VENTILATION AND YENTILATOES. 
407 
volume of the outer atmosphere. It is, however, sometimes con- 
venient to distinguish the two modes, especially in describing 
mechanical arrangements, and hence we find it usual to designate 
the “ removal” the vacuum method, and the “ introduction ” the 
plenum mode. It may, at first sight, appear to the reader that 
the introduction of 3,000 cubic feet per head per hour into any 
ordinary room would be attended with serious inconvenience ; 
but practically it is not found so. Indeed, if we take any 
room provided with a chimney and fire, we find that the quan- 
tity of air introduced per hour is much greater than we should 
have supposed. By means of a sort of scientific windmill, 
technically styled an anemometer , we are enabled (by counting its 
revolutions per minute) to estimate the velocity of currents of 
air ; and then, the calibre of the shaft through which the 
draught passes being known, we obtain, by a little calculation, 
the exact quantity of air per minute supplied by any aperture. 
We mention this here because the anemometer has been 
placed in the chimney of an ordinary room when the fire was 
burning, and its revolutions showed beyond all question that 
1,004 cubic feet of air per minuxe, or upwards of 60,000 cubic 
feet per hour, passed out of the room, and must have been 
replaced by an equal amount which entered by the usual 
channels. Thus, in this case at least twenty people might have 
been supplied with a healthy atmosphere, provided the air was 
not heated to too high a point. It is customary with writers 
on ventilation to speak of natural and artificial systems of ven- 
tilation ; but as in most cases a fire exists in what is termed the 
natural arrangement, the division is more empirical than cor- 
rect. Without, then, employing this distinction, let us consider 
the condition of one of our sitting-rooms in winter. The fire 
burns brightly, and, as a consequence, several thousand cubic 
feet of air are hourly drawn up the chimney. Whence comes 
the air to replace this loss? The chinks in the door and windows 
are constantly admitting a stream of cold air, and thus ventila- 
tion is effected at the expense of draughts, which produce 
chilled feet, catarrhs, and so forth. Still, ventilation takes 
place. We are now supposing that the lamps have not been 
lighted ; and we think everyone’s experience will show that most 
rooms in which a fire burns well are tolerably well ventilated 
( quoad the amount of air) till, say, the gas is lit. The moment 
the chandelier comes into operation (supposing it to contain five 
ordinary fish-tail burners), the state of things is changed, and in 
the course of half-an-hour or so, this change becomes distress- 
ingly perceptible. Why ? — People never ask themselves this 
question. Because more than twenty additional pairs of lungs 
have begun to use up the air, each burner in use being equiva- 
lent to nearly five persons. This is the great defect of our modern 
