90 TERRESTRIAL MAGNETISM. 



to KreiTs extensive investigation of the Moon's magnetic 

 influence no sensible difference according to the varying 

 phases of the Moon, or to her different distances from the 

 Earth, has yet been found. It would seem as if the near 

 proximity of the Moon, as compared with the Sun, does not 

 compensate for the smallness of her mass. The principal 

 result ( 86 ) of the examination which has been made into the 

 magnetic influence of our satellite, which, according to 

 Melloni, shows only a faint trace of thermal influence, is 

 that the magnetic declination observed at the surface of our 

 globe undergoes, in the course of a lunar day, a regular 

 variation, attaining two maxima and two minima. Kreil 

 remarks very justly, that "if the Moon produces at the 

 surface of the Earth no sensible increase of temperature" 

 (t. e. no increase cognisable by our ordinary thermometers), 

 " she cannot produce any variation in the Earth's magnetic 

 force by thermal agency ; and if, therefore, we do, neverthe- 

 less, find such a variation, we must conclude that it is 

 produced by some mode of action other than thermal." In 

 all actions which do not present themselves at once as the 

 unmistakable results of a single cause, we can only, as here 

 in the case of the Moon, recognise their independent exist- 

 tence after carefully excluding, or eliminating, many extra- 

 neous disturbing elements. 



But although at the present time the magnetic variations 

 of largest amount, and of most decided character, are not, it 

 must be admitted, satisfactorily explained by the maxima 

 and minima of the variations of temperature, it is yet scarcely 

 possible to doubt that when, at some future but- not dis- 

 tant period, there shall be a more complete and deeper 

 insight into the whole process of magnetic action, the great 



