74 



VETEEINAEY HYGIENE 



Fig. 13.— Boyle's 

 air - pump ex- 

 tractor. 



curved bell -mouth arrangement ' B,' in which a large 



body of air is collected and forced into the narrow annular 

 space ' C,' at which point, owing to the com- 

 pression of the air, which cannot otherwise 

 escape or free itself, it attains a greatly ac- 

 celerated velocity, and, in passing over the 

 slip ' D,' creates an induced current, exhaust- 

 ing the air with rapidity from the central 

 chamber ' E.' 



The foul air immediately rushes up the 

 shaft to supply the place of the air extracted, 

 and is in its turn drawn off, thus creating 

 a continuous upward current in the shaft 

 connected with the place being ventilated. 

 An exhaust cowl constructed on these prin- 

 ciples may also be employed in ship venti- 

 lation. 

 The extracting power of this and similar contrivances lies 



in the size of the head ; the head should always be about 



double the diameter of the tube to which it is connected. 

 Tubes or shafts may be used either as inlets or outlets. 



The golden rule is never to employ them if they can be 



avoided, and never to use a long 



one if a short will serve the same 



purpose. 



Air passing along a tube 



suffers loss from friction, and 



in consequence its velocity is 



diminished ; the narrower the 



tube the greater the friction. 



Angles or bends in tubes are 



more important causes of loss ; 



for example, if a current be pass- 

 ing along a tube at a velocity of 



10 feet per second, on meeting with a right angle its 



velocity is reduced to 5 feet per second, a second right 



angle will reduce it to 2^ feet per second. 



Thus a right angle diminishes the velocity by half, this is 



Fig. 14.— Plan of Boyle's air- 

 pump. 



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