The evolution of the most efficient illuminating apparatus for the lightship has been a most perplexing problem to the lighthouse engineer. What is applicable for the masonry tower is not necessarily adapted to its floating contemporary, since the conditions are so dissimilar. The United States service has adopted electric lighting on all its steam-driven vessels, the current being easily obtainable in this instance. On the whole, however, oil is the most popular form of illuminant, the burners--there are several lamps arranged in a ring round the mast--being fitted with two circular wicks, one within the other; while behind the lamp an ordinary parabolic reflector is placed in order to increase the intensity of the light produced. These reflectors are disposed in such a manner around the mast that the concentrated beam of light from one lamp just overlaps the rays which are projected similarly from the lamp placed on either side, the result being that a fixed white light of equal luminosity throughout the circle is projected. But, unlike the illuminant in the lighthouse, the light is not stationary in its vertical plane; it is swung from side to side and up and down in rhythm with the movement of the vessel. Under these circumstances, at one moment the light would project a short ray owing to the declination of the beam in relation to the line of the water, thereby bringing it below the horizon, while the next moment, when the ship lurched in the opposite direction, the ray of light would be thrown into the air and above the horizon. The problem is to keep the light at one steady angle, irrespective of the motion of the vessel, and this end is achieved by hanging each reflector upon gimbals, so that the rolling practically is counteracted, the reflectors maintaining a constant vertical position.