164 Prof. J. Prestwich. Underground Temperatures, [Feb. 12, 



on the contrary that those experiments in which it is stated that the 

 thermometer has been left in the rock for a period of a week, a 

 month, or two years withont any change of temperature, affords 

 prima facie evidence of error, inasmuch as it shows that the rock has 

 so far lost heat as to remain in a state of equilibrium w T ith the air 

 at the lower temperature in constant circulation. 



Another cause of the loss of heat which requires some notice is the 

 escape of the gas, which exists in the coal either in a highly com- 

 pressed, or, as the author thinks more probable, in a liquid state. A 

 strong blower of gas has been observed to render the coal sensibly 

 cooler to the touch. In another case whereas the temperature of the 

 coal at the depth of 1269 feet was 74° P., at the greater depth of 

 1588 feet in a hole with a blower of gas it was only 62°. One wit- 

 ness observed that "the coal gives out heat quicker than the rock." 

 There is generally a difference of 2° or 3° between the two. 



On the other hand, the coal and rocks when crushed and in 

 " creeps " acquire a higher temperature owing to the liberation of 

 heat by crushing. 



The effects of irregularities of the surface on the underground 

 isotherms, although unimportant in many of our coal-fields, produce 

 very decided results in the observations on the same level in the mines 

 among the hills of South Wales. Sections are given to show how the 

 temperature rises under hills and falls under valleys, showing that it 

 is often essential to know not only the depth of the shaft but the 

 depth beneath the surface at each station where the experiments are 

 made. 



The author therefore considers that to assign a value to an observa- 

 tion we should know — 1. Height of pit above sea level. 2. The 

 exact mean annual temperature of the place. 3. Depth beneath the 

 surface of each station. 4. Distance of the stations from the shaft. 

 5. Temperature and columns of air in circulation. 6. Length of 

 exposure of face. 7. Whether or not the coal is gassy. The dip of 

 the strata and the quantity of water are also to be noted. 



Yery few of the recorded observations come up to this standard, 

 and the author has felt himself obliged to make a very restricted 

 selection of cases on which to establish the probable thermometric 

 gradient for the coal strata. Amongst the best observations are those 

 made at Boldon, North Seaton, South Hetton, Rosebridge, Wakefield, 

 Liege and Mons. These give a mean gradient of 49i feet for each 

 degree F. The bore-holes at Blythswood, South Balgray, arid Creuzot 

 give a mean of 508 feet. 



Mines other than Goal. — The causes affecting the thermal conditions 

 of these mines are on the whole very different to those which obtain 

 in coal mines. Ventilation affects both, but in very unequal degrees. 

 In mineral mines it is much less active, and the cooling effects are 



