2 .-- The force of gravity which tends to any one planet is reciprocally as the square of the distance of places from the planet's centre.
"COR.

3 .-- All the planets do mutually gravitate towards one another, by Cor.

1 and 2, and hence it is that Jupiter and Saturn, when near their conjunction, by their mutual attractions sensibly disturb each other's motions.

So the sun disturbs the motions of the moon; and both sun and moon disturb our sea, as we shall hereafter explain.
"SCHOLIUM "The force which retains the celestial bodies in their orbits has been hitherto called centripetal force; but it being now made plain that it can be no other than a gravitating force, we shall hereafter call it gravity.

For the cause of the centripetal force which retains the moon in its orbit will extend itself to all the planets by Rules i., ii., and iii.
"PROPOSITION VI., THEOREM VI.
"That all bodies gravitate towards every planet; and that the weights of the bodies towards any the same planet, at equal distances from the centre of the planet, are proportional to the quantities of matter which they severally contain.
"It has been now a long time observed by others that all sorts of heavy bodies (allowance being made for the inability of retardation which they suffer from a small power of resistance in the air) descend to the earth FROM EQUAL HEIGHTS in equal times; and that equality of times we may distinguish to a great accuracy by help of pendulums.