243 



nutes ; thus leaving a total decrease, from year to year, of 

 five minutes. 



When we compare the course of these changes with those 

 observed by Cassini, we find that the directions of the move- 

 ments are precisely opposite, — as, indeed, we might have been 

 led a priori to expect from the opposition in the directions of 

 the secular changes ; and we are led to generalize the law as 

 follows : 



" From the beginning of the year to the beginning or 

 middle of April, the mean declination undergoes little or no 

 change. Thence, to the beginning of August, its movement 

 is retrograde (or opposite in direction to the secular change) ; 

 and from the beginning of August to the end of the year, it 

 is direct." 



The phenomena just described are, it is manifest, the 

 resultants of two distinct changes, namely, the annual varia- 

 tion properly so called, and the progressive or secular change. 

 If the latter be subtracted, at the rate of 5.'0 per annum, or 

 0.'4 per month, the remaining numbers give the true annual 

 period. When thus considered, the annual variation exhibits 

 an increase of the declination during the first seven months of 

 the year, and a decrease during the remaining Jive months ; 

 the apparently stationary condition of the declination, during 

 the first three months, arising from the mutual compensation 

 of the periodical and the progressive changes. 



It appears, then, that the annual variation (unlike the diur- 

 nal in this respect) is a single oscillation. The minimum 

 occurs near the end of January, and the maximum in the 

 beginning of August ; and the whole range of the change is 

 &.Q minutes. 



The laws of the annual variations of the declination and 

 of the temperature, present the most complete accordance 

 in the epochs of maxima and minima, as well as those of the 

 mean values. The maximum of temperature occurs about 

 August 1, — that of declination about August 8. The mini- 

 mum of temperature takes place about January 15, — the mini- 



