MECHANICAL PHILOSOPHY. 427 



experiment of Cavendish. By an instrument called the balance 

 of torsion an instrument of the highest value in the measurement 

 of minute forces this philosopher was enabled to measure with 

 precision the attraction of large metallic globes, and to compare 

 their forces with that of the earth itself. From these investiga- 

 tions it appeared that the mean density of the earth is about 

 double of that of its superficial parts, and five and half times 

 that of water. 



I cannot terminate this subject without noticing the inter- 

 esting experiment proposed by Mr. Whewell and Professor Airy, 

 in order to determine the law of terrestrial gravity by direct 

 observation. The experiment consisted in comparing the rate of 

 a pendulum clock with that of a chronometer, at the surface of the 

 earth, and at the bottom of one of the deepest mines in England. 

 The rate of the pendulum varying with the force of gravity, 

 while that of the chronometer is independent of it, it is obvious 

 that their comparison would afford the means of determining the 

 relative forces at the two stations. The experiment, I believe, 

 was not completed. But there can be no doubt of its sufficiency ; 

 and it is a remarkable fact that Bacon proposed one altogether 

 the same in principle, and having nearly the same objects in view. 

 He observes, that if the tendency of bodies downwards be the 

 result of the earth's attraction, it must vary with the distance 

 from the earth's centre, and he proposes to determine this by 

 comparing the effects of a weight and of a spring at different 

 heights and depths. 



Such are a few of the leading facts by which the universal 

 gr a citation of matter was established. But numberless appli- 

 cations of the principle at once suggested themselves to its 

 discoverer. The inequalities in the lunar motions, some of which 

 had been detected by observation so far back as the time of 

 Hipparchus and Ptolemy, were shown to arise from the disturbing 

 attraction of the sun. The evection, the variation, and the annual 

 equation, were all traced to this cause ; and the same cause was 

 found sufficient to explain the principal of the inequalities affect- 

 ing the orbit itself, such as its change of inclination to the ecliptic 

 and the motion of its nodes. 



It had been observed by Richer, Varin, and Des Hayes, that 

 a pendulum vibrated more slowly near the equator than in the 

 higher latitudes, and the result had been a subject of much 



