426 RISE AND PROGRESS OF 



its distance from the attracted body, he showed that spheres thus 

 composed must attract according to the same law. Thus the 

 hypothesis had all the certainty that could arise from the ^agree- 

 ment of its results with established facts : its full confirmation 

 from direct experiment was reserved for a later period. 



That every particle of matter attracts every other, and is itself 

 attracted in turn, is a proposition to which, at first, we find some 

 difficulty in giving credence. We do not see the effects of such 

 attractions, among the smaller masses of matter with which we 

 are surrounded, and therefore cannot easily admit their existence. 

 But the slightest consideration must convince us that these 

 attractions, under ordinary circumstances, cannot be sensible. 

 The force is proportional to the mass of the attracting body ; 

 and as the largest mountain on the earth's surface is but as a 

 grain of sand on an ordinary globe, the attraction of the earth 

 cannot, to any considerable extent, be modified by the effects of 

 the irregular masses on its surface. Delicate observations, how- 

 ever, have rendered these comparatively minute forces perceptible. 

 In Peru, La Condamine noticed the effect of the attraction of a 

 mountain, in causing the plumb-line to deviate from the vertical. 

 Maskelyne observed a similar effect produced by the attraction of 

 Schehallien in Scotland ; and from the amount of this deviation 

 he was enabled to compare the mass of the earth, and therefore 

 its mean density, with those of the mountain. 



The effects of local attractions have been exhibited in another 

 form, in the interesting pendulum experiments of Captain Sabine. 

 This able observer found a difference in the rate of going of 

 his pendulums, amounting to ten seconds in twenty-four hours, 

 and due wholly to the effects of local attraction. The rate was 

 quickest when the soil over which the pendulum vibrated was 

 compact basalt, the heaviest of the known substances composing 

 the crust of our globe; and slowest, when it was alluvial soil, 

 which is among the lightest. From an extensive comparison 

 of these effects with the subjacent strata, he concludes that 

 the nature of the soil may in general be determined, within 

 tolerable limits, and thus the pendulum be rendered a useful 

 instrument of geological inquiry. These important conclusions 

 have, very lately, received the fullest confirmation from the 

 pendulum observations of Captain Foster. 



Time will not permit me more than to allude to the decisive 



