156 Magnetism of the Earth. 



lively in meridians 120° on either side of the Hudson's bay maximum, 

 and 60° 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 longi- 

 tude, and easterly, decreasing, for the following 30° ; so that the va- 

 riation becomes again in, or about the meridian of 102° east, which 

 is that of the Siberian maximum. 



In the sincere hope that this letter may be instrumental in promo- 

 ting this highly curious and philosophical enquiry, which would be the 

 best return I can make to M. Hansteen for his kindness in giving me 

 so early and so full an account of the progress of his discoveries, 



I remain, my dear Sir, 



very faithfully yours, 

 Edward Sabine. 



P. S. Since I wrote the above I have substituted a needle made 

 by M. DoUond for myself, for one of the two which originally be- 

 longed to M. Hansteen, and which it was my first intention to have 

 sent you. You will perceive by the memoranda which accompany 

 the needles, that No. XX. (the one I have substituted,) has remain- 

 ed perfectly steady in its magnetism, for a twelve month past, and 

 will probably, therefore, continue so. No. XI. (which I received 

 from M. Hansteen three years ago,) has increased its time of making 

 300 vibrations, from 15' 46. F' to 15' 52.7'', since June 1827, when 

 the last published observations were made with it ; Phil. Trans. 1828, 

 Art. I. page 14 ; consequently its magnetism has diminished in two 

 years, between one and two parts in one hundred. It will be pru- 

 dent, however, to treat both needles as if liable to farther changes. 



