Messrs. Babbage's and Herschel's Paper. 97 



tion of the point where the deviation clue to rotation vanishes, 

 and suggesting partial magnetism in the iron plate used as 

 one mode of accounting for the difference. At all events, by 

 an examination of the case on the ordinary supposition of in- 

 duced magnetism in the iron, he showed that a greater coinci- 

 dence between theory and fact would not result from that hy- 

 pothesis than from the one here employed. 



He then proceeded to inquire into the degree of perma- 

 nence of the polarity thus produced in iron by rotation ; from 

 which inquiry it appeared that (at least during 12 hours after 

 the plate was brought to rest) the influence of a single rota- 

 tion had scarcely suffered any diminution. It appeared also 

 that the effect is so far from depending on the rapidity of the 

 motion, that the plate can scarcely be made to revolve so 

 slowly as that the whole effect shall not be produced. 



Lastly, by a slight change in the formulae, the results of 

 computation, it is found, can be made to agree with observa- 

 tion to a degree of exactness as near as can be wished. This 

 change consists in the omission of certain terms introduced by 

 the theory, and the author regards it as very possible so to 

 modify the theory as to get rid of them. 



The author closed this communication with an appendix 

 comparing the magnetic effects produced by slow and rapid 

 rotation. The result of the comparison was, that the forces 

 exerted on the needle during rapid rotation were always in the 

 same direction as those derived from the slowest rotation, and 

 which continue to act after the rotation has ceased, but were 

 greater in intensity, and that the former effects were such as 

 might have been looked for from a knowledge of the latter. 



An Account of the Repetition of M. Arago's Exp)eriments on 

 the Magnetism developed diiring the Act of Rotation. By 

 Charles Babbage, Esq. F.R S., and J. F. Herschel, Esq. 

 Sec. R.S. 



The experiments of M. Arago having excited much interest, 

 the authors of this communication were induced to erect an 

 apparatus for their verification; and after a few trials, they 

 succeeded in causing a compass to deviate from the magnetic 

 meridian, by setting in rotation under it plates of copper, zinc, 

 lead, &c. 



To obtain more visible and regular effects, however, they 

 found it necessary to reverse the experiment, by setting in ro- 

 tation a powerful horse-shoe magnet, and suspending over it 

 the various metals, and other substances to be examined, which 

 were found to follow with various degrees of readiness the mo- 

 lion of the magnet. The substances in which they succeeded 

 Vol. 66-. No. 328. y///''. 1825. N ' in 



