[ 25'.' j 



LVII. On the Cause of the Magnetic Power of the Poles of the 

 Earth. By William Dobbie. 



To the Editors of the Philosophical Magazine and Journal. 



Glasgow, Feb. 30, m2o. 

 '' j^HE followiiig communication on the discovery of the cause 

 -■- of the magnetic power of the poles of the earth is at your 

 service, if j'xju judge it worthy of a place in the Philosophical 

 Magazine. 



In an Essay on the aurora horealis inserted in the Number 

 for September 1820, one circumstance relating to those ph«- 

 nomena is unnoticed, being either unknown or not thought of 

 by me at the time (1816) that it was first written. The fact 

 alluded to is the agitation of the iriagnetic needle during the 

 exhibition of those lights ; which fact, I presume, is established 

 by several observers, and has induced such to consider mag- 

 netism as the cause of those appearances. The above fact, 

 I confess, if it had occurred to me at the time, I could not have 

 accounted for on the principle I assumed ; but that circum- 

 stance would not have shaken my confidence as to the general 

 cause, it being too manifest to be affected by one fact unex- 

 plained. 



A curious discovery made by Professor Moriccheni of 

 Rome, probably about the same time the above-mentioned 

 f aper was written, throws much unexpected light not only on 

 this subject, but on other still more interesting phasnomena. 

 The fact discovered is, that the violet rays of the prismatic 

 spectrum possess a magnetizing power. This extraordinary 

 discovery clearly points out the connexion between the polar 

 lights and magnetism ; and also lays open the mystery of the 

 earth's magnetism, together with the deviation of the magnetic 

 from the true meridian, as I doubt not I shall be able satis- 

 factorily to explain. 



It will be recollected by those who have read the essay re- 

 ferred to, that the aurorce are explained as owing tlieir ex- 

 istence to the vast accumulation of ice around the poles re- 

 flecting the rays of the sun chiefly about the time of the equi- 

 noxes. 



The Marquis Ridolfi, in making experiments on the abo^'« 

 Professor's discov«;ry, found that a steel needle was soon com- 

 pletely magnetized by the violet rays sepai-ated by a prism, 

 and concentrated by a lens. If it is considered that a great 

 part of the space within the arctic circle is studded with prisms 

 or masses of ice, adapted for decomposing the rays of the sun 



to 



