434 THE SCIENTIFIC PAPERS OF 



hammering and rolling. Dr. Siemens confessed that he had been 

 sceptical regarding this effect until he had had light thrown upon 

 it by Professor Akerman. If pressure was the cause why steel 

 became stronger in manipulation than in chilling the outside of a 

 heated ingot, greater pressure was thrown upon the interior metal, 

 and it was easily conceived that the effect produced would be 

 similar to that brought about by hammering and rolling. The 

 experiments referred to in the paper seemed to prove this very 

 clearly. 



There was one point to which Professor Akerman also called 

 attention, and which Dr. Siemens could not allow to pass without 

 observation. They had heard much of the extension of mild steel, 

 and they knew that the results were of the most variable character. 

 Some eminent steel makers and users tested bars two inches long, 

 and obtained a marvellous elongation before rupture took place. 

 Others, again, including the British Admiralty, used bars eight 

 inches long, and the elongation, after fracture, was measured from 

 the distance of the two points that were originally eight inches 

 apart, after placing the fractured pieces end to end. Other ex- 

 periments, especially those of Mr. Barlow and the Committee 

 appointed by the Institution of Civil Engineers, dealt with very 

 long bars, and the rates of elongation were consequently much 

 less. Professor Akerman suggested they should exclude the drawn 

 portion of the bar from the total elongation, and thus obtain the 

 real elongation, and his suggestion deserved the greatest possible 

 attention. But why carry the test to rupture at all ? They knew 

 that up to a certain point the elongation continued very uniform 

 throughout the bar, and it would be quite sufficient to carry the 

 test to the point where partial tearing had set in. They should 

 thus obtain a measure of the force which a bar was capable of 

 resisting, because from the moment the slightest irregularity in 

 the line of contour occurred, the result was no longer a proper 

 test of the strength of the material, and there was a fault or error 

 created by the continuance of the general elongation after that 

 partial action had set in. But in whatever way the experiment 

 was conducted, it would be far more correct to reckon the elonga- 

 tion of a bar of steel by excluding that portion of it which had 

 been drawn, and therefore presented no portion of legitimate elon- 

 gation of the bar. 



