10& DISSERTATION SECOND. [fart u. 



would vibrate slower when carried from Europe into the 

 torrid zone. The observations of the two French astro- 

 nomers, Varin and Dc Hayes, made at Cayenne and Mar- 

 tinique, had already confirmed this conclusion. 



The problem which Newton had thus resolved enabled him 

 to resolve one of still greater difficulty. The precession, 

 that is, the retrogradation of the equinoctial points, had 

 been long known to astronomers ; its rate had been mea- 

 sured by a comparison of ancient and modern observa- 

 tions, and found to amount nearly to 50" annually, so as 

 to complete an entire revolution of the heavens in 25,920 

 years. Nothing seemed more difficult to explain than this 

 phenomenon, and no idea of assigning a physical or me- 

 chanical cause for it had yet occurred, I believe, to the 

 boldest and most theoretical astronomer. The honour of 

 assigning the true cause was reserved for the most cau- 

 tious of philosophers. He was directed to this by a cer- 

 tain analogy observed between the precession of the equi- 

 noxes and the retrogradation of the moon's nodes, a phe- 

 nomenon to which his calculus had been already success- 

 fully applied. The spheroidal shell or ring of matter 

 which surrounds the earth, as we have just seen, in the 

 direction of the equator, being one half above the plane 

 of the ecliptic and the other half below, is subjected to 

 the action of the solar force, the tendency of which is to 

 make this ring turn on the line of its intersection with 

 the ecliptic, so as ultimately to coincide with the plane 

 of that circle. This, accordingly, would have happened 

 long since, if the earth had not revolved on its axis. The 

 effect of the rotation of the spheroidal ring from west 

 to east, at the same time that it is drawn down toward 

 the plane of the ecliptic, is to preserve the inclination of 

 these two planes unchanged, but to make their intersec- 

 tion move in a direction opposite to that of the diurnal 



