SECT, xxv.] PROPAGATION OF HEAT. 259 



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 tem- 

 perature indicated by the thenrfbmeter, because, as soon as 

 the particle is more heated than the surrounding atmosphere, 

 it loses its caloric 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 re- 

 volution round the sun, from combustion, friction, fermenta- 

 tion, electricity, and an infinity of other causes, the tendency 

 to restore the equability of temperature by the transmission 

 of caloric must maintain all the particles of matter in a state 

 of perpetual oscillation, which will be more or less rapid ac- 

 cording 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 tem- 

 perature, 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, though 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 seems to be a law of matter. 



Heat applied to the surface of a fluid is propagated down- 

 wards very slowly, the warmer and consequently lighter 

 strata always remaining at the top. This is the reason why 

 the water at the bottom of lakes fed from alpine chains is so 

 cold ; for the heat of the sun is transfused but a little way 

 below the surface. When heat is applied below a liquid, 

 the particles continually rise as they become specifically 

 lighter, in consequence of the caloric, and diffuse it through 

 the mass, their place being perpetually supplied by those 



