6o GRAVITY, WEIGHT, &c. [Lesson xin. 



moved in various directions, approaching or re- 

 ceding according to the different positions of the 

 planets. 



Since, then, all bodies, terrestrial and celestial, 

 on which experiments or observations can be made, 

 arc found to gravitate towards each other, what 

 has been before said of the universal extension of 

 Gravity will be the more willingly acceded to j I 

 hope, therefore, I may now quit this part of the 

 subject, without giving any additional arguments 

 in support of the opinion. 



Before I conclude this Lesson, I cannot avoid 

 adding a few words to explain the difference be- 

 tween Gravity, freight, and Heaviness ; which 

 appear to me the more necessary, because I am 

 aware that the notions commonly entertained are 

 not very correct. 



In order to form an exact idea of the weight of 

 a body, it must be recollected, that Gravity im- 

 presses, or has a tendency to impress, on every 

 particle of bodies, in an instant, a certain velocity, 

 with which they would fall, if they were not sup- 

 ported ; and that, abstracting the influence of the 

 air, this velocity would be the same for each of 

 the particles of bodies, whatever be their substance. 

 This being observed, we must understand by the 

 weight of a body, the effort necessary to prevent 

 it from falling ; and, it is evident, that in order 

 to this, it is necessary to destroy the velocity which 

 gravity has impressed on every particle. This ef- 

 fort must, therefore, be equal to the sum of the velo- 

 cities of all the particles. Hence it may be naturally 



con- 



