©: End Nb NIE. Y 5S. 17 
warmer air which enters their tops being cooled in its 
turn, and made heavier, continues to def fecutth ; and this 
operation goes on, till the funnels are warmed by the paf- 
fing of warm air through them, or the air itfelf grows 
cooler. When the temperature of the air and of the fun- 
nels is nearly equal, the difference of warmth in the air 
between day and night is fufficient to produce thefe cur- 
rents, the air will begin to afcend the funnels as the cool 
of the evening comes on, and this current will continue 
till perhaps nine or ten o’clock the next morning, when 
it begins to hefitate ; and as the heat of the day approach- 
es, it fets downwards, and continues fo till towards even- 
ing, when it again hefitates for fome time, and then goes 
upwards conftantly during the night, as before mentioned. 
Now when {moke iffuing from the tops of neighbouring 
funnels pafles over the tops of funnels which are at the 
time drawing downwards, as they often are in the middle 
part of the day, fuch fmoke is of neceflity drawn into thefe 
funnels, and defcends with the air into the chamber. 
The Remedy is to have a fliding plate, hereafter de- 
fcribed*, that will fhut perfe&tly the offending funnel. 
g. Chimneys which generally draw well, do neverthe- 
lefs fometimes give {moke into the rooms, zt being driven 
down by ftrong winds paffing over the tops of their Sunnels, 
though not defcending from any commanding eminence. 
This cafe is moft frequent where the funnel is fhort, and 
the opening turned from the wind. It is the more griev- 
ous, when it happens to bea cold wind that produces the 
effe&, becaufe when you moft want your fire, you are 
fometimes obliged to extinguifh it. To underftand this, 
it may be confidered that the rifing light air, to obtain 
a free iflue from the funnel, muft pufh out of its way or 
oblige the air that is over it to rife. In atime of calm or 
of little wind this is done vifibly, for we fee the {moke 
that is brought up by that air rife in a column above the 
chimney. But when a violent current of air, that is, a 
C ftrong 
© See Appendix, N° Il. 
