CO SCIENCE IN SHORT CHAPTERS. 



venient distance from each other. If they are deep, it 

 becomes necessary to commence ventilation of the mere shafts 

 themselves in the course of sinking. This is done 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 between 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 U 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 fire- 

 place 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 

 stimpf. 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 toward 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 threaten- 

 ed. 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 downcast 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 proceed 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 suffocation. When this occurs, we 

 make another cross-cut, but in order that it may act, we 

 must stop the first one. So we go on until we reach the work- 



