902 VETEEINAEY HYGIENE 



higher, to prevent one taking the wind from the other. 

 They greatly reduce the speed of a vessel, and on this 

 account are disliked by those responsible for navigation. 

 There is also an objection to too many being rigged between 

 the bows and the bridge, as in some vessels they interfere 

 with the view. 



All ventilators, whether of the bell-mouthed variety or 

 windsails, require trimming every time the wind changes 

 its direction or the ship her course, and when this needs 

 doing half a dozen times day and night, it is likely to 

 become neglected, especially at night. 



Scuttles in the ship's side are available in fine weather, 

 and should always be provided with wind scoops which are 

 of the greatest use especially in the tropics, where the 

 motion of the ship is probably the only air current pro- 

 duced. 



All these contrivances depend on a wind being ahead ; 

 when it is astern the ventilation of the horse decks is 

 most defective ; when the ship is in port there is practically 

 no ventilation excepting by diffusion, and the sick list 

 increases at every stop. 



Artificial ventilation appears to be the only real solution 

 for supplying air on board a ship. A ship has been com- 

 pared to a bottle with a narrow neck ; there is plenty of air 

 of the purest kind at sea, the difiiculty is to introduce it ; 

 ships are proverbially close, stuffy, and objectionable. 



Artificial ship ventilation may be effected in many 

 ways. (1) By air conduits opening beneath the fires in the 

 stoke-hole. (2) Exhaust pipes opening into the funnel. 

 (3) Steam being ejected into exhaust pipes. (4) Jets of 

 compressed air introduced into an air conduit, acting either 

 as an extracting or propelling agent. (5) Eotary fans of 

 the centrifugal variety. All the above act either by extract- 

 ing or propelling air, and experience in ship ventilation 

 shows that of the two propulsion is superior to extraction. 



Edmonds' system of steam ejection is used as a means 

 of drawing off foul air or distributing fresh. The apparatus 

 consists of one or more ordinary lobster back or trumpet- 



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