222 Report on the Progress and State of Applied Mechanics. 



Saltash, of a class intermediate between tubular, bow-string, and sus- 

 pension bridges. 



57. The use of Suspension Beedges on Railways has been delayed 

 and limited by their flexibiUty, which might give rise to dangei-ous 

 oscillations dm'ing the passage of trains at high speeds. Were it not for 

 this objection, their lightness and economy would render them prefer- 

 able to all other structures for crossing wide spans. Nevertheless, they 

 have in some cases been used, the speed of the trains being moderated 

 in passing over them. The celebrated Niagara Railway Suspension 

 Bridge, designed by Roebling, is of the immense span of 822 feet, and 

 has two platforms, the upper for the Railway and the lower for a road. 

 In the case of a structure so large, the weight of the wire cables, sus- 

 pension rods, and platform, is so great in proportion to that of a train, 

 that the oscillation produced is but small : it would not be so in a sus- 

 pension bridge of less magnitude. It has long been the practice to stiffen 

 suspension bridges, to a certain extent, by means of a lattice framework ; 

 but when it has been proposed to stiffen them thoroughly by that 

 means, it has been objected, that auxiliary girders would have to 

 be used strong enough to carry the load by themselves, and that the 

 chains would be superfluous. Some experiments by Mr. P. W. Barlow, 

 on models, have shown that this objection was founded in error ; and 

 that auxiliary gu'ders, extremely light in comparison to what had been 

 supposed to be necessary, will be found sufficient to stiffen a suspension 

 bridce, and fit for Railway traffic at high speeds. A theoretical calcu- 

 lation, undertaken since Mr. Barlow's experiments were made, has 

 shown that mathematicians might have anticipated this result, had 

 they turned their attention to the subject, and that girders of four- 

 twenty-seventh parts of the strength required to support the entire load 

 will sufficiently stiffen a suspension bridge. That conclusion will be 

 tested in practice by the execution of a bridge, designed by Mr. Barlow, 

 at Londonderry. 



58. Although the success of Railways has diverted the public atten- 

 tion from Canals, these continue to be the most economical lines of 

 conveyance for all articles in whose transport speed is of little impor- 

 tance. Amongst the structures whose use on Canals is recent, may be 

 mentioned inclined j^^'^'-nes, for transporting boats from one level to 

 another, to save the expenditure of water at locks, and suspension aque- 

 ducts, hung by wire cables, — a kind of structure first introduced in 

 America, by Mr. Roebling. The continual and xmdisturbed unifor- 

 mity of the distribution of the weight in canab, renders the mode 

 of support by suspension from chains or cables peculiarly well suited 

 to their aqueducts; and it is remarkable that so obvious a principle 



