Notices respecting New Booh. 279 



it at pleasure. When in that position, the index line / e ap- 

 pears from behind the screen. The first part of the experi- 

 ment is made without the presence of the magnet M. When 

 the trigger is pulled, the bob falls from the point o, and per- 

 forms a certain number of vibrations before the total disap- 

 pearance of the index line I e. This number being noted, the 

 bob is again brought to the drop at o. The magnet is now 

 placed on the stage, with its poles close to the surface of the 

 disc. (The figure represents the magnet in such a position that 

 the disc may vibrate between its poles.) The trigger is again 

 pulled, and the vibrations are counted till the index line is 

 again lost sight of behind the screen. 



With a thin copper disc, eight inches in diameter, vibrating 

 between the poles of a horse-shoe magnet, the mean of six 

 trials without the magnet, and of six with the magnet, were 

 as below : 



Experiment 8. — Without the magnet... 1 GO vibrations. 

 W^ith the magnet 60 vibrations. 



Experiment 9. — With a similar disc of zinc: 



Without the magnet... 150 vibrations. 

 With the magnet 60 vibrations. 



It would appear by these two experiments, that copper 

 is more affected by the presence of the magnet than zinc. 

 This, however, is not always the case, for I have fi-equently 

 found discs of zinc more affected than copper. There is also 

 a difference even in the same kind of metal : and since a great 

 deal depends upon the power of the magnet, as well as upon 

 the character of the metal, it is plain that in all experiments 

 of this kind, especially when intended for comparing the re- 

 sults with different metals, the same magnet ought to be em- 

 ployed, and the experiments made whilst it has the same 

 standard power. With a very powerful magnet I have fre- 

 quently reduced the number of vibrations from 150 to 30, and 

 in some cases still lower. 



[To be continued.] 



XL. Notices respecting New Booh. 



South- African Quarterly Journal. No. III. From April to July, 1830. 

 Edited at the South- AJrican Institution, Cape Town, 1830. 8vo. 

 pp. 225 to 344.. 



WE have just received this Third Number of the South-African 

 Journal, of which the two preceding Numbers were noticed 

 in the Phil. Mag. and Annals for September 1830, under the head 

 of South-African Institution. It may be obtained, in London, of 



Mr. 



