[ 388 3 
Likewife the fum of the lines PM is ever the fame 
for every quarter of the folar revolution; but it has 
alternatively a contrary iign ; that is, a contrary di- 
rection. During the quarter from y to $, the altera- 
tion of the inclination of the earth’s axe to the ecliptic 
is pofitive, and the angle of the inclination increafes; 
but during the fucceeding quarter, or from £ to 
the alteration of the inclination is negative, and the 
angle of the inclination diminifhes : And as the di- 
minution from $ to ==; is equal to the augmentation 
from r to ®, it follows, that at the end of the femi- 
revolution the inclination of the earth's axe to the 
plane of the ecliptic will become again the fame, 
having undergone an ofcillation, which is completed 
in a femirevolution. It is the fame, when the fun 
paffes from to T . The angle of the inclination in- 
creafes from ^ to vp, anddecreafes from vp to T, where 
it becomes again the fame it was at -. 
And hence the inclination of the earth’s axe to the 
ecliptic may be confidered as conftant, tho’ fubjeCt 
to this ofcillation, and indeed to feveral others, which 
will be prefently explained, they being all regular, 
and performed in regular periods. 
The earth’s inclination to the ecliptic being con- 
ftant, and the motion of the pole which produces 
the preceffion, being always parallel to the plane of 
the ecliptic, the earth’s pole moves in a parallel to 
the ecliptic, about 23 degrees and a half diftant from 
the pole of the ecliptic, and the terreftrial axe de- 
fer ibes a conic furface. 
To this motion of the terreftrial axe or pole is to 
be aferibed the apparent motion of the ftars about the 
pole of the ecliptic. 
But 
