1885.] 



Conductivity of Hocks, Sfc. 



165 



proportionately less. On the other hand the loss of heat by the 

 underground waters in mineral mines is very important. In some 

 mines in Cornwall, the quantity of water pumped up does not exceed 

 5 gallons, while in others it amounts to 200 gallons per minute. The 

 Dolcoath mine used to furnish half a million gallons of water in the 

 twenty-four hours, while at the Huel Abraham mine it reached the 

 large quantity of above 2,000,000 gallons daily. The rainfall in 

 Cornwall is about 46 inches annually, and of this about 9 inches pass 

 underground. In the Grwennap district, where 5500 acres were com- 

 bined for drainage purposes, above 20,000,000 gallons have been dis- 

 charged in the twenty four hours from a depth of 1200 feet. This 

 water issues at temperatures of from 60° to 68°, or more than 12° 

 above the mean of the climate, showing how large must be the abstrac- 

 tion of heat from the rocks through which the waters percolate. 



Hot springs are not uncommon in these mines. They are due to 

 chemical decomposition, and to water rising in the lodes and fissures 

 from greater depths. The decomposition which goes on in the lodes 

 near the surface, and whereby the sulphides of iron and copper are 

 reduced ultimately to the state of peroxides and carbonates of those 

 metals, is a permanent cause of heat, especially apparent in the 

 shallower mines. On the other hand, where the surface waters pass 

 rapidly through the rocks, they lower the temperature, and give too 

 low readings. 



While ventilation, therefore, reduces the rock temperature, the 

 water which percolates through the rock, and more especially through 

 the veins and cross-courses, sometimes raise, and at other times lower 

 the temperature of the underground springs. Mr. Were Fox, who 

 for many years made observations on the underground temperature 

 of the Cornish mines, gave the preference to the rocks, while 

 Mr. Henwood, an observer equally experienced and assiduous, con- 

 sidered that the underground springs gave surer results. Both were 

 of course fully alive to all the precautions that in either case it was 

 necessary to take to guard against these causes of interference. 



Taking ten of the most reliable of Mr. Henwood's observations at 

 depths of from 800 to 2000 feet, the mean gives a thermometric 

 gradient of 42*4 feet per degree, but Mr. Henwood himself gives us 

 the mean of 134 observations to the depth of 1200 feet, a gradient of 

 41*5 feet to the experiments in granite, and of 39 feet to those in 

 slate. 



Taking the experiments of Mr. Fox in eight mines, varjdng in 

 depth from 1100 to 2100 feet, the mean of the experiments made in 

 the rock give a gradient of 43'6 feet per degree. The mean of the 

 two observers give a gradient of 43 feet per degree. 



For the foreign mines, in the absence of fuller data, and especially 

 failing in information of the depth of the station beneath the surface, 



