344 SCIENCE IN SHOUT CHAPTERS. 



by driving an air-way a horizontal tunnel from one to the 

 other, and then establishing an " upcast" in one of them by 

 simply lighting a fire there. This destroys the balance be- 

 tween the two communicating columns of air; the cooler 

 column in the shaft without a fire, being heavier, falls 

 against the lighter column, and pushes it up just as the air 

 is pushed up one leg of an \J tube when we pour water 

 down the other. Even in this preliminary work, if the pits 

 are so deep that more than one air-way is driven, it is 

 necessary to stop the upper ways and leave only the lowest 

 open, in order that the ventilation shall not take a short and 

 useless cut, as it does up our fireplace openings. 



Let us now suppose that the pair of pits are sunk down 

 to the seam, with a further extension below to form the 

 water sumpf. There are two chief modes of working a 

 coal-seam: the " pillar and stall " and the "long wall," or 

 more modern system. For present illustration, I select the 

 latter as the simplest in respect to ventilation. This method, 

 as ordinarily worked, consists essentially in first driving 

 roads through the coal, from the pits to the outer boundary 

 of the area to be worked, then cutting a cross road that shall 

 connect these, thereby exposing a "long wall" of coal, 

 which, in working, is gradually cut away towards the pits, 

 the roof remaining behind being allowed to fall in. 



Let us begin to do this by driving, first of all, two main 

 roads, one from each pit. It is evident that as we proceed 

 in such burrowing, we shall presently find ourselves in a 

 cul de sac so far away from the outer air that suffocation is 

 threatened. This will be equally the case with both roads. 

 Let us now drive a cross-cut from the end of each main 

 road, and thus establish a communication from the down- 

 cast shaft through its road, then through the drift to the 

 upcast road and pit. But in order that the air shall take 

 this roundabout course, we must close the direct drift that 

 we previously made between the two shafts, or it will pro- 

 ceed by that shorter and easier course. Now we shall have 

 air throughout both our main roads, and we may drive on 

 further, until we are again stopped by approximate suffoca- 

 tion. When this occurs, we make another cross-cut, but 

 in order that it may act we must stop the first one. So 



