40 



MR. W. FAIRBAIRN ON THE 



ley Fall stone was much less than that required to fracture 

 similar specimens from the Shipley quarries. The following 

 table gives some useful results for comparison. 



From the above it is evident that there is a considerable 

 difference between the results of Mr. Rennie's experiments 

 and those in the preceding tables. This may, perhaps, be 

 due to the different methods pursued in the experiments, or 

 from taking the first appearance of fracture as the ultimate 

 power of resistance. Whereas, there is in some cases a differ- 

 ence of nearly a third between the weight required to produce 

 the first crack, and that required subsequently to crush the 

 specimen. This is the more remarkable as all the specimens 

 did not appear to follow the same law, as in some the weight 

 which fractured the specimen by a continuation of the process 

 ultimately crushed it. Experiments of this kind require close 

 observation, and the reason just given may probably account 

 for the difference between Mr. Rennie's and my own results. 



All information respecting the strength of materials must 

 be derived from direct experiment, which is always the safest 

 and best guide; and fully aware of the importance of this 

 fact, I have deemed it expedient to append the following list 

 of the bearing powers of some other materials employed in 

 building, and to which reference may be made in any case 

 where the load is excessive, or where the material is subjected 

 to severe strain. 



The necessity of these experiments was the more apparent 

 some years since, in the construction of the Britannia and 



