Moreover, since all the parts of any planet A gravitate towards any other planet B; and the gravity of every part is to the gravity of the whole as the matter of the part to the matter of the whole; and (by law 3) to every action corresponds an equal reaction; therefore the planet B will, on the other hand, gravitate towards all the parts of the planet A; and its gravity towards any one part will be to the gravity towards the whole as the matter of the part to the matter of the whole. Q. E. D. Cor. 1. Therefore the force of gravity towards any whole planet arises from, and is compounded of, the forces of gravity towards all its parts. Magnetic and electric attractions afford us examples of this; for all attraction towards the whole arises from the attractions towards the several parts. The thing may be easily understood in gravity, if we consider a greater planet as formed of a number of lesser planets meeting together in one globe, for _hence it would appear_ that the force of the whole must arise from the forces of the component parts. If it is objected that, according to this law, all bodies with us must mutually gravitate one towards another, I answer, that since the gravitation towards these bodies is to the gravitation towards the whole earth as these bodies are to the whole earth, the gravitation towards them must be far less than to fall under the observation of our senses. Cor. 2. The force of gravity towards the several particles of any body is reciprocally as the square of the distance from the particles; as appears from cor. 3, prop. 74, book I. FOOTNOTES: [Footnote 11: Translated from the _Philosophiæ Naturalis Principia Mathematica_.] X BENJAMIN FRANKLIN 1706-1790 _Benjamin Franklin, representative of the rationalist tendencies of the eighteenth century, was born in Boston, January 17, 1706. His early life and political missions are intimately related in his “Autobiography,” a classic in American literature. Apart from his political services to the cause of American independence, he attained distinction in the field of scientific researches and experiments. In 1746 he began the experiments in electricity which resulted in his identification of electricity with lightning. He died in Philadelphia, April 17, 1790._ THE IDENTITY OF LIGHTNING AND ELECTRICITY[12]