MOTION. 29 



another body, ceasing to exist as gross, palpable motion, con- 

 tinues to exist as heat. 



Thus, let two bodies, A and B, be supposed to move in 

 opposite directions (putting for the moment out of question 

 all resistance, such as that of the air, &c.), if they pass each 

 other without contact each will move on for ever in its re- 

 spective direction with the same velocity, but if they touch 

 each other the velocity of the movement of each is reduced, 

 and each becomes heated : if this contact be slight, or such as 

 to occasion but a slight diminution of their velocity, as when 

 the surfaces of the bodies are oiled, then the heat is slight ; 

 but if the contact be such as to occasion a great diminution 

 of motion, as in percussion, or as when the surfaces are 

 roughened, then the heat is great, so that in all cases the re- 

 sulting heat is proportionate to the diminished velocity. 

 Where, instead of resisting and consequently impeding the 

 motion of the body A, the body B gives way, or itself takes 

 up the motion originally communicated to A, then we have 

 less heat in proportion to the motion of the body B, for here 

 the operation of the force continues in the form of palpable 

 motion : thus the heat resulting from friction in the axle of a 

 wheel is lessened by surrounding it by rollers ; these take up 

 the primary motion of the axle, and the less, by this means, 

 the initial motion is impeded, the less is the resulting heat. 

 Again, if a body move in a fluid, although some Jjeat is pro- 

 duced, the heat is apparently trifling, because the particles of 

 the fluid themselves move, and continue the motion originally 

 communicated to the moving body : for every portion of mo- 

 tion communicated to them this loses an equivalent, and 

 where both lose, then an equivalent of heat results. 



As the converse of this proposition, it should follow that 

 the more rigid the bodies impinging on each other the greater 

 should be the amount of heat developed by friction, and so 

 we find it. Flint, steel, hard stones, glass, and metals, are 

 those bodies which give the greatest amount of heat from 



