8 THEORY OF HEAT. 



fused notions. It brings together phenomena the most diverse, 

 and discovers the hidden analogies which unite them. If matter 

 escapes us, as that of air and light, by its extreme tenuity, if 

 bodies are placed far from us in the immensity of space, if man 

 wishes to know the aspect of the heavens at successive epochs 

 separated by a great number of centuries, if the actions of gravity 

 and of heat are exerted in the interior of the earth at depths 

 which will be always inaccessible, mathematical analysis can yet 

 lay hold of the laws of these phenomena. It makes them present 

 and measurable, and seems to be a faculty of the human mind 

 destined to supplement the shortness of life and the imperfec 

 tion of the senses ; and what is still more remarkable, it follows 

 the same course in the study of all phenomena ; it interprets them 

 by the same language, as if to attest the unity and simplicity of 

 the plan of the universe, and to make still more evident that 

 unchangeable order which presides over all natural causes. 



The problems of the theory of heat present so many examples 

 of the simple and constant dispositions which spring from the 

 general laws of nature ; and if the order which is established in 

 these phenomena could be grasped by our senses, it would produce 

 in us an impression comparable to the sensation of musical sound. 



The forms of bodies are infinitely varied ; the distribution of 

 the heat which penetrates them seems to be arbitrary and confused ; 

 but all the inequalities are rapidly cancelled and disappear as time 

 passes on. The progress of the phenomenon becomes more regular 

 and simpler, remains finally subject to a definite law which is the 

 same in all cases, and which bears no sensible impress of the initial 

 arrangement. 



All observation confirms these consequences. The analysis 

 from which they are derived separates and expresses clearly, 1 the 

 general conditions, that is to say those which spring from the 

 natural properties of heat, 2 the effect, accidental but continued, 

 of the form or state of the surfaces ; 3 the effect, not permanent, 

 of the primitive distribution. 



In this work we have demonstrated all the principles of the 

 theory of heat, and solved all the fundamental problems. They 

 could have been explained more concisely by omitting the simpler 

 problems, and presenting in the first instance the most general 

 results; but we wished to shew the actual origin of the theory and 



