10 



Prof. J. Prestwich. 



extends to the depth of about 3 feet, and it has been shown by means 

 of thermometers sunk in the grounds that at a depth of* — 



1 foot the monthly fluctuation is from 33° to 54'0° or = 21-0° 



2 feet „ „ „ 36 to 52'5 or = 16'5 

 4 „ „ „ 39 to 51-8 or = 12-8 

 8 „ „ „ 42 to 50-0 or = 8'0 



At the depth of 29 feet at Greenwich, the annual fluctuation is still 

 2° to 3 C F., and to reach the limits of range, or where it is at last 

 reduced to xgth of a degree P., we must go in this climate to a 

 depth of from 50 to 80 feet. 



Now it is evident that, owing to ventilation, there is a permanent 

 difference between the temperature of the air in a mine and the 

 temperature of the rock, analogous to the diurnal variation on the 

 surface of the ground, and that its effects will extend with equal, if 

 not greater, rapidity in the one case as in the other. 



Any surface of rock exposed to air of a lower temperature will lose 

 heat with a rapidity proportionate to the difference in temperature 

 between the two bodies and to the conductivity of the rock. There- 

 fore as convection currents and ventilation are constantly introducing 

 into the shaft and mine air of a lower temperature than the under- 

 ground rocks, these latter must suffer a loss of heat from the moment 

 they are so exposed. To avoid this, the best observers have, as just 

 mentioned, operated in freshly exposed surfaces, where the loss is 

 at a minimum : on the other hand, numerous observations have been 

 made on surfaces exposed for a length of time and often under con- 

 ditions of which we are not informed. 



Thus in the well-known case of the Dukinfield Colliery, near Staley- 

 bridge, it is stated that the bore-holes were driven to such a depth as 

 to be unaffected by the temperature of the air in the shaft, and the 

 thermometers were left in the holes from \ to 2 hours ; but we are not 

 informed of the depth of the holes, nor of the temperature of the air 

 in the shaft, nor of how long the rock had been exposed. We know 

 only that the shaft, which was a very large one and carried to a depth 

 of 2151 feet, took 10 years to sink. At this depth the temperature 

 of the rock was 75° P., and the estimated rate of increase was calcu- 

 lated to be 99 feet for 1° P. The observations were carried on during 

 the whole of the time that the work proceeded. Although we do not 

 know the temperature of the air, the action of convection currents or 

 of ventilation is clearly shown in the irregularities of the rock 

 temperatures in summer and winter, even at great depths. Thus, on 

 the 12th June, 1849, the shaft had reached a depth of 704 feet, and 

 the rock temperature was 58°. The same temperature was noted 

 with slight fluctuations until the 22nd December, at which time the 



* Prof. James D. Forbes, " Trans. Eoy. Soc. Edin.," vol. xvi, p. 189. 



