22 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Jamauy, 



twefti the top and bottom of tlie outikie frames, ami to each of tliese the 

 chains are attached, so as to have a secure hold of all the three together. 



Every provision and proper attachment being now made for the lifting, 

 there is nothing to prevent the process going on. But as the rams are only 

 capable of rising feet, another arrangement still remains to be explaiiifd, 

 and which is a curious one, and has admirably answered the purpose. Un- 

 less the rams and llieir cross-bearing beam had a very secure hold of the 

 chains, nothing would he safe ; ami yet when the rams have ascended to the 

 top of their stroke, they must let go this hold, otherwise nothing farther can 

 proceed. The chains, therefore, must be detached from their hold of the 

 hearing beams. In order to provide for this detachment, the chain, instead 

 of being bolted or fixed to the beam, is merely passed through the centre of 

 it, and the top of each link having a square shoulder formed upon it, two 

 moveable or sliding blocks are laid on tlie top of the beam, capalile of being 

 moved by screws, more or less apart, so as to come under the shoulder of the 

 link, and being screwed hard up, it forms a complete, and yet not a perma- 

 nent, attachment for the chains. When the rams, therefore, have completed 

 their lift, the chain is detached from its seat on the top of the beam by un- 

 screwing these sliding blocks ; but unless some farther provision were made 

 for supporting the chain and tube — while it is detached from its bearing on 

 the rams, the vvl-.ole would fall to the ground. A second set of sliding 

 hlocl.s, therefore, is provided, resting on the top of the malleable iron beam, 

 which carries the whole machinery, and the links of the chain being made 

 e.\actly 6 feet in length, the lower sliding blocks are placed exactly 6 leet 

 below the upper, so that the moment the rams have raised the chains 6 feet, 

 it brings the ihoulder of the next lower link level with the lower sliding 

 blocks. These being then screwed together, lay hold of the chain at the 

 Iwttom of the link, and keep firm hold until the blocks are detached from 

 the top. The rams are then allowed to descend by letting off the water 

 pressure, and having reached the bottom of the stroke, the sliding blocks 

 then become level with the shoulder of the next link lower in succession. 

 The upper blocks being then screwed up and the lower blocks detached, the 

 rams again rise by the internal pressure communicated by ihe pumps and en- 

 gine, and again carry the tube and all its appurtenances 6 feet higher, and 

 the same operation is repeated by 6 feet lifts in succession, nntd the whole 

 height is attained. When the chains ascend above the level of the rams, 

 each link, as it rises above the level of the bearing beam, is taken down and 

 removed out of the way by unscrewing the bolts. 



The opening and shutting alternately of these blocks is all contrived in- 

 geniously, so that the four ends of the two blocks, which have each separate 

 movements, are yet all made to approach or recede by the turning of a single 

 handle and piidon-winch, so as greatly to facilitate the process. On the 

 Aoglesea Tower there is only a single press, the ram being 20 inches in dia- 

 meter. The single power has one advantage, that acting in the centre of 

 the tube, this must be raised simultaneously and equally at both sides. In 

 the double power, which possesses other advantages, there is some risk of 

 the tube rising unequally at the sides, and turning off the perpendicular. To 

 avoid this, an assistant is stationed at each ram, who observes on a scale, and 

 calls out every inch as the rams ascend, and thus an equality is maintained. 

 The opposite ends of the tubes might be lifted simultaneously by having the 

 opposite engines and rams at work together, as was the case in the Conway ; 

 hut this is liable to produce an oscillating movement in the whole tube, 

 wliich it is desirable to avoid ; so the lifts are made at each end alternately. 

 As the tubes ascend at each end, care is taken to follow these up with layers 

 or plates of timber or iron, piled up uniformly to within an inch or two of 

 the bearing beam, so that in the event of anything going wrong, the tube 

 would fall and rest on this packing, and do no injury. 



In regard to the strength of the tube, Mr. Buchanan gave the result of 

 some experiments, communicated by Mr. Clarke, on the strength of malleable 

 iron. It had formerly been considered from those of Telfordand Brown that 

 malleable iron would bear 27 tons on the square inch, but these experiments 

 were made with hydraulic or lever power, which is affected by the anomalies of 

 friction. Clarke's experiments are not liable to this objection, as they were 

 made by direct tension, by heaping on masses of iron or other weights till 

 fracture took place, and from them it appeared that the average strength of 

 malleable iron cannot be reckoned greater than 20 or 21 tons per square 

 inch. 



Some remarkable experiments were also made on rivetted plates. It had 

 hitherto been considered, and very naturally, that the tensile strength of 

 rivetted plates must be diminished by a quantity equal to the aggregate 

 section of the rivet-holes, which being pierced through the metal, must, as 

 was assumed, detract from its strength. .Mr. Clarke, however, has found by 

 careful trials, that when the bolts are put in red hot, and quickly and pro- 

 perly rivetted, that the contraction of the iron in cooling is such as to com- 

 press the plates together with a pressure about 5 tons to the inch, so as to 

 require an enormous power to make the plates slide one upon another, and 

 the heads being, moreover, so closely compacted into the plate, the holts 

 also resist this sliding by the power of detrusion, — in proof of which, these 

 holts, in cases of fracture, are often seen cut clean across as if by the shears. 

 But whatever may be the cause, the result is, that he considers' the rivetted 

 portion of the plate as strong as the solid. These experiments, therefore, 

 though contrary to the r('Cei\ed notions, are highly important; they give ad- 

 ditional confiilcnce to the structure of the hrulge, and are also extensively 

 applicable in various cases of steam boilers and others. 



He then explained particularly another remarkable circumstance in tho 

 structure of the bridge — namely, the uniting of the two great central tubes 

 of the Britannia Tower. This was proposed to be done by inserting a small 

 middle portion of tube in the Britannia Tower, so as, by this connectmg link, 

 to unite the two extremities of the opposite tubes in one continuous mass ; 

 and, in order to give full etfect to the principle, it was proposed, before rivet- 

 ting the last and final joint, to lift the extreme end of the tube resting on the 

 land tower 12 inches or more, while the joint was making, and then let it 

 down again to its place. The effect would be, by the two tubes pulling 

 against one another, and distending powerfully the upper side of the tube in 

 the Britannia Tower, that the deflection in the two opposite tubes will he 

 diminished, and the strain, instead of being home by the central portions of 

 the tube, would be distributed, and shared by the whole of the section at tha 

 extremities in the Tower, where the depth is the greatest, being there 

 30 feet. 



This was a happy idea, and he had no doubt it would be successful ; it 

 would have the effect, indeed, if all the ends were united, that though ona 

 of the central tubes were cut across at the middle, it would still hang by the 

 extremities and sustain a very great load ; and he explained particularly the 

 nature and effect of the strains on a beam in this situation, which resembled 

 in fact a continuous beam or flooring deal passing over several bearings or 

 joists intermediate between the walls, or like the rails of a railway resting 

 on its chairs. It is well known that the continuous beam is much stronger 

 than if it were cut across at any of the intermediate joists, and the rails are 

 subject to greater deflection in the space next the joint chairs, which, on 

 this account, are brought closer togetiier. Now, it is important to remark, 

 in the case of such a beam, not merely supported at the ends, but fixed or 

 attached longitudinally to auother beam, that the whole of the particles on 

 the upper side of the beam are not subject to a compressive force according 

 to the general notion, but are only compressed near the centre. The 

 extremities are subject to violent distention, and the middle parts re- 

 main neutral. The true lines of the compressive force resembled exactly 

 that of the rafters of the roof relative to the tie-beam ; and this confirmed 

 what he had formerly explained, that the nearer we can approach, in the 

 form of our girders, to this simple figure of a triangular frame, the more 

 perfect would be the distribution of the tensile and compressive forces 

 throughout the material proper for bearing them. On the whole, this ar. 

 rangement would give great additional confidence in the structure of the 

 Britannia Tubular Bridge; for, in ordinary girders, if there were any imper- 

 fection or failure in the centre, nothiug could save the structure ; but here 

 we have a girder which, though it were cut through the centre, would still 

 bear up the bridge, and any load that might be upon it, by the great strength 

 remaining in the extremities. 



Great, however, as is the strength and security of this structure, it should 

 not be forgotten that bridges of this description, and of such enormous 

 spans, could not be executed without great sacrifice of materials, and should 

 not therefore be attempted, unless from absolute necessity, as in the present 

 instance. As we increase the span, the strains on the bridge arising from its 

 own weight and that of the passing loads must increase rapidly, owing to 

 the nature of the transverse strain, even if there were no increase of load ; 

 and when we consider that, in addition to this, the bridge itself must he 

 increased in all its dimensions — in length, depth, and thickness — and the 

 passing load increased also in proportion to the length, it is evident that we 

 must quickly approach a limit beyond which the mass of the structure will 

 nearly overpower its strength, and leave no remnant for either load or con. 

 tingency. This is shown very clearly when we compare the strength of the 

 model tube, as shown by the experiments of Fairbairn, with those of the 

 Conway and Britannia Biidges. The model tube weighed nearly G tons and 

 carried 92| tons in the centre before breaking, which is equivalent to 30 times 

 its own weight equally distributed. Now, the Conway or Britannia tube, cal- 

 culating from the experiments on the model tubes, and the data furnished by 

 them, could not be expected to carry more than three or four fimes their 

 own weight. As the passing load cannot, in the most extreme case, exceed 

 one-fourth part of the weight of the bridge, there is still here an ample 

 margin of strength and security; but yet it appears that if we were to 

 extend our spans much farther we would rapidly approach the limit of 

 safety. 



In answer to a question from the President, he explained the mode by 

 which provision was made to allow the tube to expand or contract by heat 

 or colli. This was done by fixing the ends in the Britannia Tower, and 

 causing the tube at the other hearings on the towers and abutments to rest 

 on numerous cast-iron rollers, on which it could easily move backwards or 

 forwards. And, in answer to a question from the Vice-President regarding 

 the means of keeping up continuity in the rails at the extremities of tha 

 tube, he did not think any inconvenience was found from this in the Conway, 

 and it was proposed to be provided for in the Britannia Bridge by sliding 

 joints. 



The thanks of the Society were voted to Mr. Buchanan for this interest- 

 ing series of expositions on the strength of materials, which were given to 

 him from the Chair. 



2. "Description and Drawing of a ilachine for Mortising, Tenoning, 

 Boring, and Ripping Timder," By Mr. William K. Douglas. 



It was stated that this machine in all its parts possesses great advantage 

 over hand-labour ; and, as all the parts are useful fur the trade, it is a saving 



