224 



MEMOIE OF EATON HODGKINSON. 



labors of Mr. Hodgkinson liimself, wlio alone is responsible for their accuracy, 

 usefulness, and general adaptation to promote the ends of physical and engin- 

 eering science ; but there were other important results obtained by other mem- 

 Ders of the commission, to which it may not be deemed out of place to refer. 



The experiments at Portsmouth dockyard, conducted by Colonel Sir Henry 

 James, and the discussion of the results by Professor Willis and Professor 

 Stokes, were also the work of the commissioners. And it would be no easy task 

 to over-estimate the value of these labors, both on account of the novel nature 

 of the experiments and the mathematical deductions to which they conducted 

 when placed in the hands of Professor Stokes. 



Colonel Sir Henry James and Captain Galton subjected cast-iron bars, placed 

 between fixed supports, to 100,000 successive deflections, at the rate of four per 

 minute, by means of a cam. When the deflections were one-third of the ulti- 

 mate deflection, the bai:s were not weakened ; when, however, the deflections 

 were one-half of the ultimate deflection, the bars were broken with less than 

 900 depressions. 



Professor Hodgkinson subjected cast-iron bars, firmly fixed between supports, 

 to 4,000 continued impacts. When the blow was such as to deflect the bars 

 one-third of their ultimate deflection, they resisted the concussion of 4,000 

 impacts \vithout injury ; but when the blow was such as to deflect the bars one- 

 half of their ultimate deflection, no bar could resist 4,000 depressions. These 

 results strikingly confirm each other. 



Colonel James and Captain Galton caused a weight equal to one-half the 

 breaking Aveight of the cast-iron bar to be drawn backwards and forwards from 

 one end of the bar to the other. The bar was not weakened by 96,000 transits 

 of the weight. No perceptible effect was produced in wrought-iron bars by 

 10,000 successive deflections, each of which was equal to that produced by half 

 the breaking weight. 



Professor Hodgkinson notices the following results which he obtainied 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 ball were pro- 

 portional 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 

 Colonel Sir Henry James and Captain 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 supports, with 

 velocities varying from to 30 miles per hour. These experiments developed 

 the singi;lar 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 important fact was confirmed in all its entirety l)y the larger experiments 

 made Ity the conmiissioncrs on the Ewell bridge, on the Epsom line, and the 

 Godstone bridge, on the Southeastern line. 



Colonel James found that when a carriage was loaded with 1,120 pounds and 

 placed at rest upon a cast-iron bar, it produced a deflection of six-tenths of an 

 inch ; when, however, the carriage moved over the bar at the rate of 10 miles 

 per hour, the deflection was increased to eight-tenths of an inch ; when the speed 

 of the carriage was increased to 30 miles per hour the deflection was increased 

 to one inch and a half, which is more than double the statical deflection. It 

 follows from this that a much less weight will 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 

 bars, when broken by a load passing over them, were fractured at points l)eyond 

 their centres, often into four or five pieces, indicating the unusual strains to which 



