ON THE condhctrh: powees of vaeious substances. 
833 
L 
mass AC'BC, suppose the depth C'L (very nearly the same as CL) to be three times as 
great as DM. Also let C denote the temperature at any point of LM, that at the 
point C, a central point at the lower sui'face of the sedimentary mass, and 4 the tempe- 
rature at D. The temperature of the surface at C' will be very nearly that of the mean 
external tempera tui’e, and may therefore be considered equal to the superficial tempe- 
rature at D. Consequently, in the particular example we are considering, we shall have 
and the difference of temperature, at L and C, 
= 4400 °-^,, 
= 4400°-(^;+48°); 
while the difference of temperatures at M and D will 
= 4400-4 
Hence if Q and Q! be the quantities of heat transmitted along LC and MD respectively 
in the same time, 
n . n' . . 4400 °- (4 + 48 ) . 4400 ° -4 . 
4 . U • MD ’ 
and since, by hypothesis, LC = 3.MD, we shall have 
Q' = 3Q 
A ery nearly. Consequently the quantity of heat which will be conducted through MD in 
a given time, will equal three times that conducted through CC' in the same time ; and 
since the conductive power in the former case is three times that in the latter, the rate 
of decrease of temperature through CC' and through MD will be very nearly equal. 
If, then, we could admit the hypothesis, that the depth of any deep internal surface 
of equal temperature beneath any point of the earth’s external surface is inversely pro- 
portional to the conductive power of the superficial stratum at that point, we could thus 
account for the equality of the rates of increase of temperature in descending through 
superficial masses of different conducthe powers. There does not appear, however, to 
be any concehable grounds for the admissibility of this very limited hypothesis, according 
to the theory of central heat ; for though irregularities in the structure and conductivity 
of tlie unstratified portion of the earth at such depths might produce considerable irre- 
gularities in the forms of the deep isothermal surfaces, it is difficult to imagine the possi- 
bility of any such relation as that above mentioned, between these irregularities of form 
