MEMOIR OF THE LATE PROF. E, HODGKINSON, F.R.S. 193 



specting the deflection of the bridges was necessary. " You 

 will sometimes find/^ he adds^ " an exceptional case occurs, 

 if the engine happen to jump on the springs, which may, 

 of course, accidentally occur ; but if it be a mere question 

 of velocity, I do not think it increases the strain upon the 

 girder. There may be a lateral strain backwards and for- 

 wards when the whole train comes into play and causes 

 a jerk.^^ 



Mr. Locke, after making many experiments with loco- 

 motives passing over bridges, arrives at the conclusion 

 that there is but little difference in the deflection between 

 high velocities and low. " If there be,^^ he remarks, " three 

 or four bad rails or joints upon the top of a bridge, there 

 is far more effect produced upon the bridge. A bad joint 

 is more serious than lo or 12 miles^ increase or diminution 

 of velocity." 



Mr. Hawkshaw's opinion is, that there would be a greater 

 deflection in a bridge by running a weight over it, than by 

 allowing the same weight to rest upon it — because there 

 is always an irregularity in the surface of the rails, and the 

 force of impact is thereby brought into activity. W. H. 

 Barlow stood under a wooden viaduct while a heavy goods 

 train passed over it. There was a slight deflection pro- 

 duced by the heavy train ; but the express, with a much 

 lighter engine, and moving at a greater speed, produced a 

 much worse effect. It seemed to produce a wave through 

 the bridge, as it ought to do from the ordinary principles 

 of dynamics. This load was passing over the bridge in a 

 very few seconds, and therefore the total deflection is per- 

 formed by the weight in a few seconds ; and it therefore be- 

 comes a kind of blow — the descent of a heavy weight — and 

 the bridge has not time to accommodate itself to the de- 

 flections required of it. These deflections are propagated 

 throughout the structure, and may prove exceedingly dan- 

 gerous and disagreeable. 



SER. III. VOL. II. o 



