90 OUTLINES OF NATURAL PHILOSOPHY. 



has been preserved ; and from it LA' PLACE dedu- 

 ces the obliquity at that time, = 23 54' 2". Con- 

 naissance des Terns, 181 1, p. 432. 



A series of observations, from the age of PYTHEAS, 

 down to the present time, confirms the same ge- 

 neral result. LA PLACE, ibid. LA LANDE, 

 Astron. 2738. ; VINCE, Astron. i. 151. 



The mean obliquity for 1 750, was determined very 

 exactly by LA CAILLE and BRADLEY, to be 

 23 28' 19". 



The obliquity, beside this progressive diminution, is 

 subject to slight periodic irregularities, which are 

 not here considered. The diminution itself, 

 though apparently progressive, will be found after- 

 wards to be really periodical, and a part of a slow 

 vibration, by which the obliquity of the ecliptic 

 alternately increases and diminishes within very 

 narrow limits. 



Apparent 



