it SPHERICAL BODTES. 357 
heat may be loft and diffipated in the boundlefs fields of va- - 
cuity, or of ether, which furround the earth, no fuch equili- 
brium can be eftablifhed. The temperature of the earth will 
then continue to augment only, till the heat which iffues from 
it every moment into the furrounding medium, become equal 
to the increafe which it receives every moment from the fup- 
pofed central refervoir. When this happens, the temperature at 
the fuperficies can undergo no farther change, and a fimilar 
effect muft take place with refpect to every one of the fphe- 
rical and concentric ftrata into which we may conceive the 
folid mafs of the globe to be divided. Each of thefe muft in 
time come to a temperature, at which it will give out as much 
heat to the contiguous ftratum on the outfide, as it receives 
from the contiguous ftratum on the infide ; and, when this hap- 
pens, its temperature will remain invariable. 
5. THAT we may trace this progrefs with more accuracy, 
let us fuppofe a fpherical body to be heated from a fource of 
heat at its centre ; and iet 4, 4’, hb’, be the temperatures at the 
furfaces of two contiguous and concentric ftrata, the diftances 
from the centre being «x, x’, #”; and let it alfo be fuppofed, that 
the thicknefs of each of the ftrata, to wit, x/—x, and x«”—12’, is 
very {mall. 
TueEn fuppofing the body to be homogeneous, the quantity 
of heat that flows from the inner ftratum into the outward, in 
a given time, will be proportional to the excefs of its tempera- 
ture above that of the outward ftratum multiplied into its quan- 
tity of matter, that is, to (4 — hb’) (#’? —x’). 
6. In 
