14 Captain Sabine's Experiments on the 



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

 made some years ago by Mr. Dollond for myself, for one of the 

 two which originally belonged to M. Hansteen, and which it 

 was my first intention to have sent to you. You will perceive, 

 by the memoranda accompanying the needles, that No. xx, 

 the one I have substituted, has remained perfectly steady in its 

 magnetism for a twelvemonth 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 vibra- 

 tions from 15' 46".l to 15' 52".7, since June, 1827, when 

 the last published observations were made with it. Phil. Trans. 

 1828, Art. 1, page 14. Consequently, its magnetism has 

 diminished, in two years, between one and two parts in one 

 hundred. It will be prudent, however, to treat both needles as 

 liable to further changes. 



Experiments on the Force of the Earth's Magnetism. By Capt. 

 Edward Sabine, R. A., Secretary of the Royal Society. 



[Communicated by the Author.] 



The preceding letter to Mr. Renwick, respecting M.Hansteen's 

 recent observations on the isodynamic magnetic curves in 

 Russia and Siberia, contains also a general notice of all exist- 

 ing observations on the same subject. Those of my own 

 making, referred to in that letter, were published in 1825, in 

 the volume containing the account of my Pendulum and other 

 Experiments made in 1822 and 1823. That account con- 

 tained the Dip of the Needle at nineteen stations, principally 

 in the Northern Hemisphere, and the times of vibration of 

 magnetic needles suspended horizontally, at the same stations. 

 The times of vibration were not corrected for the effect of dif- 

 ferences of temperature, arising from the widely different lati- 

 tudes at which the observations were made, nor were they 

 reduced to what the time would have been in infinitely small 

 arcs. It also appeared, on comparing the times of vibration 

 in London, before I left England and after my return, that the 

 needles had not all preserved their magnetism unimpaired 



