1 NOTES. 



terrestris." He ridiculed the loadstone mountains which TYacastoro, the great 

 contemporary of Columbus, supposed to constitute the magnetic polea " roji- 

 cicuda cst vulgaris opiuio de montibus magneticis aut rupe aliqua magnctica, 

 ant polo phaatastico a polo mundi distante." Ho assumed the declination of 

 the magnetic needle to be invariable at each point of the surface of the Earth 

 (" vavialio uniuscujusquc loci constans est") ; and explained the inflexions oi 

 the iso<_'onic linos by the form of the continents and the relative positions of 

 the sea-basins, which exercise a less degree of magnetic force than the solid 

 portions which rise above the le^el of the ocean (Gilbert de Magnete, ed. 1633, 

 p. 42, 98, 152, and 155). 



( u -) p. 16?. Gauss, Allgemdne Theorie des Erdmagnetismus, in den 

 Resultaten aus den Beob. des magnet. Vercins, 1838, 41, S. 56. Trans- 

 lated in " Taylor's Scientific Memoirs," Vol. ii. P. vi. Art. v. 



O p. 168. There are also perturbations which do not extend to such 

 great distances, and of which the causes are more local, and are seated, per- 

 haps, at less depths. I made known, some years ago, a rare instance of this 

 kind, in which the disturbance manifested itself in the Freiberg mines, but not 

 at Berlin. (Lettre de M. de Humboldt a S. A. R. le Due de Sussex sur le 

 moyens propres a perfectiouner la connaissance du magnet isme terrestre, 

 published in Becquercl's Trait e experimental de 1'electricite, T. vii. p. 442). 

 Magnetic storms, which were felt simultaneously from Sicily to Upsala, did 

 not extend from Upsala to Allen (Gauss and Weber, Resultate des Magnet. 

 Vereins, 1839, S. 128 ; Lloyd, in the Comptes-rendus de 1' Academic dea 

 Sciences, T. xiii. 1843, Sem. ii. pp. 725 and 827). Amongst the numerous 

 and recent examples of disturbances extending simultaneously over wide por- 

 tions of the earth's surface, which are assembled in Sabine's important work 

 (Observ. on Days of Unusual Magnetic Disturbance, 1843), one of the most 

 memorable is that of September 25, 1841, which was observed at Toronto in 

 Canada, at the Cape of Good Hope, at Prague, and partially at Van Dienien 

 Island. The English observatories suspend their operations on Sundays, and, 

 owing to the difference of longitude, the midnight of Saturday took place at 

 Van Dienien Island soon after the commencement of the disturbance, of which, 

 probably, the greater portion thus escaped observation. 



[The disturbance of Sept. 24th and 25th appears also to have extended to 

 Macao (Phil. Trans. 1843, p. 133) ; and it is worthy of remark that, from Sir 

 Edward Belcher's observations, it may be inferred that the intensity of the 

 horizontal force was increased at Macao at the same hours when it waa 

 diminished elsewhere. EDITOJI.] 



