GENERA LIZA TION. 255 



other placed an equal weight of gold. The pendulums 

 were then equal in weight and in size ; and, on setting 

 them simultaneously in motion, Newton found that they 

 vibrated for a great length of time with exactly equal 

 vibrations. He tried the same experiment with silver, 

 lead, glass, sand, common salt, water, and wheat, instead 

 of gold, and ascertained that the rapidity of motion of his 

 pendulum was exactly the same whatever was the kind 

 of matter inside them h . He considered that a difference 

 of a thousandth part would have been apparent. The 

 reader must observe that the pendulums were made of 

 equal weight only in order that they might suffer equal 

 retardation from the air. The meaning of the experiment 

 is that all the substances manifest exactly equal accelera- 

 tion from the force of gravity, and that therefore the 

 inertia or resistance of matter to force, which is the only 

 independent measure of mass in our possession, is always 

 proportional to gravity. 



These experiments of Newton were considered conclu- 

 sive up to very recent times, when certain discordances 

 between the theory and observations of the movements 

 of planets led Nicolai, in 1826, to suggest that the equal 

 gravitation of different kinds of matter might not be 

 absolutely exact. It is perfectly philosophical and desir- 

 able thus to call in question, from time to time, some of 

 the best accepted laws. On this occasion Bessel carefully 

 repeated the experiments of Newton with pendulums 

 composed of ivory, glass, marble, quartz, meteoric stones, 

 &c., but was unable to detect the least difference. This 

 conclusion is also confirmed by the ultimate agreement of 

 all the calculations of physical astronomy based upon it. 

 Thus, whether the mass of Jupiter be calculated from the 

 motion of its own satellites, from the effect upon the small 



11 * Principia/ bk. III. Prop. VI. Motte's translation, vol. ii. p. 220. 



