DOWNWARD SYSTEM OF VENTILATION. 765 



successful. Thus, the fresh air must of necessity be rendered 

 more or less impure by intermixtui-e with the rising vitiated 

 air before it reaches the occupants of the room. It must 

 therefore be conceded that under ordinary circumstances it is 

 impossible to ensure that the aii- breathed within a room shall 

 be as pure as the source from which the fresh air is obtained. 

 This has been admitted by such eminent authorities as Dr. 

 Parkes and De Chaumont, who have also shown that in 

 order to jiurify the vitiated air sufficiently to keep a room 

 occupied by healthy people in a perfectly healthy condition 

 3000 cubic feet of fresh air must be admitted for each in- 

 dividual per hour. 1 have shown that a given quantity of air 

 may be admitted and yet the room remain very imperfectly 

 ventilated ; there must, therefore, not only be the given 

 quantity of fresh air, but means must be taken to ensure that 

 it shall be admitted in such a way that perfect diffusion and 

 intermixture with the vitiated air shall take place in all parts 

 of the room, and this without any inconvenience to the 

 occupants. 



These conditions, I am of opinion, can only be fulfilled by 

 making use of the essential factor in the upward system, viz., 

 the downward movement of the fresh air. Between the 

 ceiling and the level of the heads of the occupants of any 

 I'oom there is always a space equal to, and in most cases 

 considerably larger than the sjiace between that level and the 

 floor, which can, without the sHghtest inconvenience, be made 

 use of for purifying the warm vitiated air by intermixture 

 with the cool fresh air as fast as it ascends from the lower 

 level. This could not well be accomplished until the 

 expedient was hit upon of placing an ordinary centrifugal fan 

 within the rooitf \tse\i\'loiie to the ceiling, instead of placing 

 the fan outside the room and forcing the air in through tubes. 

 This difference nuikes it easily possible, as I have proved by 

 personal experiments, to distribute the fresh air thoroughly 

 to every part of the room without any perceptible draught; 

 we need therefore have no doubts about the possibility of 

 efficiently carrying out the downward system should it be 

 determined that 1 am correct in thinking it desii'able. 



The best kind of fan to ](e used ; the way in which it shall 

 be worked ; the best form of outlet, and all other details of 

 execution may well be left for consideration at some future 

 period. 



Given, tlien, the possibility of admitting and thoroughly 

 distributing the air at the ceiling level without inconvenience, 



