BY THE PKESIDENT. 
145 
draughts can be trusted to provide for such a state of things ? 
Practically none. This difficulty is increased, obviously, by the 
facts that we can have little or no control over the temperatures 
of the external air that is to enter and displace that already within 
the room, and that those temperatures may vary greatly, especially 
in such a country as ours. The varying direction and varying 
force of the wind will also materially complicate the problem of 
satisfactory ventilation. All these difficulties are enhanced if the 
house is closely packed amongst others, or if the occupants are 
poor, or if the external air is liable to be impure. 
Of course there is another side to the foregoing picture. An 
occupied house is always warmer inside than outside, hence the 
whole hulk of air it encloses is, under the laws of heat, ever rising 
up through and out of the house, especially from its upper parts v 
while a corresponding quantity of fresh air is ever entering the 
house, especially at the ground floor. Again, under the laws of 
diffusion, the moment the air of a room is rendered impure, and 
during the whole time that it continues impure, it and the external 
pure air commence and continue slowly to interchange with each 
other, through chimneys and open doors and windows, through 
chinks and cracks, and even through ceilings, walls, and floor. 
Again, the available amount of air in a room may be, and perhaps 
generally is, under ordinary conditions, much larger than the 
amount nature demands for the health of the few persons who 
habitually occupy the room. Again, most persons, even invalids, 
can hear a considerable change of the air near them, especially if 
the hulk of air moves, without catching cold. Again, the systems 
of fairly healthy persons can stand the inhalation of even highly 
impure air for the short periods of the various kinds of gatherings 
or meetings they may have occasion to attend, without any serious 
or prolonged harm to health. Lastly, when crowded rooms become 
unbearably close, yet must continue to he crowded for some time 
longer, and windows or doors or both consequently have to be 
opened wide, there are generally some persons who can hear 
without injury, or with only temporary inconvenience, the strong 
draught of inflowing air near such openings. With all these 
mitigations granted, however, there remains the fact that a certain 
amount of ventilation is essential to the occupants of all rooms; 
while it also is certain that the purchasable contrivances termed 
11 ventilators ” cannot be efficient ventilators under all the varying 
conditions mentioned. 
What is the remedy against the imperfect ventilation of rooms ? 
Fortunately experience teaches everybody something of the working 
10 
VOL. IV.—PART V. 
