6o ESSAYS awn OBSERVATIONS 
be juftly reprefented by perpendicular afcent 
and defcent. A planet moving from the fun, 
refembles a ftone moving from the earth up- 
wards; and, in its return towards the fun, 
refembles the fame ftone returning to the 
earth, after its upward motion is fpent. And, 
indeed, by prolonging the greater axis inde- 
finitely, an ellypfe coincides at laft with a 
ftreight line; and the motion in the former 
coincides with the motion in the latter. Con- 
fidering then that gravity and the vzs zafita 
are the two powers which govern motion in 
the curve as well as in the ftreight line; it 
muft follow, that the law of motion is the 
fame in both. - 
