much higher temperature is attained, and if it
is properly worked, little or no cold air will enter the furnaces
during stoking operations. As far as possible a balance of pressure in
the cells during clinkering should be maintained just sufficient to
prevent an inrush of cold air through the flues. The forced draught
pressure should not exceed 2 in. water-gauge. The efficiency of the
combustion in the furnace is conveniently measured by the
"Econometer," which registers continuously and automatically the
proportion of CO_{2} passing away in the waste gases; the higher the
percentage of CO_{2} the more efficient the furnace, provided there is
no formation of CO, the presence of which would indicate incomplete
combustion. The theoretical maximum of CO_{2} for refuse burning is
about 20%; and, by maintaining an even clean fire, by admitting
secondary air over the fire, and by regulating the dampers or the
air-pressure in the ash-pit, an amount approximating to this
percentage may be attained in a well-designed furnace if properly
worked. If the proportion of free oxygen (i.e. excess of air) is
large, more air is passed through the furnace than is required for
complete combustion, and the heating of this excess is clearly a waste
of heat. The position of the econometer in testing should be as near
the furnace as possible, as there may be considerable air leakage
through the brickwork of the flues.

The air supply to modern furnaces is usually delivered hot, the inlet
air being first passed through an air-heater the temperature of which
is maintained by the waste gases in the main flue.

[Sidenote: Calorific value.]

The modern high-temperature destructor, to render the refuse and gases
perfectly innocuous and harmless, is worked at a temperature varying
from 1250 deg. to 2000 deg. F., and the maintenance of such temperatures
has very naturally suggested the possibility of utilizing this
heat-energy for the production of steam-power. Experience shows that a
considerable amount of energy may be derived from steam-raising
destructor stations, amply justifying a reasonable increase of
expenditure on plant and labour. The actual calorific value of the
refuse material necessarily varies, but, as a general average, with
suitably designed and properly managed plant, an evaporation of 1 lb.
of water per pound of refuse burned is a result which may be readily