SUMMARY. 



391 



ginia mines are dry. This is very true in the sense in which "dry" is used in 

 mining, /. c, tliere are many places where water does not drip from the 

 walls^but the present examination has failed to reveal rocks which are not 

 moist; indeed, the occurrence of really desiccated rock thousands of feet 

 below the surface, near vast quantities of water, would disprove the gen- 

 eralization of the perviousness of rocks, which is one of the best established 

 in geology. Unless, therefore, very strong proof to the contrary can be 

 add"uced,"ihe conduction of heat on the Comstock must be considered as 



taking i)lace in moist rock. 



Discission of the ther.no,.etHc observations.— The rclatiou of the tcmpcrature to the 

 depth from the surface is evidently one of great interest, but not entirely 

 simple. If the rock were wholly uniform in character and unfissured, the 

 relation of temperature to depth would be wholly regular and would be 

 represented by a curvilinear locus. As the source of the heat was ap- 

 proached the rate at which the temperature rose would rapidly increase, 

 and under the ideal conditions supposed, it would be possible to deduce the 

 constants of the equation and to calculate the position of the source of heat. 

 But unless the source of heat were so close to the surface that the errors 

 introduced by the presence of fissures, the lack of homogeneity of the rock, 

 and the percolation of surface water were insignificant in comparison with 

 the rate of increase of the temperature, such a calculation wo<ild not be 

 possible A careful record of temperatures has been kept at three of the 

 newer shafts to a depth of above 2,000 feet. On plotting these temperatures 

 as ordinates and the depths as abscissae no indication of regular curvature 

 appears, being wholly obscured by the fluctuations due to the disturbing 

 causes mentioned. In other words, there is as yet nothing in the observa- 

 tions to show any but local divergences from a strict proportionality between 

 depth and temperature. The source of heat nnist, consequently, he at a 

 very ^.reat distance from the surface as compared with the depth yet reached, 

 and the curve is to be regarded as still sensibly coincident with its tangent. 

 In order to eliminate the fluctuations of temperature as far as possil^le 

 Mr Reade and Dr. Barus have computed the observations made at the 

 Forman, ComUnation, and new Yelloiv Jacket shafts by the method of least 



