604 THE PRINCIPLES OF SCIENCE. [CHAP. 



showed, in opposition to the Aristotelians, that matter is 

 equally affected by gravity, irrespective of its form, 

 magnitude, or texture. All objects fall with equal rapidity, 

 when disturbing causes, such as the resistance of the air, 

 are removed or allowed for. That which was rudely 

 demonstrated by Galileo from the leaning tower of Pisa, 

 was proved by Newton to a high degree of approximation, 

 in an experiment which has been mentioned (p. 443). 



Newton formed two pendulums, as nearly as possible the 

 same in outward shape and size by taking two equal round 

 wooden boxes, and suspending them by equal threads, 

 eleven feet long. The pendulums were therefore equally 

 subject to the resistance of the air. He filled one box 

 with wood, and in the centre of oscillation of the other he 

 placed an equal weight of gold. The pendulums were then 

 equal in weight as well as in size ; and, on setting them 

 simultaneously in motion, Newton found that they vibrated 

 for a length of time with equal vibrations. He tried the 

 same experiment with silver, lead, glass, sand, common 

 salt, water, and wheat, in place of the gold, and ascertained 

 that the motion of his pendulum was exactly the same 

 whatever was the kind of matter inside. 1 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 substances manifest exactly equal 

 acceleration from the force of gravity, and that therefore the 

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

 independent measure of mass known to us, 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 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 



Principia, bk. iii. Prop. VI. Motte's translation, vol. ii. p. 220. 



