58 GAUSS AND WEBER ON TERRESTRIAL. MAGNETISM. 



It has been already stated that exceptions sometimes occur on 

 single daySj when the difference between the forenoon and after- 

 noon decUnations may have the opposite sign. But such ex- 

 ceptions are rare ; during the three years' obsei'vations only 

 fourteen cases of the kind have occurred; or, on an average, 

 one in 79 days. I give them in this place, together with the 

 amount by which, on each occasion, the declination at 8, a.m. 

 exceeded that at 1, p.m. 



Of these fourteen exceptions, twelve, as might be expected, 

 occur in the winter months, and only two in the summer months ; 

 the small regular action of the sun in the former being more 

 easily exceeded by an anomalous movement than could be the 

 case in regard to the far greater regular action in the summer 

 months. 



To try how far the secular variation might be recognised in 

 the present observations, the monthly means of the first yeai* 

 have been compared with the corresponding ones of the second, 

 and these with those of the third year. Among the forty-eight 

 comparisons thus obtained (for the incomplete month of March, 

 1834, has been excluded from this as well as from all the other 

 combinations), forty-seven give a decrease, and only one an 

 increase, which is therefore characterised in the following table 

 by the sign — . 



Yearly Decrease of the Declination. 



