170 COSMOS. 



variability, is ascribed either to the heated mass of the Eartu 

 itself,* or to those galvanic currents which we consider as 

 electricity in motion, that is, electricity moving in a closed 

 circuit, f 



The mysterious course of the magnetic needle is equally 

 affected by time and space, by the sun's course, and by changes 

 of place on the earth's surface. Between the tropics, the 

 hour of the day may be known by the direction of the needle 

 as well a-s by the oscillations of the barometer. It is affected 

 instantly, but only transiently, by the distant northern light 

 as it shoots from the pole, flashing in beams of coloured light 

 across the heavens. When the uniform horary motion of the 

 needle is disturbed by a magnetic storm, the perturbation 

 manifests itself simultaneously, in the strictest sense of the 

 word, over hundreds and thousands of miles sea and land, or 

 propagates itself by degrees in short intervals of time in every 

 direction over the earth's surface. :f In the former case, the 



* William Gilbert, of Colchester, whom Galileo pronounced " great to 

 a degree that might be envied," said " magnus magnes ipse est globus 

 terrestris." He ridicules the magnetic mountains of Prascatori, the great 

 contemporary of Columbus, as being magnetic poles : " rejicienda est 

 vulgaris opinio de montibus magneticis, aut rupe aliqua magnetica, aut 

 polo phantastico a polo mundi distante." He assumes the declination of 

 the magnetic needle at any given point on the surface of the Earth 

 to be invariable (variatio uniuscujusque loci constans est), and refers 

 the curvatures of the isogonic lines to the configuration of continents 

 and the relative positions of sea basins, which possess a weaker magnetic 

 force than the solid masses rising above the ocean. (Gilbert, de Magnete, 

 ed. 1633, pp. 42, 98, 152, and 155.) 



t Gauss, Allgemeine Thcorie des Erdmagnetismus, in the Resultale 

 aus den Beob des magnet. Vereins, 1838, s. 41, p. 56. 



There are also perturbations which are of a local character, and do 

 not extend themselves far, and are probably less deep-seated. Some 

 years ago I described a rare instance of this kind, in which an extra- 

 ordinary disturbance was felt in the mines at Freiberg, but was not 

 perceptible at Berlin. (Lettre de M. de Humboldt a Son Altesse Royale 

 le Due de Sussex sur les moyens propres a perfectionner la connaissance 

 du Magnetisme Terrestre, in Becquerel's, Traite experimental de I'Elec- 

 tricite, t. vii. p. 442.) Magnetic storms, which were simultaneously felt 

 from Sicily to Upsala, did not extend from Dpsala to Alten. (Gauss and 

 Weber, Rexultate des magnet. Vereins, 1839, 128; Lloyd, in the 

 Comptes Rendus de I'Acad. des Sciences, t. xiii. 1843, Sem. ii. p. 725 and 

 827.) Amongst the numerous examples that have been recently observed, 

 of perturbations occurring simultaneously and extending over wida 



