98 DISSERTATION SECOND. [fart ii. 



The principle of gravity which was thus fully established, 

 and its greatest and most extensive consequences deduced, 

 was not now mentioned for the first time, though for the first 

 time its existence as a fact was ascertained, and the law it 

 observes was discovered. Besides some curious references to 

 weight and gravity, contained in the writings of the ancients, 

 we find something more precise concerning it in the writings 

 of Copernicus, Kepler, and Hooke. 



Anaxao;oras is said to have held that '• the heavens are 

 kept in their place by the rapidity of their revolution, and 

 would fall down if that rapidity were to cease.'" 



Plutarch, in like manner, says, the moon is kept from 

 falling by the rapidity of her motion, just as a stone 

 whirled round in a sling is prevented from falling to the 

 ground. 2 



Lucretius, reasoning probably after Democritus, holds, 

 that the atoms would all, from their gravity, have long since 

 united in the centre of the universe, if the universe were not 

 infinite, so as to have no centre. 3 



An observation of Pythagoras, supposed to refer to the 

 doctrine of gravity, though in reality extremely vague, has 

 been abundantly commented on by Gregory and Maclaurin. 

 A musical string, said that philosopher, gives the same sound 

 with another of twice the length, if the latter be straitened bj 

 four times the weight that straitens the former ; and the 

 gravity of a planet is four times that of another which is at 

 twice the distance. These are the most precise notices, as 

 far as I know, that exist in the writings of the ancients, con- 

 cerning gravity as a force acting on terrestrial bodies, or as 

 extending even to those that are more distant. They are 



1 Caelum omnr vehementi circuitu constare, ali;:? reniission^ 



rum. (Diog. Laert. in Anax. Lib. II. Sect. 12.) 



2 De facie in Orbc Luna-. 3 Lib. I. v. 983. 



