8 THE pbesident's inatjgueal address. 



scon after careful examination, yet the hridp^c liad become 

 much weakei-; so, as a final test, a portion erf a rivetted 

 joint was cut out and examined. Externally the joint was 

 perfect. The plates a,nd the rivet-heads were all apparently 

 tight and close ; but when tlio joint was cut open for in- 

 ternal examination, it was found that the rivets themselves 

 had been corroded greatly at the joint between the plates 

 in the middle, so much in fact that they did not there retain 

 half their original strength ; hence the hidden source of the 

 weakness that had been observed. 



On the other hand there are, of course, cases of very 

 limited headroom, where iron-topped bridges arc a necessity, 

 such as that for which I erected the iron-topped bridge for 

 the Bristol Harbour Railway to pass over Victoria Street, 

 where we had only sixteen inches of headroom ; and there 

 are cases of very wide spans, where iron may be the most 

 suitable material. 



With regard to bridges of largo span, thcori(!S as to the 

 possible limit of span in iron bridges have been promnlgnted 

 from time to time, and it was reckoned that a span of 1,500 

 feet for a railway bridge might be accomplished in that 

 material. But now, by using steel instead of iron, that 

 limit has been extended in favour of the stronger material ; 

 so that we have at the present time a bridge in process of 

 construction, across the Firth of Forth, of 1,700 foot span, 

 three to the mile, an immensely bold conception. 



Talking of theories, it is a common saying that " tiicory 

 and practice don't agree." Now, I for one do not admit the 

 correctness of this apothegm. I say, on the contrary, that 

 theory and practice must agree, if the theory be only 

 properly applied to the practice. If a wrong theory be 

 applied, or if any fact be omitted from the theory, why, of 

 oour.se it does not, neither could it be expected to apply ; 



