370 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



IDkc. 



specific gravity of the water above the mercury in the bulb. Set o, of the 

 scale to this point, and open the cock *, and shut the other, and the instru- 

 ment is ready to indicate tlie speed of the vessel. 



I subjoin a table founded upon a series of experiments instituted with a 

 view of ascertaining the resistance on a plane in still water at various veloci- 

 ties, the result of which I have given in a pamphlet, entitled, " Practical 

 01)servations on the Steam Engine." This scale, however, must be tested 

 by further experiments to ensure perfect accuracy. 



Column 1, represents nautical miles, and column 2 the height of a column 

 of mercury in inches and decimals. 



November, 1847. 



G. V. GnSTAFSSON. 



AVKOUGHT-IRON TUBULAR BRIDGES. 



In consequence of the experiments made at the suggestion of Mr. Robert 

 Stephenson by Mr. Eaton Hodgkioson and Mr. Fairbairn of Manchester, 

 at Millwall, to ascertain the best form of tubular bridge for carrying the 

 Chester and Holyhead railway across the IMenai Staits, Mr. Fairbairn has 

 devoted his particular attention to carrying out Mr. Slephenson's ideas in 

 the construction of wrought-iron tubular girders for railway purposes, and 

 as they are likely to be extensively adopted, we have collected some in- 

 formation as to their cost and weight compared with solid iron girder 

 bridges. It appears that the tubular girder for spans of 60 feet and upwards 

 is in many respects superior to the arch, whether it be of iron, stone, or 

 brick, besides being much cheaper in construction. 



The arch, as is well known, is not always admissible where railways 

 have to be carried across public thoroughfares, deep ravines, navigable 

 rivers, and canals. In such situations, the hori-^ontal wrought-iron girder 

 bridge appears to be the only structure which can with safety be applied 

 to such a purpose. Cast-iron girders are applicable for spans not ex- 

 ceeding 40 feet ; beyond that point the compound trussed girders have been 

 used and in every case they are equally if not more expensive and much 

 less secure than those composed of wrought iron, and it is doubtful whether 

 the principle is a sound one. Now, in the wrought-iron girder the weight 

 is less than one-third of the cast-iron, the strength being the same, and as 

 th«se girders form the parapet of the bridge, they are particularly well 

 adapted for a bridge of considerable span. 



In order to show the commercial value of this description of bridge, we 

 give the comparative cost of one of these bridges compared with one of 

 cast-iron, from bridges actually constructed. 



Truss Girder Bridge. — The weight and cost (J a bridge of W feet span 

 having cast-iron trussed girders. 



Cast-iron work 70 tons at £12 per ton . £ 012 



Wrought-iron work as composed of truss-rods, bolts, &c., 



14 tons at £37 4s. . . . 520 16 



entirely of cast-iron riveted in parts, but the increased weight and addi. 

 tional cost would render such a strncture inadmissible for such a purpose. 



Cost of Stephenson and Fairhairns's uroiight-irnn bridge. 

 Three wrought-iron girders, each GO feet long, riveted complete, weight 

 30 tons, at 30/. per ton, 900/., which is the sum required for the girders, 

 exclusive of the cross beams and roadway as before. 



The comparative value of the two bridges will therefore be as 9 to 14, 

 irrespective of the superior strength and security of the former to that of 

 cast-iron, in whatever form it may be applied. 



The plan has already been adopted by Messrs. Stephenson, Cnbiit, 

 V'ignoles, Bidder, aud others, and Mr. Fairbairn during the earlier stages 

 of the experiments engaged, at the request of .Mr. Vignoles, to construct 

 two bridges of this kind — one to be erected over the canal aud the other 

 over the turnpike-road on the Blackburn and Bolton railway. These 

 bridges were the first constructed for the support of a railway,* and 

 although they are probably not so well proportioned as others now in pro- 

 gress, they nevertheless exhibit such extraordinary powers of resistance 

 as not only to ensure complete success, but lo lead to new and future de- 

 velopments in what may probably be considered a new era in the history 

 of bridges. Viewing the subject generally, we feel assured, from what 

 has already been done conjointly by Mr. Stephenson and Mr. Fairbairn, 

 that the present discovery is only a beginning of an extensive application 

 of this useful art. 



Since the completion of the first experiments on sheet-iron tabes, others 

 of a more conclusive character, and upon a much larger scale, have been 

 made. They indicate several new and important facts; and from the 

 greatly increased size of the model tube, with its rectangular cells, greatly 

 superior powers of resistance have been obtained by a considerable in- 

 crease lo the area of the bottom. The ratio of that part to the cellular top, 

 will now stand as 10 : 12, inslead of 3 : 5, as formerly indicated in the 

 experiments with the corrugated top. 



Through the kindness of the Editor of the Railway Chronicle, we have 

 been enabled to give drawings and an account of one of the bridges, that 

 over the canal on the Blackburn and Bolton Railway. 



" Fig. 1, represents an elevation of the side girders, each 66 feet long, 

 with a span of 00 feet. Fig. 2, a transverse section of the bridge. Fig. 3. 

 a side view and section of the cross beams ; and fig. 4, a section of one of 

 the side girders, including its suspended cross beam and platform. 



" The thickness of the plales used in the construction of these girders 

 was half an inch for the sides and top, and | inch for the bottom ; the 

 whole firmly riveted to angle iron, as shown in the sections. 



" On referring to the sections it will be observed that the wood cross- 

 beams, D, D, for supporting the roadway and rails, are screwed up to the 

 bottom of the hollow girders by the straps n a, and the vertical bolt b, which 

 perforates the top cell through the tube c, and answers as a stay for con- 

 necting the upper and lower sides of the cellular top. Since these bridges 

 were finished, a better and more efficient 'mode of construction has been 

 adopted, by forming a longitudinal shelf of plate-iron along the bottom of 

 each girder, to receive the cross beams, and also to strengthen the bottom 

 in its resistance to a tensile strain. In this construction it will be observed 

 that the cross beams may be formed of either cast-iron, wrought-iron, or 

 wood, as may be deemed expedient." 



This Blackburn and Bolton bridge has already been subjected to severe 

 tests. Before the line was opened to the public, three locomotive engines 

 each of 20 tons, and covering the span of 60 feet, were run together as a 

 train, at rates varying from 5 to '2a miles per hour. The deflection pro- 

 duced by a weight of 60 tons was -025 of a foot. This seemed to be with- 

 out any sensible alteration from the difference of velocities. Captain Cod- 

 dington, the government inspector, and Mr. Flannigan, the engineer, then 

 placed on the rails, in the middle of the bridge, two wedges of the height 

 of one inch, acting as inclined planes. The engines dropping from this 

 height when at a speed of 8 to 10 miles per hour, caused a lolal deflection 

 of -035 of a foot. Willi wedges of an inch and a half thickness, the total 

 deflection became -045, which is nearly half an inch. Altogether, it has 

 been fully proved that the bridges are strong enough to bear any force lo 

 which they may be subjected, whether brought by a dead weight or by 

 impact. 



Total for the girders, exclusive of cross beams and road- 

 way .... £ 1432 16 

 We believe it possible to make a compound girder of the above span 



* SimviUaneously «-ith these constructions, Dtr. Dockray erected, under the direction 

 of Mr. Stephenson, a similar bridge, with a cast-iron top. for carrying tlie turnpike-rvad 

 across the London and North-Wcstern Railway at Camiea Toivu. 



