999 -^^of, Cumming on the Galvanoscope, 



planes. On dissolving and recrystallising 

 this salt, I obtained crystals resembling 

 fig. 2, and others much more reduced in 

 height ; sqme of these are so thin as to 

 leave scarcely a vestige of the planes M and 

 A, and several are hemitfopes, the plane of 

 imaginary section being parallel to P. I 

 have, therefore, been induced to consi- 

 der the primary form an oblique rhombic 

 prism* 



Fig. 1 represents the ordinary shape of 

 the crystals. 



PonM,orM' 108° 



P on e, or e' . . . , 129 



PonA 121 



M on M' 76 



M on A 128 



MonA; 141 



c on g' 79 



e on k 140 



[OOT, 



Fig. 2. 



Article X. 



Description of the Galvanoscope. By the Rev. J. Cumming, 

 MA. FRS. and Professor of Chemistry in the University of 

 Cambridge. 



(To the Editor of th^ Annals of Philosophy.) 



MY PEAR SIR, Camlridge, Sept. 17, I82S. 



I HAVE found the galvanoscope, mentioned in the note of my 

 last communication, so useful in detecting minute electromag- 

 netic action, that I wish it to be more generally known than it 

 seems to be at present ; you will, therefore, obhge me by insert- 

 ing an account of it in the next number of your Annals, 



The drawing and description (Plate XXIII), are taken from 

 the first volume of our Cambridge Transactions, with the addi- 

 tion of the mode of neutralizing the needle, which I find prefer- 

 able to what I then proposed. 



Its dehcacy is such as to show a deviation of from 20° to 30° 

 by the galvanic action of zinc and copper surfaces not exceeding 

 1-1 600th of an inch. Disks of one inch diameter moistened 

 with spring water, alcohol, or sulphuric ether, give nearly the 

 same deviation. Two wires of silver and platina, each 1-lOOth 

 inch diameter, and 3 inches long, twisted together at one end, 

 and heated by a spirit lamp, gave a deviation of 90°. 



