84 



COSMOS. 



1826 to 1850), a periodically-varying frequency in the oc- 

 currence of the solar spots, showing that their maxima fell 

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

 years 1833 and 1843. "I have not had the opportunity," 

 he writes, " of investigating a continuous series of older ob- 

 servations, but I willingly subscribe to the opinion that this 

 period may itself be variable." A somewhat analogous kind 

 of variability — periods ivithin periods — is undoubtedly observ- 

 able in the processes of light of other self-luminous suns. I 

 need here only refer to those complicated changes of intensi- 

 ty which have been shown by Goodricke and Argelander to 

 exist in the light of /3 Lyrae and Mira Ceti.* 



If, as Sabine has shown, the magnetism of the sun is 

 manifested by an increase in the terrestrial force when the 

 earth is nearest to that luminary, it is the more striking 

 that, according to Kreil's very thorough investigations of the 

 magnetic influence of the moon, the latter should hitherto 

 not have been perceptible, either during the different lunar 

 phases, or at the different distances assumed by the satellite 

 in relation to the earth. The vicinity of the moon does not 

 appear, when compared with the sun,t to compensate in this 



* Op. cit., vol. iii., p. 228. 



t Though the nearness of the moon in comparison with the sun 

 does not seem to compensate the smallness of her mass, yet the al- 

 ready well-ascertained alteration of the magnetic declination in the 

 course of a lunar day, the lunar^diumal magnetic variation (Sabine, in 

 the Report to the Brit. Assoc, at Liverpool, 1854, p. 11, and for Ho- 

 hart Town in the Phil. Tr. for 1857, Art. i., p. 6), stimulates to a per- 

 severing observation of the magnetic influence of the earth's satellite. 

 Kreil has the great merit of having pursued this occupation with great 

 care, from 1839 to 1852 (see his treatise Ueher den Elnjiuss des Mondes 

 aitfdie horizontale Component der Magnetischen Erdkraft, in the Denk- 

 schriften der Wiener Akademie der Wiss. Mathem. Naturwiss, C^asse, 

 vol. v., 1853, p. 45, and Phil. Trans, for 1856, Art. xxii.). His ob- 

 servations, which were conducted for the space of many years, both 

 at Milan and Prague, having given support to the opinion that both 

 the moon and the solar spots occasioned a decennial period of decli- 

 nation, led General Sabine to undertake a very important work. He 

 found that the exclusive influence of the sun on a decennial period, 

 previously examined in relation to Toronto, in Canada, by the em- 

 ployment of a peculiar and very exact form of calculation, may be 

 recognized in all the three elements of terrestrial magnetism {Phil. 

 Trans, for 1856, p. 361), as shown by the abundant testimony of hour- 

 ly observations caiTied on for a course of «ight years at Hobart Town, 

 from January, 1841, to December, 1848. Thus both hemispheres fur- 

 nished the same result as to the operation of the sun, as well as the 

 certainty " that the lunar-diurnal variation corresponding to different 

 years shows no conformity to the inequality manifested in those of 

 the solar-diurnal variation. The earth's inductive action, refieoted 



