2 inches (the head actually used) it consumes 20.06 cubic feet per
hour. As shown by the following table, such increment of gas pressure
raises the specific intensity of the light, _i.e._, the illuminating
power per unit of incandescent surface, but it does not appreciably raise
the duty or economy of the gas. Manifestly, in terms of duty alone, a
pressure of 23.6 inches of water-column is as advantageous as the higher
Chassiron figures; but since intensity of light is an important matter in
a lighthouse, it is found better on the whole to work the generators at a
pressure of 49.2 inches. In studying these figures referring to the
French lighthouse, it is interesting to bear in mind that when ordinary
six-wick petroleum oil burners wore used in the same place, the specific
intensity of the light developed was 75 candle-power per square inch, and
when that plant was abandoned in favour of an oil-gas apparatus, the
incandescent burner yielded 161 candle-power per square inch;
substitution of incandescent acetylene under pressure has doubled the
brilliancy of the light.
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| | Duty.
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