556 



by Professor W. Weber ; but, notwithstanding his known skill and 

 accuracy as an experimenter, his results did not command general 

 conviction. The author sketches the arguments that have been 

 urged against the inferences which M. Weber has drawn from his 

 experiments, and the conditions laid down by those who urged 

 these arguments, for the rigorous demonstration of diamagnetic 

 polarity. In the present paper these conditions are accepted and 

 fulfilled. 



To arrive at an exact notion of the value of M. Weber's experi- 

 ments, the author thought it best to operate with an instrument 

 similar to that used by M. Weber himself. He has to thank the 

 latter philosopher for the plan of an apparatus more delicate than 

 any which has been hitherto used, which plan was carried out in an 

 efficient manner by that able mechanician, Leyser of Leipzig. 

 The instrument consists essentially of two spirals of covered copper 

 wire, about eighteen inches long, firmly attached to a massive slab 

 of mahogany. The slab is attached by brass bolts to the solid 

 masonry of the Royal Institution, so as to have the spirals in a 

 vertical position. Above the spirals is a wooden wheel, with a 

 grooved periphery, and below them a similar one. The wheels are 

 united by an endless string, which communicates motion from one 

 of them to the other. To this string the cylinders submitted to 

 examination are attached ; and by turning the lower wheel with a 

 suitable key, the cylinders can be caused to move up and down 

 within the spirals. Two steel bar magnets are arranged astatically, 

 connected by a rigid brass junction, and so suspended that the 

 magnets are in a horizontal plane. The two magnets have the two 

 spirals between them, and have their poles opposite the centres of 

 the spirals. When therefore a current is sent through the spirals, 

 it exerts no more action upon the magnets than the central, or neu- 

 tral point of a magnet would do. If the bars within the spirals 

 be perfectly central, they also will present these neutral points to 

 the suspended magnets, and hence exert no action upon them. 

 But if the key be so turned that the two ends of the diamagnetic 

 bars shall act upon the magnets, then, if these bars be polar, the 

 intensity and character of their polarity will be indicated by the 

 deflections of the magnets. Here, then, we have not only the 

 action of the earth neutralized, but a turning force is brought to 



