WEIGHT AND MATERIAL. 



the heat developed on striking, the resistance 

 of the fluid through which motion takes place, 

 and so on. 



This latter method of measuring quantity 

 depends on the fact that any material particle 

 in motion tends to keep moving straight on at 

 the same speed, unless interfered with by 

 resistance of air, gravity, or some other cause. 

 When the speed and all other circumstances 

 are the same, then the force with which material 

 tends to resist stopping appears to be pro- 

 portional to the number of ulti- 

 mate corpuscles or small par- 

 ticles of which it is made up 

 that is, its total quantity, 

 just as the weight does. This 

 tendency to resist stopping, or 

 any interference with or modifi- 



FIG. 22. CENTRI- .. P .. .. . n i 



FUGAL FORCE. cation of its motion, is called 

 the inertia of the material. 



Now when anything is moving in a circle, 

 its tendency at any moment is, in accordance 

 with this law of inertia, to go straight on not 

 curvingly on, in continuation of the circle, but 

 straight on, in the line which mathematicians 

 call the tangent at the point where it is at the 

 moment. 



Thus, referring to Fig. 22, if a body be re- 

 volving in the circular path A, B, C, D, in the 

 clockwise direction, when it is at A it has a ten- 

 dency to continue in the direction A E, at B 



93 



