WHEWELL'S WRITINGS AND CORRESPONDENCE. 531 



Thus experiments on the fall of bodies may be regarded as experiments of 

 research into the laws of gravity. We find by careful measurements of times 

 and distances that the intensity of the force of gravity is the same whatever 

 be the motion of the body on which it acts. We also ascertain the direction 

 and magnitude of this force on different bodies and in different places. All 

 this can only be done by careful measurement, and the results are affected by 

 all the errors of observation to which we are liable. 



The same experiments may be also taken as illustrations of the laws of 

 motion. The performance of the experiments tends to make us familiar with 

 these laws, and to impress them on our minds. But the laws of motion cannot 

 be proved to be accurate by a comparison of the observations which we make, 

 for it is only by taking the laws for granted that we have any basis for our 

 calculations. We may ascertain, no doubt, by experiment, that the acceleration 

 of a body acted on by gravity is the same whatever be the motion of that 

 body, but this does not prove that a constant force produces a constant accele- 

 ration, but only that gravity is a force, the intensity of which does not depend 

 on the velocity of the body on which it acts. 



The truth of Dr Whewell's principle is curiously illustrated by a case in 

 which he persistently contradicted it. In a paper communicated to the Philo- 

 sophical Society of Cambridge, and reprinted at the end of his Philosophy of 

 the Inductive Sciences, Dr Whewell conceived that he had proved, a priori, that 

 all matter must be heavy. He was well acquainted with the history of the 

 establishment of the law of gravitation, and knew that it was only by careful 

 experiments and observations that Newton ascertained that the effect of gravi- 

 tation on two equal masses is the same whatever be the chemical nature of 

 the bodies, but in spite of this he maintained that it is contrary not only to 

 observation but to reason, that any body should be repelled instead of attracted 

 by another, whereas it is a matter of daily experience, that any two bodies 

 when they are brought near enough, repel each other. 



The fact seems to be that, finding the word weight employed in ordinary 

 language to denote the quantity of matter in a body, though in scientific language 

 it denotes the tendency of that body to move downwards, and at the same 

 time supposing that the word mass in its scientific sense was not yet sufficiently 

 established to be used without danger in ordinary language, Dr Whewell en- 

 deavoured to make the word weight carry the meaning of the word mass. Thus 

 he tells us that "the weight of the whole compound must be equal to the 



weights of the separate elements." 



' 672 



