If the chamber or lantern be not perfectly ventilated, the substances produced by combustion are diffused through the air, so that in winter or damp weather the water condenses on the cold glass windows, which, if the light be a fixed one, greatly impairs its brilliancy and efficiency, or, if the light be a revolving one, tends to confound the bright and dark periods together. The extent to which this may go, may be conceived, when it is considered that some lighthouses burn as much as twenty, or more, pints of oil in one winter's night, in a space of 12 or 14 feet diameter, and from 8 to 10 feet high, and that each pint of oil produces more than a pint of water; or, from this fact, that the ice on the glass within, derived from this source, has been found in some instances an eighth, and even a sixth of an inch in thickness, and required to be scraped off with knives.
The carbonic acid makes the air unwholesome, but it is easily removed by any arrangement which carries of the water as vapour. One pound of oil in combustion produces about 1.06 pound of water and 2.86 pounds of carbonic acid.
The author's plan is to ventilate the lamps themselves by fit fines, and then the air inside the lantern will always be as pure as the external air, yet with closed doors and windows, a calm lantern, and a bright glass.
In lighthouses there are certain conditions to which the ventilating arrangement must itself submit, and if these are not conformed with, the plan would be discarded, however perfect its own particular effect might be. These conditions are chiefly, that it.should not alter the burning of the oil, or charring of the wicks,—that it should not interfere with the cleaning, trimming, and practice of the lamps and reflectors,—that it should not obstruct the light from the reiectors, —that it should not, in any sudden gust or tempest, cause a downward blast or impulse on the flame of the lamp,—that, if thrown out of action suddenly, it should not alter the burning; and, added to these, that it should perform its own ventilating functions perfectly.
Lighthouses have either one large central lamp, the outer wick of which is sometimes 3¾ inches in diameter, or many single Argand burners, each with its own parabolic reflector. The former is a fixed lamp; the latter are frequently in motion. The former requires the simplest ventilating system, which may be thus described:—
