MECHANICAL PHILOSOPHY. 425 



calculation. Unfortunately, one of the data necessary to the 

 calculation the magnitude of the earth's circumference was 

 hitherto very erroneously determined ; and the result, in conse- 

 quence, appeared to be inconsistent with the assumed law. In 

 the true spirit of philosophy, Newton at once abandoned his 

 hypothesis, the moment that it appeared at variance with facts ; 

 and it was not until some years afterwards that his thoughts 

 seemed to have been recalled to the subject by the remarkable 

 speculations of Hooke. The measurement of a meridional arc, 

 undertaken by Picard, was soon after completed, and the new 

 determination of the earth's radius happening to be the subject 

 of discussion at the Royal Society, one day that Newton was 

 present, he instantly perceived its bearing upon his own in- 

 quiries, and noting down the result, he hurried home to resume 

 his calculations. He had scarcely substituted the new value in 

 his formula, when the greatness of the anticipated conclusion 

 overpowered him, and he was unable to proceed : a friend who 

 happened to come in at the moment completed the calculation, 

 and the theory of Newton was verified. 



Such was the first step to the theory of universal gravitation. 

 The law was now ascertained, and the analogy which Newton had 

 already remarked, between the moon and earth and the planetary 

 system, led him at once to apply the same great principle to the 

 latter. Accordingly he next inquired what must be the paths, 

 velocities, and periodic times of the planets, on the hypothesis that 

 each of them had received an original impulse in any direction, 

 and was at the same time urged towards the sun with a force 

 varying inversely as the square of the distance. The result of 

 this inquiry was the complete confirmation of the principle. The 

 known motions of the planets, and even of the comets, were shown 

 to flow immediately from the law of gravitation ; and the famous 

 laws of Kepler were among its first consequences. 



It remained to prove that the forces by which these vast 

 bodies are actuated, and which they exerted in turn, arose from 

 the combined attraction of all their parts ; and that the law 

 which was thus manifested among the great bodies of the solar 

 system was, in fact, a universal property of matter. Many 

 strong arguments induced Newton to take this for granted ; and 

 assuming that every particle of matter attracts with a force 

 directly proportional to its mass, and inversely as the square of 



