766 PROCEEDINGS OP SECTION H. 



either by one or several fans acrorcling to the size of the room, 

 are there any disadvantages outweighing the great advantage 

 to be gained by its adoption .'* 



The argument urged against the system that it is unscientific 

 to bring down the impure air to be re-breathed is answered 

 ])y the fact tliat the impure air is not brought down to the 

 level of the occupants as such, for it is, as I have stated, 

 purified by a thorough intermixture with fresh air near the 

 ceiling level, so that when reaching the level of the occupants 

 where it may be re-breathed it would be as pure as it is 

 possible to obtain air in an enclosed and occupied space. We 

 may therefore, I think, without error, affirm that in all cases 

 of ventilation for healthy persons (leaving out of the question 

 special ventilation required for noxious trades, infectious 

 diseases, &c.) the air surrounding the occupants in a room 

 ventilated by the downward system will be as pure, and there 

 is a great probability of its being purer, than the air sur- 

 rounding them in a room well ventilated by the ujDward system. 



It may be put forward that it is a wasteful expenditure of 

 energy to use the mechanical power in opjjosition to the 

 natural upw^ard movement of the warm vitiated air. There is, 

 of course, a certain amount of power required to overcome 

 this law, but even under the most extreme conditions the 

 power that the heated air can exert is very slight as compared 

 with that capable of I)eing exerted by a good form of fan. A 

 well constructed fan can be worked easily and economically 

 so that it shall exert a pressure equal to two or three inches 

 of water, while the pressure exerted by a column of warm air 

 80 feet high, at a temperature of 70° Fahr., and with the 

 external air at 32^ Fahr. is, by the formula — 



T - t X H ^ p 



491 



Where T = Temperature of warm air. 

 t = Temperature of cold air. 

 H = Height of column. 

 P = Pressure or head. 

 Hence 70 - 32 x 80 ^ 3040 ^ g . .-, 

 491 m 



And to obtain the velocity due to this head we have 

 8v/P = V, where V equals the velocity in feet per second. 

 Thus, 8^6*2 = 8 x 2'd = 20 feet, a velocity which gives, 

 according to Hutton's experiments, a pressure in inches of 

 water of '1446. 



