The famous Pharos of Alexandria, built about 285 B.C. is the first light of undoubted record. The light-house at Corunna, Spain, is believed to be the oldest existing light-tower. This was built in the reign of Trajan, and in 1634 it was reconstructed. The erection of the Eddystone Light-house, off Plymouth, England, formed an era in the construction of light-houses. The masonry was 76 feet 6 inches, and the top of the lantern 93 feet, above the foundation. It was completed in 1759. The various courses were so dovetailed into each other, and the whole fifty so secured together, that the tower was almost as solid as if cut out of the solid block. Immense difficulties had to be overcome from the first landing on the rock on April 5, 1756, to the laying of the first stone, June 12, 1757, and the last, on August 24, 1759. But strong as it was, it became necessary to take it down and rebuild it on a neighboring rock, as that on which it was founded was weakened from the constant assaults on the sea. This was safely done within our own time. The Wolf Rock Light-house, off Land's End, Cornwall, England, is the last great British work, and both in its structure and its illumination it combines all the refined improvements. A survey was made in 1861, and the foundation commenced in March, 1862. In the first season only eighty-three hours of work could be done, and between that and its completion, on July 19, 1869; there were in the eight working seasons two hundred and ninety-six landings on the rock, and the time occupied was equal to about one hundred and one working days of ten hours each. The cost was £62,726. The great distinction between the later towers and their predecessors is that the stones of each course are dovetailed together laterally and vertically, so that the use of metal or wooden pins is needless. This method was first used at the Hanois Rock, Guernsey. On the upper face and at one end of each block is a dovetailed projection; and on the under face and at the other end is a dovetailed indentation. The upper and under dovetails are made just to fall into each other, and when the hydraulic cement is placed on the surface it so locks the dovetailing that the stones cannot be separated without breaking. Thus, when the cement is set and hardened, the whole of the base is literally one solid mass of granite. The lower courses for the first 39 feet of the Wolf Rock Light-house have fillets on their outer edges, into which the upper course is stepped, and this prevents the action of the waves from penetrating the joint.