OP IRON BEAMS. 529 



If, in these two last cases, we throw away 

 the decimal, and call the value of c = 26, or 

 26, as the case may be, the negative error in 

 the preceding table will be very small. 



71. Hence we may in future take 



— j— 5 or — r-y 



(according as the beams were cast erect or on 

 their sides), for the measure of the ultimate 

 strength in tons, in our best forms of beams, 

 and with the iron we used. . ^^^.^ 



It would perhaps however have been better 

 to have called the multipliers 26 and 24, as I 

 conceive the iron from which the 25 was derived 

 to have been somewhat stronger than that fro»^ 

 which the 26 was obtained. ^^^ 



Example. — What weight laid on the middle 

 of one of the main beams, in the rail-road bridge 

 crossing Water-street, Manchester, would be 

 required to break it, supposing it cast erect,* 

 and of the same iron we have used ; the dimen- 

 sions from the model now constructing by 

 Messrs. Fairbaim and Lillie being as follow : — 



* The bettns were cast on their sides; but there wu a 

 little additional matter in their vertical part, which would 

 perhaps make up the small deficiency in strength, arising 

 from that mode of casting. 



3 X 



