358 On th PROGRESS of HEAT 
6. In the fame manner, the heat which goes off from the 
fecond ftratum in the fame time, is proportional to (h'—bh”) 
(x3 — x’) ; and thefe two quantities, when the temperature of 
the fecond ftratum becomes conftant, muft be equal to one 
another, or (b— 4’) («’? —x°) = (b'— b”) («#3 — x”). 
Bur becaufe 4—d’, and «’—x are indefinitely fmall, 
b—b =h, and x—x> = 3x°x3 therefore 4x ga'x ma 
given quantity; which quantity, fince * is given, we may re- 
an a x 
prefent by a'4°3 fo that b= 22 = 4 or, becaufe b is 
tor wre ys 
negative in refpect of «, being a deccrement, while the latter is an 
mike 
4 » a x a* 
increment, = — aa and therefore 4 = C-+ ae 
+. To determine the conftant quantity C, let us fuppofe 
that the temperature at the furface of the internal nucleus of 
ignited matter is = H, and y= radius of that nucleus. Then, 
in the particular cafe, when x =7 and 4=H, the preceding 
2 2 
equation gives H=C + Gc 3; fo that C= H— ce and confe- 
2 
quently = H— “+ am or b=H+> CG->) 
3 
8, Ir is evident, from this formula, that for every value of. 
y there is a determinate value of 4, or that for every diftance 
from the centre there is a fixed temperature, which, after a 
certain time, muft be acquired, and will remain invariable as 
long 
