attained, and affords a basis of calculation which engineers may
safely adopt in practice. Many destructor steam-raising plants,
however, give considerably higher results, evaporations approaching 2
lb. of water per pound of refuse being often met with under
favourable conditions.

From actual experience it may be accepted, therefore, that the
calorific value of unscreened house refuse varies from 1 to 2 lb. of
water evaporated per pound of refuse burned, the exact proportion
depending upon the quality and condition of the material dealt with.
Taking the evaporative power of coal at 10 lb. of water per pound of
coal, this gives for domestic house refuse a value of from {1/10} to
{1/5} that of coal; or, with coal at 20s. per ton, refuse has a
commercial value of from 2s. to 4s. per ton. In London the quantity of
house refuse amounts to about 1 1/4 million tons per annum, which is
equivalent to from 4 cwt. to 5 cwt. per head per annum. If it be
burned in furnaces giving an evaporation of 1 lb. of water per pound
of refuse, it would yield a total power annually of about 138 million
brake horse-power hours, and equivalent cost of coal at 20s. per ton
for this amount of power even when calculated upon the very low
estimate of 2 lb.[1] of coal per brake horse-power hour, works out at
over L123,000. On the same basis, the refuse of a medium-sized town,
with, say, a population of 70,000 yielding refuse at the rate of 5
cwt. per head per annum, would afford 112 indicated horse-power per
ton burned, and the total indicated horse-power hours per annum would
be

70,000 x 5 cwt.
--------------- x 112 = 1,960,000 I.H.P. hours annually.
20

If this were applied to the production of electric energy, the
electrical horse-power hours would be (with a dynamo efficiency of
90%)

1,960,000 x 90
-------------- = 1,764,000 E.H.P. hours per annum;
100

and the watt-hours per annum at the central station would be

1,764,000 x 746 = 1,315,944,000.

Allowing for a loss of 10% in distribution, this would give
1,184,349,600 watt-hours available in lamps, or with 8-candle-power
lamps taking 30 watts of current per lamp, we should have

1,184,349,600 watt-hours
------------------------ = 39,478,320 8-c.p. lamp-hours per annum;
30 watts

39,478,320
that is, ------------