364 Scientific Proceedings, Royal Dublin Society. 



College of Science, also took a set of careful observations; the 

 number he independently obtained was somewhat lower, namely, 

 16,470 kilogrammes per square millimetre. 



The mean of these three series of experiments is the same 

 number as the mean of the first fifteen experiments, viz. : — 



16,800 kilogrammes per square millimetre, 



or 1680 x 10 6 grammes per square centimetre, which may be taken 

 as the modulus of hard manganese steel icire. The diameter of the 

 wire used was - 98 millimetres, and the length under observation 

 3*455 metres. 



The soft manganese wire was now tried. Six sets of experi- 

 ments were made with three or four elongations in each, the mean 

 of twenty elongations giving a modulus of 



16,710 kilogrammes per square millimetre, 



slightly below that of the hard wire. 



These numbers are lower than I expected. Iron has a modulus 

 of 18,610 kilogrammes per square millimetre. Steel wire varies 

 from 18,810 up to pianoforte wire, which is 20,490 kilogrammes 

 per square millimetre. 



But this comparatively high rate of extensibility of manganese 

 steel is for many purposes a considerable advantage, as it enables 

 the material to give under a sudden stress without fracture. 



Breaking stress. — Experiments in the breaking stress of the 

 wire were now made. The dynamometer I used was tested 

 and found correct. A comparative experiment was made with 

 pianoforte steel wire, 0-027 inch diameter. This broke at a stress 

 of 150 lbs., corresponding to a breaking stress of 117 tons per square 

 inch. Ordinary steel wire has a breaking stress of 54 to 63 tons 

 per square inch. The tenacity of the best pianoforte steel wire is 

 the highest known, and amounts to 150 tons per square inch. 



The soft manganese steel wire. No. ]9 S.W. Gr., or 0*96 milli- 

 metres in diameter (that is an area of 0'00125 square inch), broke 

 at a stress of 124 lbs., with 18 per cent, elongation : the elongation, 

 in fact, was remarkable, being 4 centimetres in 22 centimetres. 

 This breaking stress is equivalent to 48-8 tons per square inch. 



The hard icire of the same size had a far higher tenacity and 

 far less elongation. The first experiment gave a breaking stress 



