6*14 LECTURE XLIX. 



xinertia is that property of matter, by which it retains its state of rest" or 

 of uniform motion, with regard to a quiescent space, as long as no foreign 

 cause occurs to change that state. This property depends on the intimate 

 constitution of matter; it is generally exhibited by means of the force of 

 repulsion, which enables a body in motion to displace another, in order to 

 continue its motion, or by means of some attractive force, which causes two 

 bodies to approach their common centre of inertia with equal momenta. 



Another universal property of matter is reciprocal gravitation, of which 

 the force is directly in the joint proportion of the quantities of matter attract- 

 ing each other, and inversely as the square of their distance. In order to 

 prove that the gravitation towards a given substance, for instance, the 

 weight of a body, or its gravitation towards the earth, is precisely in pro- 

 portion to the mass or inertia of the moveable matter of which it consists, 

 Sir Isaac Newton made two equal pendulums, with hollow balls of equal size : 

 in order that the resistance of the air might be the same with respect to both, 

 he placed successively within the balls a variety of different substances, and 

 found that the time of vibration remained always the same; whence he inferred 

 that the attraction was proportional in all cases to the quantity of matter 

 possessing inertia. For if any of these substances had contained particles, 

 capable of receiving and communicating motion, yet without being liable to 

 gravitation, they would have retarded the vibrations of the pendulum, by 

 adding to the quantity of matter to be moved, without increasing the moving 

 force. The law of gravitation, which indicates the rario of its increase with 

 the diminution of the distance, is principally deduced from astronomical 

 observations and computations: it is the simplest that can be conceived for 

 any influence, that either spreads from a centre, or converges towards a centre; 

 for it supposes the force acting on the same substance to be always propor- 

 tional to the angular space that it occupies. 



Newton appears to have considered these laws of gravitation,, which he 

 first discovered, rather as derivative than as original properties of matter; 

 and although it has often been asserted that we gain nothing by referring 

 them to pressure or to impulse, yet it is undoubtedly advancing a step in the 

 explanation of natural phenomena, to lessen the number of general principles; 

 and if it were possible to refer either all attraction to a modification of re- 



