8. Webber on Ventilation. 318 
‘In the season of the year when it is most oe to have 
doors and windows closed, artificial warmth is needed, and. this 
supplies also a power for propelling a current of air ‘naltale the 
ventilators. A common oper st ete witha sve: cconstruoted 
sera either for burning wood on the hea ‘coal 1 
grate a little elevated, is one of the bose ‘of einilasings panne 
‘tuses. It will answer a good purpose even without a n 
ever the room is warmer than the outward air, and from the low 
level of its opening, being generally even with the floor, or only 
‘slightly elevated, it is particularly fitted to carry off the lower 
air containing the largest quantity of carbonic acid gas. An 
Opening into the flue of the chimuey above the fire-place, and 
near the ceiling is the best situation for the upper ventilator, as 
it not only acts well. there’ itself in favoring the escape of the 
Warmer and lighter impure-air, bes by its transmission upwards 
increases the draft wnbsce the opening of the fire-place below. 
This opening however may .be very much smaller than that 
below, and indeed if as below be large and the draft of air 
upwards through it good, it is-but little needed unless when the 
numberof persons in the room ‘is eens, or when namer- 
ous seen are burning. 
But a free draft of air ap the chimney cannot take lade ssithee 
out an equally free admittance of fresh air into the room to take 
the place of that which ascends the chimney. If the room be 
tight so that no external air can enter by the sides, the chininey 
will have to serve for both currents, and there will be a column 
and presses upwards into the flue of the chimney already oceu- 
pied by colder air, that air and the walls of the chimney which 
must be of the same temperature, will withdraw the caloric more 
or less from the warm air, thus condensing and cooling some por- 
tion of it which is then forced back into the room: again with the 
ronarew i air, and this process will go on, till the chimney 
has ab o much caloric eee the warm air einenail to 
chinmey must coleneali lose more of rein than that hiile 
in the center, it seems to follow that the ing current of 
