28 CORRELATION OF PHYSICAL FORCES. 



hand : the motion, which has apparently ceased, is taken up 

 by the air, from the air by the walls of the room, &c., and 

 so by direct and reacting waves, continually comminuted, but 

 never destroyed. It is true that, at a certain point, we lose 

 all means of detecting the motion, from its minute subdivi- 

 sion, which defies our most delicate means of appreciation, 

 but we can indefinitely extend our power of detecting it ac- 

 cording as we confine its direction, or increase the delicacy 

 of our examination. Thus, if the hand be moved in uncon- 

 fined air, the motion of the air would not be sensible to a per- 

 son at a few feet distance ; but if a piston of the same extent 

 of surface as the hand be moved with the same rapidity in a 

 tube, the blast of air may be distinctly felt at several yards 

 distance. There is no greater absolute amount of motion in 

 the air in the second than in the first case, but its direction 

 is restrained, so to make the means of detection more facile. 

 By carrying on this restraint, as in the air-gun, we get a 

 power of detecting the motion, and of moving other bodies at 

 far greater distances. The puff of air which would in the 

 air-gun project a bullet a quarter of a mile, if allowed to es- 

 cape 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 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 expo- 

 nent 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 associa- 

 ted with the moving body, and which, when this impinges on 



