136 TERRESTRIAL MAGNETISM. 



borealis." We know from his own statement that prior to 

 1716 Halley had never seen either a northern or a southern 

 Aurora ; although, in his voyages for determining the decli- 

 nation lines, he had advanced to the 52nd degree of south 

 latitude : and southern Auroras are assuredly sometimes 

 seen within the tropics, as in Peru. It would appear, there- 

 fore, that Halley had never himself observed the remarkable 

 disturbances and fluctuations of the needle in conjunction 

 with seen (or unseen) northern or southern Auroras. 

 Olav Hiorter and Celsius, at Upsala, were the first who, 

 in 1741 (before Halley 's death), confirmed by a long 

 series of observations the connection between a visible 

 Aurora and disturbance from the normal march of the 

 magnetic needle, which was only conjectured by Halley. 

 This meritorious undertaking gave occasion to the first 

 concerted simultaneous observations in conjunction with 

 Graham in London : extraordinary disturbances of the de- 

 clination accompanying displays of Aurora were examined 

 specially by Wargentin, Canton, and Wilke. 



Observations which I had the opportunity of making 

 together with Gay-Lussac in 1805, at Rome, on the Monte 

 Pincio, and more particularly a more extensive series which 

 I was thereby led to undertake in conjunction with Oltmanns, 

 at the equinoxes and solstices of 1806 and 1807, in a large 

 and well-detached garden at Berlin (employing a magnetic 

 telescope of Prony's, and a distant point of reference well illu- 

 minated by a lamp), made me early aware that that part of 

 the telluric magnetic activity described under the general name 

 of " extraordinary disturbances," which acts so powerfully 

 at particular periods and in no merely local manner, deserved, 

 and from its complicated character required, a persistent 



