MOTION. 21 



the weight, the wheels move, the pendulum is kept oscillating, 

 heat is generated at each point of friction, and the surrounding 

 air is set in motion, a part of which is made obvious to us by 

 the ticking sound. But it will be said, if instead of allowing 

 the weight to act upon the machinery, the cord by which it 

 is suspended be cut, the weight drops and the force is at an 

 end. By no means, for in this case the house is shaken by 

 the concussion, and thus the force and motion are continued, 

 while in the former case the weight reaches the ground quietly, 

 and no evidence of force or motion is manifested by its im- 

 pact, the whole having been previously dissipated. 



If the initial motion, instead of being arrested by the impact 

 of other bodies, as in friction or percussion, is impeded by 

 confinement or compression, as where the dilatation of a gas 

 is prevented by mechanical means, heat equally results : thus 

 if a piston is used to compress air in a closed vessel, the com- 

 pressed air and, from it, the sides of the vessel, will be heated : 

 the air being unable to take up and carry on the original 

 motion, communicates molecular motion or expansion to all 

 bodies in contact with it ; and, conversely, if we expand air 

 by mechanical motion, as by withdrawing the piston, cold is 

 produced. So when a solid has its particles compressed or 

 brought nearer together, as when a bar of iron is hammered, 

 heat is produced beyond that which is due to percussion 

 alone. In this latter case we cannot very easily effect the 

 converse result, or produce cold by the mechanical dilatation 

 of a solid, though the phenomena of solution, where the par- 

 ticles of a solid are detached from each other, or drawn more 

 widely asunder, give us an approximation to it : in the case 

 of solution cold is produced. 



We are from a very extensive range of observation and 

 experiment entitled to conclude that, with some curious 

 exceptions to be presently noticed, whenever a body is com- 

 pressed or brought into smaller dimensions it is heated, i.e. 

 it expands neighbouring substances. Whenever it is dilated 

 or increased in volume it is cooled, or contracts neighbouring 

 substances. 



Mr. Joule has made several experiments for the pur- 

 pose of ascertaining what quantity of heat is produced 



