Page:Elementary Text-book of Physics (Anthony, 1897).djvu/86

 proportional to the mass of the body, since all bodies have the same acceleration. He further brought forward, as the most satisfactory theory which he could form, the general statement that every particle of matter attracts and is attracted by every other particle.

The experiments necessary for a complete verification of this last statement were not carried out by Newton. They were performed in 1798 by Cavendish. His apparatus consisted essentially of a bar furnished at both ends with small leaden balls, suspended horizontally by a long fine wire, so that it turned freely in the horizontal plane. Two large leaden balls were mounted on a bar of the same length, which turned about a vertical axis coincident with the axis of rotation of the suspended bar. The large balls, therefore, could be set and clamped at any angular distance desired from the small balls. The whole arrangement was enclosed in a room, to prevent all disturbance. The motion of the suspended system was observed from without by means of a telescope. Neglecting as unessential the special methods of observation employed, it is sufficient to state that an attraction was observed between the large and small balls, and was found to be in accordance with the law as above stated.

59. Centre of Gravity.—The forces with which the earth attracts the particles of an ordinary body are parallel and proportional to the masses of the particles, so that the sum of their moments about any axis passing through the centre of mass will vanish, because the corresponding sum of the products of the masses and their respective distances from any plane containing that axis vanishes by the definition of the centre of mass. Gravity will, therefore, have no tendency to produce rotation in a free body or system of particles. It will cause a translation of the body, if it be rigid, such as would be produced if a force equal to the sum of all the forces acting on the particles were applied at the centre of mass. This point of application of the force is called the centre of gravity of the body. If the forces acting on the particles be not parallel, the body will, in general, have no centre of gravity.