578 SPECIAL METHODS OF DISCOVERY. 



absence of magnets that Fisher, in 1818, discovered that 

 their rate was affected by the proximity of a magnet. In 

 1820 Brewster, by comparing, discovered the coincidence 

 of position between the geographical points of maximum 

 cold and the terrestrial magnetic poles. Kreil, of Prague, 

 in 1839, and Plantamour, of Geneva, in 1842, discovered, 

 by comparison of the positions of the moon with the 

 movements of the magnetic needle, that the former in- 

 fluenced the magnetic declination. 



Comparisons of coincident phenomena and observations 

 often render manifest new truths. It was observed by 

 Mr. Carrington and by Mr. Hodgson, at different places 

 simultaneously, that a very bright spot suddenly appeared 

 upon a particular part of the sun's surface, and lasted 

 about five minutes ; other persons remarked that the 

 self-recording magnetic needles at Kew Observatory were 

 strongly affected at precisely the same moment ; and it 

 was further observed that strong electric currents were 

 produced at the same time in the telegraph wires nearly 

 all over the earth all these circumstances indicating a 

 terrestrial magnetic storm coincident with the solar out- 

 burst, although the distance between the sun and the 

 earth is about ninety-two millions of miles. It was whilst 

 endeavouring to obtain a fixed measure of the positions of 

 lines in luminous spectra, by placing two spectra together 

 and comparing them, that Kirch off and Bunsen discovered 

 that the black line D in the solar spectrum exactly 

 coincided in position with the bright yellow arc of sodium ; 

 and, by passing sunlight through the flame or heated 

 vapour of sodium, Kirchoff found that the dark line D was 

 blacker than before, and thus was led to conclude that the 

 black line D in the solar spectrum was caused by the white 

 light of the sun having passed through vapour of sodium 

 in the solar atmosphere, and thus had its yellow rays 



