COAL MINES OF EASTERN VIRGINIA. 537 



The former of these resuhs, it will be seen, is slightly above 

 the temperature of the Black Heth new shaft, previously cited, 

 though this shaft was fifty feet deeper. This difference is doubt- 

 less o^ving to the very large amount of cooler water which de- 

 scended in drippings to the bottom of the Black Heth shaft, 

 causing a decided reduction of temperature in the rock, in which 

 the thermometer was inserted. 



Assuming 56.° 75, the mean temperature of Shockoe hill, 

 Richmond, as a sufficiently near approximation to that of the 

 region of these mines, which is only about tAvelve miles from the 

 city, the following Tables will exhibit the resuhs of my observa- 

 tions, together with the excess of the subterranean temperature, 

 at each depth, over the average heat of the surface. 



Table I. Observations in Mines in operation at the time. 



Depth. 



Greenhole Pit, . 100 feet, 



Mills and Reed's Pit, 318 " 



" " " " 375 " 



420 " 



Black Heth Engine, 412 " 



Wills and Michael's, 386 " 



570 " 



Table II. Observations in Shafts either just completed 

 or in progress. 



Depth. Temperature. Ezceii. 



Black Heth new shaft, 380 feet, 61.°5 . 4.°75 



Mid Lothian (spring), 330 " . . 61. 75 . 5. 00 



(bottom), 600 " . 66. 25 . 9. 50 



(deep), 780 " . . 68.75 . 12.00 



Making no deduction for the depth of the invariable plane, the 

 results embraced in the first table give, as the rate of increase of 

 temperature with the depth, 1° for 74 feet. 



Assuming sixty feet for the depth of this plane, a distance 

 which, from the open and moist condition of the rocks, I think 

 35 



