ON GEOLOGICAL TIME. 57 



ably, last midsummer's heat is now about reach- 

 ing thirty feet below this place. Principal Forbes 

 instituted experiments on the Calton Hill, at the 

 Craigleith Quarry, and the Experimental Gardens, 

 and in these three places the observations were con- 

 tinued for several years, the temperature being ob- 

 served every week. From these observations, he 

 calculated the conducting powers of the different 

 strata, and his results were, I believe, the. first ob- 

 tained of an accurate kind, regarding the conduct- 

 ing power of rock in its natural condition in the 



o 



earth's crust. Angstrom made similar experiments 

 in Sweden, and deduced results on the same 

 principles. Similar observations were made at 

 Greenwich, and calculated by Dr. Everett ; so from, 

 these results we may consider the conductivity 

 of ordinary surface rocks as generally very well 

 known. 



28. But the question, how much does tempera- 

 ture increase downwards from hundred feet to 

 hundred feet, is one which has been but very 

 imperfectly investigated indeed. Observation of 

 temperature in mines, as Schwarez points out, and 



