[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. [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.