136 COSMOS. 



lination attained its minimum between 1843 and 1844, and 

 its maximum from 1848 to 1849. After the declination has 

 thus increased for five years, it again diminishes for a period 

 of equal length, as is proved by a series of exact horary ob- 

 servations, which go back as far as to a maximum in 1786^.* 

 In order to discover a general cause for such a periodicity in 

 all three elements of telluric magnetism, we are disposed to 

 refer it to cosmical influences. Such a connection is indeed 

 appreciable, according to Sabine's conjecture, in the altera- 

 tions which take place in the photosphere, that is to say, in 

 the luminous gaseous envelopes of the dark body of the sun-t 

 According to the investigations which were made throughout 

 a long series of years by Schwabe, the period of the greatest 

 and smallest frequency of the solar spots entirely coincides 

 with that which has been discovered in magnetic variations. 

 Sabine first drew attention to this coincidence in a memoir 

 which he laid before the Royal Society of London^ in March, 

 1852. "There can be no doubt," says Schwabe, in the re- 

 marks with which he has enriched the astronomical portion 

 of the present work, "that, at least from the year 1826 to 

 1850, there has been a recurring period of about ten years in 

 the appearance of the sun's spots, whose maxima fell in the 

 years 1828, 1837, and 1848, and the minima in the years 

 1833 and 1843."t The important influence exerted by the 

 sun's body, as a mass, upon terrestrial magnetism is con- 

 firmed by Sabine in the ingenious observation that the period 

 at which the intensity of the magnetic force is greatest, and 

 the direction of the needle most near to the vertical line, 

 falls, in both hemispheres, between the months of October 

 and February ; that is to say, precisely at the time when the 

 earth is nearest to the sun, and moves in its orbit with the? 

 greatest velocity.§ 



I have already treated, in the Picture of Nature, || of the 



period of magnetic declination which coincides with the frequency of 

 the solar spots must be estimated at H*l years. 



* See page 74. 



t Sabine, in the Phil. Transact, for 1852, pt. i., p. 103, 121. See 

 the observations made in July, 1852, by Rudolph Wolf, above referred 

 to in page 76 of the present volume ; also the very similar conjectures 

 of Gautier, which were published very nearly at the same time in the 

 Bibliotheque Universelle de Geneve, t. xx., p. 189. 



X Cosmos, vol. iv., p. 85-87. 



§ Sabine, in the Phil. Transact, for 1850, pt. i., p. 216. Faraday, 

 Exper. Researches on Electricity, 1851, p. 56, 73, 76, § 2891, 2949, 

 2958. 



II Cosmos, vol. i., p. 191 ; Poggend., Annalen, bd. xv., s. 334, 335; 



