MEMOIR OF THE LATE PROF. E. HODGKINSON, F.R.S. 181 



The same distinguished engineer says_, with respect to the 

 question of the effect of vibrations on materials, " as to the 

 change being produced in wrought iron, which is a very 

 popular and almost universal theory now, I have not 

 known one single instance in which I have traced it to its 

 origin, where the reasoning is not deficient in some im- 

 portant link." On the whole, Mr. Stephenson attaches 

 but little importance to the question of vibration in a prac- 

 tical sense. 



Mr. Brunei, in answer to the question whether the in- 

 ternal structure of an iron beam becomes altered by a 

 succession of slight blows at a low temperature, as in rails 

 long used, railway-axles, or springs of carriages, says, ^^ I 

 have turned my attention a good deal to this inquiry, and 

 I have long acted on the assumption that iron is so changed ; 

 but I must confess that I have doubts as to the fact. And 

 I believe that if the subject were thoroughly examined, it 

 would be found that the different appearances shown by 

 iron when broken arise from the combinations of the causes 

 producing fracture as often as from any change in the 

 texture of the material itself. This opinion was strength- 

 ened by various specimens of irons broken, some with a 

 fibrous fracture by means of a slow heavy blow, and some 

 with a crystalline fracture by means of a sharp, short blow. 

 Mr. May refers to the case of a cast-iron beam of a steam- 

 engine, vibrating hundreds of thousands of times per annum, 

 being as good at the end of 20 or 30 years as when first 

 put up. In this case, though the strain has been in opposite 

 directions, and constantly varying, still the vibrations have 

 not weakened the beam. On the other hand, he says, I 

 have seen a cast-iron gun absolutely broken across by many 

 years^ dropping pig-iron upon it." 



In order to facilitate the calculation of the strength of 

 short pillars, Mr. Hodgkinson has given the crushing- 

 strength of a great variety of timbers used in practice. 



