1 8. CORRELATION OF PHYSICAL FORCES. 



distances. The puff of air which would in the air-gun project 

 a bullet a quarter of a mile, if allowed to escape without its 

 direction being restrained, as by the bursting of a bladder, 

 would not be perceptible at a yard distance, though the same 

 absolute amount of motion be in both the cases impressed on 

 the surrounding air. 



It may, however, be asked, what becomes of force when 

 motion is arrested or impeded by the counter-motion of another 

 body ? This is generally believed to produce rest, or entire 

 destruction of motion, and consequent annihilation of force: 

 so indeed it may, as regards the motion of the masses, but a 

 new force, or new character of force, now ensues, the exponent 

 of which, instead of visible motion, is heat. I venture to 

 regard the heat which results from friction or percussion as a 

 continuation of the force which was previously associated with 

 the moving body, and which, when this impinges on another 

 body, ceasing to exist as gross, palpable motion, continues to 

 exist as heat. 



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

 opposite directions (putting for the moment out of the 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 res- 

 pective direction with the original 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 

 resulting 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, 



