MECHANICS AND USEFUL ARTS. 21 



more crystalline fracture than the same iron warmed a little. The 

 same effects are by some supposed to be extended to cast-iron. 



" We have endeavoured to examine this question experimentally in 

 various ways. A bar of cast-iron, 3 inches square, was placed on 

 supports about 14 feet asunder. A heavy ball was suspended by a 

 wire 18 feet long-, from the roof, so as to touch the centre of the side 

 of the bar. By drawing this ball out of the vertical position at right 

 angles to the length of the bar in the manner of a pendulum, to any 

 required distance, and suddenly releasing it, it could be made to strike 

 a horizontal blow upon the bar, the magnitude of which could be ad- 

 justed at pleasure, either by varying the size of the ball or the distance 

 from which it was released. Various bars (some of smaller size than 

 the above) were subjected by means of this apparatus to successions 

 of blows, numbering in most cases as many as 4,000, the magnitude 

 of the blow in each set of experiments being made greater or smaller 

 as occasion required. The general result obtained was, that when 

 the blow was powerful enough to bend the bars through one half of 

 their ultimate deflection (that is to say, the deflection which corre- 

 sponds to their fracture by dead pressure), no bar was able to stand 

 4,000 of such blows in succession ; but all the bars, when sound, re- 

 sisted the effects of 4,000 blows, each bending them through one third 

 of their ultimate deflection. 



" Other cast-iron bars, of similar dimensions, were subjected to the 

 action of a revolving cam, driven by a steam-engine. By this they 

 were quietly depressed in the centre, and allowed to restore them- 

 selves, the process being continued to the extent even, in some cases, 

 of 100,000 successive periodic depressions for each bar, at a rate of 

 about four per minute. Another contrivance was tried, by which the 

 whole bar was also, during the depression, thrown into a violent 

 tremor. The results of these experiments were, that when the de- 

 pression was equal to one third of the ultimate deflection, the bars 

 were not weakened. This was ascertained by breaking them in the 

 usual manner with stationary loads in the centre. When, however, 

 the depressions produced by the machine were made equal to one half 

 of the ultimate deflection, the bars were actually broken by less than 

 900 depressions. This result corresponds with and confirms the for- 

 mer. By other machinery, a weight equal to one half of the breaking 

 weight was slowly and continually dragged backwards and forwards 

 from one end to the other of a bar of similar dimensions to the above. 

 A sound bar was not apparently weakened by 96,000 transits of the 

 weight. i 



" It may on the whole, therefore, be said, that, as far as the effects 

 of reiterated flexure are concerned, cast-iron beams should be so pro- 

 portioned as scarcely to suffer a deflection of one third of their ultimate 

 deflection. And, as it will presently appear that the deflection pro- 

 duced by a given load, if laid on the beam at rest, is liable to be con- 

 siderably increased by the effect of percussion, as well as by motion 

 imparted to the lond, it follows, that to allow the greatest load to be 

 one sixth of the brenking weight is hardly a sufficient limit for safety, 

 even upon the supposition that the beam is perfectly sound. 



