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



period of more than twelve montlis_, the bar of wrought iron 

 dropped 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 de- 

 finite. One thing, however, seems clear, viz. that wrought 

 iron is more affected by vibrations than cast. The evidence 

 given before the Commissioners on this important question 

 is very striking, and contains all the practical information 

 which 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 be before it breaks.^' 



According to my 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 stability, of the existence and destructive nature of vibra- 

 tions. Notwithstanding these views on the effect of con- 

 tinued vibrations, there are not wanting engineers of great 

 eminence who think the subject of but little practical im- 

 portance, however interesting it may be in a scientific and 

 philosophical sense. 



The late Robert Stephenson refers to the beam of a 

 Cornish engine, and states that it receives a shock eight or 

 ten times a minute, equal to about 55 tons, during a period 

 of 20 years, without the slightest perceptible change in its 

 structure and strength. 



The connecting-rod of a locomotive engine is another 

 illustration in point : '' one 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 change in the strength of 

 the connecting-rod.'^ 



