34 HOW TO BUILD A CHMINEY. 
is only the pressure of 9 or 6 inches on the bottom, 
while elsewhere there is a foot. Now an eddy in the 
same way may take place with the wind, blowing 
over and past a house, and though, as air is an expans- 
ive fluid, it does not leave a vacancy entirely, still 
the pressure of the air that is in the eddy is lessened 
in proportion to the force of the current. Nowif the 
door or window through which the pressure is applied 
and tke current flows, to carry up the smoke, should 
happen to be in such an eddy, the pressure might be 
greatly lessened, even to the extent of entirely coun- 
teracting the effect of the lesser pressure in the chim- 
ney, and causing a current in the opposite direction, 
z.e., down the chimney and owt at the door or win- 
dow, or the effect of the eddy is often easily detect- 
able by an aneroid or barometer. Of course the only 
remedy is to close that door or window, and open 
one somewhere else. ‘The shape and construction of 
the ‘‘old wife” and ‘‘sailor’s hat”’ forms of chimney- 
tops gives them the advantage of this eddy, in re- 
moving by so much the pressure of air at the top of 
the chimney and thereby increasing the force of the 
current up the chimney. 
Such are a few facts and philosophies which may be 
useful, let it be hoped, in producing fire and warmth 
without smoke. 
P.S.—In building a fire-place, two objects are to 
be kept in view. Allowing the heat to come out 
into the room and tbe smoke to escape up the chim- 
ney. To effect the first, openness is required, the 
latter closeness. A convenient compromise may be 
made by making the back and sides of the fire-place 
of the same width as the chimney, or only a very 
little wider at the back and contracting gradually 
into it. The sides of the same width as the sides 
0! the chimney, so that the whole forms as it were 
a continuation of the chimney with the front want- 
ing. From the line of the front the sides widen out 
on a slope as much as possible to allow the heat 
to radiate through the room. The advantage of this 
is that the air only gets the opportunity of rising 
with the smoke on one side, 7.e., the front, and thus 
the volume of smoke is not increased, and what is 
is kept at the back and must go up the chimney. 
