POLAR LIGHT. 151 



Doskino, not far from Nishneinowgorod, and consequently 

 had advanced 24 45' westward in the course of 113 years. 

 Is the line of the Azores, which Christopher Columbus deter- 

 mined on the 13th of September, 1492, the same, which, 

 according to the observations of Davis and Keeling, in 1607, 

 passed through the Cape of Good Hope r" and is it identical 

 with the one which we designate as the Western Atlantic. 

 and which passes from the mouth of the river Amazon to 

 the sea- coast of North Carolina ? if it be, we are led to ask 

 what has become of the line of no variation which passed in 

 1600 through Konigsberg, in 1620 (?) through Copenhagen, 

 from 1657 to 1662 through London, and which did not, ac- 

 cording to Picard, reach Paris, notwithstanding its more 

 eastern longitude, until 1666, passing through Lisbon some- 

 what before 1668 ? 10C Those points of the earth at which 

 no secular progression has been observed for long periods of 

 time are especially worthy of our notice. Sir John Herschel 

 has already drawn attention to a corresponding long period 

 of cessation in Jamaica, 1 while Euler* and Barlow 3 refer to a 

 similar condition in Southern Australia. 



Polar Light. 



We have now treated fully of the three elements of ter- 

 restrial magnetism in the three principal types of its mani- 

 festation, namely, Intensity, Inclination and Declination, in 

 reference to the movements which depend upon geographical 

 relations of place, and diurnal and annual periods. The ex- 

 traordinary disturbances which were first observed in the dip, 

 are as Halley conjectured, and as Dufay and Hibrter recog- 

 nised, in part forerunners, and in part accompaniments of the 



99 Sabine, Magn. and Meteor. Observ. at the Cape of Good Hope, vol. i, 

 p. Ix. 



00 In judging of the approximate epochs of the crossing of the line of 

 no variation, and in endeavouring to decide upon the claim of priority 

 in this respect, we must bear in mind how readily an error of 1 may 

 have been made with the instruments and methods then in use. 



1 Cosmos, vol. i, p. 174. 



2 Euler, in the Mem. de I'Acad. de Berlin, 1757, p. 176. 



3 Barlow, in the Phil. Transact, for 1833, pt. ii, p. 671. Great un- 

 certainty prevails regarding the older magnetic observations of St. 

 Petersburg during the first half of the 18th century. The variation 

 seems to have been always 3 15' or 3 30' from 1726 to 1772 ! Hart- 

 eteen, Magnetismus der Erdc, s. 7, p. 143. 



