THE OPERATIVE CHEMIST. 
24 
It seems, therefore, adviseable in all cases to make the 
vent or vents equal in area to that of the free space left be 
tween the bars of the grate. 
The situation of the vent is usually at the top or back of 
the furnace ; but there results a very great inconvenience 
from its being situated in the latter position; since, when 
the feeding or stoking-doors are opened to supply fresh fuel, 
or manage the fire, a strong indraught of cold air takes 
place, which rushes over the surface of the fire, and not 
only cools the whole interior of the furnace, and prevents 
the accension of the vapour from the raw fuel, thus causing 
the production of smoke and soot, but also cools the ves 
seis and materials exposed to the action of the fire ; and 
when the vessels are made of glass, pottery-ware, or cast 
iron, frequently cracks them, unless they are defended by a 
thick coating of lute, which necessarily diminishes the heat 
that can be applied to the materials contained within them. 
Mr. Losh, already mentioned as a considerable improver 
of the construction of furnaces, has therefore proposed to 
remove the vent to the front of the furnace, immediately 
over the feeding or stoking-door, and to conduct the burn 
ed air, through channels made in the masonry, into the 
flue of the chimney. A great advantage attends this con 
struction, that when either of the entrances into the fire 
room are opened, the indraught of air, instead of rushing 
over the surface of the burning fuel and striking against 
the vessels and materials, instantly passes up the vent, and 
does not enter at all into the interior of the furnace, 
whence this is much less cooled than in the furnaces of the 
usual construction. 
As the entrance of air into the furnace is regulated by 
sliders and other contrivances, so in many furnaces, where 
this is neglected, its outlet is regulated by a damper or 
slider placed at the vent, by which its opening into the flue 
is altered at pleasure, and may be even stopped entirely : 
but it is far preferable always to have a door to the ash 
room, or entrance for the air, and regulate the fire by it. 
The Chimney or Flue. 
The chimney or flue is one of the most important parts 
of a furnace; and yet, in general, the least attended unto ; 
being.usually made much too large in its horizontal area. 
By making it thus large the draught through it is much 
diminished, and the soot collects and becomes troublesome. 
For when the sides of the flue contain a larger surface than