VENTILATION. 443 



and therefore becomes lighter than air at an ordinary tempera- 

 ture. 



Were it not that man has taken advantage of this principle, 

 there could not be a deep mine in England. In any deep 

 excavation, even though it be a well, foul air, mostly composed 

 of carbonic acid gas, always collects, and, being much heavier 

 than atmospheric air, lies at the bottom of the pit as surely as 

 hydrogen would rise out of it. To breathe this air is as certain 

 and as sudden death as to take prussic acid, and no mine can 

 be worked as long as " choke-damp " is in it. 



In coal mines there is an additional source of danger, namely, 

 the coal gas, which is nearly identical with our coal gas of the 

 streets, and takes fire when brought into contact with flame. 

 To rid the mines of these gases, a simple, ingenious, and 

 effectual remedy is used. A ventilating shaft is made, which 

 reaches from the bottom to the mouth of the pit. At the 

 bottom, diagonal shafts are made, entering the main shaft, as 

 shown on the right hand of the illustration. One of these is 

 connected with a furnace, and the other, or others, open into 

 the mine. 



The heat of the furnace rarefies the air in the shaft, 

 causing it to rush upwards with great violence, and so, by 

 creating a partial vacuum, to force the air in the shaft to follow 

 it. The loss of air thus caused is supplied by fresh air from 

 above, which, by the law already described, is obliged to take 

 the place of that which was driven out. Thus a complete 

 circulation of air is kept up, and a well-managed mine has a 

 fresher atmosphere than many houses in which the windows are 

 mostly kept shut, and the only ventilation is accomplished by 

 occasionally open doors. 



The " draught " of our domestic chimneys is owing to this 

 principle, and the reason why factory chimneys are built of such 

 enormous height is, that the column of heated air may be 

 increased, and consequently that the draught may be stronger, 

 and the heat of the furnace made fiercer. 



The " Steam-blast," by which the escape steam of engines 

 is sent into the chimney, is another example of this principle, 

 the steam taking the place of the hot air. 



Further examples of the weight of the atmosphere are given 



