94" On the Magnetism developed by Rotatio?i. 



the rotation of the earth's magnetic poles, and knowing at the 

 same time that Mr. Christie had found a permanent change in 

 the magnetic state of an iron plate by a mere change of posi- 

 tion on its axis, it seemed to him highly probable that this 

 change, due only to a simple inversion, would be increased by 

 rapid rotation. On trial, however, it was found that the effect 

 produced was merely temporary. The experiments at first 

 were made with a thirteen-inch mortar-shell fixed to the man- 

 drel of a powerful turning lathe, worked by a steam-engine in 

 the royal arsenal at Woolwich. 



This being made to revolve at the rate of 640 turns per mi- 

 nute, the needle was deflected out several degrees, and there 

 remained stationary during the motion of the ball, but re- 

 turned immediately to its original position on ceasing the ro- 

 tation. On inverting the motion of the shell, an equal and 

 contrary deflection took place. 



As the law of the phaenomena was not evident with this dis- 

 position of the apparatus, and the shell was found too heavy 

 for perfect safety, a Shrapnell shell of eight inch diameter was 

 mounted in a proper apparatus (described in the paper), and 

 a number of experiments made, the law of which, however, 

 still seemed anomalous, till the idea occurred of neutralizing 

 the earth's action on the needle; when the anomalies disap- 

 peared, and the general law of the effect was placed in evidence. 

 The needle being made a tangent to the ball, if the ball was made 

 to revolve towards the needle (whatever was the direction of the 

 axis of rotation), the north end of the latter was attracted, and 

 if the contrary way, repelled. In the two extremities of the 

 axis there was found no effect; while in two opposite points at 

 right angles to the axis, the effect was a maximum, and the 

 direction of the needle was to the centre of the ball. 



The author then proceeded to show how the results, which 

 before appeared anomalous, agree with this general view, and 

 closed his communication with some theoretical views of their 

 general bearing on the subjects of the earth's magnetism, 

 which he thought there were strong reasons for believing to be 

 of the induced kind; and although it appeared to him doubtful 

 whether the anomalies observed in the variation of the needle 

 on the earth's surface can ultimately be referred to this cause, 

 yet he observed that one condition essential to the pi'oduction 



in these papers has determined us to give in a connected form the official 

 abstracts of them, as read before the Royal Society, from the Quarterly 

 Journal of Science. We have added, to complete the series, Mr. Marsh's 

 account of Mr. Barlow's repetition of M. Arago's late experiments, from 

 the Edinburgh Philosophical Journal. 



