158 APPENDIX. 



the Phil. Trans, for 1823 it is shown that, hy reducing the 

 directive power of the needle by means of artificial magnets, 

 the daily variation may be increased from about 1^' to seve- 

 ral degrees, and the effect which was in this case only pro- 

 duced artifically, is, with such considerable dips, produced 

 naturally, as has been already sufficiently explained. It 

 will of course be seen that, notwithstanding the incapability 

 of the needle to take up a determined direction, its actual 

 magnetic strength was not diminished, and it M-as conse- 

 quently liable to be disturbed from any external cause act- 

 ing upon it. The recent science of electro-magnetism proves 

 the great disturbing power of electric currents ; and Mr. 

 Dalton, many years back, observed the influence of the 

 Aurora upon a magnetic needle, even in these latitudes, 

 where its directive power is considerable. It is therefore 

 by no means astonishing that, in latitudes where the Auroras 

 are stronger, and the directive power of the needle so much 

 less, than in England, the disturbance of the needle from 

 this cause should be so perceptible. The remark, that these 

 phenomena and motions in the needle were not observed till 

 a certain hour in the evening, although the sun had been 

 set some hours, may be ultimately of importance in tracing 

 out the connexion of these phenomena with each other, but 

 at present it appears to be inexplicable. 



The change of latitude, nearly 3°, is amply sufficient to 

 account for the diminished error here noticed. 



(m) It is obvious that, if the power of the vessel on the 

 needle were equal to that of the earth, in the most northern 

 part of the voyage, which has been shewn to be probable, 

 and the variation having been nearly nothing at the same 

 time, it would follow of course that, with the ship's 

 head at east, the compass would show n.e., as here stated; 

 but this error would diminish on the return to the south- 



