The curvature of the earth requires that a light-house on a coast should have a considerable elevation, otherwise its summit would not appear above the horizon, unless the mariner were very near. To attain this elevation, the architects usually take advantage of some hill or cliff, or rocky eminence near the shore. There was, however, no opportunity to do this at Pharos; for the island was, like the main land, level and low. The requisite elevation could only be attained, therefore, by the masonry of an edifice, and the blocks of marble necessary for the work had to be brought from a great distance. The Alexandrian light-house was reared in the time of Ptolemy Philadelphus, the second monarch in the line. No pains or expense were spared in its construction. The edifice, when completed, was considered one of the seven wonders of the world. It was indebted for its fame, however, in some degree, undoubtedly to the conspicuousness of its situation, rising, as it did, at the entrance of the greatest commercial emporium of its time, and standing there, like a pillar of cloud by day and of fire by night, to attract the welcome gaze of every wandering mariner whose ship came within its horizon, and to awaken his gratitude by tendering him its guidance and dispelling his fears. The light at the top of the tower was produced by a fire, made of such combustibles as would emit the brightest flame. This fire burned slowly through the day, and then was kindled up anew when the sun went down, and was continually replenished through the night with fresh supplies of fuel. In modern times, a much more convenient and economical mode is adopted to produce the requisite illumination. A great blazing lamp burns brilliantly in the center of the lantern of the tower, and all that part of the radiation from the flame which would naturally have beamed upward, or downward, or laterally, or back toward the land, is so turned by a curious system of reflectors and polyzonal lenses, most ingeniously contrived and very exactly adjusted, as to be thrown forward in one broad and thin, but brilliant sheet of light, which shoots out where its radiance is needed, over the surface of the sea. Before these inventions were perfected, far the largest portion of the light emitted by the illumination of light-house towers streamed away wastefully in landward directions, or was lost among the stars.