258 Farther Observations on Experiments on the 



needle by the influence of iron balls in rotation, became the 

 subject of conversation at the Royal Society. Upon hearing 

 this result, Mr Babbage, on the authority of magnetical ex- 

 periments of a totally different kind, stated that he should have 

 expected it, and that the effect depended essentially on the 

 time that iron and steel took to receive and give up magne- 

 tism. This opinion was not received at the time as possess- 

 ing much. plausibility, but, on the following day, Mr Herschel 

 and himself tried Mr Barlow's experiment with the wheel of 

 one of Mr Babbage's turning lathes, and they found the 

 deviations to accord with Mr Babbage's expectations, and his 

 explanation to be satisfactory. It is of course manifest, that any 

 mass of iron becoming magnetic by induction, will, if moved 

 rapidly round, not immediately lose its axis of polarisation, 

 but this axis will continue to exist with diminished force for a 

 short time, and thus, during rotation, the virtual axis of mag- 

 netism will be deviated a little from its direction while at rest. 

 At this stage of the inquiry, a report reached London in the 

 spring of 1825, that M. Arago had produced rotation in a 

 needle by making a plate of copper revolve under it, and 

 that other metals, and even glass, wood, and other substan- 

 ces, produced the same effect. It was also reported, that when 

 the plates were cut by slits from the centre, no effects were pro- 

 duced.* It immediately occurred to Mr Babbage, that these 

 effects depended on the influence of time, but the statement re- 

 specting the effect of cutting seemed to present great difficulties. 

 His explanation, however, was, that the substances tried by 

 Arago were all slightly susceptible of magnetism (as Coulomb 

 had proved of the metals before,) that they acquired magnetism 

 more rapidly than they parted with it when once acquired, 

 and, consequently, that if a plate revolve under a needle, the 

 difference of the attractions on the two sides will cause the 

 needle to advance in the direction of the plate. 



At this time Mr Herschel proposed to Mr Babbage to enter 

 upon a course of experiments on this subject ; and the results 

 which they obtained have been published in the Philosophi- 

 cal T?-ansactions. 



* The latter part of this report was not quite correct- 



