PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. I19 
| (which is equivalent to the expression in the Theorie de la Chaleur, p. 499, Eq. (26) ), 
gives the following numerical result :-— 
Trap. Sand. Sandstone. 
14:124 16°137 24-750 
but if referred to the French Metre instead of foot as unity (the centigrade de- 
gree has been already employed), they become 
a 4:588 5:242 8:040 
which are comparable with Porsson’s result, 5-11655 for the Observatory of Paris. 
Now, the specific heat c having been found in the last section, we may eliminate 
it, and obtain the following numerical values of /, the conducting power of the 
strata, which it may be presumed has rarely been so accurately determined for 
any kind of matter. 
Trap. Sand. Sandstone. 
k 11°120 8:260 29-884 
There is another constant 6 employed by Poisson, which involves the character 
of the recipient surface of the ground as well as the interior conductivity, and 
which is determinable from the retardation of epochs by equation (27) of page 499 
of the Theorie de la Chaleur. 
ie | cot [2 (0 + )-2ave | 360°—1 \ 
where 6 and 6, are the epochs of maximum and minimum temperature at any 
given depth, reckoned from the 21st March in fractions of a year (= 1), the metre 
being also the unit. Instead of taking observations at a single depth, we may 
take the epoch for 24 French feet from the interpolating lines in Plate X., which 
represent not merely the observations at that depth, but the result of their com- 
bination with all the others. 
Trap. Sand. Sandstone. 
Maxima at 24 F. ft., aly 8 = Sit ui ialridiy=f498 May 6.=234 
Minima, é ; : Jan. 4.=1-008 Dec. 26.=:984 Noy. 4.=°841 
Mean reckoned from Ist Jan., : 461 740 591 
Reckoned from 21st Mar.=3 (0+ 0,) +545 “594 “BIT 
Substituting the values of z=7°7961 metres (24 F. ft.), and of a before found, we 
obtain 
Trap. Sand. Sandstone. 
b 0:4972 _ 01007 0-0772 
M. Potsson finds for 6 at the Paris Observatory, the value 1:057. If we examine 
the circumstances which influence the value of }, we shall admit that its determi- 
nation in this manner is liable to so great errors as to render it almost worthless. 
