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The former of thefe two motions gives the pre- 
ceffion with refpedt to the ecliptic, and has its di- 
rection always the fame way. 
The latter motion has two oppofite directions, in 
the two femirevolutions of the pole of the lunar orbit 
round the pole of the ecliptic, and caufes an ofcilla- 
tion of the terreftrial axe on the plane of the ecliptic, 
which is compleated in a revolution of the pole of 
the lunar orbit round the pole of the ecliptic. 
From all that has been laid, it follows, that there 
are five diftinCt motions of the pole of the earth ; 
namely, two of preceflion, which are parallel to the 
plane of the ecliptic, and three of ofcillation on the 
plane of the ecliptic. 
The two of preceffion are caufed, the one by the 
fun, the other by the moon. That, which is caufed 
by the fun, is conftantly the fame at every quarter of 
the time of the revolution of the fun round the earth, 
that is, every three months : That which is caufed by 
the moon, is conftantly the fame at every quarter of 
the time of the revolution of the moon round the 
earth ; that is, about every feven days. 
Of the three motions of ofcillation, one is caufed 
by the fun, and is completed in the time of the femi- 
$evolution of the fun round the earth, taken from one 
equinox to the following one; that is, in fix months. 
The other is caufed by the moon ; and each ofcil- 
lation is completed in the fpace of a femirevolution 
of the moon round the earth ; that is, in about 1 4 days. 
The third is caufed likewife by the moon, and 
arifes from the plane of her orbit being different from 
the plane of the ecliptic, and from the pole of the 
lunar orbit making its revolution about the pole of the 
. ecliptic 
