TERRESTRIAL MAGNETISM. 135 



being 53 S.) ; and the Sun of course never passes through 

 its zenith. ( 166 ) We already possess several years of published 

 hourly observations, from which it appears that at this station, 

 almost precisely as at St. Helena, the needle from May to 

 September moves westward from its extreme eastern position 

 at 19^h. until 23-Ui.; and from October to March the 

 movement is, on the contrary, eastward from 204h. to l^h. 

 or 2h. The discovery of this phenomenon, of which the 

 existence is so well demonstrated, while at the same time 

 its causal connection is still veiled in such profound obscurity, 

 shows forcibly the importance of the system of hourly ob- 

 servations continued uninterruptedly for several years. 

 Disturbances which, as we shall presently see, deflect the 

 needle persistently, at one time to the west and at another 

 to the east, would prevent any well-assured inferences being 

 derived from isolated observations by travellers. 



The extension of navigation, and the use of the compass 

 in geodesical surveys, led very early to the remark of oc- 

 casional extraordinary disturbances in the direction, accom- 

 panied by fluctuating, starting, or tremulous movements, of 

 the needles employed. This used to be ascribed to a certain 

 condition of the particular needle : it was very characteris- 

 tically termed, in the French nautical language, "Taffolement 

 de 1'aiguille," and it was recommended that " une aiguille 

 affolee" should be magnetised afresh, and more powerfully. 

 Halley was the first who stated the Aurora to be a magnetic 

 phenomenon ( 167 ), on the occasion of his being asked by the 

 Royal Society of London, to explain the " great meteor" of 

 the 6th of March, 1716, seen over the whole of England, and 

 which, in his reply, he considered to be " analogous to that 

 to which Gassendi in 1621 had first given the name of Aurora 



