CORRECTIONS AND ADDITIONS 



BY THE AUTHOR. 



PAGE 32. LINE 12. 



A far more important result in reference to the density of the earth 

 than that obtained by Baily (1842) and Reich (1847 1850) has been 

 brought out by Airy's experiments with the pendulum, conducted with 

 such exemplary care in the Mines of Harton, in the year 1854. Accord- 

 ing to these experiments, the density is 6'566, with a probable error of 

 0-182 (Airy in the Philos. Transact, for 1856, p. 342). A slight modi- 

 fication of this numerical value, made by Professor Stokes on account 

 of the effect of the rotation and ellipticity of the earth, gives the den- 

 eity for Harton, which lies at 54 48' north latitude, at 6'565, and for 

 the Equator at 6' 489. 



PAGE 76. LINE 10. 



Arago has left behind him a treasury of magnetical observations 

 (upwards of 52,600 in number) carried on from 1818 to 1835, which 

 have been carefully edited by M. Fedor Thoman, and published in the 

 (Euvres Completes de Francois Arago (t. iv, p. 498). In these observa- 

 tions, for the series of years from 1821 to 1830, General Sabine has 

 discovered the most complete confirmation of the decennial period of 

 magnetic declination, and its correspondence with the same period, in 

 the alternate frequency and rarity of the solar spots (Meteorological 

 Essays, London, 1855, p. 350). So early as the year 1850, when Schwabe 

 published at Dessau his notices of the periodical return of the solar 

 spots (Cosmos, vol. iv, p. 397), two years before Sabine first showed the 

 decennial period of magnetic declination to be dependent on the solar 

 spots (in March, 1852, Phil. Tr. for 1852, p. i, pp. 116121 ; Cosmo*, 

 vol. v, p. 76, note), the latter had already discovered the important 

 result, that the sun operates on the earth's magnetism by the magnetic 

 power proper to its mass. He had discovered (Phil. Tr. for 1850, p. i, 

 p. 216, Cosmos, vol. v, p. 140), that the magnetic intensity is greatest, 

 and that the needle approaches nearest to the vertical direction, when 

 the earth is nearest to the sun. The knowledge of such a magnetical 

 operation of the central body of our planetary system, not by its heat- 

 producing quality, but by its own magnetic power, as well as by changes 

 in the Photosphere (the size and frequency of funnel-shaped openings), 

 gives a higher cosmical interest to the study of the earth's magnetism 

 and to the numerous magnetic observatories (Cosmos, vol. 1, p. 184 ; 

 vol. v, p. 73) now planted over Russia and Northern Asia, since the 

 resolutions of 1829, and over the colonies of Great Britain since 1840 

 1850. (Sabine, in the Proceedings of the Roy. Soc. vol. viii, No. 25, p. 400 

 aud in the Phil Trans, for 1856. p. 362). 



