----- = 563 8-c.p. lamp hours per annum per
70,000 population head of population.

Taking the loss due to the storage which would be necessary at 20% on
three-quarters of the total or 15% upon the whole, there would be 478
8-c.p. lamp-hours per annum per head of the population: i.e. if the
power developed from the refuse were fully utilized, it would supply
electric light at the rate of one 8-c.p. lamp per head of the
population for about 1{1/3} hours for every night of the year.

[Sidenote: Difficulties.]

In actual practice, when the electric energy is for the purposes of
lighting only, difficulty has been experienced in fully utilizing the
thermal energy from a destructor plant owing to the want of adequate
means of storage either of the thermal or of the electric energy. A
destructor station usually yields a fairly definite amount of thermal
energy uniformly throughout the 24 hours, while the consumption of
electric-lighting current is extremely irregular, the maximum demand
being about four times the mean demand. The period during which the
demand exceeds the mean is comparatively short, and does not exceed
about 6 hours out of the 24, while for a portion of the time the
demand may not exceed {1/20}th of the maximum. This difficulty, at
first regarded as somewhat grave, is substantially minimized by the
provision of ample boiler capacity, or by the introduction of feed
thermal storage vessels in which hot feed-water may be stored during
the hours of light load (say 18 out of the 24), so that at the time of
maximum load the boiler may be filled directly from these vessels,
which work at the same pressure and temperature as the boiler.
Further, the difficulty above mentioned will disappear entirely at
stations where there is a fair day load which practically ceases at
about the hour when the illuminating load comes on, thus equalizing
the demand upon both destructor and electric plant throughout the 24
hours. This arises in cases where current is consumed during the day
for motors, fans, lifts, electric tramways, and other like purposes,
and, as the employment of electric energy for these services is
rapidly becoming general, no difficulty need be anticipated in the
successful working of combined destructor and electric plants where
these conditions prevail. The more uniform the electrical demand
becomes, the more fully may t