each block being formed wedge-shaped so as to be easily adjusted to any required height. There is considerable variety in the shape and arrangement of the side walls of graving docks, but they must be strong enough to bear any pressure that may be applied to them as training walls. The faces are constructed in steps, or altars as they are termed (Fig. 5), on which rest the shores which support a vessel in the dock. Steps are constructed in suitable positions for access to the bottom, also slides down which timber and other materials can be lowered. These side walls are usually of concrete faced with stone; in some docks the facing is formed of concrete of extra quality, and in others of blue bricks with copings of stone. The entrances are constructed with side walls and sills to suit the method adopted for closing the dock, gates requiring a different arrangement from that provided for a caisson. Culverts fitted with penstocks are built in the side walls near the entrances, for filling the dock with water. For emptying, centrifugal pumps seem on the whole the most convenient and the least liable to be affected by the passage of small chips of wood, which cannot always be avoided even with the greatest care. When arrangements have to be provided for keeping the entrance clear of mud occasional dredging is generally resorted to. At Liverpool an extensive system of sluicing is adopted, the head of water retained in the dock or closed basin as the tide falls being utilized to produce a strong current which carries off the mud. At Tilbury jets of water under pressure, combined with an arrangement of harrows, are applied at ebb tide to the bottom of the dock and entrance, from an apparatus attached to a floating craft which can be moved to any part required; the mud stirred up by the harrows is set in motion by the jets, and carried away by the ebb tide.

docks,

Hitherto most of the dry docks for the United States Government have been constructed of timber, those of masonry having been considered too expensive. Timber In the navy yards of Charlestown, Brooklyn, and Norfolk there are masonry docks, the sills of which are 25 feet below high-water spring tides; in the timber docks at the various yards the sills are from 15 to 22 feet below high water. A report on the relative value of timber and masonry for dry docks by Commodore Endicott, U.S. Navy, chief of the Bureau of Yards and Docks, states that timber docks are temporary structures, and that the wood of which they are built requires extensive renewal at the end of 25 years, practically amounting in some instances to rebuilding; also that accidents have occurred from the partial collapse and bursting of the floor and sides of some of the timber docks. He considers that, in view of the increased depth required in new docks to accommodate vessels of large draught, the risk to their stability will be largely increased, as they will be subjected to more unfavourable conditions, in particular to a much greater hydrostatic pressure, and that freedom from such risk can only be obtained from a masonry structure designed to resist by its own weight the dangerous force to which it is subjected. He recommends that any new docks should be constructed, not of timber, but of masonry and concrete combined; and that instead of being constructed by day labour and by Government employees, they should be carried out by contract, which would be the means of greatly reducing their cost. Dock gates are constructed of timber or iron. The two doors are exactly similar, each being rather longer than half the width of the entrance, so that when Dock gates. they meet in closing their faces form two sides of a triangle whose base is the straight line drawn from one heel post to the other (Fig. 6). The distance from the apex or meeting-point to the base, which is termed the rise of the gates, is a matter of considerable importance,

hollow quoin; at the bottom is fixed a cast-iron socket which rests upon a cast-iron pivot firmly secured to the masonry of the gate platform, the top being held by an iron strap passing round it and securely anchored to the masonry of the side walls. The main beams of each gate are built into and secured to the heel post at one end and to the mitre post at the other end; the two mitre posts constitute the meeting face of the pair of gates, and should form a tight and even joint from top to bottom. The timber generally used for gates is greenheart, which is durable and not liable in Britain to the attacks of seaworms. When gates are made of iron the closing faces at sills, mitre posts, and heel posts are formed of timber to ensure close-fitting joints. The gates when open fit back into recesses formed in the side walls. Timber gates are less liable to injury or twisting, and are more easily fitted together than iron gates; on the other hand, the latter can be easily rendered buoyant by the introduction of air chambers, which reduce the strain on the hinges and the weight on the rollers supporting the outer end. These rollers are fixed so that they can be adjusted as required to support the gate, and work on a circular iron way attached to the masonry of the gate platform; they are not necessary for gates where the entrance is less than 60 feet wide. The pressure on a gate is due to the difference in level of water on the inside and outside surfaces; this has to be withstood and conveyed to the side walls by the structure of the gates. The strains in any gate are those of an arch with the load applied in radial lines and equally distributed (Fig. 6). A pair of gates should form part of a circular dam, and should be so designed that the line of pressure approximates to a line forming the arc of a circle passing through the centres of the heel posts and the centre of the meeting face of the mitre posts, to ensure that the gates are subject to compressive stress only. For convenience a pair of gates are usually shaped so that their outside faces when closed form a continuous curve, with the inside faces straight. The subject of strains in dock gates has been most fully dealt with in a paper by Mr A. F. Blandy (Proc. Inst. Č. E. vol. lviii.), and by Mr J. M. Moncrieff (ib. vol. cxvii.), to which the reader is referred for information. For moving dock gates an opening and a closing chain is attached to the outer and inner faces respectively of each gate as low down as can be arranged. The chains are conducted to the back of the side walls through openings, and are worked mostly by hydraulic power. At Barry, Leith, and other ports a more rapid and simple arrangement has been introduced by attaching the piston of a hydraulic engine direct to the back of each gate (Fig. 7). The gate thus closed is held up firm against any undulation of the water outside, and the chains are done away with, together with the necessary guiding sheaves. Instead of gates caissons S. III. - 62

caissons are that they save room by shortening the length of the entrance, they dispense with hollow quoins, they facilitate maintenance and repairs, and, by being capable of carrying a road or railway along their top, they do away with the cost and inconvenience of a swing bridge across the entrance. The deck forming the roadway on top of the sliding caisson can be so arranged that it falls automatically as the caisson is drawn into the chamber, and rises again when

Fig. 7.

1. BRITISH.

Floating and pontoon docks are constructed in various forms, but their function is in every case to lift a vessel out of the water and support it by means of their buoyancy. They can only be used in completely sheltered positions. Circumstances, such as a bad foundation, which might render the construction of a solid dock almost if not quite impracticable, would favour the adoption of a floating dock, but each case must be judged on its own merits.

Reference may be made to Proc. Inst. C. E.-C. COLSON, Notes on Docks and Dock Construction.-L. E. VERNON-HARCOURT, Harbours and Docks.-WILLIAM SHIELD, Principles and Practice of Harbour Construction. (W. E.)

Recent Structural Developments. The extensions of H.M. dockyards at Portsmouth and Chatham, described in the ninth edition article on DOCKYARDS, marked the most important stage in the history of dockyard construction which had been reached up to the date of their completion and prior to the year 1880. It is proposed to describe briefly in the present article the extent to which H.M. dockyards have developed since that date, with special reference to the great works which were sanctioned by the Naval Works Acts of 1895 and subsequent years. The works proposed under these Acts are classified under three heads, viz., (a) the enclosure and defence of harbours against torpedo attack; (6) adapting naval ports to the present needs of the fleet; (c) naval barracks and hospitals. Under the first heading are included the defensive harbours at Portland, Dover, and Gibraltar; under heading (6) are included the deepening of harbours legislation. and approaches, the dockyard extensions at Gibraltar, Keyham, Simons Bay, and HongKong, with sundry other items. Under heading (c) are included the naval barracks at Chatham, Portsmouth, and Keyham; the naval hospitals at Chatham, Haslar, and Haulbowline; the colleges at Keyham and Dartmouth, with other items. The total estimated cost of these works, as stated in the Act of 1899, amounts to upwards of twentythree and a half millions sterling, and they will form, when completed, the most important additions yet made at any one period to the dockyard and harbour works required to meet the necessities of the fleet. These works will now be briefly described under the headings of their various localities, together with the principal additions to other dockyards carried out between 1880 and 1901.

Under

special

(a) Enclosure and Defence of Harbours against Torpedo Attack.

Portland. The existing harbour of refuge at this station was commenced in 1847 under the direction of the Admiralty, and completed about fifteen years later in the form and extent in which it remained up to the date of

the commencement of the additional works now being carried out. The original design consisted of the partial enclosure of an extensive area of water bounded on the south and west by the island of Portland and the adjacent coast, and sheltered thereby from winds from those quarters, and on the east by a breakwater of large dimensions starting from the north-east corner of Portland island. This breakwater consisted of a rubble mound of stone quarried by convict labour at the summit of the island, lowered by a wire-rope incline to the sea-level, and deposited by means of staging in its position in the mound. The breakwater consisted of two portions, an inner arm terminating in a masonry head and fort, and an outer or detached breakwater terminating in a circular fort at its northern extremity, an entrance for shipping being left between the two. The enclosure thus formed afforded a magnificent sheltered roadstead of great extent, with a depth of water of not less than thirty feet over a large proportion of the area.

The developments of naval strategy and the necessity of greater protection from torpedo attack have, however, led to additional works. It will have been observed from the foregoing remarks that the harbour, while protected on the west, south, and east sides, was still open on the north to Weymouth bay and the Channel. It is therefore to the further and complete enclosure of the roadstead on this northern side that the additional works have been directed (Fig. 1). The distance from the circular fort at the extremity of the eastern breakwater above described to the Bincleaves rocks on the western shore of the harbour is about two miles. This opening is now closed by two additional breakwaters, one portion being detached and about 4465 feet in length, the other starting from the Bincleaves shore and about 4642 feet in length. Each breakwater consists of a rubble mound of stone, quarried as before on the summit of the island, lowered by wire-rope incline, shipped into specially designed hopper barges, towed out and deposited on the site of the breakwaters, which have thus been brought up from the sea bottom to above the water-line in depths of water ranging from 30 to 50 feet. Each section of breakwater

is terminated by a masonry head with beacon light, and two entrances, each 700 feet in width, serve for the navigation of shipping in entrance and exit. The defensive harbour thus completely enclosed, partly by natural means, partly by the great artificial works now carried out, covers an area of 2200 acres to the one-fathom line, of which 1500 acres have a depth of not less than 30 feet at low water, and is probably one of the finest artificial harbours in the world. There is no dockyard at Portland, but the watering and coaling appliances for the supply of the fleet are of considerable importance and extent, the latter having lately been greatly improved by the construction of a coaling jetty and camber for the storage of both sea-borne and landborne coal and its handling by modern hydraulic appliances.

Dover. The Admiralty pier at Dover, so familiar to passengers by this route to or from the Continent, was the first instalment of the scheme for a large harbour of refuge recommended by the Royal Commissions of 1844-45. This pier was commenced in 1847, and practically completed in 1871, having a total length of about 2000 feet. As an engineering structure it has proved successful, as, with the exception of the destruction of the parapet in 1877, no material damage has been caused by the heavy seas to which it has constantly been exposed. The remainder of the scheme was not, however, completed. The Royal Commission of 1881 on the employment of convicts again recommended the construction of a large harbour at Dover, but beyond the erection of a convict prison nothing further was done for some years. In the meanwhile the Dover Harbour Board, with a view to improving the existing condition of the port, commenced the scheme indicated on the plan and entitled the "commercial harbour," consisting of an east pier, which runs parallel to the general direction of the Admiralty pier, and will ultimately enclose in conjunction with the latter a sheltered water area of some 75 acres. This work was commenced in 1893, and is now approaching completion. The enclosed harbour for the accommoda

Union Workhouse,

of the harbour; the last starts from the shore at a point somewhat east of the convict prison, and runs southerly for a length of 3320 feet.

These three breakwaters, with a united length of rather more than 12 miles, are each built of massive concrete blocks in the form of a practically vertical wall, founded upon the solid chalk and rising to a quay level of 10 feet above high water Two entrances, one 800 feet and the other

NEYMOUTH HARBOUR

PORTLAND HARBOUR.

WEYMOUTH AND

LAND RAILWAY

WEST

BAY

FATHOM

tion of H.M. navy, for which funds were voted by Parlia- | ment in 1895 and subsequent years, is indicated on the plan. It will be seen that the water area enclosed amounts to 610 acres, exclusive of the commercial harbour, of which 322 acres have a depth of not less than 30 feet at low water. The enclosing breakwaters are three in number, the most westerly one being an extension in a south-easterly direction of the existing Admiralty pier for a length of 2000 feet; the southern breakwater is isolated, and is 4200 feet in length, curving round shorewards at its eastern end to accord with the direction of the third breakwater, which forms the eastern boundary

PLAN SHEWING

NEW BREAKWATERS.

100 0

BREAKWATER

600 feet in width, are provided in the positions shown on the plan. The foreshore at the foot of the cliffs between the Castle jetty and the root of the eastern breakwater is in process of reclamation by means of a massive sea-wall founded upon the chalk. These great works were begun in 1897 (Fig. 2).

Gibraltar. The details of the defensive harbour here are closely connected with the general scheme of dockyard extension, and will be found described under the reference to Gibraltar which follows. The enclosed water area at Gibraltar is less than that at Dover in the proportion of about 440 to 610 acres, but neither harbour is comparable with Portland in area.

(b) Adapting Naval Ports to the Present Needs of

the Fleet.

Gibraltar. The history of British naval dockyards is in general one of continuous development, extending over considerable periods of time, and reflecting in the various stages of that history the naval requirements, the engineering skill, and the mechanical resources and appliances of the day. As these have developed and increased, so the older and more obsolete portions and appliances of the yards have been from time to time renewed and readapted to meet modern exigencies; and the result exhibits the constantly increasing effort to meet the demands which modern naval architecture and strategy are as constantly renewing. In the case, however, of the great works at Gibraltar we have the somewhat unique example of the creation of a modern dockyard of the first class, as it were, at a stroke, as the small existing yard will have become entirely absorbed in the new extension works. The absence of any dry dock at Gibraltar had been for some time the occasion of considerable public discussion, and a commission was appointed in 1890 to consider the question. But the recommendations of this commission became greatly amplified in the scheme finally approved by the Admiralty, for which funds were provided in the Naval Works Loan Act of 1895 and subsequent years. This scheme may be said to consist of two principal parts: the construction of a dockyard with ample dry dock accommodation, with the necessary workshops, stores, and offices; and the construction of a great defensive harbour of sufficient extent to shelter a powerful fleet, and yielding largely increased coaling and wharfage accommodation. Combined with the defensive harbour, and forming an integral part thereof, is the new commercial mole, a portion of the cost of which is borne by the colony (Fig. 3).

The necessary space for dockyard purposes is exceedingly limited at the Rock of Gibraltar, and thus, with the exception of the small area of the old yard, the area required has been obtained by artificially reclaiming the land over a considerable sea-space, of which the soundings range from low-water mark to 40 feet in depth. The total area thus created for dockyard purposes is about 64 acres. The material required for this reclamation is obtained chiefly from quarries on the eastern side of the Rock by means of a tunnel pierced for that purpose. The graving docks are three in number, alike in cross section and in depth over the sill, but of unequal lengths, to accommodate battleships and cruisers. Dock No. 1 is 850 feet in length, 95 feet in width at entrance, and has a depth of 35 feet over the sill at low-water springs. It is divided into two portions by an intermediate sliding caisson, and is capable of docking two ships simultaneously. Dock No. 2 is 550 feet, and Dock No. 3, 450 feet in length, and of similar dimensions in width of entrance and depth over sill. All the docks will have their entrances closed by steel sliding caissons, and are lined throughout with granite and limestone. The most important dockyard buildings are the pumpingengine house, the workshops of the chief constructor's and chief engineer's departments, the naval stores and offices. ing-engine house will contain the steam machinery for the emptying of the dry docks, which will be capable of pumping out the contents of the largest dock, amounting at high water to 105,000 tons, and discharging into the sea in five hours. The workshop for the chief constructor's department is an extensive building, 407 feet in length and 322 feet in width. The shops of the chief engineer's department are comprised within a building of similar dimensions. The staff captain's department and other stores are comprised within buildings having a total floor-space of about 300,000 superficial feet. At the northern end of the yard are the administrative offices and a series of slipways for hauling up destroyers, together with a slip for small craft, a wharf for ordnance purposes, and an auxiliary boat camber. reclaimed area of the dockyard is faced seawards by a wharf wall constructed of interlocked concrete blocks, upon a new system, built in the sea without the aid of a dam, and having an unbroken line for wharfage purposes of about 1600 feet lineal, with a depth of water alongside of 33 feet at low water. Upon this wharf are erected powerful shears and cranes for the use of vessels alongside.

The pump

The

The older appellation of "harbour of refuge" has, from the point of view of the naval strategist, acquired a new and extended

meaning, and while the modern defensive harbour may still.serve as shelter from bad weather, it further fulfils the not less important function of protection from torpedo attack. The defensive harbour now in course of construction at Gibraltar is formed by the enclosure of a water area of about 440 acres, of which some 250 acres will have a minimum depth of 30 feet at low water. This enclosure is formed by three several works of considerable magnitude: the New Mole Extension forming the southerly boundary, the Commercial Mole on the north side, and the Detached Mole forming the westerly boundary, while two entrances, each 200 yards wide, form the approaches to the harbour from the sea. The New Mole, so called in distinction from the Old Mole or Devil's Tongue at the northern end of the town, is supposed to have been commenced during the Spanish occupation in the year 1620. Both New and Old Moles are frequently referred to in Drinkwater's History of the Siege of Gibraltar, and the successful assault upon the former by the officers and men of the British fleet forms a leading incident in the capture of Gibraltar by Admiral Sir George Rooke in 1704. Additions to the length of this mole were made at various times, and in 1895 the total length of the artificial mound was about 1400 feet. The extension now in progress will add 2700 feet to this length.

This mole is formed of a rubble mound, with a top width of 102 feet, the stone being obtained from adjacent quarries and deposited in the sea by barges. The harbour side is faced by a continuous wharf wall, having 30 to 35 feet depth of water

The

alongside, and the total length of wharfage available for ships of war for coaling or other purposes will be 3500 feet. Detached Mole, forming the westerly boundary of the harbour, is of a different type of construction. It is a vertical wall formed of massive concrete blocks, the greater number of which are of 32 tons in weight, arranged upon what is known as the sloping block system, and founded upon a rubble mound of stone deposited from barges and levelled for the reception of the blocks by divers. As this breakwater stands isolated, some half a mile from the nearest point on shore, in from 45 to 65 feet depth of water, and has at no time during its construction been connected with the land by any temporary staging, a brief description of the method employed for commencing its constuction will not be without interest. A box-shaped steel caisson, 33 feet wide, 74 feet in length on the top, 101 feet in length at the bottom, and 48 feet high, fitted with compartments and with ends sloped to the angle of the blockwork, was constructed in this country, shipped to Gibraltar, re-erected, launched, towed to its correct position, and sunk, by the admission of water, on the rubble mound before mentioned. Concrete was then filled in as rapidly as possible until the entire mass, weighing about 9000 tons, had formed, so to speak, an artificial rock or island in the sea, being, in fact, a completed section of the breakwater itself. Upon this foundation were erected two block-setting Titans (see TITAN CRANES), capable of setting 36-ton blocks at a radius of 75 feet, by which means this mole has been rapidly extended north and south to its full length of 2720 feet. The blocks, having been transported from the block-yard by rail and then shipped in barges to the site of the mole, are laid in their sloping position by means of special apparatus designed and patented by the

writer.

The Commercial Mole, forming the northerly boundary of the harbour, is intended to serve the commercial requirements of the colony, while at the same time it forms an integral portion of the defensive scheme. This mole starts from the neighbourhood of the Waterport wharf, alongside the Old Mole or "Devil's Tongue," an appellation given to this portion of the defences of the fortress during the siege of Gibraltar in 1779-83, on account of the annoyance to the besiegers caused by its flanking fire. The mole consists of a rubble mound projecting in a westerly direction, furnished with five jetties, lying north and south, and terminating in a western arm lying parallel to the jetties. Each jetty and the western arm is faced with a concrete wharf wall, and the total length of wharfage available for mercantile coaling or other purposes amounts to 7000 feet lineal, with a depth of water alongside ranging from 20 to 30 feet. Extensive coal and bonded stores are provided, and an open viaduct is constructed in the connecting arm, leaving a waterway for the purpose of giving circulation to the enclosed waters of the harbour, which is being dredged to secure the depths already referred to.

Devonport and Keyham.-Prior to the period dealt with in this article, the works at Keyham described in the ninth edition article on DOCKYARDS had been completed; railway communication with the Cornwall line was established, and a tunnel formed a connexion between the two yards. Keyham yard in 1880 consisted substantially of two basins, the north basin of 9 acres and the south basin of 7 acres in extent; three docks, having floor-lengths of