THE TLANETS. 125 



indeed, also assumes only narrow limits for the alternating 

 variation in the obliquity of the ecliptic, but considers it 

 more advisable not to assign to it a determinate number. 

 11 La diminution lente et seculaire de l'obliquite de l'eclip- 

 tique," says he, " offre des etats alternatifs qui produisent 

 une oscillation eternelle, comprise entre des limites fixes. La 

 theorie n'a pas encore pu parvenir a determiner ces limites ; 

 mais d'apres la constitution du systeme planetaire, elle a de- 

 montre qu'elles existent et qu'elles sont trcs peu etendues. 

 Ainsi, a ne considerer que le seul effet des causes constantes 

 qui agissent actuellement sur le systeme du monde, on peut 

 affirmer que le plan de l'ecliptique n'a jamais coincide et ne 

 coincidera jamais avec le plande l'equateur, phenomene qui, 

 s'il arrivait, produirait sur le Terre le (pretendu !) printemps 

 perpetuel."* — Biot, Traite cV Astronomic Physique, 3d ed., 

 1847, torn, iv., p. 91. 



While the nutation of the Earth's axis discovered by Brad- 

 ley depends merely upon the influence of the Sun and the 

 Earth's satellite upon the oblate figure of our planet, the in- 

 crease and decrease in the obliquity of the ecliptic is the con- 

 sequence of the variable position of all the planets. At the 

 present time, these are so situated that their united influence 

 upon the Earth's orbit produces a diminution in the obliquity 

 of the ecliptic. This obliquity amounts, according to Bessel, 

 to 0"-457 annually. At the end of many thousand years, the 

 situation of the planetary orbits and their nodes (their points 

 of intersection with the ecliptic) will be so different, that the 

 advance of the equinoxes will be converted into a retrogres- 

 sion, and consequently an increase in the obliquity of the eclip- 

 tic. Theory teaches us that these increases and diminutions 

 occupy periods of very unequal duration. The most ancient 

 astronomical observations which have come down to us, with 

 accurate numerical data, reach back to the year 1104 before 

 Christ, and testify to the extreme antiquity of Chinese civil- 

 ization. The literary remains are scarcely a century more 



* "The slight and secular variation of the obliquity of the ecliptic 

 presents alternating states, which produce an eternal oscillation com- 

 prised within fixed limits. Theory has not been able to determine 

 those limits; but, according to the constitution of the planetary system, 

 it has been proved that they exist, and that they are of very slight ex- 

 tent. Thus, to consider only the effect of the permanent causes which 

 act upon the system of the world, it may be affirmed that the plane of 

 the ecliptic never has and never will coincide with the plane of the 

 equator, a phenomenon which, if it took place, would produce upon the 

 Earth the (pretended !) eternal spring. 



