316 'THE SCIENTIFIC PAPERS OF 



tained the form of the diagram would be more nearly represented 

 by straight lines crossing each other in the neutral axis. 



He was ready to admit that the limit of elasticity was not an 

 absolute point ; that there was a slight set produced in straining a 

 bar for the first time ; and that the ultimate limit would be more 

 correctly represented by a bend in a curve than by a sudden 

 change of direction. But, nevertheless, he maintained that the 

 position of this bend in the curve, denoting elongation and com- 

 pression in each material, was of great importance, and could not 

 be ignored without arriving at erroneous conclusions. 



The resisting force of cast steel a material that would here- 

 after enter largely into air mechanical construction was nearly 

 three times greater than that of iron up to the limit of elasticity. 

 He believed that the Eailway Inspectors of the Board of Trade 

 would be willing to acknowledge the greater strength of cast steel, 

 if that material could be readily distinguished. No doubt, at first 

 sight, it was difficult to ascertain whether the plates of a bridge 

 were of cast steel or of wrought iron ; but this he would suggest 

 might readily be ascertained from the specific gravity of the metal. 

 He had, in his own laboratory, submitted a number of specimens 

 of steel and iron to this test ; and he found that in all cases 

 wrought iron was 2 per cent, or 3 per cent, lighter than cast steel 

 of nearly the same chemical composition. Fused wrought iron 

 had a specific gravity of 7'87 ; but if 2 per cent, of carbon was 

 added to it the specific gravity was reduced to 7'79 ; common bar 

 iron had a specific gravity of 7*55 ; and puddled slab 7'53 only. 

 The specific gravity of puddled iron of the greatest purity never 

 reached 7'6 ; while that of mild cast steel, with carbon varying 

 from nil to 4 per cent., always exceeded 7'7, a distinctive difference 

 that could be easily recognized. There was an easy way of deter- 

 mining roughly the specific gravity of metal : chip off the corner 

 of a plate, suspend it from the arm of a balance, and weigh it 

 both in and out of water ; divide its weight in the air by the loss 

 of weight in water and the result was the specific gravity of the 

 metal. But if it were said this was too much trouble for engineers 

 or government inspectors to undertake, the Board of Trade might 

 appoint inspectors for the purpose. The manufacturer might pay 

 the inspector's expenses, and the latter might stamp upon each 

 plate, which he had seen made and properly annealed, a mark 



