in SPHERICAL BODIES. 357 



heat may be loft and diffipated in the boundlefs fields of va- 

 cuity, or of ether, which farround the earth, no fuch equili- 

 brium can be eftablifhed. The temperature of the earth will 

 then continue to augment only, till the heat which ifTues 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 refpedt 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 mult 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 let h, h', b", be the temperatures at the 

 furfaces of two contiguous and concentric ftrata, the diftances 

 from the centre being x, x', x" ; and let it alfo be fuppofed, that 

 the thicknefs of each of the ftrata, to wit, x' — x, and x" — x', is 

 very fmall. 



Then 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 (h — ti) (V 3 — x 5 ). 



6. In 



