INSTRUMENTS ILLUSTRATING KL 



when it is going at the same rate it must contain twice a 

 motion as one of the small ones ; or, as we now say, with the same 

 velocity it has twice the momentum. Now the change of mo- 

 mentum is found to be the same for all bullets, when the momen- 

 tum is reckoned as proportional to the quantity of matter in the 

 bullet as well as to the velocity. The quantity of matter in a body 

 is called its mass; for bodies of the same substance it is, of 

 course, simply the quantity of that substance ; but for bodies of 

 different substances it is so reckoned as to make the rule hold 

 good. The rule for this case^may then be stated thus ; the change 

 of momentum of a body (that is, the change of velocity multi- 

 plied by the mass), depends on the state of strain of adjoining 

 bodies. Regarded as so depending, this change of momentum is 

 called the pressure or tension of the adjoining body, according to 

 the nature of the strain ; both of these are included in the name 

 stress, introduced by Rankine. 



But in the case of projectiles, the acceleration is found to be 

 the same for all bodies at the same place ; and this rule holds 

 good in all cases of planetary motion. So that it seems as if the 

 change of velocity, and not the change of momentum, depended 

 upon the position of distant bodies. But this case is brought 

 under the same rule as the other by supposing that the mass of 

 the moving body is to be reckoned among the " circumstances." 

 The change of momentum is in this case called the attraction of 

 gravitation, and we say that the attraction is proportional to the 

 mass of the attracted body. And this way of representing the 

 facts is borne out by the electrical and magnetic attractions and 

 repulsions, where the cliange of momentum depends on the posi- 

 tion and state of the attracting thing, and upon the electric charge 

 or the induced magnetism of the attracted thing. 



Force, then, is of two kinds ; the stress of a strained adjoining 

 body, and the attraction or repulsion of a distant body. Attempts 

 have been made with more or less success to explain each of these 

 by means of the other. In common discourse the word " force " 



