So far as the first condition is concerned, the proper height of a lighthouse-tower is easily ascertained, when the distance is determined at which its rays should be visible. This distance will necessarily depend on the character of the neighbouring seas, and the nature of the reef, rock, or shallow from which the lighthouse is to warn the navigator; but, once determined, the elevation of the tower will easily be calculated by means of the known relations existing between the form of the earth, the effects of atmospheric refraction, and the proper height of an object which is to be seen from a given distance. The state of the atmosphere at any particular point is also an important point of consideration. It is quite possible that the lighthouse, when erected at what seems a suitable elevation, may be rendered useless by a prevalence at that elevation of dense mists and heavy fogs. In 1785, the Trinity Board commenced the erection of a lighthouse on the summit of St. Catherine’s Down, in the Isle of Wight, and from so lofty an altitude it might well be supposed that its radiance would illuminate the Channel for leagues around. But, unfortunately, the crest of St. Catherine’s is, for the greater part of the year, enveloped in cloud and mist, which effectually prevents the escape of a single ray of light; and, consequently, the Trinity Board were compelled to abandon their design. The shell of the building still crowns the bleak summit of the down, as a warning to future lighthouse-builders.
Still more recently, the lighthouse on the Needles Down (also in the Isle of Wight), which for years had pointed out the dangerous character of the western entrance to the Solent, has been abandoned on account of the mists so frequently obscuring its lustre; and a new lighthouse has been erected on the outermost of the celebrated Needle Rocks, in a position of far greater utility.
The question regarding the interior accommodation of the tower must, in like manner, be answered by the nature of the locality where it is erected. Where it is easily accessible, and its stores can be replenished with ease at very short intervals, obviously the interior accommodation may be reduced within very narrow limits. But in exposed situations, as, for instance, on an isolated rock, whose communication with the mainland may be cut off for weeks at a time, room must be provided for ample supplies, and conveniences for the keepers must be arranged on a liberal scale. In the long and dreary nights of winter, where, in the northern parts of Great Britain, it is necessary to keep the light burning for about seventeen hours, not even for a moment is it left without the watchful care of at least one keeper; and thus, as he will require an interval of repose, its superintendence will occupy two persons; but in open, exposed places like the Eddystone, the Bell Rock, the Wolf, and the Skerryvore, where it is frequently impossible to communicate with the mainland for three, four, and even six weeks, circumstances have rendered it desirable that there should not be fewer than three men on duty. Hence, sleeping apartments have to be provided, as well as receptacles for sufficient supplies of water, food, fuel, and other matters.
The second condition to which we have adverted is, that the building shall be capable of resisting the force of the wind and waves. The wind is baffled with comparative ease, but the sea is a far more formidable, and, moreover, is an ever-present foe. Even in the summer months the pressure of the waves is very considerable, averaging, perhaps, about 611 lbs. per square foot of surface exposed to it. In the winter, however, the average rises to 2086 lbs. per square foot; while, during stormy weather, the force has amounted to no less than 4335 lbs.[23] To oppose this immense pressure, not only must the masonry be of the solidest description, but such a form must be given to the building as will expose that masonry to the least possible stress. From various experiments it has been found that the most effective form is that of the cylinder; and with certain modifications, the cylindrical is now almost universally adopted in the erection of lighthouses. Scientifically speaking, however, it is not so much a cylinder as the union of frustra of different cones, with a curve osculating the outline of the successive frustra. To the youthful reader this may not be very intelligible, and we will, therefore, refer him to the form of the Eddystone as an illustration of what we mean. Smeaton himself relates that it was suggested to him by the trunk of an oak, but there seems reason to believe that this was an after-thought, intended for the benefit of the large class of minds which cannot appreciate scientific reasonings.
Let us now pass into the interior of a lighthouse, and take notice of its general arrangements.
And, first, observe the massive door of bronze which opens to admit us into the lowermost story. Here are collected the stores of wood, cordage, oil, and water; and here too is placed the carpenter’s shop. On the next story we find the kitchen and the dining-room. Then we ascend to the sleeping-rooms of the three keepers; they are exquisitely neat and clean, but in other respects do not call for notice. On the highest story we enter that portion of the structure more particularly destined for the special service of the tower. It contains numerous vessels of oil, lenses, lamps, a thermometer, a barometer, and a chronometer. The spiral staircase by which we have hitherto ascended terminates at this point, and to reach the lantern we must climb a ladder before us. Entering the cupola, which enshrines the magic light, we are surprised by its exquisite propriety of arrangement. The form of the lantern is light and graceful; and to avoid the necessity of painting it, the framework is made of gun metal, and the dome of copper. A lantern for a light of the first order is twelve feet in diameter, and its glass frames are two feet high. The glazing is thick, and great care is exercised in fixing it that the plates may not be broken during high winds. Panes glazed in frames padded with cushions, and capable of being temporarily fixed in a few minutes, are always kept ready for use in Scotland. These are called storm-panes. The total cost of a lantern such as we have been describing is about £1260.
