I18 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 
Poisson to express the ratio “f) ‘ where k is the conductivity of the soil and c is 
c 
specific heat. Whence, if the whole quantity B be known, and c the specific heat 
be deduced from direct experiment in the laboratory, £ may be found. [In the 
present instance, it is to be recollected that the Pench foot is taken as the unit. | 
M. Exre bE BEAumonrt, who has taken much interest in the experiments de- 
scribed in this paper, very obligingly requested M. Recnavutr of Paris (whose 
skill in this matter is well known) to determine the specific heat of specimens 
taken from the grounds of the Observatory, Experimental Garden, and Craigleith 
respectively ; and M. Reanautr had the goodness promptly to submit them to 
experiment, and he communicated to me the following results :— 
Specific Heat. 
Porphyry of the Calton Hill, . j , : 0:20654 
Another Experiment, : : : : 0°20587 
Mean, , 0-20620 
Sand of the Experimental Garden, : : : 0°19432 
Sandstone of Craigleith Quarry, i f 3 0°19257 
‘Another Experiment, : : ; ; 0°19152 
0-19205 
Some correction would, no doubt, require to be made for the moisture con- 
tained in the soil, but this appears difficult to apply, and probably would be in- 
considerable. The above results evidently represent specific heats referred to 
unit of weight of the body, but that referred to in the theoretical investigation, is 
taken with respect to unity of volume.* The above results require, therefore, to 
be multiplied by the specific gravities (water being the standard in each case) 
which I have found to be, when reduced to 60° F. 
Trap. Sand. Sandstone. 
Specific gravity, : : . : ; 2-562 1-547 2-408 
Whence we have speeific heat referred to unit of volume, 0:5283 0°3006 0:4623 
G. Final Results. 
The value of Porsson’s constant a, expressive of the ratio a o being obtained 
C; 
from our constant B by means of the relation 
T 
= log e 
a= 
* Poisson, Théorie de la Chaleur ; Suppl., p. 4. 
+ Mean of two experiments, 1°556 and 1538. It is evident, that since it is required to find the 
specific heat of unit of volume of the mass to be heated or cooled, we must take the aggregate of sand 
as we find it in the soil, and not the specific gravity of the individual grains. Accordingly, the specific 
gravity was determined by comparing the weights of closely packed sand and of distilled water contained 
in a stoppered phial. 
