MEMOIR OF EATON HODGKIXSON. 219 



1,456 pounds during' a period of from five to six months, and then liroke. The 

 second broke with l,oOO pounds hiid on immediately. From this experiment 

 Mr. llodfjkinson inferred that time has hut litth', if any, inlhience on the 

 streng-th of cast iron. 



This inference seems, at least to me, to be theoretically coiTect. If the weight 

 laid on the beam and its molecular forces be statically equal, the ft)rces will 

 remain in this state of equilibrium until the molecular forces are weakened by the 

 influence of unequal temperature or other causes. Our knowledge, however, of 

 this practical subject is indeed very limited. The inquiry would ami)ly repay 

 any one who has the ability, opportunity, and means to pursue it. Mr. Ilawk- 

 shaw has made some admira))le remarks on this subject in his evidence before 

 the royal counnissioners in 1847. (See Report, page 296.) 



The opinion of experienced engineers appears to be that vibrations ])roduced 

 by continual impact and change of temperature aflect the strength of iron to a 

 g'reater extent than a continued strain, which preserves the molecules of the iron 

 in the same fixed position. Mr. Rastrick, in his evidence before the commis- 

 sioners, gives the result of an experivnent made l)y a friend, bearing on this ques- 

 tion, at Pontypool Iron Works. He hung a bar of iron, an inch square, up by 

 one end perpendicularly, and contrived a small hammer to be continually ham- 

 mering it ; after a period of more than 12 months the bar of wrought iron 

 drojiped in two. 



That the internal structure of iron becomes changed by continued vibrations 

 is commonly believed by engineers of experience ; but in what way this change 

 is produced, both in speciality and magnitude, does not appear to be very definite. 

 One thing, however, seems clear, viz., that wrought iron is more atl'ected by 

 vibrations than cast. The evidence given before the commissioners on this 

 important question is very striking, and contains all the practical information 

 whicli has been recorded or known on the subject. Mr. Fairbairn states '* that 

 if you take any material whatever, and destroy its original form, and repeat the 

 changes, it is only a question of time how long it will bo before it breaks." 



According to rny view, this statement from an engineer of so great experience 

 should convince those whose duty leads them to the application of iron, timber, 

 and stones to the erection of structures, the first characteristic of which is stabil- 

 ity, of the existence and destructive nature of vibrations. Notwithstanding 

 these views on the effect of continued vibrations, there are not wanting engineers 

 of great eminence who think the subject of but little practical importance, how- 

 ever interesting it may be in a scientific and ])hilosophical sense. 



The late Robert Stephenson refers to the beam of a Cornish engine, and 

 states that it receives a shock 8 or 10 times a minute equal to about 55 tons, 

 during a period of 20 ^-ears, without the slightest perceptible change in its struc- 

 ture and strength. 



The connecting-rod of a locomotive engine is another illustration in point : 

 " Oue I know," says Mr. Stephenson, " which has run .50,000 miles, and received 

 a violent jar eight times per second, or 25,000,000 vibrations, and yet there is 

 not the slightest appearance of changt^ in the strength of the connecting-rod." 



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 important link." On the whole, Mr Stephenson attaches 

 but little importance to the question of vibration in a i)racti(;al sense. 



^Ir. Brunei, in answer to the question whether the internal structure of an 

 iron beam Ijecomes 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 1 have long acted on the assunqjtion 

 that iron is so changed; but I must confess that I have doubts as to the fact. 



