[Illustration: CUPOLA OF A LIGHTHOUSE OF THE FIRST CLASS.]
To secure a good and efficient light it is necessary that the lantern should be well ventilated. Otherwise its sides will be continually covered by the water of condensation produced by the contact of the ascending current of heated air, and the glass, thus obscured, impedes the passage of the rays and diminishes their power. To prevent such an evil an excellent system of mechanical ventilation was devised by the late Professor Faraday.
The ventilating pipe or chimney is a copper tube four inches in diameter, divided into three or four lengths; the lower end of each for about an inch and a half being opened out into a conical form, about five inches and a half in diameter at the lowest part. When the chimney is put together, the upper end of the bottom piece is inserted about half an inch into the cone of the next piece above, and fixed there by three ties or pins, so that the two pieces are firmly held together; but there is still plenty of airway or entrance into the chimney between them. The same arrangement holds good with each succeeding piece. When the ventilating chimney is fixed in its place, it is so adjusted that the lamp chimney enters about half an inch into the lower cone, and the top of the ventilating chimney into the cowl or head of the lantern.
With this arrangement (I use the Professor’s own words) it is found that the action of the ventilating flue is to carry up every portion of the products of combustion into the cowl; none passes by the cone apertures out of the flue into the air by the lantern, but a portion of the air passes from the lantern by these apertures into the flue, and so the lantern itself is in some degree ventilated.
The important use of these cone apertures is, that when a sudden gust or eddy of wind strikes into the cowl of the lantern, it should not have any effect in disturbing or altering the flame. It is found that the wind may blow suddenly in at the cowl, and the effect never reaches the lamp. The upper, or the second, or the third, or even the fourth portion of the ventilating flue might be entirely closed, yet without influencing the flame. The cone junctions in no way interfere with the tube in carrying up all the products of combustion; but if any downward current occurs, they dispose of the whole of it into the room without ever affecting the lamp. The ventilating flue is, in fact, a tube which, as regards the lamp, can carry everything up but conveys nothing _down_.