184 ' Messrs, Babbage and Herschel on the [Sept. 



might not beset in rotation if freely suspended over a revolving 

 magnet. In order to make this experiment, we mounted a 

 powerful compound horse-shoe magnet, capable of lifting 20 

 pounds, in such a manner as to receive a rapid rotation about 

 Its axis of symmetry placed vertically, the line joining the poles 

 being horizontal and the poles upwards. A circular disc of 

 copper, 6 inches in diameter and 0*05 inch thick, was suspended 

 centrally over it by a silk thread without torsion, iust capable of 

 supportmg it. A sheet of paper properly stretched was inter- 

 posed, and no sooner was the magnet set in rotation than the 

 copper commenced revolving in the same direction, at first 

 slowly, but with a velocity gradually and steadily accelerating. 

 The motion of the magnet being reversed, the velocity of the 

 copper was gradually destroyed ; it rested for an instant, and 

 then immediately commenced revolving in the opposite direc- 

 tion, and so on alternately, as often as we pleased. 



The rotation of the copper being performed with great regu- 

 larity, it was evident that by noting the times of successive 

 revolutions, we should acquire a precise and delicate measure of 

 the intensity of the force urging it, provided we took care to 

 neutralize the torsion of the suspending thread. To make the 

 experiment strictly comparable proved however a matter of much 

 delicacy, as the slightest change in the distance of the plate 

 from the magnet was found to produce a material alteration in 

 the time of its gyration. 



Our first inquiry was directed to ascertain the effect of the 

 interposition of difF<=*rent bodies as screens in cutting off or 

 modifying the peculiar rotatory effect. The substances tried 

 were paper, glass, wood, copper, tin, zinc, lead, bismuth, anti- 

 mony, and tinned iron plate. 



The metallic plates here interposed, as also the wooden ones, 

 were circular discs of 10 inches in diameter and half an inch in 

 thickness, the metals being all cast for the purpose, the wooden 

 disc serving for a pattern. It was found that the various sub- 

 stances examined exert no sensible interceptive power. Glass 

 in like manner had no effect; but when the substance inter- 

 posed was iron, the case was widely different, the magnetic 

 influence being greatly diminished by one, and almost annihi- 

 lated by two thicknesses of common tinned iron plate. When 

 the poles of the revolving magnet were connected by a piece of 

 soft iron, the rotation of the copper disc was in like manner 

 almost entirely annihilated. 



Resuming now the original form of the experiment, the copper 

 disc of 10 inches diameter and half an inch thick, was placed 

 on the vertical axis, and made to revolve with a velocity of 

 seven turns in a second, a velocity which it was found conve- 

 nient to give, and easy to maintain, corresponding as it did with 



