1872.] 



between Explosions in Collieries and Weather. 



303 



and under conditions similar to those we have described, some time, 

 possibly several days, must elapse after the causes which have led to its 

 formation have disappeared before the contents of such a cavity shall have 

 been rendered innocuous again. 



We do not propose to treat specially of sudden "eruptions' 1 of gas. 

 Such occurrences, at least of any serious magnitude, are very rare. When 

 they do take place in collieries where naked lights are used, the gas 

 takes fire at the nearest such light, and at most one or two men are 

 injured. If safety -lamps are employed, the gas may pass those which 

 are in good order and travel on until it meets a defective lamp, when the 

 whole explodes. 



A few words on the subject of the dependence of ventilation on tem- 

 perature above ground may not be undesirable. 



When the temperature of the air at the surface is less than that in the 

 mine, the phenomenon called Natural Ventilation ensues. The colder air 

 which descends the downcast shaft is heated nearly to the temperature of 

 the workings on its passage through the air-courses and along the working 

 faces, and when it reaches the upcast shaft it has a temperature which is 

 nearly constant if the workings are extensive. The temperature of the 

 workings increases with their depth from the surface : thus, speaking 

 generally, it is 55° at 50 fathoms, 60° at 100, 65° at 150, 70° at 200, and 

 so on*. Now, natural ventilation ceases when the temperature at the 

 surface is the same as that in the workings, and, moreover, as the tempe- 

 rature at the surface rises above that point there is an increasing resistance 

 to artificial ventilation. 



The amount of the force which produces natural ventilation is still fur- 

 ther modified by the changes in the hygrometric state of the atmosphere. 



For instance, if the tension of aqueous vapour in the air is less than 

 that due to the temperature, water will be evaporated in a wet downcast 

 shaft, and the air will not be able to rise in temperature as it descends, 

 and may be actually much colder at the bottom than at the top of the 

 shaft. These considerations will show why shallow mines are much less 

 easily ventilated, and also much more affected by surface temperature 

 than deep ones. Whatever be the artificial means adopted for producing 

 ventilation at any mine, the quantity of air passing through the workings 

 must vary with every variation of the natural force, unless the artificial 

 power be changed at the same time. It follows, therefore, that if there be 

 no means for ascertaining what is the actual quantity of air supplied to 

 the workings at every instant t, a slight decrease, sufficient to bring the 

 colliery into a dangerous state, may take place without being noticed. 



* Report of the Commissioners appointed to inquire into several matters relating to 

 Coal, 1871, vol. i. p. vii, and Report A, p. 86. 



t The Select Committees on Accidents in Mines, 1835 and 1852, were aware of this, 

 and were anxious to introduce instruments whereby a constant check might be kept 

 over the veDtilation. 



