\STOVE AND FURNACE HEAT. 68 
atively small, the sectional area of the air-shafts should be at least twenty square 
inches per head. 
Open fire-stoves combine the advantages of the open fire-place and the close 
stove, provide for fresh air, and economize heat. The important improvements 
in stoves of this kind have not only well-nigh supplanted the open fire-grate in 
supplying all that was ever claimed for it, but excel it in all the requisites of 
economy and comfort. 
Hot-air furnaces are simply inclosed stoves placed outside the apartments to 
be warmed, and usually in cellars or basements of the buildings in which they 
are used. The manner of warming is virtually the same as by indirect steam- 
heat — by the passage of air over the surface of the heated furnace or steam-heated 
pipes, as the case may be, through flues or pipes provided with registers. The 
most essential condition of satisfactory warming by a hot-air furnace is a good 
chimney-draught, which should always be stronger than that of hot-air pipes 
through which the warmed air is conveyed into the rooms, and this can be meas- 
ured by the force with which it passes through the registers. A chimney-draught 
thus regulated effectively removes all emanations; for, if the chimney draught 
exceeds that of the hot-air pipes, all the gaseous emanations from the inside of 
the furnace, and if it have crevices, or is of cast-iron and overheated, all around 
it on the outside, will be drawn into the chimney. Closely connected with this 
requirement for the chimney-draught is the regulating apparatus for governing 
the combustion of fuel — the draught of the furnace. This should all be below 
the grate ; there should be no dampers in the smoke-pipe or chimney, and all 
joints below and about the grate should be air-tight. The fire-pot should be 
lined with brick and entirely within the furnace, but separate from it, so that 
the fresh air to be warmed cannot come in contact with the fuel-chamber. 
It should go without saying that the air which passes from furnaces into 
living-rooms should always be taken from out-of-doors, and be conveyed in per- 
fectly clean air-tight shafts to and around the base of the furnace. Preferably, 
the inlet of the shaft, or cold-air box, should be carried down and curved at a 
level (of its upper surface) with the bottom, and full width of the furnace. Thus 
applied, the air is equally distributed for warming and ascent through the hot-air 
pipes to the apartments to be warmed. On the outside, the cold-air shaft should 
be turned up several feet from the surface of the ground, and its mouth protected 
from dust by an air-strainer. A simple but effectual way is to cover the mouth 
with wire cloth, and over this to lay a piece of loose cotton wadding. This may 
be kept in place with a weight made of a few crossings of heavy wire, and it 
should be changed every few months. And here, too, outside the house, should 
be placed the diaphragm for regulating the amount of coid-air supply, and not, 
as commonly, in the cellar. 
As the best means of regulating the temperature and purity of the atmos- 
phere from hot-air furnaces, it is necessary to provide sufficiently large channels 
for both the inlet of fresh air and its distribution through the hot-air pipes. The 
area of the smallest part of the inlet (or inlets, for it is sometimes letter to have 
