GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



57 



It will be perceived that, not only in the mean values, but 

 also in each of the separate years, the diiFerence has been smallest 

 in December ; and this is what we might expect, as those changes 

 which vary according to the time of the day must necessarily be 

 ascribed to the action of the sun, although as yet we know not 

 how this action is effected. It may at first appear surprising, 

 on the other hand, that the differences are not greatest at the 

 time of the summer solstice, but appear smaller in June and 

 July than in April, May, and August, especially as the coinci- 

 dence of all three years in this circumstance affords a presump- 

 tion that it is not accidental. It must not, however, be over- 

 looked, that in the months immediately following the solstice, 

 the time of the minimum of the declination is earher, and there- 

 fore the whole increase would be sensibly greater than the change 

 reckoned from 8 o'clock. 



It is further obseiwable that in each month the differences are 

 greater in the second year than in the first ; and again, in the 

 third year greater than in the second. But these differences are 

 by far too great in amount to admit of our considering them as 

 pai-ts of a secular increase, and it is rather to be expected that 

 by continuing the observations for several years we shall not 

 fail to discover a fluctuation. But, in any case, we hereby learn 

 that one year may differ from another in respect to the effect 

 of the sun on the earth's magnetism, somewhat in the same 

 way that one summer or one winter differs from another in 

 temperature. On this account also we shall only arrive at an 

 accurate determination of the mean values by observations con- 

 tinued for several years. 



