, 1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



349 



jetties, which are huilt to form the sides of the apparatus. A chain connected 

 with all these rollers traverses in each suspended railway groove, and the 

 power of a steam-engine and wheel-work, being applied after the vessel is 

 floated on the platform and made fast, the frames raise the platform and 

 vessel together gradually out of the water, permitting free access all round 

 the ship ; and when the repairs are completed, the whole is again lowered 

 into the water. It is contended that many practical advantages would arise 

 from this system — that the ship would not be strained, that time would be 

 gained, and that it is superior to the ordinary methods now practised. The 

 calculations of the leverage, the division of the load over the fixed points, 

 &c., are given in detail, and the paper is illustrated by a series of elaborate 

 drawings and a complete model of the apparatus. 



Remm-ks. — Mr. Rendel thought that credit was due to Mr. Mallet for the 

 science and the practical skill combined in the production of the contrivance 

 under discussion ; it was perhaps imperfect in some of the details, but he was 

 inclined to beUeve that, in certain situations, and for vessels of moderate 

 size, it might be adopted. Its construction would certainly be more expen- 

 sive than that of a patent slip, but it would be less costly than a graving 

 dock, and not liable to injury from hydrostatic pressure, to guard against 

 which frequently constituted a main portion of the expense of a graving 

 dock. The foundation of this structure might be simple, as the weight was 

 distributed over so many points ; he conceived, however, that unless it was 

 established where the rise of tide was considerable, the foundations must be 

 laid at a depth of 5 or 6 feet under low-water mark, to allow for the thick- 

 ness of the frames and tlie platform beneath the ship's bottom. He was of 

 opinion that a modificatian of the plan might be advantageously employed 

 for canal lifts. 



Mr. Hawkins agreed with Mr. Mallet that a ship must be strained while 

 on a patent slip, because the timbers were all bearing a weight at an angle ; 

 but more particularly when leaving the slip, as the stern floated whilst the 

 stem was still on the cradle of the slip. 



Mr. Palmer did not admit the advantages of the proposed plan over grav- 

 ing docks, for, as they are now constructed, they possess every requisite con- 

 venience for examining and repairing vessels ; the gates are made to exclude 

 the water perfectly, and the machinery for pumping is so effective, that a 

 very short time suflices to lay the dock dry. The plan might possess some 

 advantage over Morton's slip in retaining the vessel in a vertical position, 

 but it would be more expensive to construct, and he was not at all convinced 

 that the objections urged against the patent slip were well founded. 



Mr. Gordon observed that the position of a ship upon a patent slip was 

 exactly that in which it was built ; he could not therefore understand why it 

 should be so very injurious ; besides, if the stern cradles were elevated, as 

 was the case on some of the slips proposed by Captain Brown, the vessel re- 

 mained nearly on an even keel. Another improvement introduced by Captain 

 Brown was, substituting solid rollers for the wheels of Morton's slip, the 

 axles of which frequently twisted and prevented the progress of the vessel. 

 Among the modes of examining the bottoms of vessels enumerated by Mr. 

 Mallet, he had omitted the " gridiron," which consisted of a strong frame of 

 horizontal timbers resting upon the heads of piles a Uttle above low-water 

 mark ; over this frame the vessel was moored, and on the tide receding was 

 shored up, resting upon chocks. When it was dry the bottom could be ex- 

 amined, and any slight repair made before the returning tide floated the ship 

 off. " Gridirons " existed at Liverpool, at Havre, and at many other ports. 



The President observed, that, like the form of breakwaters, much depended 

 upon locality. Where timber was cheap, and the rise of tide considerable, 

 the plan might be applicable ; at Liverpool, where the tide rose 30 feet, and 

 in the Channel Islands, where the rise was 40 feet, the platform might be 10 

 feet above low-water mark, and still accommodate any ordinary vessel. It 

 certainly appeared to avoid some of the main expences of the graving dock, 

 in which so many precautions must be taken for preventing the springs rising 

 and blowing up the bottom. The Institution was much indebted to Mr. 

 Mallet for the great pains he had bestowed on the communication, for the 

 complete drawings and model illustrating it (which were presented to the 

 Institution), and he deserved credit for the ingenuity displayed in the con- 

 trivance. 



*' Account of the Machinery and Apparatus /or compressing and using Gas 

 for Artificial Illumination at the Portable Gas Works of London, Edinburgh, 

 Manchester, and Paris." By Charles Denroche, Grad. Inst. C. E. 



This paper gives an account of the improvements introduced by Mr. David 

 Gordon into the syphon forcing-pumps, reservoirs, &c., whereby the requisite 

 degree of compression was obtained for rendering gas portable for the pur- 

 poses of illumination, and of the arrangements adopted in the works at Edin- 

 burgh, Manchester, London, and Paris. A description is given of the various 

 kinds of apparatus which were tried before a pressure could be obtained of 

 30 atmospheres, or 450 lb. per square inch. The portable lamps, with their 

 ingeniously contrived graduated cocks are also described, with the several 

 parts composing the apparatus. It appears that, owing to the cost of com- 

 pression, which was 3s. 6(/. per thousand cul)ic feet, and that of delivery, 

 which amounted to lO.v. per thousand cubic feet, the speculation was unsuc- 

 cessful in a mercantile point of view, although most of the mechanical difti- 

 culties were overcome. The paper was accompanied by a series of detailed 

 drawings of every part of the apparatus. 



Mag 3. 



" Description of the Tunnels, situated between Bristol and Bath, on the 

 Great Western Kailwag, with the methods adopted for executing the works." 

 By Charles Nixon, Assoc. Inst. C. E. 



The works described in this paper comprised a large quantity of heavy 

 earth-work in tunnels, &c. ; they were commenced in the spring of the year 

 1836, and terminated in the year 1840. The whole of the tunnels are 30 ft. 

 in height from the hue of rails, and 30 feet in width ; they are curved to 

 a radius of about 120 chains ; the gradient of that part of the' line is 4 ft. per 

 mile. The strata through which they were driven consisted generally of 

 hard gray sandstone and shale, with the gray and dnn shiver, &c. ; in a few 

 places only, the new red sandstone and red marl were traversed. Every pre- 

 caution was taken for securing the roofs, by lining them with masonry where 

 the nature of the strata demanded it, and in some places invert arches were 

 turned beneath. Driftways were driven before the tunnels were commenced, 

 and shafts were sunk to enable the work to proceed at several points simul- 

 taneously. The modes of conducting the works by these means are fully 

 described, with all the difliculties that were encountered. The construction 

 of the centres is given, with the manner of lining the arches with masonry, 

 which is stated to be what was termed " coursed rubble ; " but was of a very 

 superior description, and in every respect similar to ashlar-work. The author 

 offers some remarks with regard to the expense of working tunnels by means 

 of centre driftways. He states this plan to be costly, and in many instances 

 without corresponding advantages, on account of the difliculty of keeping the 

 road clear for the wagons. He recommends that when driftways are used, 

 they should be on the lower side of the dip of the strata, as the excavation 

 would be facihtated, and the road would be kept clearer. In long tunnels 

 he has found the cheapest and most expeditious mode of working to be by 

 excavating the centre part from shafts, and both the ends (together if pos- 

 sible) from the extremities after the open cuttings are made. The drawing 

 accompanying the paper gave a longitudinal section of all the tunnels, and 

 showed to an enlarged scale several transverse sections of them, where the 

 variations of the strata rendered either partial or entire lining necessary. 



Remarks. — In answer to questions from Mr. Vignoles and other members, 

 Mr. Nixon explained that the extra number of shafts had been required in 

 order to enable the woiks to be completed within a given time ; there had 

 not been any accidents during his superintendence, but subsequently one of 

 the shafts had collapsed. The cost of driving the driftways, the dimensions 

 of which were 7 feet wide by 8 feet high, was ten guineas per yard lineal. 

 He then described more fully his proposed plan of cutting the driftways on 

 the lower side, instead of the centre of the tunnel, and stated the advantages 

 chiefly to consist of a saving in labour and gunpowder, as a small charge 

 sufficed to hft a considerable mass of rock when acting from the dip ; the 

 road was also less Uable to be closed by the materials falling into it when the 

 enlarged excavation proceeded from one side instead of upon both sides. 



Dr. Buckland, after returning thanks for his election as an honorary mem- 

 ber of the Institution, expressed his gratification at the prospect of a more 

 intimate union between engineering and geology, which could not fail to he 

 mutually beneficial, and cited examples of this useful co-operation in the 

 cases of railway sections, and models that had recently been furnished by 

 engineers to the Museum of Economic Geology. He then proceeded to re- 

 mark upon the geological features of the Soutb-Western Coal Field near 

 Bristol and Bath, which had been described by Mr. Conybeare and himself, 

 in the Transactions of the Geological Society of London (1824). Some of 

 the tunnels near Bristol are driven in the Pennant Grit of the coal formation, 

 where it is thrown up at a considerable angle, and composed of strata yield- 

 ing slabs and blocks of hard sandstone used extensively for pavement. Iii 

 traversing such inclined and dislocated strata, the engineer's attention should, 

 he conceived, be especially directed to the original joints that intersect the 

 beds neatly at right angles to their planes of stratification, and also to the 

 fractures produced during the movements they have undergone. These natu- 

 ral divisions and partings render such inclined stratified rocks unworthy of 

 confidence in the roof of any large tunnel, and liable to have masses sud- 

 denly detached. Inclined strata of a similar sandstone are ])erforated by 

 many tunnels on the railway near Liege, in nearly all of which the roofs are- 

 supported by brick arches. It has been found impossible to make the tunnels 

 through Lias and Red Marl without continuous arches of masonry. In any 

 of the tunnels which have been carried through strata of the great oolite, the 

 parts left unsupported by masonry would, in his opinion, be peculiarly liable 

 to danger, because even the most compact beds of oolite are intersected at 

 irregular intervals by loose joints at right angles to the planes of the strata, 

 and occasionally by open cracks ; and it is to be feared that the vibration 

 caused by the railway carriages would tend eventually to loosen and detach 

 these masses of stone. He apprehended still greater danger would exist in 

 tunnels cut through the loosely jointed strata of chalk, unless they are lined 

 throughout with strong masonry ; and even that, in a recent case, had been 

 burst through by the weight of the incumbent loose chalk coming suddenly 

 upon the arch. In open cuttings through chalk, where the numerous inter- 

 stices and the absence of alternating clay beds prevent any accumulation of 

 water, there is little chance of such frequent landslips as occur where beds of 

 stone, gravel, or sand rest on beds of clay ; but until the side walls of chalk 

 are reduced to a slope at which grass will grow, they will be subject to con- 

 tinual crumblings and the falling down of small fragments, severed by the 

 continual expansion and contraction of the chalk, under the destructive force 

 of atmospheric agents, and chiefly of frost. In open cuttings, where the 



