136 Mr. C..Hood on Changes in the Structure of Iron. 



not an essential element in the operation ; that the change, 

 under certain circumstances, may take place instantaneously ; 

 and that an axle may become crystallized in an extremely 

 short period of time, provided that vibrations of sufficient 

 force and magnitude be communicated to it. This circum- 

 stance would point out the necessity for preventing as much 

 as possible all jar and percussion on railway axles. No 

 doubt one of the great faults of both engines and carriages 

 of every description — but particularly the latter — is their pos- 

 sessing far too much rigidity; thus increasing the force of 

 every blow produced by the numerous causes incidental to 

 railway transit; by causing the whole weight of the entire 

 body in motion to act by its momentum in consequence of the 

 perfect rigidity of the several parts and the manner of their 

 connection with each other, instead of such a degree of elas- 

 ticity as would render the different parts nearly independent 

 of one another, in the case of sudden jerks or blows ; and which 

 rigidity must produce very great mischief, both to the road 

 and to the machinery moving upon it. The looseness of the 

 axles in their brasses must also be another cause which would 

 greatly increase this evil. 



Although I have more particularly alluded to the change 

 in the internal structure of iron with reference to the effects 

 on railway axles, it need scarcely be observed that the same 

 remarks would apply to a vast number of other cases, where 

 iron, from being more or less exposed to similar causes of ac- 

 tion, must be similarly acted upon. The case of railway axles 

 appears to be of peculiar and pressing importance, well de- 

 serving the most serious consideration of scientific men, and 

 particularly deserving the attention of those connected with 

 railways, or otherwise engaged in the manufacture of railway 

 machinery, who have the means of testing the accuracy of the 

 theory here proposed. For if the \iews I have stated be 

 found to harmonize with the deductions of science, and to co- 

 incide with the results of experience, they may have a very 

 important effect upon public safety. It may be observed, on 

 the other hand, however, that at the present time all railway 

 axles are made infinitely stronger than would be necessary for 

 resisting any force they would have to sustain in producing 

 fracture, provided the iron were of the best quality ; and to 

 this circumstance may perhaps be attributed the comparative 

 freedom from serious accidents by broken axles. The neces- 

 sity for resisting flexure and the effects of torsion, are reasons 

 why railway axles never can be made of such dimensions only 

 as would resist simple fracture ; but it would be very desi- 

 rable to possess some accurate experiments on the strength of 



