On Underground Temperatures. 



23 



hours, after which the thermometers were left in them for twenty-four 

 hours, so that the coal was exposed for at least for three days, if not 

 more, to a current of 5000 cubic feet of cooler air per minute. Further, 

 it is stated that there was no difference in the temperature after the 

 lapse of a fortnight. I infer from this that the coal here again had 

 reached a cooling point at which the equilibrium between the air and 

 coal had been temporarily established. 



The experiments at South Hetton (No. 136) have especial value, as 

 they were made in a bore-hole drilled into the strata at the bottom of a 

 shaft, 1066 feet deep, to a further depth of 878 feet. The first 

 reading made on the completion of the hole, and while yet dry, gave a 

 temperature of 96° F. Three years later the experiments were repeated, 

 but in the meantime the hole had become filled with water, and silted up 

 to the depth of 234 feet. The thermometer was, however, pushed down 

 in the silt to the depth of 26 feet, and a temperature of 77° recorded. 

 Whilst no doubt the first readings were rather too high, as the heat 

 caused by the tools could not have been all lost,* it is equally probable 

 that convection currents may have cooled the mud in the bore-hole. 

 Taking, however, the temperature in the bore-hole between the depths 

 in it of 100 and 670 feet (1166 and 1736 feet below surface), we 

 have a difference of 11°, which gives a rate of increase of 52 feet per 

 degree. 



The strong ventilation, and the uncertainty about the length of 

 exposure, are objections to the otherwise careful records of the Pendle- 

 ton Colliery (No. 104), and the same doubts attach to the other deep 

 works of this district. It is true that many of the observations are 

 said to have been made in newly-opened ground, but we do not know 

 whether this means an hoar, a day, or a week, and as in all newly- 

 opened mines the ventilation must be well kept up for the sake of the 

 workmen, the rate of cooling must be rapid. 



The Wakefield pit (No. 120) bad been newly opened. Ventilation 

 was stopped as much as possible at the stations, and the temperature 

 of the air was very near the normal ; the hole in the coal was 6 feet 

 deep, and distant 600 yards from the shaft. In the Barnsley pit 

 (No. 119) the station was too near the shaft, and the air was much 

 below the normal. 



The observations, both in the Radstock and Kingswood Collieries 

 (Nos. 163 and 155), were made in holes only 2 feet deep, and the ther- 

 mometers allowed to remain respectively two, five, and seven days — 

 long enough for the rock to cool, as shown by the fact that after 

 another week the thermometer at Kingswood gave the same reading. 

 The temperature of the air is not given. 



I pass over the Upper Daffryn, Tredegar, Cwyn Neol, and other 



* The boring was only stopped about twenty minutes before the experiments 

 were made. 



