218 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



PoissoN to express the ratio /_ where k is the conductivity of the soil and c is 



specific heat. Whence, if the whole quantity B be known, and c the specific heat 

 be deduced from direct experiment in the laboratory, k may be found. [In the 

 present instance, it is to be recollected that the French foot is taken as the unit.] 

 M. Elie de BEAUiioNT, Avho has taken much interest in the experiments de- 

 scribed in this paper, very obligingly requested M. Regnault of Paris (whose 

 skill in this matter is well known) to determine the specific heat of specimens 

 taken from the grounds of the Observatorj^, Experimental Garden, and Craigleith 

 respectively ; and M. Regnault had the goodness promptly to submit them to 

 experiment, and he communicated to me the following results : — 



Specific Heat. 

 Porphyry of the Calton Hill, .... 020654 



Another Experiment, .... 0-20587 



Mean, . 020620 



Sand of the Experimental Garden, . . . 0'19432 



Sandstone of Craigleith Quarry, . . . 0-19257 



Another Experiment, . . . . 0-19152 



0-19205 



Some cori'ection would, no doubt, require to be made for the moisture con- 

 tained in the soil, but this appears difiicult to apply, and probably would be in- 

 considerable. The above results evidently represent specific heats referred to 

 unit oi 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 (Avater being the standard in each case) 

 which I have found to be, when reduced to 60° F. 



Trap. Sand. Sandstone. 



Specific gravity, ..... 2-562 l-547t 2-408 



Whence we have specific heat referred to unit of volume, 0-5283 0-3006 0-4623 



G. Final Results. 

 The value of Poisson's constant a, expressive of the ratio J— being obtained 

 from our constant B by means of the relation 



a=\\oge 



* PoissoN, Theorie de la Chaleur ; SuppL, p. 4. 



f 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. 



