& 
PRECESSION OF THE EQUINOXES. 815 
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 very 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. 
