PRECESSION OF THE EQUINOXES. 315 



just rule : Consider as certain only what has been de- 

 monstrated. The demonstration of the Newtonian con- 

 ception of the precession of the equinoxes was, then, a 

 great discovery, and it is to D'Alembert that the glory of 

 it is due.* The illustrious geometer gave a complete 

 explanation of the general movement, in virtue of which 

 the terrestrial axis returns to the same stars in a period 

 of about 26,000 years. He also connected with the 

 theory of gravitation the perturbation of precession dis- 

 covered by Bradley, that remarkable oscillation which the 

 earth's axis experiences continually during its movement 

 of progression, and the period of which, amounting to 

 about eighteen years, is exactly equal to the time which 

 the intersection of the moon's orbit with the ecliptic 

 employs in describing the 360 of the entire circumfer- 

 ence. 



Geometers and astronomers are justly occupied as 

 much with the figure and physical constitution which the 

 earth might have had in remote ages as with its present 

 figure and constitution. 



As soon as our countryman Richer discovered that a 

 body, whatever be its nature, weighs less when it is 

 transported nearer the equatorial regions, everybody per- 



* It must be admitted that M. Arago has here imperfectly repre- 

 sented Newton's labours on the great problem of the precession of 

 the equinoxes. The immortal author of the Principia did not merely 

 conjecture that the conical motion of the earth's axis is due to the 

 disturbing action of the sun and moon upon the matter accumulated 

 around the earth's equator: he demonstrated by a ver^y beautiful and 

 satisfactory process that the movement must necessarily arise from 

 that cause; and although the means of investigation, in his time, 

 were inadequate to a rigorous computation of the quantitative effect, 

 still, his researches on the subject have been always regarded as 

 affording one of the most striking proofs of sagacity which is to be 

 found in all his works. Translator. 



