66 THEORY OF HEAT. [CHAP. I. 



3rd. All the specific coefficients which regulate the action 

 of the heat, that is to say, g, K, H and h, compose, with the 



dimension e, in the value of m n a single element f + 77+ fr&amp;gt; 



whose value may be determined by observation. 



If we doubled e the thickness of the boundary, we should 

 have the same result a&amp;gt;s if, in forming it, we employed a sub 

 stance whose conducibility proper was twice as great. Thus the 

 employment of substances which are bad conductors of heat 

 permits us to make the thickness of the boundary small; the 



o 



effect which is obtained depends only on the ratio - . 



4th. If the conducibility K is nothing, we find 

 that is to say, the inner air assumes the temperature of the 

 source : the same is the case if H is zero, or h zero. These con 

 sequences are otherwise evident, since the heat cannot then be 

 dispersed into the external air. 



5th. The values of the quantities g, H, h, K and a, which 

 we supposed known, may be measured by direct experiments, 

 as we shall shew in the sequel ; but in the actual problem, it 

 will be sufficient to notice the value of m n which corresponds 

 to given values of cr and of a, and this value may be used to 



determine the whole coefficient j- + ^ + jj. , by means of the equa- 



ii/ j\. jj. 



tion m n (a n}-p~ (1 +- p] in which p denotes the co 

 efficient sought. We must substitute in this equation, instead 



of - and a n, the values of those quantities, which we suppose 

 s 



given, and that of m n which observation will have made 

 known. From it may be derived the value of p, and we may 

 then apply the formula to any number of other cases. 



6th. The coefficient H enters into the value of m n in 

 the same manner as the coefficient h; consequently the state of 

 the surface, or that of the envelope which covers it, produces 

 the same effect, whether it has reference to the inner or outer 

 surface. 



We should have considered it useless to take notice of these 



