HORARY VARIATION. 81 



the variable numerical relations of space, as well as to extend 

 and complete that mathematical theory of terrestrial mag- 

 netism, which was so nobly sketched by Gauss, is to pro- 

 secute simultaneous and continuous observations of all the 

 three elements of the magnetic force at numerous well se- 

 lected points of the earth's surface. I have, however, else- 

 where illustrated by example the sanguine hopes which I 

 entertained of the great advantages that may be derived from 

 the combination of experimental and mathematical investi- 

 gation. 7 * 



Nothing that occurs upon our planet can be supposed to 

 be independent of cosmical influences. The word planet 

 instinctively -leads us to the idea of dependence upon a 

 central body, and of a connection with a group of celestial, 

 bodies of very different masses which probably have a. similar 

 origin. The influence of the sun's position upon the mani- 

 festation of the magnetic force of the earth, was recognised 

 at a 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 planets 

 were magnetically directed towards one portion of the uni- 

 Terse. 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." 77 The attraction of masses 

 and gravitation appeared at that time under the semblance 

 of magnetic attraction. Horrebow. 78 who did not confound 

 gravitation with magnetism, was the first who called the 

 process of light a perpetual northern light, produced in the 

 solar atmosphere by means of magnetic forces. Nearer our 



76 See p. 6. 



77 Kepler, in Stella Martis, pp. 32 34 (and compare with it his 

 treatise, Mysterium Cosmogr. cap. xx, p. 71). 



78 Cosmos, vol. iv, p. 386, where, however, in consequence of an 

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

 Clavis Astronomic^. The passage ( 226) in which the luminous process 

 of the sun is characterised 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 Malhematico-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 William and Sir 

 John Herschel (Cosmos, voL iii, pp. 39, 40). 



VOL. V. Q 



