TRANSACTIONS OF THE SECTIONS. 



423 



Ott the direct tensile Strength of Cast Iron. By 



E. HODGKINSON. 



The absolute strength of this metal, notwithstanding the 

 extensive use made of it in the arts, is still a matter of doubt. 

 If we turn for information to authors, we find Mr, Tredgold 

 and Dr. Robison making it nearly three times as great as Mr. 

 Rennie or Captain Brown, and the advocate of the greater 

 strength (Tredgold) attributing the less strength, as found by 

 the others, to the straining force not having been kept in the 

 centre of the pi-ism. For supposing the extensions and com- 

 pressions to continue always equal from equal forces (which they 

 are under slight strains), a small deviation from a central strain 

 would make a great reduction in the strength ; and if the force 

 were applied along the side of a square piece, the strength 

 would be reduced to one fourth. {Tredgold, Art. 61, 62. 234.) 



The above contrariety of opinion was the cause of the fol- 

 lowing experiments, in which the utmost care was taken to 

 keep the straining force along the centre of the castings, which 

 had their transverse sections of the form -|- , except in the 

 last two experiments, in which the section of the castings was 

 rectangular, and the force applied exactly along the side. The 

 iron was of a strong kind, the same as in the author's expe- 

 riments on 'heaims{Manchester Memoirs, \o\. v.), and was broken 

 by a machine on Captain Brown's principle for testing iron 

 cables. 



Force up the middle. 



