16 



Prof. J. Prestwich. 



Even in the two pits off work, we have in the one instance 

 44,550 cubic feet of air passing out per minute, with a gain in tem- 

 perature above that of the outer air of 17° F., and in the other case 

 25,500 cubic feet pass out with a gain of 18° F. 



In these instances the temperature of the outer air was only in one 

 observation under 50° F. At other seasons of the year, with the outer 

 thermometer lower, and with the difference of temperature between 

 the circulating air and the strata greater, the loss of heat would 

 proceed still more rapidly. Mr. Forster mentions that he has known 

 ice form at the bottom of a coal shaft, and Mr. Bald, speaking of the 

 Whitehaven Collieries, states that the circulation of air sometimes 

 causes the water to freeze on the sides of the shaft, and " even form 

 icicles upon the roof of the coal within the mine," whilst the air from 

 the rise-pit issues in a dense misty cloud. 



In consequence of the allowance necessary to be made for these 

 effects of ventilation, some of the witnesses, while taking the actual 

 rate of increase at about from 50 to 60 feet for each degree F., con- 

 sidered that the normal rate might not be more than 50 feet per 

 degree. 



As a rule, the deeper the mine the greater must be the ventilation, 

 and therefore, the more rapid must be the cooling of the underground 

 strata. But the amount of ventilation depends also not only on depth, 

 but on the quantity of gas present in the coal. Mr. "Warington 

 Smyth says,* that " in round numbers 100 cubic feet of air per 

 minute may be required for the health and comfort of each person 

 underground, or for 100 men, 10,000 cubic feet ; but if fire-damp be 

 given off — say at the rate of 200 cubic feet per minute — we should 

 need at the very least thirty times that amount of fresh air to dilute 

 it, or 6,000 cubic feet in addition. Increase the number of men and 

 liability to gas, and 40,000 to 60,000 cubic feet of air may be indis- 

 pensable for safety." 



It is evident, therefore, that the rate of cooling from ventilation in 

 different coal mines and at different depths is likely to be very vari- 

 able. The greater the heat and the more gas in the coal, the more 

 rapid will be the cooling, and this may possibly account for the great 

 discrepancies between the thermometric gradients at different col- 

 lieries. Until, however, we are in possession of all the collateral 

 conditions, it will not be possible to assign in each instance, the precise 

 weight to be attached to this disturbing cause. f 



Other Causes which may affect the Temperature of the Goal Strata in 

 Underground Workings. — Apart from the chemical decomposition of 



* " Coal and Coal Mining," p. 205. 



f Mr. W. Warington Smyth, gives, in his Anniversary Address to the Geological 

 Society in 1868 (pp. 79 — 87), some interesting particulars respecting ventilation 

 and other causes affecting underground temperatures. 



