308 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Oct. 





which are united to form the side 



[j-r'i*. '»'' ^ici if th: tarsi port'oa 



plates *- ■*_,," 



HelRht of rectangular cells at top and bottom 

 Breadth of ditto 



(Excepting those at the angles of the tube which are in breadth 

 Thickness of plates forming the cells 



n 



9 

 4 



2 8 



In all the experiments on the large model the weights were suspended at 

 the centre of it, from its lower side. In the first experiment the tube 

 broke with a weight of 35^ tons. It was then thoroughly repaired and 

 the sides were stiffened by vertical ribs attached at intervals throughout 

 their length : by these means and the addition of another plate at the bot- 

 tom the breaking weight was increased to 56 tons. It was observed in 

 one case that the lop of the tube bulged out transversely. The form of 

 joint which was intended for the top of the tube was made by laying ihe 

 ends of the plates together without uniting them ; they were then lo be 

 simply overlapped by pieces of metal above and below to prevent tliem 



islippiug past each other , . Instead however of adopt- 



fng this'form, the maker of the model had by mistake used a joint of the 



loUowiug form — — , bolting the two plates where they over- 



apped. In consequence of this arrangement and ihe great pressure to 

 ■which the upper side of the model was subjected, Ihe plates slipped some 

 considerable distance past each other, and slits were cut in tliem by the 

 bolts. The section of the Conway bridge given above, shows one of the 

 vertical stiffening plates attached to each of the sides of the tube. These 

 stiffening plates are T iron J ioch thick. 



Mr. Eaton Hodgkinson said that his own decided conviction was that 

 the best form for the top of the bridge was a series of long hollow cylinders 

 such as those first described by Mr. Fairbairn. There were three forms 

 proposed for the top — solid iron, rectangular compartments and cylindrical 

 compartments. He certainly thought the latter form was the best. In 

 fact the superiority was so great tliat no difficulty or expense in the manu- 

 facture ought for an instant to weigh as an objection lo its adoption. The 

 great object was perfect security, and to this every minor consideration of 

 trouble or preliminary outlay ought to be sacrificed. 



From the proportions which had been chosen he estimated that the 

 strength to resist lateral pressure was one-fourth of the vertical strength. 

 In his own models the proportions for the side plates were different, and 

 he had laid his tubes on their side to compare the lateral to vertical 

 strength. He found the latter was to the former in the proportion 26 ; \5. 

 This comparison he had instituted in order to ascertain the capability of 

 the bridge for resistance to the action of wind. As however the propor- 

 tions actually chosen for the Conway bridge differed from his own, he had 

 suggested that the strength of the side plates should be increased, and this 

 suggestion had been partially acceded to. He did not think that the top 

 plate should be arched between the abutments as in that case the steam of 

 the engine would rise to the top and have no means of escape. [This by- 

 the-by is a very insufficient objection, and one which might be easily re- 

 moved by some simple mechanical contrivance.] 



Mr. Clark, who had been present during the performance of all the ex- 

 periments, next read a paper. Each of Ihe longer tubes of the Menai 

 bridge would be 462 feet long, and weigh 1200 tons. It was intended to 

 put all the parts of tube together at a place about a quarter of a mile from 

 tlie intended position of the bridge. When the structure was finished it 

 •nould be floated to its place on large barges, and finally raised by hydrau- 

 lic presses al the two ends simultaneously. The Conway bridge had to be 

 raised only 18 feet, and this work would be completed first. Mr. Clark 

 then read some calculations respecting the strength of the bridge, but they 

 bore so great an analogy to those which have been already published in 

 this Journal, in the Notes on Engineering, that it is not necessary to re- 

 peat them. He said also that the tensile strength of the bottom 

 would be proportional to the sectional area, and independent of either the 

 width or thickness considered separately ; so that provided the sectional 

 area contained a given number of square inches, it was immaterial what 

 proportion the thickness of the bottom plate bore to its width. [This is 

 one of several grave errors which have been committed in the contrivance 

 of the Tubular Bridge. We do not however consider it necessary to enter 

 on the demonstration here, as it will be given in the Notes on Engineer- 

 ing.] 



From the experiments it would appear that the sectional area of the two 

 sides together ought to be equal to that of the bottom plate. 



Mr. Fairbairn expressed some surprise that Mr. Hodgkinson should 

 take to himself the credit of suggesting the cellular form for the top of the 

 tube. He could assure him that he was mistaken on this point, as experi- 

 ments on tubes with cellular tops had been commenced at Millwall before 

 he arrived there. Wilh respect to the pressure of the wind, it would be 

 found that if it be taken at 50 lb. per square fool, Ihe total lateral pressure 

 on one side of the bridge amounts to 300 tons. The proposition of sup- 

 porting Ihe bridgH by suspension chains had been finally abandoned. In 

 order to deaden as much as possible the effect of vibrations from the pass- 

 age of a train, the rails would be laid on vulcanised india-rubber 2 inches 

 thick. 



Dr. RoDiNsfiN called attention lo the difference between laying a load 

 gradually on the bridge and drawing a load rapidly over it. If Ihe im- 

 pulses from the train were synchronous wilh Ihe vibrations, the latter 

 would be increased to a frightful extent. He himself had ascertained 

 the vibration of a train at a distance of 15,000 feel by observing the effect 

 on quicksilver. He thought that in the model different loads ought to be 

 drawn through very rapidly to test effects corresponding to those of the 

 passage of trains. 



Mr. Fairbairn said he would certainly endeavour to follow out the 

 suggestion. 



Mr. Scott Eussell said that it appeared to him that the principal ques- 

 tion had not yet been discussed. The point lo be considered was — not 

 whether the bridge could be made strong enough to bear the weight on it ; 

 for there was no doubt that the engineers would keep on adding metal till 

 they removed all chance of fracture — but the real question was whether 

 the requisite strength was attained with a minimum quantity of metal: 

 whether, in fact, the metal was disposed in the most advantageous manner. 

 He had some doubts on this point. If two spans of the bridge were united 

 at the bottom pier, it was clear that any deflection in one span would com- 

 municate its effect to those which adjoined it ; so that there would be points 

 of contrary flexure, and the line of the bridge would be alternately convex 



and concave. In this case the top plate of the bridge would, in the neigh- 

 bourhood of the pier, be subject to tension instead of compression, and 

 conversely for the bottom plate of the bridge. 



Mr. Hodgkinson said the consideration of vibrations from the rapid 

 transit of a traiu was of the utmost importance. The effect of a train ia 

 motion could scarcely be calculated accurately. " I confess," added Mr. 

 Hodgkinson. " that when I consider this subject my mind misgives me, 

 and I cannot help feeling a few tremors." This announcement produced 

 a sensation. 



Mr. Roberts said that vibrations might be checked by altering the time 

 of the impulses. It was a common practise of engine-drivers, when they 

 found the engine began to " kick," as they term it, lo accelerate or retard 

 the train, and this generally destroyed the vibrations. 



Mr. Fairiiairn allowed that rectangular compartments for the top of the 

 tube were not theoretically the best, but they were so practically. It was 

 necessary to provide means for the entrance of workmen into every com- 

 partment to make any repairs that might be required, or to renew defective 

 plates. 



Dr. Robinson said that what was to be apprehended during the passage 

 of trains was not a sudden impulse, but the accumulation of vibrations. It 

 was well known that even a child might set a great bell in motion by con- 

 tinued efforts properly timed. If the impulses communicated by the bell- 

 rope were synchronous with the vibrations of the bell itself, the extent of 

 those vibrations might be increased till the bell got its full swing. 



Mr. Hodginson said that it had been suggested that the top plates 

 should be of cast iron. He objected to this proposition on the following 

 grounds. From his own experiments on the strength of cast iron, and 

 wrought iron columns to resist pressures applied in the direction of their 

 lengths it appeared that the relative strength of the two materials depended 

 entirely on the length of the columns. The columns might fail either by 

 bending or by crushing. With short columns cast iron bore to wrought 

 iron the strength of 17 : 10, but with long columns the strength of cast iron 

 to that of wrought iron was at 14 : 50. This showed the superiority of 

 the latter material for the upper plates of the tubular bridge. 



MICHAEL ANGELO. 



A correspondent of the Literary GazeUe has furnished to that paper 

 some particulars relating to the discovery of certain works by Michael 

 Angelo and other artists of renown. The correspondent is described as a 

 lady married into a collateral branch of the family of the great Italian. 

 A minute search through the Buonarotti Gallery has led to the following 

 results ; — " Upon opening the door I found Michael Angelo's own original 

 wax model of his superb David, looking even more majestic and imposing 

 than the well-known gigantic statue on the Piazza del Gran Duca, which 

 may be imputed to the fact of his having been stinted in the size of the 

 block of marble. Besides this masterpiece, the cupboard further con- 

 tained two other wax models by Michael Angelo, one his Giorno, the other 

 his Crepuscolo, — both of which are in the chapel called Cappello di Michel 

 Angelo in the Church of St. Lorenzo. There were likewise in this same 

 closet models in clay by Giovanni di Bologna ; they are his first concep- 

 tions of his most celebrated groups and statues. Another in wax, by Bac- 

 cio Bandinelli, besides another by an unknown hand. Then, at tho bottom 

 of all, under a thick veil of cobweb, I perceived a quantity of fragments, 

 which appeared to me of surpassing beauty. I collected the pieces, and 

 joined them together with boiling wax, thread, &c. ; when, to my great 

 delight, 1 found my fragments ussumed the form of the torso and legs of a 

 Satyr, which is one of the finest works of Art that can be imagined. The 

 celebrated engraver Jesi and another artist chanced to call upon us, and 

 both proclaimed the Satyr to be the work of Michael Angelo ; — but then, 

 we had no proof of such being the case. Now comes what I deem the 

 marvellous part of the story. The following morning, I again occupied 

 myself groping and poking about the gallery, particularly in an old cabinet 

 or closet whicii they say Michael Angelo used to write in. At length, I 

 pulled out a drawer, in which was a letter, dated 1660, from one Covr. 

 Pansani, who begs the Proc. Buonarotti to accept a Torso di Satiro, the 

 work of his great ancestor Michael Angelo.* * "—In this extraordinary 

 Banner have I obtained the proof of the authenticity of my Satiro, 



