204 SCIENCE IN SHORT CHAPTERS. 



as the cold air of the downcast shaft will travel by a de- 

 scending road to the workings, and then after becoming 

 heated will simply obey the superior pressure of the heavy 

 column behind, and proceed by an upward road to the up- 

 cast shaft. As the impelling force of the air current will 

 be the difference between the weight of the cool column of 

 air in the downcast shaft and roads and the warm column 

 in the upcast, the available force of natural ventilation and 

 cooling will increase just as demanded, i.e., it will increase 

 with the depth of the workings and the heat of the rocks. 

 A mining engineer who knows what is actually done with 

 present arrangements, will see at once that with the above- 

 stated advantages a gale of wind or even a hurricane might 

 be directed through any particular roads or long- wall work- 

 ings that were once opened. Let us suppose the depth to 

 be 5000 feet, the rock temperature at starting 133, and 

 that of the outer air 60, we should have a torrent of air, 73 

 cooler than the rocks, rushing furiously downwards, then 

 past the face of the heated strata, and absorbing its heat 

 to such an exent that the upcast shaft would pour forth 

 a perpetual blast of hot air like a gigantic furnace chim- 

 ney. 



But this is not all; the heat and dryness of these deep 

 workings of the future place at our disposal another and 

 still more efficient cooling agency than even that of a 

 hurricane of dry-air ventilation. In the first part of the 

 sinking of the deep shafts the usual water-bearing strata 

 would be encountered, and the ordinary means of "tub- 

 bing" or "coffering" would probably be adopted for tem- 

 porary convenience during sinking. Doorways, however, 

 would be left in the tubbing at suitable places for tapping 

 at pleasure the wettest and most porous of the strata. 

 Streams of cold water could thus be poured down the sides 

 of the shaft, which, on reaching the bottom, Avould flow 

 by a downhill road into the workings. The stream of air 

 Sashing by the same route and becoming heated in its course 

 would powerfully assist the evaporation of the water. The 

 deeper and hotter the pit, the more powerful would be these 

 cooling agencies. 



As the specific heat of water is about five times that of 



