274 PROPAGATION OF HEAT. SECT. XXVII. 



to the earth by the sun. On the contrary, dense bodies, 

 especially metals, possess the power of conduction in the 

 greatest degree, but the transmission requires time. If a bar of 

 iron twenty inches long be heated at one extremity, the heat 

 takes four minutes in passing to the other. The particle of 

 the metal that is first heated communicates the heat to the 

 second, and the second to the third : so that the temperature of 

 the intermediate molecule at any instant is increased by the 

 excess of the temperature of the first above its own, and 

 diminished by the excess of its own temperature above that of 

 the third. That however will 'not be the temperature indicated 

 by the thermometer, because as soon as the particle is more 

 heated than the surrounding atmosphere it loses its heat by 

 radiation, in proportion to the excess of its actual temperature 

 above that of the air. The velocity of the discharge is directly 

 proportional to the temperature, and inversely as the length of 

 the bar. As there are perpetual variations in the temperature 

 of all terrestrial substances, and of the atmosphere, from the 

 rotation of the earth, and its revolution round the sun, from 

 combustion, friction, fermentation, electricity, and an infinity 

 of other causes, the tendency to restore the equability of 

 temperature by the transmission of heat must maintain all the 

 particles of matter in a state of perpetual oscillation, which 

 will be more or less rapid according to the conducting powers of 

 the substances. From the motion of the heavenly bodies about 

 their axes, and also round the sun, exposing them to perpetual 

 changes of temperature, it may be inferred that similar causes 

 will produce like effects in them too. The revolutions of the 

 double stars show that they are not at rest ; and although we 

 are totally ignorant of the changes that may be going on in the 

 nebulae and millions of other remote bodies, it is hardly possible 

 that they should be in absolute repose ; so that, as far as our 

 knowledge extends, motion is a law of the universe and the 

 immediate cause of heat, as in the sunbeam so also in all 

 terrestrial phenomena. 



This is by no means hypothetical, but founded upon fact and 

 experiment. Heat is produced by motion and is equivalent to it, 

 for we measure heat by motion in the thermometer. The heat 

 evolved by percussion is proportional to the force of the blow : 

 by repeated blows iron becomes red hot ; and the quantity of 



