POLAR LIGHT, OR AURORA. 153 



which we now see in the western part of the Atlantic 

 Ocean, passing from the mouth of the Amazons to the 

 coast of North Carolina ; we are led to ask what has 

 become of the line of no declination which passed in 

 1600 through Konigsberg; in 1620? through Copen- 

 hagen; in 1657 1662 through London (and yet, ac- 

 cording to Bicard, only in 1666 through Paris, which is 

 to the east of London) ; and rather before 1668 through 

 Lisbon ? ( 20 ) There are particular parts of the globe 

 in which, throughout a long interval of time, no secular 

 change has been remarked. Sir John Herschel has 

 called attention ( 201 ) to such a long period of suspension 

 of change in Jamaica, as Euler ( 202 ) and Barlow ( 203 ) had 

 already done to a similar one in South Australia. 



Polar Light, or Aurora. 



I have treated in detail the three elements of Ter- 

 restrial Magnetism, or the magnetic "Declination," "In- 

 clination," and "Force," in their connection with 

 geographical position, and their variations according to 

 hour and season. The " extraordinary disturbances," or 

 perturbations, which were first observed in the decli- 

 nation, are, as Halley conjectured, and as Dufay and 

 Hiorter recognised, partly the precursors and partly the 

 accompaniments of the magnetic polar light, or aurora 

 borealis or australis. In the "Eepresentation of Nature" 

 contained in the first volume of my work, I have dis- 

 cussed with some degree of fullness the peculiarities 

 belonging to this " telluric luminous process," often so 

 remarkable by its beautiful display of colours; and 



