MEMOIR OF THE LATE PROF. E. HODGKINSON^ F.R.S. 191 



Prof. Hodgkinson notices tlie following results, which 

 he obtained from his experiments on the impact of cast- 

 iron bars : — 



All cast-iron bars of the same sectional area require the 

 same blow to break them in the middle. 



The deflections of wrought-iron bars produced by the 

 striking baU were proportional to the velocity of impact ; 

 but in cast-iron bars the deflections were greater than the 

 proportion to the velocity of impact. 



The most striking and novel experiments, however, were 

 those made by Col. Sir Henry James and Capt. Galton at 

 Portsmouth Dockyard. These gentlemen constructed a 

 large apparatus by which weights could be made to move 

 over cast-iron beams placed horizontally between fixed sup- 

 ports, with velocities varying from o to 30 miles per hour. 

 These experiments developed the singular fact, at variance 

 with the impressions of the most eminent engineers, that 

 a train passing over a bridge at a given speed will produce 

 a greater deflection than that produced by the train being 

 placed upon the bridge in a state of repose. This im- 

 portant fact was confirmed in all its entirety by the larger 

 experiments made by the Commissioners on the Ewell 

 Bridge, on the Epsom line, and the Godstone Bridge, on 

 the South-eastern line. 



Col. James found that when a carriage was loaded with 

 1 1 20 lbs. and placed at rest upon a cast-iron bar, it pro- 

 duced a deflection of six- tenths of an inch ; when, how- 

 ever, the carriage moved over the bar at the rate of ten 

 miles per hour, the deflection was increased to eight-tenths 

 of an inch ; when the speed of the carriage was increased 

 to thirty miles per hour, the deflection was increased to 

 one inch and a half, which is more than double the stati- 

 cal deflection. It follows from this that a much less weight 

 wiU break a bar of cast iron when it moves over it at a 

 great speed, than if it be placed at rest upon the bar. The 



