31 8 Captain Wilson's experiments in China and St Helena, 



grath Selb, and now in the possession of the Grand Duke of 

 Baden. He ascertained, however, that the hemiprismatic 

 gypsum-haloide agrees exactly in form with pharmacolite. He 

 saw distinct crystals of the latter, having the shape of Figure 

 4 of the paper above referred to. They are four-sided and 

 eight-sided prisms, with an inclined base, and exhibit the same 

 disproportionate enlargement of one of the faces of the prism^' 

 Like other species which consist of arsenic acid, lime, and 

 water, their origin depends on the oxidation of arsenical py- 

 rites, or other minerals containing arsenic, and on the reac- 

 tion of the arsenic acid so formed on calcareous spar. 



The crystallized specimens of pharmacolite from St Marie 

 aux mines, in Alsace, which have been lately found in con- 

 siderable number, do not possess any very distinct forms ; but 

 in some of the more regular crystals the faces peculiar to the 

 prismatic gypsum-haloide are perceptible. The single dis- 

 tinct cleavage, and the slight degree of flexibility of the lami- 

 nae, two characteristic properties of the same species, may like- 

 wise be observed. 



The third species was recognized by Mr Haidinger in se- 

 veral crystalline fragments in the Royal Museum of Berlin ; 

 but their locality is unknown. 



Art. XXXIV. — An Account of Magnetical Experiments 

 made in China and St Helena, with a view of determining 

 the Position of the Plarte of no deviation in those places. 

 By Captain J. P. Wilson of the H. E. I. C. Ship Hythe. 

 Communicated by Peter Barlow, Esq. F. R. S. Mem. 

 Imp. Ac. Petrop. &c. 



It is known, that, in the commencement of my experiments, I 

 had conceived (for the purpose of generalizing and simplifying 

 magnetic results) an ideal sphere to circumscribe the iron ball 

 or shell on which the experiments were performed ; and, ac- 

 cording to my deductions, it seemed to follow that this sphere 

 would occupy different positions on different parts of the 

 globe. I also expressed a hope that the truth of this deduc- 

 tion might be verified by experiments. To meet my views 



