Observations in Siberia. 13 



Christiania, as their base. The needles should be kept apart 

 from each other, and from contact with iron, and particularly 

 with magnetised iron. 



I do not attempt in this letter to enter at any length on the 

 consideration of the curves of Dip and of Variation. M. Han- 

 steen has shewn, in the treatise already named, the general 

 conformity of these phenomena to such an arrangement of 

 magnetic attraction, as is indicated by the course of the isody- 

 namic curves. His observations in Siberia, in as far as they 

 go, confirm this view. Thus, for example, in the parallel of 

 55° north, the Dip, which in tracing the parallel to the east- 

 ward progressively decreases from Labrador, where it exceeds 

 80°, is found by M. Hansteen to attain a minimum of 67°J, 

 about the 42nd degree of longitude east of Greenwich ; from 

 thence it increases, until the intersection of the parallel with 

 the meridian of the Siberian maximum of intensity (102° E.), 

 when its amount is 70°|: from that meridian it again decreases 

 to a second minimum, by the observations of Russian officers, 

 in the meridian of Kamtchatka (163° east). Hence, as regards 

 the dip in the parallel of 55° N., there are two points of maxi- 

 mum and two of minimum ; those of maximum are in the same 

 geographical meridians, or nearly so, as the points of maximum 

 intensity ; and those of minimum occur respectively in meri- 

 dians 120° on either side of the Hudson's bay maximum, and 

 G0° on either side of the Siberian maximum. In like manner, 

 the variation, in the 55th parallel, is in the longitude of the 

 minimum of Dip, 42° east ; is, easterly increasing, for the next 

 30° of longitude, and easterly decreasing, for the following 30° ; 

 so that the variation becomes again in or about the meridian 

 of 102 east, which is that of the Siberian maximum of inten- 

 sity. 



In the sincere hope that this letter may be instrumental in 

 promoting this highly curious and philosophical enquiry, which 

 would be the best return I can make to M. Hansteen for his kind- 

 ness in giving me so early and so full an account of the progress 

 of his discoveries, 



I remain, my dear Sir, 



Very faithfully your's, 



Edward Sabine. 



