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DRY DOCKS. :

THE subjoined report on this Bubject was presented to tho Legislative Assembly on Tuesday by the Hon. John

Sutherland :

Inattention to Mr Secretary Sutherland's directions that I should furnish him with a report on the subject ol dooks gene i ally, as bearing upon the proposal submitted by Government to l'ardament to construot a new flrst-rlsss dock at Cuckatoo Island, which would be capable of taking in the largest class of shifs, 1 have the honour to lay before him the following; short report upon the subject, Illustrated by a few sketches ot the dilferent kinds ot docks which have been brought into use tram time lo

time at otber places.

Var lou« methods have been had recourse to for cleaning an4 repairing the immersed parts of boats and vessels. In early times they were probably hauled up OD tho beach for this purpose, or, in situations w here there was sufficient ribo and fall of tide, the} were placed at high water on the beach, where the receding tide would leave them high and dry; but as the requirements of navigation demanded the eoiploj mont of larger vessels, different and less simple methods bad lo bo resorted to. Careening 01 heaving down, forming gridirons on the flat shore, where the rise ana fall ot tide was sufficient, graving docks, and patent Blips, balance docks, sectional docks, and hydraulic lifts hive been tried, where the circumstances seemed to favour eaoh particular pian; and it is worthy of remark that m this harbour aro to bo found examples of nearly all these dilferent appliances.

Careening-a practico nsw nearly obso'cte-was brought to a considerable degree of perfection by the Dutch towards th'e end of the last century, ana was also extensively practised in the French arsenals as well as In those of 1,'ngland. It was performed in «Re or other oí the following ways :-file ves el to ba oximinod having firBt beon partly lightened, as much of the cargo or ballast as was considered safe was moved to ono side, causing her to heel over, and exposing to view a considerable portion of the other side, which could then bo cleaned, caulked, or repaired. Tills waB, however, a dangerous practice, as was shown by the cap itelug and foundering of the lloyal Ocorga, at Spithesd, in

AuguBl, 1782) wi h Admiral Kempcmeldt and his whole crew of

eCO mm.

In addition to tho danger attending this method of careening, there was also the objeolion that It wa« frequently attended by a distortion of the shape of tho ship, which was csuscd by the unequal stress brought on the cipo'cd sida of the r<.eiel during ibo operation, and which was rendered permanent by the driving

of caulking into the seams.

Cuterning or beavlng-down was also performed by bringing tue vessel alongside another vessel or wharf, whereon suiiab o cap- stans and other appliances had been provided, and, by means of ropes or tackles fastened to the ma«ts, hoaving her down to one 6loc until the other sido was clear of the water, in which posi'ion she was sccuicd while being repaired or coppered ; the operation being then icpcatcd for the other side

In Moisman'a Buy aro still to be seen thc~whtrfs. with the re- mains of capstans and other appliances of the heaving down es- tablishment formerly carried on lhere by Captain Mossman, who gavehiBname lo the place. In the early times and indeed for many years after its seulement, this was the only repairing estab- lishment In thiB colony; but as its trade advanced, and tho requirements of the port increased, it fell into disuie and cima to be succeeded by the other, and more perfect methods adopted tn

ibe present day.

The ordinary " dry " or " graving dooks " as they aro gone-ally designated-tho latter name being supposed lo bs derived from the french word orî'ie, which signifies a fUt beach or shore these docks being geuerally formed in such positions-aro of a very ancient date. From a mere excavation in the river's bank, open to the tide, and occasionally lined with timber to support the soft earth, to the granite docks introduced by the late Mr. Bennie, the chango has been gradual and progressive; as has also been the advance from the wooden floating docks which were found on the Thine» and dee where about ihebf ginning of the present century, to the largo Iron floating docks of Larthugena and Ferrol, in later times, designed

by another member ot the same distinguished family of en-

gineers.

As far back as 1623 there was a graving dock at Deptford, and In 1667 there was another at Rocheford for the repair,, of the French ships of war. Since then they have been constructed in almost every important port of Great Britain, Prance, and America, where the formation of the ground or the character of

the soil was suitable for such works.

Thete »re, however, nany situations where, owing to the occurrence of unsuitable soils, such as quicksands mud, peat, or gravel of great depths, and charged with wver, or rock much fissured or disturbed, the onstruotion ol dry docks ol muanry is difficult sad expensive. Of this a striking example Is to be found in the history of the United States Dry Dock at New York. Ia ' other situations where the ground Is suitable, as is the eise at

Liverpool and Birkenhead, and markedly so ia this port, I their construction becomes almost reduced to a simple qieation of excavation. At Liverpool, the old;red sandstone formation in which the docks are excavated, being perfectly homogeneous, and almost free from water, very little more waa required than the mete scooping oat of tbe dook to the required form aad building the pier heads ; the great rise and fall of the tide In the ¡River 1 Mersey rendering pumping al_oat unnecessary. , I

Ia Part Jackson, a*regards theformellon of the rook, the opera- tion of dock excavation ia equally stsip'e, tha «aadstoo* harai haine 4«IU aaaat-d ladU-tnmkakfjt-UMi* tfcersaaUMWM

at Liverf ool ; but. In consequence of the »mall rise and fall ot trio ' tide, wliitli i« on)y from 4 to s lect, pumping machinery to em n> ihe duck is required ji. ,u.

.Iho it mouilles wh ch I have referred to us being found la th ? 0 nstiuctiun of nia-onry ducks in sime tiiuatiuns, to withst mil the ixtcn ni pressure of the water, and bo porlectly wa ertigh *' all linns, and which shill a)»o bo able to Bustain the weinh' ot hiavy ships upon the block", has led to the invention *f mnv uiLMiiiou» substitute-, a few of which, and tho pcou iant cs of tn< cucurn.tances which have ltd to their adop ion, X shall mw

ccdeavcur to dcsciibc.

FioATivo DOCKS

The earliest examples ot ti mung ducks som»whit re>cmblod Uvgo boxes, »ith «ates to the open cndB, walch on being closet ti d he rfocks pumped out gave acetas to tho bottoms nt the vet uls in them TIRBO docks were very common in furmer years on the Ï humes, at Portsinou h, ana at moa. oi the duckya d cttablishmeniB and they »re still employed in some ot the Frenen por s A dock oí his dcsciptlon may bo seen in thia har lour at promt where it baa been woi kid wah csmp ote success for n st j j, ars, and where it hBS proved itself lo be a most valuab e

»ni men ion .

1 he balance docks and tcctiona docks constructed of timber, which arc iroprovinunts by Mr Ollbeit, .( th* U 8 Nivy, on lkc box floatiijg dock 1 ist described, h ive been priuoipilly uaed in Anirica The sectional duck ii compostd of a number of pontoor s ai d f mines, capable of being juined tug the , as insny liol toms biing uaed as may bo requited to eui. the siro of the teseel to bo lifted Euch siouon conaistB of a water- tight rectangular wooden box or pontoon, secured io whicn are vc,Heal frames supporting watertight boxes, wnioh hliile up and down, and c*n be fixed at any do ired height by means of a rack-pln'on motion Too oompto o duct thus ouuais a of tight (H) ten (IO), ir as many moro independent sections or pontoon« as may be required, each one having ita two mivable air chamber« secured witnin iho frames, one on each side The Philadelphia Sectional Duck conalsts ot nine secti ins or pontoons, inch 115 feet in length by 32 feet in width and IO ieet d ep Whin united, they constitute a duck floor of 3 0 feet long »nd 10V na wide. ïhe toUl déplacement when drawing IO feet of wa cr i« 4600 tonB The total cutt uf the dook is B-ated to have been about ¿170,000, - a large »um for a s ruotu e of BO peiisbable a material aa wood, ot which it is built, fnese pontoons and air chambers however, being lo a greut extent in depmdim of one another, neither ia able to auppo t tho adjo ning oins, which is undoubtedly a greit objection to ducis of this description, and is stated to have been toe cause ol tbo destruc- tion ot thai at Callao, on whioh, while a ship «as being Hfto I, in coi icqucncc ot nome of tho boxe» being lowero I too quiokly, the 6h,p'B keel pot off tto block«, and she bede I over auu Bank, wittt the dock. Serioua damage was also sustains I by the dan frun- cí, co dock aid II M B. Terangan! marly capsized while beii g lilted on it. A floating pontoon of similar kind cap ei/id «nd tank in the London Docks, a nearly similar accident happened to the dock constructed tor dourab¿ya, and the largo dock erected at Rio was found to be utterly uactoaa The at

Tin maa' Dock, constructed of iron, al<o sank, and rcm lined for «onie jcarB at the bottom of the harbour, it has, however, been lately raited, and is now in use. In cons qucnoe of these and s «me other accidents these docks have lately como to bo regarded with

distrust.

Sectional docks aro Bometimcs used in oonneotion wi'h pitent slip?, the veaeel after having been ruiaed by the pontoons being hau'ed fr«m them on to a patent slip, when the duck becjmea available for other iliipB

An illustration of the United 8tatcs Government Dock at Pctla dclphia of this kind 1B given

q he bsUnce docks, aluo mainly the invention of Vr Gilbert, aro a great improvement on the sectional dock», both as regards ino safety of the vieseis on them, and of the doLks themselves. They aro to be found In moat of tbe dockyards of the United ätutes, whfic they a>e worked with g eat BOOCCBS

The Un ted btateB Uovcrnment attaching considerable import- ance to buving iheir cbipa of war ou. of water »nd under cover when laid up in o dinar), the docks of Pcnsicola and Portsmouth, and other potts in the United Stales, aro »o arranged that the pontoons on which the vessels arc litten arc brougnt olosc to the slip-ways, on which the vessel» can be drawn These slips are oi the UBual form of construction The di i.ensiona ot the Pensaoola Dock are-length 350 feet, breadth It» feet 4 inches, dep h.38 feet 8 Inches Its coat is stated to have been as follows .-Floating dock, £111,HO; buln and slip-way, ¿70,615 , caisson, £2600, total, ¿184,385

In 1858 the Austrian Oovernment employed Mr Gilbert to con- struct a dock of thia kind for tbo dockyatd establishment of Po'a tho cnaracier ot the rock adjoining the arsenal being such as to preclude all prospect of const »elinga graving dock of the ordi t ary kind, except at a ver) great cost The dock wa» built at %enice, and was towed to Pola, where it hu since been worked with BuccesB In connection with the floating dock there are th'colines of slipway to receive tho ships off the docks, when laid np for extensive repairs, the dock being thui lott unoccupied i so as to be able to receive other vessels and place them in turn on

' the other slips Borne very beivy BhipB have been raised on thl

dock, ihe hne-of-hattle ship Kaisc , of 3125 tons, the lton-cUd, Max, of 3C66 tons, and ihc Don Juan d'Austria, of 2128 tons, anti 1 many others, having been successfully raised on it and repaired

The mode of working this dock is aa follows -The d jen being I moored in a sheltered and secute basin, in about 40 feet water,

and sunk to the required depth b¿ admitting water into the

bottom compartment, the ship to bo repaired is 11 >ated in, the water ia then pumped out by steam power and the dock uses carrying the ship with it till the surface of tho lower compart- ment is clear of the water , the dock ia then hau ed into ashailou basin and grounded at the end of one of the lines of railway, which are UV feet in length, and rise with an inclination of ab mt 0 inches in the 100 feet Each line of raits would hold two first class men-of-war or a greater number of small vessels lot absence of tide in the Adriatic, an i the completely landlocked character of the harbour of Pola, affo ded peculiar facilities for (he working of a floating dock of this description.

Carihagesa and Ferrol Docks-lhe Spanish Oovernment having likewise found that lhere would be great difficulty in con. Btruciiog stone graving docke at their naval arsenals of Carthagenu and Ferrol, consulted Mr Hennie in reference to the construotiun of floating docka which should be capable of lifting their ironclad «hips, the weight of which Mould be from 5000 to 600u tons T*j were accordingly constructed, one for Carthagena and a somewhat larger one for Ferrol, of the following dimensions -

Carthagena. Ferrol.

Length... 324 feet 350 feet. Breadth . .. 105 " 105 " Height (outside) . . 48 " 50 "

The lifting chamber of the Carthagena Dock is 324 feet long, that of rcrrut 85« feet They arc li>5 feet broad in both oisea and II feet 6 inches and 12 feet 6 inches deep respectively

Both docks are constructed of iron plates, and are divided by a number of longitudinal ano transverse bulkheads , in the former lhere being twenty and in the latter twenty-two watertight com- partments, by which means the stability of the structure is insured and 11 flotation regulated with the utmost accuracy. The w.tor s admitted to any or all tho compartments when it is deslrab o to ink the dock for the reception of a vessel, and is pumped out by means of the ordinary pumping maohmery when it is deBired to aise it. The basin at Catthagcna, wherein the dock la moored, measures about 1800 feet in length by 1120 fee. ia wld h, an I

has been excavatod by dredging machinery to a uni- form depth of about 50 feet At the south-west oorner of this buln another shallow* basin or dook has been cons'ruoted to receive the floating dock. This latter baaln ia of a uniform depth of 16 feet 6 inches, and is 38 J feet In length on Ihe north and 845*feet on the south side, the inner end being curved to a ? adina of 320 feet where theend of the dock, when in place, joins the Blips, which i adíate in three horizontal line* from the diok, so that after the dock has been grounded in the btiin opposite to either of the lines of wuy the ship to be repaired may be drawn at once on to the slips, thus relieving the d ick, which may then b« floated oat, and kept in readiness to receive another Teasel. The slips are 725 feet in length, 45 feet broad, and aro constructed

with altars similar to those in the ordinary d icks, to admit of the ships being securely shored up on them Each Blip wl 1 hold two first-elate, or a greater number of smaller vessels

The accompanying sketch of the Carthagena dock, whioh shows a ship undergoing repairs in it, will illustrate the design and afford some idea of tbe general construction and modo of wotking it and the llips The shallow basin in which the dock is grounded may be cloeea by means of a caisson and the water pumped out,

should it he necessary to repair or paint the dock Itself. '

The weight of the Carthagena Dock is 440« toni, and its cost was about £160,000, exclusively of she cost of the bain, slips caisson, and other appliances, which would probably bring th' whole cost np to about £350,1,00 The Spanish iron-clad ship Numancla, weighing over 5000 tons, and o her ships of the sime class, have been lifted on it and rep «Ired with perfect sa'ety

Callao Floating Dock - An iron floating dock, similar in moat respecte to that just described was constructed and sea

to Callao. Its principal dimensions are-total length, 300 feet, breadth over all, 100 feet, w iden inside, 76 feet, depth, 30 feet, width on lloor, 50 feet, depth of bottom compartment, 12 feet This dock is rcpiesented on the accompanying sketch, which gives a perspective view of one side and end of It ¡the pumping i ngines are shown on the platform« on each side, the keel blocks allors, ladders, £.0, aro shown in the inside Preparatory to receiving a ship in the dock, the water is admitted into the bottom compartment, and the dock sinks to the line indicate I by the letters BB, or as low aB nny bo necessary when the ship to be docked Is hauled in, placed upon the blocks, and i hored up from the sides The pumps are then sot ia raoiion and the dock pumped out till tho floor rises above the water-line, when free access ma) be obtained to all parts of the vessel to be

examined

Bermuda Floating Dock -In 1821 the British Government having retcrmincd on constructing docks for their ships of war at Bermuda, an engineer vi as Fent to examine the Bite, but the rock was found to be so porous and fissured that the inten- tion to construct masonry dockB was abandoned. Again, in 1851, a second examination was made, with the same unfavourable result, and it was subsequen ly de crmined to construct an iron floating dook instead of a per- manent masonry dock The harbour ia so perfectly landlocked that no danger was to bo appreüended of the dock being injured in bad vi oathcr. Accordingly, a dock on what has come to bo designated tho U pnnciplo was constructed, and with s >m> diffi- culty towed out to Bermuda,. The accompanying sketches illus- trate this dock in various positions The m inner of working it is aa follows --When a Bhip ia to bo docked, tho water is admitted into tho ppper chambers, the dook then sinks and »Bsumes iho position shown at the sketch A, ready to recaive a ship, which is floated in and grounded on the kcol-blocke ¡ the water Is ihon discharged from the upper chambers, whereupon the di-k n5es 10 feet or thereabouts, the pumping Is then commaacei till the

vessel is lifted clear of the water

Sketch B represents the dock cloBOd with the caiBson and pumped out C Bbows the dock pumped out and tho ship inBldo dry. D and K show the dock careened for ilie purpose of being painted or rcpal ed. lins '.s done by admitting water into the compartments on one side, and pumping it out on tbo other.

About 8000 tons of iron were consumed in its construction

One of the objections to this deacripiion of dock was ex- perienced whi'e lying at Chatham, »hen during a g ile she dragged all ber anchors, and it was at one time a question whether she would not be wrecked in the river before she ooutd ho se it to

Bermuda.

Speaking of the iron dook at Bermuda, Sir John Packington as picsident of the Inttltution of Naval Architects, at one of their meetings, (aid " The discussion, to which he had listened with very great interest, impressed him with ihe belief that the ques- tion of floating docks was vory much in its infancy, and, wolle it gave bim that Impression, it confirme 1 his previous belief that it was a most important miner. It was important from a financial view . . » Another important consideration was, whether or not the success of ibitlng dockB would not enable them to aoootnmodate the flee s of this country with dock accommodation where there were physical Impediments to dooks at tho present time Ber- muda was a most pointed illustration of thle. Ho always under- stood that the nature of the Bermuda rock was such that it was next to impossible to corstruct a dock there in the usual mode of constructing dooks on shore. Bermuda was a most Inmotttnt station for the British Navy, and his belicf.was, that if they were to have a dock there stall it must be a floatlag dook, tie was sorry to hear tbo discouraging language ol his bon friend, Cap tain de Ilorel), who seeme to disparage floating dooks al ogether, and his authority on all subjects mia very high one."

The following is a list of some of the principal floating dooks as

yet constructed :

5

oö s a

«

Portsmouth ... United 8Utea.1(51 850 105 4 Penskola .,. " . .1851 350 105-4 Pola ... latría. 1857 300 108 Havannah ... Cuba..1858 300 79 Carthagena.., Bpain.1859 324 los .Ferrol ... ,v .1859 850 Haurabeya ... Java .1861. 250 Saigon , ," Cochin China ...- - Callao ". Peru.1887 (00

( St.Thoo«» ... r?e»t ladies ......186« soo _ ".", .

Bermuda ... .I860 (81 121» 16,700 | ¡|

I , HYDaauiic LIFTS-Gmlvino DOCKS.

I A yen bMutital and ingenious airaogentest for Utting VLMU

líqultínf »«nair. Mt« US» w*Wr, »4 pUftln, ttvMi M tttaw

tor pontoons bytnearaof hydraullcrsms.wssinventedsomrycarB .ince by Mr idwin Clark, M Inst. CE. The apparatus ?,» elected in ihs repairlrg basin of tho Lond m "V ictorU I) ick u im puny, wheie it has bein worked for some ycara wi.h consider i1! i

Tue ei'uation selec'cd for the hydraulic lift was aliw-lyinir extent of marsh land between tho Viotoria Dooks and tho Thv nos which boin» beow high-water-uiark was parucu arly lavourib e, as it it.voWid no excavation for the shallow docks into which in pontoons wilh iho vessclB on them are floated, the onlj excavation of importance was for tho lift pit and tho cntranco the dip h of water required in the latter being J7 teet, and in ihe former only 6 feet, 'i he area of shallow water is 16 acres, whiou «ffurdf sufficient space for floating 15 or 20 yontoons.

The dockn g ol a vessel by this process consists of two opera- tions; first, tie din« raisingof the vcsseland pontoon; secondly, the transporting of the vessel to any convenient po tuon m one of the eight shallow basins for its repair on the pontoon

The pontoon with the vessel on it, Is rai ed by u.e»ns or the lijdrnu io process, of whiih there aro two rows 6U Kct apiri 1-aoh row coDrists of 16 cast-iron columns, 4 feet io diameter and ¿ti leet apart from cen re to centre. 'Iho length of the dock is ibu* 30u feet, and l's width 6u fict ; but as vetsols may o-ern mir .he pontoon at either end, ihe e is practically a wjrking length of 35' feet The columns wore sunk to a depth of 12 feet into the ¿round, and the lo«cr parts filled with concre o The upper pontina aie occupied by the hvdraullo rams. Tu each ram IB icurtd a powetful cross head from either side of wh cb. d pend

.be side tods to which the lowir trussed frames or cross giruers which conuco' each column with ita corresponding one un tho »ipoalte side are scoured. The by mulla rams, which aro 10 neues diameter ard 25 feet stroke, aro raised or lowered or maintained at any height wilh tho utmost accuracy by mo ns uf the hydraulic pumpa which are worked by a steam-engine on

lbe operation of lifting a vessel out of water is performef in tbe following manner :-A puntooa of sufficient aUe to suppjrt i he vessel being piactd on tbe CIOBS gilders and tbo keel aud bllgu

ilccke adjus eu to her fo m, tre whole is allowed to sink to tue biiltom by letting ihe water escape from unuer the rams ; the ies6tl to be docked is then brought over the pontuon and property ecured over i be bocas, the pumps being set in motion, the ,iontoon is gradually raised till iho vessel rests on the seel bljcks, «Len tbc bl ge b oika being drawn in and secured, the opera ion uf lifting le continued ml vessel and pont jon aro c ear of water, mean Sine the water having been allowed to esoipe from he Inside of tbo pontoon, tho rams are again lowered and the pontoon and vessel «re floated out into one of tho shallow latins, and another pontoon being p aced between tho columns ina lowcied, ano hrr vessel may be immediately taken up, and so ho operation lepeated as oltcn as may . e required. I will thus ne seen that wilh one hydraulic apparatus, lbe number of vessels *hicb may be undei going ri pairs at any one time is simply de , endent on the number of pontoons available, and the area of -hallow basin, or the number of the shallow docks ready to

receive them.

.J be hydraulic lift at the Victoria Pocks, with its machinery, 1» aid to bave cost about £30,(00, but the seven iron ponioons and .he shallow basins for receiving them, brought the company's

expenditure on the whole establishment up to about £i9>,UuO j

en illustration of it with a vessel raised out of the wa'er on one uf the pontoons is gives.

Another hydraulic lift dock, similar to that just d-scribed, but , larger and nure powerful, has recently been oonstruoled at Bom- i bay for the Imperial Government Tncrc are eighteen columns ' in each row, lb leet opart at the centre in the longitudinal direc- tion, Incrrasirg to 24 feet at the ends, and 83 feet apart tr ms < erie ly. bach column 1B 7 feet 6 Inches in diameter at base and 6 tier 6 inches at the upper part. Home uf the columns are 109 feet in 1er gib, and they use 86 feet above high-water. Tue nee and ?all el tide at Bombay is about 16 fee , and the dep h of water where the dock Is built variosirotn 50 to 60 feet Toe hydraulic presses in each column are Si feet stroke, and the rams 14 inchc« in diameter. The transverse girders suspended from ihe cross

heads, ,by which the pontoons are Ulled, are 10 feet 8 inches)

deep.

'I here is bnt one pontoon at present attached to this dock, i which is fit ed in the ordinary manner with keel and bilge blocks

It Is 58" feet long by Í6 feet broad, 9 feet 6 Inches deep ; it weighs1 1610 tons, and is capable of supporting a vessel weighing 65 I) tone. The quantity of ii on-work in the lift and pontuon is said to exceed "noli ions, and the cost, 1 have been informed, without the pontoon was about ¿300,000,

A dock on ina hydraulic lift principle is proposed to bo con- structed al Barrow in Furness, estimated to oost£lr,3,000, with -ix pontoons '

Another hydraulic lift dock has recently been constructed at Malla, for the repairs of small vessels: and a contract is stated to bave been entered into by Mr. Clark with the Italian Government, tor the construction of another at tirindisi, to oust £.47,001.

1 he dtmensioi s of tbe ibree principal docks ol the kind as yet

on6trncted arc as follows .

ViotonaDotk. Malta. Bombay.

Length .. . 310 feet Sil feet 350 feet ;

Width between rowa of

columns ... ... 62 " 62J ,, 88 " \vailable draught of water at

ordinary high water ... l8 " ill ,, 30¿ " ,

Number of columns in each

row .. . 16 " , 16 " l8 "

Diameter of columns ... 4 " 5 " 6J " { vuniber of presses... T 11 ,, H " 72 ,,

Diameter ol ram ... ... 10 Inches 14 inches 14 Inches ..tioke of rams ,.. ... 25 feet 30 feet 35 feet Heaviest vessels intended to

be lifted ... ... 3000 tons 4000 tons 8000 tona I

GBAVIKO DOCKB OP MASON&Y.

Chatham Cock -The accompanying illustrations show the plan, longitudinal and cross sections, of one of the tour new graving docks lately constructed at Chatham, on the south side uf the repairing basin, from the design of sir Andrew Clark, It j:. C B Tbeir dimensions arc as follows .-Leng h on the floor, 416 leet 6 inches, 456 feet at the coping, 108 feet in width at the coping, bu leet wide at the entrance, and nave 31 feet 6 inches ol water uo the sill at h'gh waier spring tides, and 28 feet 6 inches at high water neup tides. These docks will thus be capable of taking in helarge<tof the non-clad ships of the navy The masonry ol he foundation and side walls waa laid on a bed of gravel found underlying the Medway clay.

In toe construe tion of these works convict labour has been ve-y < xter.elvely used ihe number of convicts employed ranging from I3C0 to l.'tO atid upwards ; and it has been remarked that ao far irom the woik performel by the convicts being of an inferior character, it bears favourable comparison with that executed under contract by free labour.

Portsmouth Docks -The accompanying Illustrations, showing he plan, longitudinal and cross sect ons of the Portsmouth docks,

»ill give some iaca of the modo of constructing graving dooks in I .-oil« of the kind net with in this instance, and which consisted of mud for about 30 feet in depih, with a substratum of sound yellow clay ULderlying it,' the clay furnishing'tbe male ml lor the bricks so largely u°ed in the construction ot the works, and which were made enti e y by convict labour

At the coamencement of the works about 4-0 convicts were con- stantly employed, but more recently tho number has been in- creased lo over boo At fl st they Were only employed on the rndcr kinds of work, hut latterly they have been, as in tbe case oi Chatham, not only employed In brick-making, pile-driving, Sea , but also in constructing the roads and railways in the dockyard, as well as building portions of the masonry of tbo biala walls, but they hat e been used with greatest p oh. io making the bricks, lor which the clay found In the excavation is admirably tilted ,

The new Portsmouth dookB arc of the following dimensions rt -pectiTtly : The double dsck 636 feet long, 83 feet wide, with 27 teet of w atcr en the sill, the two lock dooks are 418 feet long, 83 feet wide, with 32 feet at high-water on the sill. The deep ducks are io be four in number, i3i feet on the floor, 110 feet between copings, 63 feet in width of entrance, and 81 feet of water on the

ill.

Brest Dock.-The new double dock at Brest, having a division in the centre to admit of either one very large or two or more rmallcr vessels being docked at one time, is Til feet long, 9i lect wide, and has 55 feet at high-water spring tides on the sill

Ihe new Somerset Dock at Malta is 428 feet long on the floor, 468 feet over all, 104 feet wide, at coping, 83 feet width of en- trance ; and the depth of wat» on the sill is 33 feet 6 inches. The total cost of the dock and entrance was £150,000, and the cost'ot emporary pumping, for machinery, coats, oil, &o , came io orci iSo.iOO. . '

1 he Alf red Dock at Melbourne, constructed from the designs and under lbe superintendence of W, W. Wardell, Esq., M. mat C.E , Inspector General of Public Wo: ks, is 43u feet ou the fluir, 65 feet over all, 07 feet wide between the coning«, and 60 feet ui entrence, with 24 feet 6 inches at low water and 26 lect 6 incae-

on the sill at high tide. It is stated to have cost £123,uOO Pur he dock proper, and (20,000 for pumping machinery and caisson 'Ibis dock is undoubtedly one ol the finest out of England, it nut in the world; and for convenience in its general arrangements and excellence of workmanship, in every detail, it stands seoond

to none.

Bavins; In the foregoing psges given a brief description of solus of the principal kinds ol dockB which have been constructed showing what modifications in thcoiiginal typo of masonry doc«< have bien adopted by engineers to meet tho peculiar circuin stances of each case with which they were called upon to deal. 1 shall now endeavour to show as conclecly as possible the kind oi work which, in my opinion is best suited to the peculiar circum stances of this port. But I would first invite attention to the tw o graving docks already conetrue'ed here

The \\ atcrvitw Dock, constructed by Mr. Mort, Is of the very plainest kind of work, nothing more In faot having been required than a simple evcavation in the solid sandstone rock, resembling somewhat in foim the ships it is intended to receive. The natura iockfor,ms the sides and floor of the dock, which, while sutil Liently Eoft to bo caBily wo'ked, is yet thoroughly sound and watertight, so that no expensive masonry lining (absolutely neces- sary when the soil is cha ged wilh water, and which involves the greater part of the expenses as well as the chief difficulty m the t xecution of such works) is required, almost tbo only masonri in ihe Waterview Dqck being the plorheads at the entranoe for the support of the caisson.

It will bo observed that no work could be more simple or easy of constrnetion, and no dook could be more convenient or s lie I am enabled to speak from personal experience In this matter as to the foimer, having acted for Mr. Mort as consulting engineer

during its construction.

The Fitzroy Dock has also been scooped out of tho solid rock in a manner very similar to the Waterview Dook ; but in conse- quence of the occurrence of a bed of shale at the bottom, u was lound necessary to cover the floor with an invert of solid masonrj

On some portions of the sides masonry for the altars was kl&u icqulred where the rock was unsound, or where tno sha'e-blnds showed themselves. Iho pier-heads at the entrance and the sill, wLere the caisson rest, aie all of heavy masonry.

Ko kind of ground could be moro suitable for dock construction than the sandstone of this harbour, which, while ao soft as to be worked with ease, 1B strong enough to stand without masonry lining, and ia all but perfectly wa'ertight.

lo attempt, theiiforc, to introduce here any of those complex arrangements or forms of dock which have been forced on en- gineers at other places by the peculiar difficulties thev had to conletd with would be ns manifestly nnwUe as unnecessary.

In devising the most suitable lorm of dock for any locality, the engineer first considers the character of ihe ground, or tho material he has to work In. It it bo tock-ltssu ed or permeated by water to a great degree, or If it oonslst ot gravel, quicksands, or toft boggy soil, in which there would be difficulty m obtaining a foundation, such as Is commonly to be met with ou the banks of rivers or shores of harbours, be will find himself compelled perhaps to n'lopt some form of floating, ba'ance, or sectional dock or hydraulic lift such as 1 have described. Hut if, on the other band, the ground be cuitab e for the commotion ol peimanent docks, the many advantages which they posses >, in the economy, facility, and safety with which ships can be docked and repaired in them, will always g\ve tnem a preference. It was probably tho many difficulties cncountoiol in cons ruotini the New 'iotk Dry Dock, arising from the enormous qu»ntlty of water met with, whioh retarded its cotnple'ion for about ten years end brought ita Cost up to half a million sterling, coupled with the abundance and cheapness of timber in the United States, which induced the Government engineers of that country to tuin their attention to the construction of tho timber sectional and floating docks which thev have adopted, but which tewevcr designed, must always bo worked at great disadvantage and bo costly to maintain, from the perishable nature of tho material of which they are constructed. It was simlUr difficulties, 1B the cases of Cartbagena, Ferrol, St Thomas, Bermuda, and Callao, whioh led to the adoption of Iron floating docks at these places, to which similar objections as to their non-permanence apply, though not pei naps In as great a degre«. The iron plates, however carefully painted, must corrode, and the rate of corrosion in the warm sea water ol these low latitudes is rapid enough to be an oblootion to its sppllcatlon In this way.

The corrosive action of sea water on cast-iron ia not nearly so rapid as It is on wrought-Iron ¡ except therefore, in so f »r aa sppied to the pontoons, the foregoing observations would not apply with much force to the cylinders and other apparatus of a hydraulic lift ; but there wonld bVother objections to tha adop- tion in thlt »ort of tbia latter form of dock of even greater moment, ausb as the difficulty of safely securlag a larc« ship on a pontoon in any part of tala harbour, exposed aa it la to the' force of tie aouthtriy and easterly gales, of which we have all had ex- perience. ' ' v l .

In order to work a hydrauho lift with safety, It would bo accessary to form ? dock or docks for the pontoons, «blah wonld'

1 bo atarijr U aotqaUeastxpwaivo as weald boa piala jjmSif

?

dock alone for a larger ship. There is no suitable plaoo in the i el« bbuutbuod of the Dockyard Establishment of Cockatoo where u hjdiaulic lift cou'd be easily placed, and where the pontoons .'could be taf. ly moored. 8' me shtl'eted spots might no doubt be li/und at ihe beads of some of tho coves or bays within the laibour; but tbiB would email either the removal of thedock jard citibiithmtnt, tools, &c, end abandoning the pieseni dook -.»bichioeije, I apprehend, wunld r< commend-or thoincon * cii.ci.ee and expense of working two distinot and separate esta.

blubnieniB

On flin considering the subject of a first-class dock at Cook atoi, 1 was disposed to think that it would be botter to enlarge tt.epiesint ? eck iban cenatiuct anew one altogether; but alter in«.ti e y cocudciing ihe subject in all its bearii g«, I have been con pi lltd to chango my opinion, and I am now eatittled, for the lollewing, emonael other r'casune, thal it will be belter to con- struct a new dock at the western end of the island, with its hoad I o lilli g taatward towal d s tlie present duck.

In the first place the ritz Boy Dock aa it is at presen', is of a mott convenient sien for geneial purposes, and we e it to bo ? maigiu it would necessitate closing it lor a considerable lime »lie the alteiati-n was being made; for although ii might ' | ci baps be lind occasionally ruing the interval, it would, if the

. n siutn.tnt weie carried out by contract bo considered better tu o'ose it altogether during the ptogrc-s of the work.

Kcciidlj, to deepen the nil to a« feet and widen the entrance to Mi le« t w hie h are the minimum dimensions for a first-class dook, it «uu d be necessary to endure the pier heads with a coftor-d im, ivh'ch would bo a »oik of some considerable difficulty and . xiii.K', owing to the sand having .been removed and tho look laid bare lor blasting out 'be pieseni entrance.

In the third place, the deepening of the channel outside the dook ni.uld Invulve much under water lock excava iun as well as a lui ge amount of i ridging.

And lastly and, lu m¡ opinion, the moBt formidable objection li- that I lie tuurtc ot tbe currents, both of flow and ebb, set across be line of tin axis of the present dock nearly at right angles. Ihe inconvenience, and indeed dunger to very heavy ships i rom i bib cruse was maco plain when tho F.cnch iron-clad At., ne le was bell, g dockid. Being a Btrong flood (ships should dus} s be dockiu ut ft eui half lo th te-quarter flood), the current cm.glu ihe vessel on the staibuaid quarter and slcwid her out of the line of Hie i ock, and it was with the utmost dilitcuity that even with her poncrtul crew, she could bo got and kept in the lil opt r lii.c fur taking the keel blocks. Tho line ot the axis of the dick is shoal a considerable ungle with the north-easterly as »el se southerly windB which at times blow with considerable loree, and duiii g the prevalence of which great difficulty is ex- pel ii need in docking lui gc vessels.

li tbe picreni dock were en'argeda now caisson would bo re- quired, lur I find, OB carefully going into the manor, that the cost of cuttit g, lengthening. taiBing, and strengthening the pre- sent caisson tu make it suit the wide- and deeper cutrance would make it ainiOB'., ii not quite, BB costly as a new one.

As lcgards the site ut the proposed new duck at the western side of the Uland, it is completely landlocked, being sheltered on lue scutti by bi kennend Point and Schnapper Island: on the »ist, noith-west, at.d noith by Spectacle Island, Pulpit Point, ' und Hunter's ilnl; and on the north-east and eaBt by Cookutoo

j Is'And Itsc.f, The head of the dook poins east north-

east, so that the prevailing summer winds, whioh blow i 1 ironi nuith-cast, would tend to keep a ship entering the. , dock in the proper line for the blocks, and the flood-tide on the

south side or the island, passing up almost in a line with tho

1 dock's axis, would also tend to inaietain a ship in the proper ; 1 une for being docked. The direction of the flood and ebb our-'

rents will he seen on reference to the current churt which accom-, panics thia paper. Some of the ebb currents will bo observed to. i un seroBs the axis of the dock, but this ia unimportant, as vessels

are not, except In oases o( extrema emergency, docked on an, ebbing tide. .

'1 here is ample depth of water off this part of tho island for ships of the heaviest draft, with good anchorage. The deep WaterJ comes close in BIIOIC near tho propoBed entrance to tho dock, and! there being a sufficient depth of Band and mud overlying the roc e, at this point, tntie would be Utile difficulty in constructing a' corio-darn fur building the pier heads and breaking out the entrance l'he ruck ou-of which the dock would bo carved ia' tout d. and may be said to bo almost tree from water to a depth, of 43 feet, or about 10 feet below the fluor of the dock. Tais Is soo* ni) the trial shafts which I huve had put du wa along the ccnue line o( the proposed dock. The leakage, which is mainly i.tsh water off the island, is insignificant.

In deleimin'.ng the dimensions of the new dock, I have been ,'UiCca by those of the new flret-clasB docks at Chatham, Ports- mouth, and Malta, and the Alired dock, Melbourne ; but as any increased dimensions in our proposed cock would bo simply a matter of a little more or less . xcavation, involving little or no iiiaconiy,! have thought it should slightly exceed the dimensions ol those oilier docks described, so that we m<ty ituly boast of Having in Port Jackson tbe Wrgist single dock ia existence.

The list given in an appendix, of the dimensions of some of ihe principal dry docks, is taken ftom Span's -'Dictionary uf f.nginicruig."

E. O. MORIARTY, ; Engineer-in-Chief for Harbours and Rivers. ¡

Digitisation generously supported by
Vincent Fairfax Family Foundation
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