HORARY VARIATION. 81 



very early period. The most distinct intimation of this 

 relation was afforded by the discovery of horary variation, 

 although it had been obscurely perceived by Kepler, who, a 

 century before, had conjectured that all the axes of the plan- 

 ets were magnetically directed toward one portion of the uni- 

 verse. He says expressly, " that the sun may be a magnetic 

 body, and that on that account the force which impels the 

 planets may be centred in the sun."* The attraction of 

 masses and gravitation appeared at that time under the 

 semblance of magnetic attraction. Horrebow,| who did not 

 confound gravitation with magnetism, was the first who 

 called the process of light a perpetual northern light, pro- 

 duced in the solar atmosphere by means of magnetic forces. 

 Nearer our own times (and this difference of opinion is very 

 remarkable) two distinct views were promulgated in refer- 

 ence to the nature of the influence exerted by the sun. 



Some physicists, as Canton, Ampere, Christie, Lloyd, and 

 Airey, have assumed that the sun, without being itself mag- 

 netic, acts upon terrestrial magnetism merely by producing 

 changes of temperature, while others, as Coulomb, believed 

 the sun to be enveloped by a magnetic atmosphere,t which 

 exerts an action on terrestrial magnetism by distribution. 

 Although Faraday's splendid discovery of the paramagnetic 

 property of oxygen gas has removed the great difficulty of 

 having to assume, with Canton, that the temperature of the 

 solid crust of the earth and of the sea must be rapidly and 

 considerably elevated from the immediate effect of the sun's 

 transit through the meridian of the place, the perfi&t co-or- 

 dination and an ingenious analysis of all the measurements 

 and observations of General Sabine have yielded this result, 

 that the hitherto observed periodic variations of the magnetic 

 activity of the earth can not be based upon periodic changes 



* Kepler, in Stella Martis, p. 32-34 (and compare with it his treat- 

 ise, M'ysterium Cosmogr., cap. xx., p. 71). 



f Cosmos, vol. iv., p. 77, where, however, in consequence of an 

 error of the press, in the place of Basis Astronomies, we should read 

 Clavis Astronomies. The passage ( 226) in which the luminous pro- 

 cess of the sun is characterized as a perpetual northern light does not 

 occur in the first edition of the Clavis Astr., by Horrebow (Havn., 

 1730), but is only found in the second and enlarged new edition of 

 the work in Horrebow's Operum Mathematico-Physicorum, t. i., Havn., 

 1740, p. 317, as it belongs to this appended portion of the Clavis. 

 Compare with Horrebow's view the precisely similar views of Sir Will- 

 iam and Sir John Herschel (Cosmos, vol. iii., p. 34). 



t Mtmoires de Math&n. et de Phys. presentes cl I'Acad. Roy. des Sc., 

 t. ix., 1780, p. 262. 



D2 



