On Underground Temperatures. 



21 



of temperature due to the fall of the surface is apparent, though the 

 temperature at Station No. 2 is too low, which the witness attributed 

 to a strong current of air passing at the time. Outside the thermo- 

 meter marked 59° F. 



Fig. 4. — Section at Yochrrw Dowlais, Aberdare District. 



It will be noticed that the observations in all these pits were made 

 when the thermometer outside stood high, so that the cooling effects 

 of the outer air would be less at the stations near the shaft than at 

 other seasons. 



There is no other coal-field in Britain so hilly as that of South 

 Wales ; still it frequently occurs that a shaft is sunk in a valley, and 

 the works pass under adjacent hills (or the reverse), which though 

 not so high as those of Wales, are of sufficient height to influence the 

 temperature more or less. 



Many of these are points which have commonly not been noticed in 

 the discussion of this question, yet they must not unfrequently have 

 influenced the observations. In all coals there is an escape of gas 

 when first opened out, and in some it is very large, as in several of 

 the South Wales pits ; others are merely called " gassy," and there 

 fewer precautions are required. Amongst the pits in England, where 

 recorded temperature observations have been made, and which are 

 situated in districts sufficiently hilly to be affected by irregularity of 

 surface, are the Dukinfield and some of the pits in the Manchester 

 district. 



Selection of Observations. — It will be apparent from the existence of 

 these many disturbing causes, that not only should we know that the 

 instruments employed are properly constructed and properly used, but 

 we should also in all cases know, — 1st. Height of the ground above 

 the sea level. 2nd. The exact mean annual temperature of the surface. 

 3rd. Depth beneath the surface of the ground of each station where 

 the observations are made. 4th. Distance of those stations from the 

 shaft. 5th. Temperature of the air and volume in circulation at the 

 stations. 6th. Exact length of time that the face of the coal or rock 



