PORTION OF THE COSMOS. THE PLANETS. 331 



diminution lente et seculaire de Fobliquit6 de I'ecliptique 

 offre des etats alternatifs qui produisent une oscillation eter- 

 nelle comprise entre des limites fixes. La theorie n'a pas 

 encore pu parvenir a determiner ces limites ; mais d'apres 

 la constitution du systeme plantaire, elle a demontre 

 qu'elles existent et qu'elles sont tres peu et endue 8. Ainsi a 

 ne eonsiderer que le seul effet des causes constantes qui 

 agissent actuellement sur le systeme du monde, on peut 

 affirmer que le plan de Tecliptique n'a jamais coincide 

 et ne coin cider a Jamais avec le plan de Fequateur, phe- 

 nomene qui, s'il arrivait, produirait sur la terre le (pre- 

 tendu!) printemps perpetuel" (Biot, Traite d'Astronomie 

 Physique, 3me ed. 1847, T. iv. p. 91). 



While the Nutation of the terrestrial axis discovered by 

 Bradley depends solely upon the influence of the Sun and 

 the Earth's own satellite upon the compressed form of our 

 planet at its poles, the increase and decrease of the ob- 

 liquity of the ecliptic is a consequence of the varying posi- 

 tions of all the planets. These are at present so distributed 

 that their joint action on the Earth's path or orbit produces 

 a diminution of the obliquity, which diminution amounts at 

 the present time, according to Bessel, to 0"*457 annually. 

 After the lapse of several thousand years the places of the 

 planetary orbits and their nodes (points of intersection on 

 the ecliptic) will be so different that the advance, or preces- 

 sion, of the equinoxes will be changed into a retrogression, and 

 thereby produce an increase of the obliquity of the ecliptic. 

 Theory teaches that this increase and decrease occupy periods 

 of very unequal duration. The oldest astronomical obser- 

 vations which have been preserved to us with exact nume- 

 rical data extend back to the year 1104 B.C., and testify the 



