194 MEMOIR OF THE LATE PROF. E. HODGKINSON^ F.R.S. 



Mr. Rastrick always considered that when a weight passed 

 rapidly over a structure^ there would be less deflection than 

 if it were stationary. He takes the example, for compari- 

 son_, of a man skating upon ice, and states that if he re- 

 main stationary for a length of time, he would soon go 

 through the ice ; but he may skate over it without any 

 danger of going through, because the ice has no time to 

 break. 



Mr. BrunePs impression was that where the rails are 

 perfect the deflection is, as it ought to be, less with a 

 weight passing rapidly over it than when it rests upon it ; 

 '^but the experiment is so diflicult to make, from the 

 number of interfering causes, that perhaps my impression 

 is still only prejudice rather than positive information." 



Mr. Cubitt, engineer of the Great Northern Railway, 

 could perceive no difference in the deflection of a large 

 girder between the weight being stationary upon it and 

 passing over it at a great speed. The experiment was made 

 upon a girder 47 feet span, and a heavy locomotive engine, 

 the deflection being a tenth of an inch. 

 . The opinion of Mr. Charles Fox, engineer, is very de- 

 cided on this point. He states positively that, if the rails 

 have been carefully laid over the portion of the line resting 

 upon the bridge, less deflection is caused in the girder by a 

 load passing at a high speed than at a low one, and that 

 there is less deflection with any rate of speed than when 

 the weight is stationary. " I imagine this arises, in a great 

 measure, from the short time there is to overcome the 

 inertia of the mass ; of course the higher the velocity the 

 less time is expended in the train passing over the bridge.^^ 



Mr. Glynn, of Butterly Iron Works, Derbyshire, thinks 

 that, if the strength of the beam were not great in propor- 

 tion to the stress it had to sustain, the weight, being sta- 

 tionary upon it, would tend to deflect it permanently more 

 than a weight passing rapidly over it. " This opinion is not 



