ART. 19. _Universality of the Attractive Force._--The principle upon which Universal Attraction rests is found in the words: "Every particle of matter in the universe attracts every other particle." It must, however, be admitted that this statement has never actually been proved. The smallest body that Newton used to prove his Law of Attraction was our satellite the moon. Cavendish, however, in 1798, by a series of experiments, conclusively demonstrated that the force of Gravitation existed in small bodies. He took two small leaden balls of a certain weight, and fixed them at the ends of a rod about six feet long, the rod being suspended by a piece of wire in the air. Large leaden balls were then brought near the small ones, and great care was taken to see if there were any twist in the wire by which they were suspended. It was found that the wire had become twisted on the approach of the large leaden balls, and thus he was able to prove that every particle of the attracted and attracting body are mutually concerned in the Attraction of Gravitation. There is abundant evidence of the application of this force in relation to our earth, as we shall see later on. The universality of the Attraction of Gravitation is a fact that has been proved in a thousand ways, and a thousand times. All stars and suns, and all planets, satellites and comets and nebulae are subject to this universal law. Astronomy teaches us that its power extends across the vast abysses of space, and that stars situated at distances that cannot possibly be measured, are subject to this world-wide law. Some of the greatest discoveries in astronomical science were due to the operations of this wonderful law, the gravitating influences of certain planets indicating their existence, although their discovery had not yet been made. The discovery of Neptune through the mathematical calculations of Le Verrier and Mr. Adams in 1846 was the crowning proof of the Law of Gravitation. Mr. Adams in England had noticed that the planet Uranus was being pulled out of the course by some unknown power, and so set to work to calculate the position of the body which thus influenced the motion of Uranus in its orbit. He located the position of the supposed influencing body strictly by mathematical calculations, and then took his results to the Astronomer Royal. Delay, however, occurred in the search for the supposed new planet, and nothing was done further in the matter for many months. Meanwhile Le Verrier in France, unknown to Mr. Adams, had been making similar calculations with reference to the perturbations of Uranus, and had arrived at similar results.