22 



Prof. J. Prestwich. 



has been exposed. 7th. Whether or not there is much gas given off 

 from the coal. Besides these more essential points, it is well to know 

 the temperature of the air outside, the dip of the strata, their hydro - 

 geological conditions, together with any local causes tending to 

 increase or lower the temperature. 



It is only when all these essential conditions are known and can 

 be taken into account, that we can hope to arrive at a more exact 

 estimate of the real rate of increase of temperature with depth 

 in Coal Mines. It is to be feared that very few of the observa- 

 tions recorded in Table II satisfy this standard. While they 

 present many points of interest, and confirm the general fact of an 

 increase of temperature with depth, they fail, I think, to give the 

 more precise information required. For these reasons I feel that only 

 a very small selection can be made, and I doubt whether those even 

 give sufficiently true readings, though they may give the best approxi- 

 mation that can at present be obtained. 



In making the selection, the points I have looked to are (L) accu- 

 rate mean surface temperature, (2) height of ground, (3) temperature 

 of air in gallery — or, when that is not given, such a distance from 

 shaft as will ensure the minimum of difference between the tempera- 

 ture of the air and the rock* — (4) distance of the working face from 

 the shaft, and (5) depth of trial hole. Permanence of temperature at 

 a station is not, as it has been often considered, a sign of its correct- 

 ness, or of its being the true normal temperature. On the contrary, 

 when it is stated that the thermometer in the same hole continues to 

 give the same reading for a long period, it is evident that, instead of 

 having a more definite value, it cannot represent the normal tempera- 

 ture of the rock, but that of the rock or coal after cooling to a point 

 at which an equilibrium has been established between the temperature 

 of the strata and that of the circulating air. Such readings there- 

 fore are too low. 



Amongst the best recorded observations are those at Boldon in the 

 Newcastle coal-field (No. 150). The holes in the rock were there 10 feet 

 deep, the temperature of the air in the gallery was nearly the same as 

 that of the rock, and the mean annual temperature well known. 



At the North Seaton Mine (No. 217), the station being half a mile 

 from the shore, and more than that distance from the shaft, a certain 

 uniformity of temperature between the rock and the air was necessarily 

 ensured, although that is not mentioned. 



The experiments at Hetton Colliery (No. 116) seem to fulfil the re- 

 quired conditions, but I do not place implicit reliance on them, because 

 the holes were first filled with water and then left for forty-eight 



* It is said that at about the distance of a mile from the shaft the temperature 

 of the air approximates to that of the coal, but this of course is a yery variable 

 result. 



