190 



MR. WILLIAM FALRBAIRN ON 



Other construction ; and, considering the risk of oxidation 

 arising from neglect in attending periodically to the clean- 

 ing and painting of the girders, I am satisfied I am not 

 wrong in making such a provision, and in substituting this 

 large power of resistance for the strength of the principal 

 parts of the structure. It is for these reasons that I have 

 assumed for a double line of rails 12 tons per lineal foot as 

 the ultimate strength of a Tubular Girder Bridge, calculated 

 to ensure permanency, and to meet all the requirements of 

 railway traffic. I have done so in order to meet the various 

 contingent forces of the weight of the bridge itself, the 

 maximum rolling load, and the various other conditions to 

 which railway bridges are subjected, such as vibration or 

 the force of impact acting injuriously upon the bridge. 



Amongst other considerations which have engaged the 

 attention of the commissioners on railway structures, is that 

 of impact, and the effect of vibration upon bridges composed 

 of cast-iron, either in the shape of the single or the com- 

 pound trussed girder. The elaborate investigations on this 

 subject, recently published, are exceedingly valuable ; and, 

 although they indicate several new and important proper- 

 ties in the strength of materials, they do not, so far as my 

 own investigations extend, give the correct law as respects 

 the effect of the impinging forces by which these structures 

 are assailed. I believe Professor Willis (whose high stand- 

 ing as an acute mathematician is a sufficient guarantee for 

 the accuracy of the experiments) is perfectly aware of this 

 fact, and has qualified the experiments made at Portsmouth 

 on cast-iron beams, nine feet long, by others upon existing 

 bridges of not less than 50 feet span. These latter experi- 

 ments are more satisfactory than those at Portsmouth, and 

 approximate much nearer to those made by myself, and 

 other experiments of a similar character. 



The effects produced upon a girder bridge by a heavy 

 body, such as a locomotive engine rolling over its surface 



