MAGNETISM. 615 



cnenon which no one would have guessed to be jonnected with terres- 

 trial magnetism, namely, the spots in the Sun. M. Schwabe, of 

 Dessau, had observed the Sun's disk with immense perseverance for 

 24 years : often examining it more than 300 days in the year ; and 

 had found that the spots had, as to their quantity and frequency, a 

 periodical character. The years of maximum are 1828, 1838, 1848, 

 in which there were respectively 225,* 282, 330 groups of spots. The 

 minimum years, 1833, 1843, had only 33 and 34 such groups. This 

 c-.urious fact 5 was first made public by M. de Humboldt, in the third 

 volume of his Kosmos (1850). The coincidence of the periods and 

 epochs of these two classes of facts, was pointed out by General 

 Sabine in a Memoir presented to the Royal Society in March, 1852. 



Of course it was natural to suppose, even before this discovery, that 

 the diurnal and annual inequalities of the magnetic element at each 

 place depend upon the action of the sun, in some way or other. 



Dr. Faraday had endeavored to point out how the effect of the solar 

 heat upon the atmosphere would, according to the known relations of 

 heat and magnetism, explain many of the phenomena. But this new 

 feature of the phenomena, their quinquennial increase and decrease, 

 makes us doubt whether such an explanation can really be the true 

 one. 



Of the secular changes in the magnetic elements, not much more is 



o o / 



known than was known some years ago. These changes go on, but 

 their laws are imperfectly known, and their causes not even conjec- 

 tured. M. Hansteen, in a recent memoir, 6 says that the decrease of 

 the inclination goes on progressively diminishing. With us this rate 

 of decrease appears to be at present nearly uniform. We cannot help 

 conjecturing that the sun, which has so plain a connexion with the 

 diurnal, annual, and occasional movements of the needle, must also 

 have some connexion with its secular movements. 



In 1840 the observations made at various places had to a great 

 extent enabled Gauss, in connexion with W. Weber, to apply his 

 Theory to the actual condition of the Earth ;' and he calculated the 

 Declination, Inclination, and Intensity at above 100 places, and found 



4 In 1837 there were 333. 



5 The observations up to 1844 were published in Poggendorfs Annalen. 



6 See K. Johnstone's Physical Atlas. 



T Atlas des Erdmagnetismus nach den Elementen der Theorie Enticorfen. Set 

 Preface. 



