ON THE INFLUENCE OF FORM ON STRENGTH. 423 



sufficiently well known to experienced smiths to induce a prudent workman 

 to hold up, by grasping it with his leathern apron, the screw A while the 

 weld was being made at C ; and this precaution was generally found suffi- 

 cient to prevent the fracture. 



When, however, the fracture did occur, it was usually attributed to the 

 iron having been originally bad, or to its having been injured by the great 

 amount of local hammering in reducing it from the size of the collar to that 

 of the screw, or to its haraig been " burnt," or to some other such cause. 



Enough has probably been said to give an idea of the state of engineer- 

 ing practice and knowledge on this subject of "form" in the pre-railway 

 times. 



The introduction of railways, however, caused machinery to live a very 

 fast life, and now subjects an axle in the course of two or three years' work 

 to the reception of alternating strains and shocks which it would have re- 

 quired half a centiu'y to inflict upon the crank-shafts of steady-going old- 

 fashioned engines. On the first establishment of railways the loads were 

 lighter and the pace was slower than the loads and pace of the present day ; but 

 more than twenty-five years ago attention was directed to the fact that rail- 

 way axles, which appeared from their dimensions to be amply strong, were, 

 nevertheless, frequently broken after one or two years' work ; and accidents 

 to passengers arose from these breakages, which accidents caused engineers 

 to consider the subject of the fractures. 



Fig. 3, Plate III., exhibits, in a somewhat exaggerated manner, the construc- 

 tion of axle then in use on railways, where A A are the bearings or joiirnals, 

 B B enlarged parts to receive the wheels, and at C C there were left projec- 

 tions against which the backs of the wheel-bosses abutted. The journals were 

 made with collars (D D) at their outer ends, and the junctions of these collars 

 and of the enlarged parts B with the journals were made (as in the case of 

 the old form of crank-shaft shown in Plate IV.) by right angles. 



It was found that axles thus constructed were liable to fracture at the 

 junctions of the journals A A -with the parts B, and also at the terminations 

 of the enlarged parts B up against the shoulders C C. 



Such a fracture always exhibited evidence of a crack of long standing round 

 about the axle, which crack had reduced the section of sound metal to but a 

 fraction (three-fourths to one-half) of the original area. (See fig. 3 a.) 



On one or two occasions instances of these fractures and the consideration of 

 their causes made the subject of papers brought before the Institution of Civil 

 Engineers. In the papers so presented (with the exception of Mr. Bankine's, 

 to be hereafter mentioned), and in the discussions which ensued upon the 

 reading of those papers, many curious causes, including magnetism and elec- 

 tricity, were assigned for the fractures ; the prevalent opinion, liowcver, 

 appeared to be that the iron had been deteriorated by the rapid vibration. 



In 184.3 Mr. Eankine read a paper before the Institution of Civil Engi- 

 neers, in which he combated the deterioration theory, and attributed the 

 fractures to the fact of the fibres of the iron not following the outline of the 

 axle, they having been stopped short at the square shoulder by the turning 

 down of the journal out of the solid, and to the fact that, on the axle being 

 subjected to impact, the inertia of the particles on the outside of the enlarged 

 parts B caused a greater strain upon the outer part of the journals A. Mr. 

 liankine recommended that the axle should be forged to the shape so as 

 to ensure the fibre following the outline, and further recommended that the 

 junction between the two different sizes should be made by a curve. 



Mr. Eankine gave instances of the beneficial results obtained with certain 



