be mentioned. Longitudinal and Cross Dredging.-In river dredging two systems are pursued. One plan consists in excavating a series of longitudinal furrows parallel to the axis of the stream, the other in dredging cross furrows from side to side of the river. It is found that inequalities are left between the longitudinal furrows when that system is practised, which do not occur, to the same extent, in side or cross dredging; and the writer invariably finds cross dredging to leave the most uniform bottom. To explain the difference between the two systems of dredging it may be stated that in either case the dredge is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredges are wound round windlasses worked by the engine, so that the vessel can be moved ahead or astern by simply throwing them into or out of gear. In longitudinal dredging the vessel is worked forward by the head chain, while the buckets are at the same time per forming the excavation, so that a longitudinal trench is made in the bottom of the river. When the dredge has proceeded a certain length, it is stopped and permitted to drop down and commence a new longitudinal furrow, parallel to the first one. In cross-dredging, on the other hand, the vessel is supplied with two additional moorings, one on each side; and these chains are, like the head and stern chains, wound round barrels wrought by the engine. In commencing to work by cross dredging we may suppose the vessel to be at one side of the channel to be excavated. The bucket frame is set in motion, but, instead of the dredge being drawn forward by the head chain, she is drawn to the opposite side of the river by the side chain, and, having reached the extent of her work in that direction, she is then drawn a few feet forward by the head chain, and, the bucket frame being still in motion, the vessel is hauled across by the opposite chains to the side whence she started. By means of this transverse motion of the dredge a series of cross furrows is made; she takes out the whole excavation from side to side as she goes on, and leaves no protuberances such as are found to exist between the furrows of longitudinal dredging, even where it is executed with great care. two systems will be best explained by reference to fig. 1, where A and B are the head and stern moorings, and C and D the side moorings; the arc ef represents the course FIG. 1. The of the vessel in cross dredging; while in longitudinal dredging, as already explained, she is drawn forward towards A, and again dropped down to commence a new longitudinal furrow. Blasting combined with Dredging.-In some cases, however, the bottom is found to be too hard to be dredged until it has been to some extent loosened and broken up. Thus at Newry, Mr Rennie, after blasting the bottom in a depth of from 6 to 8 feet at low water, removed the material by dredging at an expense of from 4s. to 5s. per cubic yard. The same process was adopted by Messrs Stevenson at the bar of the Erne at Ballyshannon, where, in a situation exposed to a heavy sea, large quantities of boulder stones were blasted, and afterwards raised by a dredger worked by hand at a cost of 10s. 6d per cubic yard. Sir William Cubitt also largely employed blasting in connection with dredging on the Severn, of which an instructive account is given in the Minutes of Proceedings of the Institution of Civil Engineers, from which the following particulars are taken : "It appears that a succession of marl beds, varying from 100 yards to half a mile in length, were found in the channel of the Severn, which proved too hard for being dredged, the whole quantity that could be raised being only 50 or 60 tons per day, while the nachinery of the dredgers employed was constantly giv ing way. Attempts were first made to drive iron, rods into the marl bed, and to break it up; a second attempt was made to loosen it by dragging across its surface an instrument like a strong plough. But these plans proving unsuccessful, it was determined to blast the whole surface to be operated on. The marl was very dense, its weight being 146 Ib per cubic foot; and it was determined to drill perpendicular bores, 6 feet apart, to the depth of 2 feet below the level of the bottom to be dredged out. The bores were made in the following manner, from floating rafts moored in the river. Pipes of th inch wrought iron, 3 inches diameter, were driven a few inches into the marl. Through these pipes holes were bored, first with a 1 inch jumper, and then with an anger. The holes wero bored 2 feet below the proposed bottom of the dredging, as it was expected that each shot would dislocate or break in pieces a mass of marl of a conical form, of which the bore-hole would be the centre and its bottom the apex; so that the adjoining shots would leave between them a pyramidal piece of Earl where the powder would have produced little or no effect. By carrying the shot holes lower than the intended dredging, the apex only of this pyramid was left to be removed; and in practice this was found 1 Clay weighs about 109 Ib, and sandstone about 155 fb per cubia foot. VII. 59 - rafts a few inches. Mr Edwards says that not one in a hundred shots missed fire, and these shots were generally saved by the following singular expedient:-The pointed end of an iron bar, inch diameter, was made red hot, and, being put quickly through the water, and driven through the tamping as rapidly as possible, was, in nine cases out of ten, sufficiently hot to ignite the gunpowder and fire the shot. The cost of each shot is calculated as follows: Use of material.............. Pitched bag for charge. 3 lb of powder at 54d. of a penny ....... 0 3 3 0 0 3 0144 009 0 0 41 Cost per shot......£0 7 0 Each shot loosened and prepared for dredging about four cubic yards, so that the cost of blasting was 1s. 9d. per yard. The cost of dredging the material, after it had been thus prepared, was 2s. 3d., making the whole charge for removing the marl 48. per cubic yard." One of the most recent successful combinations of blasting and dredging was that completed in 1875 by Mr John Fowler of Stockton at the river Tees, to whom the writer is indebted for the following particulars. The chief novelty was in the barge upon which the machinery was fixed. It was 58 feet by 28 feet by 4 feet, and had eight legs which were let down when the barge was in position. The legs were then fixed to the barge, so that on the tide falling it became a fixed platform from which the drilling was done. The holes were bored and charged, and when the tide rose the legs were heaved up and the barge removed, after which the shots were discharged. There were 24 boring tubes on the barge, and that was the limit which could at any time be done in one tide. The surface over which the blasting was done measured 500 yards in length by 200 yards in breadth, a small part of that surface being uncovered at low water. The depth obtained in midchannel was 14 feet at low-water, the average depth of rock blasted being about 4 feet 6 inches. The holes, which were bored with the diamond drill, varied in depth from 7 to 9 feet, the distance between them being 10 feet. Dynamite in tin canisters fired by patent fuse was used as the explosive, the charges being 2 b and under. The rock is Oolite shale of variable hardness, and the average time occupied in drilling 5-feet holes was twelve minutes. The dredger raised the blasted rock,-the cost for blasting, lifting, and discharging at sea being about 4s. per cubic yard, including interest on dredging and other plant employed. The dredger sometimes worked a face of blasted material of from 7 to 8 feet. The quantity blasted was 110,000 cubic yards, and the contract for blasting so as to be lifted by the dredger was 3s. 1d. per cubic yard. Dredging in Exposed Situations.-In some cases dredging has to be conducted in exposed situations such as the deepening of the "flats" at Londonderry and the bar at Carlingford. Messrs Stevenson found that dredging at the Foyle could not be conducted when the height of the waves exceeded 2 feet; and Mr Barton at Dundalk so far confirms this, as he estimates a swell of 2 feet as the highest to work in. Dredging on the River Clyde.-An important point connected with this subject is the cost at which dredging may be done when conducted on a large scale. This, of course, must depend on the character of the stuff to be raised and other circumstances; but the following information, kindly communicated by Mr James Deas, the engineer to the Trustees of the Clyde Navigation. cannot fail to be both interesting and useful. Mr Deas says truly that the Clyde Trustees employ probably the largest dredging fleet of any trust in the kingdom, in maintaining and still deepening and widening the river to meet the ever-increasing demands of the shipping trade. In the year 1871, for example, 904,104 cubic yards, or about 1,130,000 tons, were dredged from the river, of which 689,560 cubic yards were carried to sea by steam hopper barges, and 214,544 cubic yards deposited on land by means of punts. Of this 904,104 cubic yards, 345,209 cubic yards were deposit from the higher reaches of the river and its tributaries, and from the city sewers, and 558,895 cubic yards new material. The total cost for dredging and depositing was £35,448, or about 9.41 pence per cubic yard. Owing to the difference in power of the dredging machines employed, and the character of the material lifted, the cost of dredging varies much. In 1871 the most powerful machine, working 2420 hours, lifted 430,240 cubic yards of silt and sand at a cost of 2-60 pence per yard; and this was deposited in Loch Long, 27 miles from Glasgow, by steam hopper barges, at 5:46 pence per yard. On the other hand, another dredger, working 2605 hours, lifted only 26,720 cubic yards of hard gravel and boulder clay, at the cost of 20-8 pence per cubic yard, which was deposited on the alveus of the river at the cost of 17-46 pence per cubic yard; another, working 1831 hours, lifted 122,664 cubic yards of silt, sand, and sewage deposit, at the cost of 5-67 pence per cubic yard, which was deposited on land at the cost of 16:40 pence per cubic yard; and another, working 2233 hours, lifted 65,160 cubic yards of till, gravel, and sand, at the cost of 5-89 pence per cubic yard, which was deposited on the alveus of the river at the cost of 9.83 pence per cubic yard. The total quantity dredged from the river during the twenty-seven years prior to 1872 amounts to 13,617,000 |
