544 THE STRENGTH OF IRON BEAMS. 



been offered as an improvement upon them. The 

 form of the common beams has, as might be 

 expected, undergone modifications and improve- 

 ments since their introduction, which w^as 1 beHeve 

 by Messrs. Boulton and Watt; and they, I under^ 

 stand, were the earhest to apply them to render 

 buildings fire-proof, which was first done by 

 them in 1800, at Messrs. Philips and Lee's 

 Cotton mill, in Salford. 



84. We may therefore consider the beams 

 experimented on by Mr. Fairbairn, as a sample 

 of pretty nearly the strongest in section that have 

 hitherto been in use; but if we suppose other 

 beams made of the same length, depth, and 

 quantity of section as these (the section being 

 formed as in experiment 19, where upwards 

 of f of its area was in the bottom rib) ; supposing 

 moreover the strength of the imagined beams to 

 be estimated by the rule in article 71, it would 

 indicate a considerable increase of strength above 

 what is given by the common beams above; 

 leaving a gain indeed much higher than we 

 found in our experiments (19 to 21), arising, 

 perhaps, from some difference in the metal in the 

 two cases. 



