HEAT PHENOMENA. 229 



always more or less fissured, and a shaft or well of any depth commonly 

 drains the surrounding country, so that water from a higher level is almost 

 invariably present at the bottom. If a shaft is kept pumped out, the equi- 

 librium of waters at a lower level may be disturbed, and currents from 

 greater depths will then rise into the excavation. Even when the surface 

 is unbroken it is well known that there are usually subterranean cur- 

 rents, the course of which is determined by the structiire of the rock, 

 and which locally interfere with the regularity of the isogeotherms. While 

 absolute uniformity in the increase of temperature is nowhere to be expected, 

 a vast number of observations show that the variations are usually confined 

 to comparatively thin belts, and that they vibrate about a rate of 1° F. to 

 from 50 to 60 feet of depth. Sir William Thomson makes an increase of 

 1° F. for every 51 feet of descent the basis of his calculations on the secular 

 cooling of the earth. The marked exceptions occur in regions where there 

 are other evidences of an abnormal temperature, furnished by traces of 

 recent volcanic action or by the presence of hot springs. 



Disturbing effect of local causes in mines. If the obserVatlOnS takcU iu VCrtical 



openings of small diameter, such as artesian wells and mining shafts, are 

 subject to fluctuations from local causes like those above mentioned, this 

 must be to a much greater extent the case in an extensive and complex 

 system of mines, such as those which are being worked on the Comstock 

 Lode. The country is honeycombed to a depth of 3,000 feet. Above 150 

 miles of galleries have been driven, besides stopes of a very extensive 

 character, and in many of these artificial ventilation has been going on for 

 years. On account of the great heat, the ventilation is naturally rapid, and 

 is artificially stimulated to the greatest possible extent. The air leaves the 

 mines nearly saturated with aqueous vapor, at an average temperature, 

 according to Mr. Chiirch, of 92° F. In this way an enormous quantity 

 of heat has been abstracted from the rock. Although before the opening 

 of the mines the country was almost absolutely dry, about 7,000,000 tons 

 of hot water are now yearly pumped from the Lode. Mr. Church esti- 

 mates that the heat annually abstracted from the Lode by drainage and 

 ventilation, without considering evaporation, is as great as 55,472 tons of 

 anthracite produce in the best manufacturing usage. The disturbance of 



