MAGNETIC VARIATION. 85 



respect for the smallness of its mass. The main result of 

 the investigation, in relation to the magnetic influence of the 

 earth's satellite, which, according to Melloni, exhibits only a 

 trace of calorification,* is that the magnetic declination in 

 our planet undergoes a regular alteration in the course of a 

 lunar day, during which it exhibits a two-fold maximum and 

 a two-fold minimum. Kreil very correctly observes, " that 

 if the moon exerts no influence on the temperature on the 

 surface of our earth (which is appreciable by the ordinary 

 means of measuring heat), it obviously can not in this way 

 effect any alteration in the magnetic force of the earth ; but 

 if, notwithstanding, an alteration of this kind is actually ex- 

 perienced, we must necessarily conclude that it is produced 

 by some other means than through the moon's heat." Ev- 

 ery thing that can not be considered as the product of a sin- 

 gle force must require, as in the case of the moon, that all 

 foreign elements of disturbance should be eliminated, in or- 

 der that its true nature may be recognized. 



Although hitherto the most decisive and considerable va- 

 riations in the manifestations of terrestrial magnetism do not 

 admit of being satisfactorily explained by the maxima and 

 minima in the variations of temperature, there can be no 

 doubt that the great discovery of the polar property of oxy- 

 gen in the gaseous envelope of our earth will, by a more 

 profound and comprehensive view of the process of the mag- 

 netic activity, speedily afford us a most valuable assistance 

 in elucidating the mode of origin of this process. It would 

 be inconceivable if, amid the harmonious co-operation of all 

 the forces of nature, this property of oxygen and its modifi- 

 cation by an increase of temperature should not participate 

 in the production and manifestation of magnetic phenomena. 



If, according to Newton's view, it is very probable that 

 the substances which belong to a group of celestial bodies (to 

 one and the same planetary system) are for the most part 

 identical,! we may, from inductive reasoning, conclude that 



from the moon, must be of a very little amount." (Sabine, in the 

 Phil. Tr. for 1857, Art. i., p. 7, and in the Proceedings of the Royal 

 Soc, vol. viii., No. 20, p. 404.) The magnetic portion of this volume 

 having been printed almost three years ago, it seemed especially nec- 

 essary, with reference to a subject which has so long been a favorite 

 one with me, that I should supply what was wanting by some addi- 

 tional remarks. 



* Kreil, Einfluss des Mondes avf die Magnetisclie Declination, 1852, 

 s. 27, 29, 46. 



t Cosmos, vol. i., p. 133, 134 ; also vol. iv., p. 206. 



