Fig. 2. is a section of the dome-roof of the library, the parapet-walls of the light-room, the lantern and its cupola. a a is the balcony, shewing the position of the Alarm-Bells, and cast-iron rail; which last is made in the form of meshes, with a massive coping of brass; f f the parapet-wall of the light-room; g g the cast-iron sill of the lantern; h h, &c. the angular pillars or mullions; i i, &c. the astragals; k k the grated trimming-path; l l the smoke-tube passing through the cupola; n n, &c. the reflector frame, on which 20 reflectors, marked o o, &c. with their fountains p p, &c. are ranged; q q the upright shaft upon which the reflector-frame is built, and which is connected with the revolving-machinery; r r, &c. the copper smoke-tubes from the respective reflectors; s s the cross-bars supporting the upright shaft at t t; and u u a copper receiver for any accidental drop from the upper or central ventilator marked v´ v´ v´, while the letters v v v v mark the exterior line of the cupola; w w the external cornice of the lantern, used as a gutter or eaves-drop, from which the rain-water is conducted off the roof in small pipes to the parapet-wall of the Light-room; x x the handles which the light-keepers lay hold of; and y y the exterior grated path on which they stand when cleaning the outside of the windows; z z z the steps fixed to the parapet of the Light-room and lantern, by which they ascend to the cupola; á á the copper-tube, used as a flag-staff; b´ the signal-ball, hoisted daily at the Light-house, between the hours of 9 and 10 o’clock in the morning, when “All is Well:” The ropes for raising this ball pass over two pulleys fixed in the top of the flag-staff, and are let down through the interior of it: c´ c´ part of the cornice of the building, shewing the mode of connecting the horizontal and perpendicular joints of the Library-roof; d´ d´, &c. shew the zone or bond-belt worked in the beds of the courses, as mentioned in the description of [Plate VII.] Fig. 7., [Plate XVI.] Fig. 2., and [Plate XIX.] Fig. 10.; e´ e´, &c. from the enlargement of the scale, shews more distinctly than any of the other diagrams, the mode in which the horizontal joggling of the joints of the Light-room and other floors of the building is accomplished; f´ the door leading from the Light-room to the balcony; g´ g´ g´ the case and machinery for causing the Reflecting-apparatus to revolve, so as to distinguish this Light from others upon the coast; h´ h´, &c. the connection-rods, with their various wheels of communication, for changing the direction, and giving motion to the Bell-hammers; i´ i´, &c. the brass tubes for supplying fresh air to the Light-room; k´ the Time-piece, set upon brackets fixed in the wall, for regulating the Revolving-machinery, and for the direction of the light-keepers in changing the day and night watches; l´ a bracket of cast-iron attached to the machinery-case, for supporting the foot on which the upright shaft works; m´ the bevelled wheels connected with the interior machinery and upright shaft, which can be thrown out and into gear at pleasure.
In describing the Reflecting-apparatus delineated in Fig. 3., we may notice, that, agreeably to the laws of optics, almost inconceivable effects are produced by the use of these specula, by which portions of a sphere are illuminated, whose radii seem to be proportionate to the elevation of the radiant point, and the particular state of the atmosphere. Taking these conditions into view, the Revolving-light at the Bell Rock has been seen, from an elevated position on the land, at the distance of about 35 miles. According also to observations which have been made upon Sumburgh-Head Light, in Shetland, which is elevated about 300 feet above the medium level of the sea, it has been seen from a ship’s deck at the distance of about 27 miles. Similar effects are also expected to be produced with light refracted through glass-lenses. These, it is believed, are about to be made trial of in the Tour de Corduan, at the entrance of the Garonne, with what are termed Polygonal Lenses, being one large lens, built or composed of a number of small lenses, as suggested by Dr Brewster, in the Edinburgh Encyclopædia, in the year 1811, under the article “Burning Glass.”
Fig. 3. a a a represents one of the reflectors, measuring 25 inches over the lips, in a finished state, as now in use at the Bell Rock and Northern Light-houses in general. They consist of a circular sheet of copper, measuring, when flat, 26¼ inches in diameter; weighing 11½ lb. on an average; and plated with silver in the proportion of 6 oz. to each pound avoirdupois of copper. These plates are formed into the Parabolic curve by a very nice process of hammering, and afterwards set into a bezil or ring of brass; b is the lamp, being an Argand-burner, with a circular wick, of about three-fourths of an inch in diameter. If the parabolic figure of the reflector could be constructed and its form preserved with perfect accuracy, the magnitude of the flame of the lamp employed for its illumination would be of less importance. From certain experiments now in progress, the writer is in expectation that considerable improvements may be introduced, in the construction of reflectors, and that additional modes of distinguishing the Light-houses on the coast will be obtained. c is a cylindrical fountain of brass, which contains 24 oz. of oil, suitable for the consumpt of the lamp for about 18 hours, or equal to its expenditure during the longest night in Shetland. A great improvement upon the Burner-apparatus has lately been introduced into the Northern Lights, by affording the light-keeper an opportunity of cleaning the reflector without the obstruction of the burner: the oil-pipe and fountain of the burner is connected with the rectangular frame d, and moveable in a perpendicular direction upon the guide-rods e and f, by which it can be let down and taken out of the reflector, by simply turning the handle g, as will be more fully understood by examining Fig. 5. In this way, the necessity of lifting the reflector, or deranging its focus, is avoided. Although a properly trimmed lamp, especially upon the Argand principle, does not emit smoke, yet the heat and effluvia arising from it, are sufficient to oxydise or sully the upper part of the reflector: an aperture of an elliptical form, measuring about 2 inches by 3 inches, is therefore cut in the upper part of the reflector, to which the copper-tube h is attached. Letter i, immediately behind the reflector, refers to a cross section of the main-bar of the chandelier or frame on which the reflectors are ranged, each being made to rest upon three knobs of brass, one of which, as seen at k k, is soldered upon the brass band l, that clasps the exterior of the reflector.
To distinguish the Bell Rock Light from others on the coast, as noticed at page [401], it is made to revolve, and to exhibit periodically a bright light of the natural appearance, alternating with a red coloured light. This last effect is produced by means of shades of red glass, one of which, represented at m m, is also set in a brass bezil, and made to slide horizontally upon two rods marked n n, into corresponding sockets, fixed upon the reflector-frame at o, so as to enable the light-keeper to trim the lamp without entirely removing the red coloured shade. In the present aspect of the shade, the rods are drawn out to their full extent; but when set for use, the shade is pushed in till its bezil and that of the reflector are within half an inch of each other.
Fig. 4. is a section of the reflector a a, shewing the position of the burner b, with the glass chimney accented b´ b´, and oil cup b, or receiver for any drop that may fall from the lamp. The letters c, d, f, and g, in reference to the corresponding letters in Fig. 3., shew the oil-fountain in its place when the burner is in use.
Fig. 5. has reference to the apparatus for moving the burner up and down, so as to admit freely of the reflector being cleaned. In the present position, c, the fountain, is moved partly down; d d, &c. shows the rectangular frame on which the burner is mounted; e e the elongated socket-guides; f the rectangular guide-rod, connected with the sockets on which the perforated check-handle g slides; accented c´ is a cross section of the oil-pipe, where it joins the valve of the fountain.
Figs. 6, 7, and 8. These three diagrams represent a different mode of withdrawing the burner from the reflector, for the conveniency of cleaning it. In Figs. 3, 4, and 5., this is supposed to be done by a perpendicular motion of the burner; but in Figs. 6, 7, and 8. it slides horizontally. Fig. 8. is a horizontal section of the reflector a a a, and its apparatus, the letters in which correspond with those of Figs. 3, 4, and 5. The advantages attending this mode, are the rendering of the apparatus more steady, by its having two points of support upon the bars i i and k k; while its motion is more direct. The part of the reflector cut out for the admission of the burner in this case, being that which is partly intercepted by it, little of the reflecting surface is thus lost; and this mode may therefore be considered an improvement upon the perpendicular manner of lifting and lowering the burner.
FRONTISPIECE.
Pl. XXI.
[This scene], which is intended to represent the Bell Rock after a storm at north-east, will hardly admit of any adequate description, from the evanescence of the form of the waves upon these occasions. It is, however, founded upon observations made at the Light-house, between the years 1810 and 1822, or during twelve successive winters. The great rise of the sea during a gale, as delineated in the Vignette to the Narrative of the Edystone Light-house, being so surprising, the writer was at much pains to ascertain the effect also at the Bell Rock; and the maximum height of the sprays hitherto observed has been about 105 feet, or as high as the central part of the Light-room windows.