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.
[23] These figures are the results of experiments made with an instrument invented by Mr. Thomas Stevenson, and called the Marine Dynamometer.
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.