418 THE STRENGTH 



less than as above. — I have little doubt that these 

 quantities are much too high ; the lower parts 

 of a bar would have strength enough to render 

 it straight again w^hen the w^eight was removed, 

 though the elasticity of the top part was a little 

 injured. Besides, the full cohesion of cast iron, 

 from the experiments of Captain Brown and 

 Mr. Rennie, is but from 16 to 20,0001bs. per 

 inch. Mr. Tredgold was aware of this, and 

 quotes the experiments, and yet supposing the 

 rule^ applicable at the time of fracture, he 

 makes the value of /, or the full cohesion of 

 cast iron = 482001bs. per inch, a strength nearly 

 equal to that of malleable iron; and in the 

 deductions from other experiments, he makes 

 it nearly equal to this. The mistake seems to 

 arise from his having supposed the neutral line 

 to have retained its place in the middle of the 

 bar; but it must necessarily have shifted and 

 gone lower before fracture, on account of the 

 high resisting power of this metal when over 

 compressed, and of the stretching, which no 

 doubt takes place in the top part of the beam 

 then ; leaving much more of 5 in a full state of 

 tension, and increasing the strength far beyond 

 what is given by the rule. 



11. It will be proper, before proceeding further, 



• Mr. T. uses the same theorem as above for a joist. 



