y a standard candle
in one hour, and candle-hours in any case are obtained by multiplying
the candle-power of the source by the hours of burning. In a similar
manner, lumens output multiplied by hours of operation give the
lumen-hours. A standard candle may be considered to emit an amount of
light approximately equal to 10 lumens. A wax-candle will emit about as
much light as a sperm candle but will consume about 10 per cent. less
weight of material. A tallow candle will emit about the same amount of
light with a consumption about 50 per cent. greater. The tallow-candle
has disappeared from use.

With the appearance of kerosene distilled from petroleum the camphene
lamp came into use. The kerosene cost about 80 cents per gallon during
the first few years of its introduction. The price of kerosene averaged
about 55 cents a gallon between 1865 and 1875. During the next decade it
dropped to about 22 cents a gallon and between 1885 and 1895 it sold as
low as 13 cents.

Artificial gas in 1865 sold approximately at $2.50 per thousand cubic
feet; between 1875 and 1885 at $2.00; between 1885 and 1895 at $1.50.

The combined effect of decreasing cost of fuel or electrical energy for
light-sources and of the great improvements in light-production gave to
the householder, for example, a constantly increasing amount of light
for the same expenditure. For example, the family which a century ago
spent two or three hours in the light of a single candle now enjoys many
times more light in the same room for the same price. It is interesting
to trace the influence of this greatly diminishing cost of light in the
home. For the sake of simplicity the light of a candle will be retained
as the unit and the cost of light for the home will be considered to
remain approximately the same throughout the period to be considered. In
fact, the amount of money that an average householder spends for
lighting has remained fairly constant throughout the past century, but
he has enjoyed a longer period of artificial light and a greater amount
of light as the years advanced. The following is a table of approximate
values which shows the lighting obtainable for $20.00 per year
throughout the past century exclusive of electricity:

Hours Equivalent of Candle-hours
Year per night light in candles per night per year
1800 3 5 15 5,500
1850 3