204 



TROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



VII. Results of the Observations. 



A. Mean Temperature at different chpths. 



It has been observed at Brussels and elsewhere,* that, even at depths less than 

 25 feet, the mean annual temperature indicated by the lower thermometers is 

 greater than that of those nearer the surface. This appears to be also clearly 

 established by the observations at the three stations near Edinburgh, as contained 



The cause of the increased mean temperature below, is by no means clear. 

 From its UTegularity, it is most probably due to several causes, of which the cen- 

 tral heat of the earth is perhaps one ; its effect at 25 feet need not be insignifi- 

 cant, since the average rate of increase at great depths is 1' Fahr. for from 40 to 

 50 English feet. In the present case, the increase is not uniform, and it is also 

 decidedly different in the different soils, and wiU appear by the sequel to be inti- 

 mately connected with the conducting power of the strata. The order of magni- 

 tude of the increase is this, 1. Observatory, 2. Experimental Garden, 3. Craigleith ; 

 — or, Trap, Sand, Sandstone, which is also the order of the conducting power. The 

 following table shews that, in every instance but one (from 6 feet to 12 feet at 

 Craigleith), the increase was apparent. 



QuETELET, Aunales de I'Observatoire Boyale de Bruxelles, iv. 150. 



