VE^fTILATION OF MINES OR HOSPITALS. 335 



The pipes which conduct the air to such an engine oughit 

 not to be less than six.inch bore. 



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

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

 to adapt a capacious air-vessel to the pipes near 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 a stream of water 

 supplied by an inch and a half bore falling twelve feet, is 

 sufficient to keep it regularly working. 



A small engine to pump out two gallons at a stroke. Small engine. 

 which would be sufficient in many cases, could be worked 

 by a power equal to raising a xary 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 Ventilator ap- 



was my object to ventilate, was driven into the hill to a f^^®^ !'' ^j^f, 

 •' •' ' tunnel of th6 



distance of near three hundred yards from any opening to Tavistock 

 the surface, and being at a depth of one hundred and*^^'^^-' 

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

 would have been attended with an enormous e^tpense; 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 through a 

 length of near half a mile, and on the side of the hill where 

 it first became necessary to apply it, no larger stream of 

 water to give it motion could be relied on, than such a one 

 as I have mentioned after the description of the engines; 

 and even that flowed at a distance from the shaft where the 

 engine was to be fixed, which made a considerable length of 

 ronnexion rods necessary. 



Within a very short time after the engine began to work, It^ action. 

 the superiority of its action over those formerly employed 

 was abundantly evident. The whole extent of the tunnel, 

 which had been' uiiinterruptedly clouded with smoke for 

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

 Jaeyer rould dme o«tj ivow became speedily so cl^ar, that the 



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