NOTES. XXI 



each other, as well as upon the Earth ; and lastly, act in combination with the 

 Earth upon an imaginary needle, placed anywhere within or without the atmo- 

 sphere. We may compare the oxygen surrounding the Earth to an armature of 

 soft iron applied to a natural or to a steel magnet ; the magnet being supposed 

 spherical like the Earth, and the armature a hollow sphere like the envelope of 

 oxygen in the atmosphere surrounding the Earth. The degree of magnetic power 

 which each particle of oxygen can receive from the Earth's constant magnetism, 

 diminishes with increasing temperature and rarefaction of the gas. Now as the 

 sun's course from east to west is followed by a constant increase of temperature 

 and expansion, this must produce a modification in the magnetic relations of the 

 Earth and its oxygenous envelope, which, in Faraday's opinion, is the source of a 

 part of the variations of the observed elements of terrestrial magnetism. Plucker 

 finds that, inasmuch as the degree of force with which a magnet acts upon oxy- 

 gen is proportioned to the density of that gas, the- use of the magnet affords a 

 simple eudiometric means of recognising the presence of one or two hundredth 

 parts of free oxygen in a mixture of gases. 



(76) p> 86. Kosmos, Bd. iv. S. 10 and 11 (English edition, p. 10 and 11). 



(") p. 87. Kepler, in Stella Martis, p. 32 and 34. Compare therewith his 

 " Mysterium cosmogr." cap. 20, p. 71. 



( 78 ) p. 87. Kosmos, Bd. iii. S. 416, Anm. 23 (English edition, Note 482), 

 where, both in the original and in the translation, " Basis Astronomiae " has been 

 printed instead of, as it ought to be, " Clavis Astronomic." The passage 

 ( 226) in which the solar evolution of light is termed " a perpetual aurora," is 

 not to be found in the first edition of the Clavis Astr. of Horrebow (Havn. 1730), 

 but only in the later edition, contained in Horrebow's " Operum mathematico- 

 physicorum," t. i. Havn. 1740, pag. 317, where there is a Second Part of the 

 Clavis in which the passage occurs. Compare with Horrebow's view the entirely 

 accordant views of Sir William and Sir John Herschel, Kosmos, Bd. iii. S. 45, 56 

 (Anm. 22), 256 and 262 (English edition, p. 35, xi., Note 68), 176, and Note 

 289. 



( ra ) p. 87. Me'moires de Mathe'm. et de Phys. prdsente's a 1'Acad. Eoy. des 

 Sc. t. ix. 1780, p. 262. 



( M ) p. 88. " So far as these four stations (Toronto, Hobarton, St. Helena, and 

 the Cape), so widely separated from each other and so diversely situated, justify 

 a generalisation, we may arrive at the conclusion, that at the local hour of seven 

 and eight A.M. the magnetic declination is everywhere subject to a variation of 

 which the period is a year, and which is everywhere similar in character and 

 amount, consisting of a movement of the north end of the magnet from east to 

 west between the northern and the southern solstice, and a return from west to 



