124 On Ventilating Mines or Hospilahf 



the area of the bore of the cyhnder, and the length of the 

 stroke. 



The dimensions which I have found sufficient for large 

 works an; as follow: 



The bore of the exhausting; cylinder two feet. 



The length six feet, so as Fo afford a stroke of four feet. 



The pipes which conduct the air to such an engine ought 

 not to he less than six inch bore. 



The best rate of working is from two to three strokes a 

 minute; but if required to go much faster, it will be proper 

 to adapt a capacious air-vessel to the pipes iiear the machine, 

 which will equalize the current pressing through them. 



Such an engine discharges more than two hundred gallons 

 of air in a minute; and I have found that astreain of water 

 supplied l)y an inch and a half bore falling twelve feet, is 

 sufficient to keep it regularly workino-. 



A small engine fo pump out two gallons at a stroke, which 

 would be sufficient in many cases, could be worked bv a 

 power equal to raising a very few pounds weight, as the 

 whole machine may be put into complete equilibrium before 

 it begins to work, and there. is hardly any other friction to 

 overcome but that of the air passing through the pipes. 



The end of the tunnel of the Tavistock canal, which it 

 was my object to ventilate, was driven into the hill to a 

 distance of near three hundred yards froln any openino to 

 the surface ; and being at a depth of one hundred and twcniy 

 yards, and all in hard schistus rock, air-shafts would 

 have been attended with an enormous expense; so that the 

 tunnel being a long one, it was most desirable to sink as 

 few as possible, and of course at considerable distances from 

 each other. Thus a ventilating machine was required, which 

 should act with sufficient force thiouoh a length of near 

 half a mile ; and on the side of the hill where it lirst became 

 necessary to apply it, no larger strearri of water to give it 

 motion could be relied on, than such an one as I have men- 

 tioned after the description of the engine, and even that 

 Howcd at a distance from the shaft v. here the enjiine was to 

 be fixed; which made a considerable length of connexion- 

 rods necessary. 



Withm a very short time after the engine began to work, 

 the superiority of its action over those formerly emploved 

 was abundantly evident. The whole extent of the tunnel, 

 which had been uninterruptedly clouded with smoke for 

 some months before, and vyhich the air that was forced in 

 never could drive out, now became speedily so clear, that 

 the day-light and even objects at its mouth were distinciljr 



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