454 COSMOS. 



tion du systemc planetaire, clle a demontre qu'clles existent 

 et qu'elles sont tres peu ef endues. Ainsi, a ne considercr que 

 le seul efFet des causes constantes qui agissent actuellemcnt 

 suv le systeme du monde, on peut affirmer que le plan de 

 Tecliptique n'a jamais coincide et ne coincidera jamais avec 

 le plan de 1'equateur, phenomene qui, s'il arrivait, produirait 

 .sur le Terre le (pretendu!) prin temps perpetuel."* Biot, 

 Traite d' Astronomic Physique, 3rd ed. 1847*, torn. iv. p. 91. 



While the nutation of the Earth's axis discovered by 

 Bradley depends merely upon the influence of the Sun and 

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

 increase and decrease in the obliquity of the ecliptic is the 

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

 the present time these are so situated that their united in- 

 fluence 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 retrogression, and consequently an increase in the 

 obliquity of the ecliptic. Theory teaches us that these in- 

 creases and diminutions occupy periods of very unequal 

 duration. The most ancient astronomical observations which 



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

 ecliptic presents alternating states which ,produce an eternal 

 oscillation comprised within fixed limits. Theory has notbeen 

 able to determine those limits ; but according to the consti- 

 tution of the planetary system it has been proved that they 

 exist, and that they are of very slight extent. Thus, to con- 

 sider 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. 



