586 SPECIAL METHODS OF DISCOVERY. 



Probably all discoveries of great laws or principles 

 require the employment? of this method. About the year 

 1666, the question occurred to Newton, why do the 

 planets move round the sun, and the moon move round 

 the earth, instead of moving in a straight line ? and he 

 inferred that the earth pulls the moon, and the sun pulls 

 the planets, and holds them by an invisible power of 

 attraction ; and he concluded that the earth pulls every- 

 thing towards it, by what he called the ' force of gravita- 

 tion.' He then calculated, and found the result wrong by 

 a sixth part. He next waited sixteen years, namely, until 

 1682, when he heard of the result of Picard's measure- 

 ment of an arc of the meridian in France, viz., that the 

 earth was larger than had previously been supposed. He 

 then repeated his calculations, and the results agreed with 

 his inference of the action of gravity. 



Hooke appears to have been the first to infer that any 

 variation of the force of gravity at different altitudes 

 might be determined by observing the rate of a pendulum 

 at those places. By means of inference from theory, 

 Newton concluded that a plumb-line would be deflected by 

 the proximity of a mountain. Bougier verified this, in a 

 rough sort of way, in Peru, during the year 1738 ; and 

 Maskelyne, in 1774, confirmed it more perfectly at the 

 mountain Schehallien, near Loch Tay, in Perthshire. Dr. 

 Hutton, also, ascertained the specific gravities of specimens 

 of parts of the mountain, and from the results he obtained 

 he inferred that the average specific gravity of the mountain 

 was about two-and-a-half times that of water, and one-half 

 of that of the entire Earth, and therefore that the Earth was 

 about five times the density of water ; and this is nearly 

 the same as Cavendish inferred from the results of pendu- 

 lum experiments. Bode having discovered, by study and 

 comparison of the distances of the planets from the sun, 



