J23 THE MATHEMATICAL PRINCIPLES [BOOK 111 4ih Prop, of this Book we found, that, if both earth and moon were revolved aoout their common centre of gravity, the mean distance of the one from the other would be nearly 60^ mean semi-diameters of the earth : and the force by which the moon may be kept revolving in its orbit about the earth in rest at the distance PT of 60^ semi-diameters of the earth, is to the force by which it may be revolved in the same time, at the distance of 60 semi-diameters, as 60