VENTILATION 



83 



excepting under very special conditions, both from its cost 

 and the probabiUty of a break-down ; yet when horses are 

 stabled underground, or in tunnel-shaped structures with 

 no openings above, or on board a ship, it may prove a very 

 useful system. 



Its convenience of adoption largely depends upon what 

 motive power is available. On board a ship this should 

 not be difficult to arrange either by steam or electricity, 

 but on shore it is likely that special apparatus may have to 

 be supplied for this system of ventilation, and this adds 

 greatly to the expense. 



Fans of different forms have been used for many 



years for propelling air, in ,. .. ...^.^^^-v, :-,. 



mines. Since the intro- 

 duction of electricity they 

 have been commonly em- 

 ployed in crowded places 

 either for propelling or ex- 

 tracting, and a considerable 

 quantity of air can be 

 delivered or removed by 

 these means. 



A well - known form of 

 fan is that seen in Fig. 25, 

 known as the Blackman 

 Air - Propeller. Depending 

 on its size, from 6,000 to 

 minute can be delivered for one horse-power expended in 



driving it. 



Ventilation by propulsion is considered more satisfactory 

 than that by extraction; but there is no necessity to be 

 limited to one system. Both may be advantageously 

 combined, and with such natural ventilation as may be 

 available, it should be possible to keep the air of a place, 

 even that of a horse- ship, comparatively pure. This 

 subject will be touched on again in dealing with marine 

 hygiene, when reference will also be made to extraction of 



air by means of a steam jet. 



6—2 



a»73 



Fig. 25. — Blackman Air-Propeller. 



12,000 cubic feet of air per 



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