Mr. Christie's Paper. — Mr. Barlow's Experments. 99 



by rotation. He corroborated by his own experience the re- 

 sults obtained by Mr. Herschel, in which a disc of copper was 

 set in rotation by the rotation of one or more magnets beneath 

 it, both in the case where poles of the same name were imme- 

 diately below the disc, and when of a contrary name. The 

 actions appeared equally intense in both cases ; and from this 

 circumstance, he concludes the magnetism thus communicated 

 to the copper to be extremely transient. The experiment was 

 varied by combining the revolving magnets differently, and 

 the results were stated. 



The next experiments of Mr. Christie were directed to the 

 determination of the law according to which the force dimi- 

 nishes as the distance between the disc and magnets increases. 

 It seems to follow from these experiments, that when a thick 

 copper plate is made to revolve under a small magnet, the 

 force tending to deviate the needle is directly as the velocity, 

 and inversely as the fourth power of the distance; but that 

 when magnets of considerable size are made to revolve under 

 thin copper discs, the diminution follows more nearly the 

 ratio of the inverse square of the distance, or between the 

 square and the cube, though not in any constant ratio of an 

 exact power. 



The author then investigated the law of force when copper 

 discs of different weights are set in rotation, which, for small 

 distances, appear proportioned to the weights of the discs, but 

 for smaller ones appear to vary in some higher ratio. 



Accouni of the Repetition qf_ M. Arago's Experiments on the 

 Magnetism developed during the Act of Rotation. By Messrs. 

 Barlow and Marsh. 



The experiments by Mr. Barlow on the magnetism imparted 

 by I'otation, described in p. 93, were begun in Dec. 1 824 ; and it 

 was not until April 1825 that he was informed of M. Arago's 

 rotative experiments on copper and other metals. " The latter 

 were not known in England," Mr. Marsh states, " until M. Gay- 

 Lussac's visit to London at the time above stated." — " I am 

 not aware," he continues, " of the precise nature of these ex- 

 periments; and shall, therefore, only endeavour to describe 

 those which I have assisted Mr. Barlow in making, and which 

 he founded on the description he had received : they may, 

 therefore, be considered as the experiments of M. Arago re- 

 peated, and varied as different circumstances occurred to sug- 

 gest new ideas. The account he had of M. Arago's experi- 

 ment, was that, by placing a copper plate upon a vertical spin- 

 dle, the plate being horizontal, and then placing just above it 

 a light compass needle, but independent, of course, of the 



N 2 plate; 



