Note 2 — BOOK II. THE DYNAMICAL PERIOD

It must be noted that these words, in which the laws of gravitation are always summarised in histories and text-books, do not appear in the Principia; but, though they must have been composed by some early commentator, it does not appear that their origin has been traced. Nor does it appear that Newton ever extended the law beyond the Solar System, and probably his caution would have led him to avoid any statement of the kind until it should be proved. With this exception the above statement of the law of universal gravitation contains nothing that is not to be found in the Principia; and the nearest approach to that statement occurs in the Seventh Proposition of Book III.:— Prop.: That gravitation occurs in all bodies, and that it is proportional to the quantity of matter in each. Cor. I.: The total attraction of gravitation on a planet arises, and is composed, out of the attraction on the separate parts. Cor. II.: The attraction on separate equal particles of a body is reciprocally as the square of the distance from the particles.