152. But if, whilst the projectile force carries the Planet from _B_ to
_b_, the Sun’s attraction (which constitutes the Planet’s gravitation)
should bring it down from _B_ to I, the gravitating power would then be
too strong for the projectile force; and would cause the Planet to
describe the curve _BC_. When the Planet comes to _C_, the gravitating
power (which always increases as the square of the distance from the Sun
_S_ diminishes) will be yet stronger for the projectile force; and by
conspiring in some degree therewith, will accelerate the Planet’s motion
all the way from _C_ to _K_; causing it to describe the arcs _BC_, _CD_,
_DE_, _EF_, &c. all in equal times. Having it’s motion thus accelerated,
it gains so much centrifugal force, or tendency to fly off at _K_ in the
line _Kk_, as overcomes the Sun’s attraction: and the centrifugal force
being too great to allow the Planet to be brought nearer the Sun, or
even to move round him in the Circle _Klmn_, &c. it goes off, and
ascends in the curve _KLMN_, &c. it’s motion decreasing as gradually
from _K_ to _B_ as it increased from _B_ to _K_, because the Sun’s
attraction acts now against the Planet’s projectile motion just as much
as it acted with it before. When the Planet has got round to _B_, it’s
projectile force is as much diminished from it’s mean state about _G_ or
_N_, as it was augmented at _K_; and so, the Sun’s attraction being more
than sufficient to keep the Planet from going off at _B_, it describes
the same Orbit over again, by virtue of the same forces or laws.
[Sidenote: Fig. IV.