348 ON PHYSICAL DATA 



of the particle, is, I presume, decidedly mechanical. 

 There is no doubt that heat, or caloric, has great 

 influence in disturbing the equilibrium of the 

 attractive and repulsive forces. 



The attractive force which acts upon all bodies 

 is mutual, and varies in its intensity reciprocally, 

 as the square of the distance from the attractive 

 particle to the attracted one ; as was first dis- 

 tinctly stated and enforced by Newton, in his 

 Principia, book iii. prop. 7, in consequence of 

 the facility with which many of the phenomena 

 observed in practical astronomy were explained. 

 There are two ways in which this attractive, or 

 central force, may be supposed to exist in every 

 material particle ; the first, which is the one 

 above stated, where rays of force are supposed 

 to emanate from the particle, in every possible 

 direction, with a variable magnitude that is some 

 function of the distance ; and another way, which 

 is, that the attractive forces which surround a 

 particle are constant in intensity, and unlimited in 

 the distance to which they exercise their influence. 

 If we examine the eff'ects of the latter of these 

 two modes of considering attractive forces, the 

 former being well known, we shall find that the 

 law of gravitation and the inertia of bodies are a 



