Hence in the
comparative statement of the cost of different methods of lighting, oil
will be taken at the cheapest rate at which it could ordinarily be
obtained, including delivery charges, at a country house, when bought by
the barrel. This rate at the present time is about ninepence per gallon.
A higher price may be paid for grades of mineral oil reputed to be safer
or to give a "brighter" or "clearer" light; but as the quantity of light
depends mainly upon the care and attention bestowed on the burner and
glass fittings of the lamp, and partly upon the employment of a suitable
wick, while the safety of each lamp depends at least as much upon the
design of that lamp, and the accuracy with which the wick fits the burner
tube, as upon the temperature at which the oil "flashes," the extra
expense involved in burning fancy-priced oils will not be considered
here.
The efficiency (_i.e._, the light yielded per pint or other unit
volume consumed) of oil-lamps varies greatly, and, speaking broadly,
increases with the power of the lamp. But as large or high-power lamps
are not needed throughout a house, it is fairer to assume that the light
obtainable from oil in ordinary household use is the mean of that
afforded by large and that afforded by small lamps.
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