Magnetism of the Earth. 145 



transparency, and arc sometimes levelled upon their acute lateral 

 edges. 



4. Tabula?' Spa7' and Idocrase at Moriah, (A''. ,Y.) 



These minerals were found by Dr. Lewis Heermann and myself, 

 during a mineralogical excursion upon the western shores of Lake 

 Champlain. The Tabular Spar we found in loose blocks, in a wood 

 by the road side, about one mile west of Major Dallarby's. It oc- 

 curs in foliated prismatic masses of considerable dimensions, highly 

 translucent, having a greyish white color with a tinge of olive. In- 

 termingled with it is a brown mica, and more sparingly, scales of 

 Plumbago. We did not find it in place ; but, from the nature of 

 the rocks in the vicinity, we were disposed to consider it as coming 

 from the primitive limestone. The Idocrase is found near Maj. Dal- 

 larby's mills, and occurs among the rocks thrown out in forming a 

 mill-sluice. It is of a dark reddish brown color, and in masses of 

 considerable size, one of which we obtained, presenting several crys- 

 talline facets. It is associated with iron pyrites, calcareous spar, and 

 the delicate hair brown hornblende, which elsewhere in the United 

 States, so frequently accompanies the spinelle. 

 Yale College, Sept. 8th, 1829. 



Art. XVI. — Observations on the Magnetism of the Earth, especial- 

 ly of the Arctic Regions ; in a letter from Capt. Edioard Sabine, 

 to Professor Renwick. 



Communicated for this Journal. 



TO PROF. RENWICK. 



My dear Sir, — I received a few days ago, a letter from Professor 

 Hansteen, of Christiania, dated from Irkutsk, in Siberia, in April last. 

 M. Hansteen is travelling, as you know,, at the expense of his King, 

 and witli the permission of the Emperor of Russia, for the purpose 

 of observing the Magnetic Dip, Variation, and Intensity, over the 

 whole of the north of Europe, and of Asia ; and of comparing the 

 actual phenomena with the system of terrestrial magnetism, propound- 

 ed by himself, in his celebrated treatise, entitled " Magnetismus der 

 Erde." 



Vol. XVIL— No. L 19 



