218 SCIENCE PROGRESS. 



M. Barba enunciated some years ago certain laws in 

 regard to this point. He stated that in cylindrical test 

 pieces of similar form the percentages of extension at 

 fracture are the same for similar material, and that in 

 the case of bars of the same length but varying in their 

 cross-sectional area, or of bars of the same cross-sectional 

 area but varying in their length, the percentages of exten- 

 sion vary as the ratio of length to diameter, and decrease 

 as this ratio increases. 



These laws have been very fully investigated by Mr. L. 

 H. Appleby in a paper contributed to the Proceedings of the 

 Institution of Civil Engineers (vol. cxviii., p. 395), embody- 

 ing the results of a long series of experiments carried out at 

 University College, London. The materials experimented 

 with were two qualities of Bessemer steel, a hard and a 

 soft quality, both specially prepared for the experiments. 

 Briefly, the results of the experiments may be said to con- 

 clusively verify Barba's laws. They also showed very 

 distinctly that the local contraction before fracture was 

 enormously affected by local hardness or impurities, a point 

 before observed by Bauschinger and others. 



There can be little doubt that much better results for 

 the sake of comparison, and even for judging the quality of a 

 given material, would be obtained, if the final extension on 

 some fixed short length which includes the fracture were 

 discarded, and only the rest of the ultimate extension given 

 when recording any experiment. Probably the most con- 

 venient standard for cylindrical bars would be obtained by 

 making the ratio of length to diameter 10. The whole 

 question was very fully thrashed out as regards English 

 engineers, in the discussion on the late Mr. Hackney's 

 paper on " Forms of Test Pieces " (Inst. Civil Engrs. Pro- 

 ceedings, vol. lxxvi., p. 70) ; it was then pointed out that 

 there were great practical difficulties in the way of the 

 adoption of a fixed ratio of length to diameter, owing to 

 the fact that in testing gun steel, tire steel, etc., the speci- 

 mens must of necessity be made very short, hence with a fixed 

 ratio of length to diameter such as 10, the cross-sectional 

 dimensions of these specimens would be unsuitably small. 



