416 PKOCEEDINGS OF THE AMERICAN ACADEMY 



weight will be too great. From what has been stated above, however, 

 it may be inferred that, if the pillars are cast straight and sound, they 

 will, in general, be as strong when made of the cheaper material and 

 by the cheaper mode of casting, as the experimental pillars of Ilodg- 

 kinson. 



We have seen, also, that when the formulas were applied to experi- 

 mental pillars of somewhat larger dimensions than those from which 

 the formulas were deduced, cast from different kinds of iron, that the 

 actual breaking weights generally exceeded the breaking weights com- 

 puted by the formulas. 



In practice pillars are subject to the effects of vibration and time, 

 which tend to cause them to fail when loaded with less than the com- 

 puted breaking weights. 



What allowance should be made to cover all unfavorable influences, 

 and to afford sufficient surplus strength to insure complete security 

 against failure, is a matter of opinion, and one in which the most ex- 

 perienced and skilful may differ widely. 



In 1847, a commission was appointed by the British government to 

 inquire into the application of iron to railway structures. This com- 

 mission obtained the opinions of the most eminent English engineers 

 and constructors, and in the appendix to their report* they give the 

 following analysis of the evidence relating to cast-iron beams or 

 girders : — 



" There appears to be a considerable difference of opinion as to the 

 proportion lietween the greatest load which a girder should be allowed 

 to bear and the breaking weight. There are two conditions under 

 which the weight may be applied, viz. : 1st, when stationary, as in the 

 case of water-tanks, floors, &c. ; 2d, when the weight moves so as to 

 cause concussions and vibrations, as in railway bridges. In girders re- 

 quired for tlie first case, iMr. Fox and Mr. T. Cubitt consider that the 

 breaking weight should be three times the greatest load ; Mr. P. W. 

 Barlow, four times ; and Mr. Glynn would not make it less than five 

 times the load. 



" In girders for railway bridges, Mr. Brunei states that he allows the 

 load to be 4, or |, of the breaking weight ; but he considers that the 

 rule he adopts for calculating the dimensions of his girders gives more 

 than the usual strength. Mr. Grissell and Mr. Charles May consider 



* Report of the Commissioners appointed to inquire into the Application of Iron 

 to Railway Structures. London, 1849. 



