Force of the Earth's Magnetism* 27 



The dips which I have employed are those which I observed 

 myself, except at Madeira, where an accident prevented my 

 using the needle on which I could place most reliance ; and I 

 have reason to think that the needle I did use made the dip a 

 few minutes too great ; I have therefore taken it at 20' less. 



There are two other series of observations on the magnetic 

 intensity of principal note : one made by M. de Humboldt, 

 consisting chiefly of observations during his celebrated voyage 

 to the equinoctial parts of the American continent, at the 

 close of the last and beginning of the present century, twenty- 

 four years before mine ; the other made by M. Hansteen, and 

 gentlemen who have used instruments prepared by him, in the 

 northern parts of the old continent. These latter are either 

 cotemporaneous or subsequent to mine. 



M. de Humboldt's series includes two stations, at which I 

 have also observed, and which are contained in the preceding 

 list. M. Hansteen's series also includes two of the stations 

 visited by me. It is desirable to examine how far the obser- 

 vations correspond. In order to do this, we must first have an 

 accurate comparison of the magnetic intensity at the stations 

 which have served as bases to the respective series. M. de 

 Humboldt's observations were made relatively to the intensity 

 at Paris ; M. Hansteen's, to that at Christiana; and mine, to 

 that at London. In the Philosophical Transactions for 1828, 

 I have given an account of the comparison of the intensities at 

 London and Paris, effected by means of six needles frequently 

 interchanged between the stations ; and the intensity at Chris- 

 tiana and London has since been compared by M. Hansteen 



