46 
Mr Tredgold on the Principles and Practice 
when there is only a small difference between the temperatures 
of the external and internal air. When the diflPerence does not 
exceed 10°, and the internal air is at 60°, then, 
.95 N 
N 
irr the area of the ventilator in feet or — — area with 
h 
sufficient accuracy. 
There will be much advantage in dividing this area, so that 
the air may rise through several outlets instead of one ; and, 
consequently, operate more uniformly in ventilating the room. 
When the cold air enters, the apertures should be. not less than 
double the area of the outlets for hot air. 
The same rule applies to the ventilation of churches, courts 
of justice, and the like ; and it is exceedingly simple and easy of 
application. 
It is not difficult to cause the ventilators to open or close in 
proportion to the temperature of the room. The difference be- 
. tween the expansion of iron and zinc rods might be made the 
means of opening the registers, whenever the temperature rose 
above the intended degree. The same thing may be done by 
expansion of mercury ; and, perhaps, still easier by the expan- 
sion of air. Attendants seldom think it necessary to open ven- 
tilators till the heat has become oppressive; the influx of cold 
air is then dangerous; and, therefore, it is desirable that ventila- 
tion should be self-acting. They should begin to open as soon 
as the temperature exceeds 54° of Fahrenheit, and be quite 
open at 70°. 
In cases where the ventilation is likely to be interrupted by 
winds, it may be much assisted by placing a lamp in the upper 
tube, the heat of which will serve to maintain an ascending cur- 
rent; but it will in most cases be quite sufficient to depend on the 
heat generated by the individuals in the room ; which must, at 
least, be sufficient to raise the temperature of four cubic feet of 
air 10° in a minute for each individual The advantage to 
be derived from using a lamp, consists in establishing a current 
at first, and, by that means, preventing the cool walls from con- 
* It is shewn, in the “ Principles of Warming and Ventilating Buildings,” 
p. 161, that the breath alone will warm 2 ^ cubic feet 5° per minute, and the heat 
given off by the surface of the body will be equivalent to the rest. 
