Smeaton commenced the undertaking by visiting the rock in the spring of 1756, accurately measuring its very irregular surface, and in order to ensure exactness in his plans, making a model of it. In the summer of the same year he prepared the foundation by cutting the surface of the rock in regular steps or trenches, into which the blocks of stone were to be dovetailed. The first stone was laid in June 1757, and the last in August 1759. Of that period there were only 431 days when it was possible to stand on the rock, and so small a portion even of these was available for carrying on the work, that it is calculated the building in reality occupied but six weeks. The whole was completed without the slightest accident to any one; and so well were all the arrangements made, that not a minute was lost by confusion or delay amongst the workmen. The tower measures 86 feet in height, and 26 feet in diameter at the level of the first entire course, the diameter under the cornice being only 15 feet. The first twelve feet of the structure form a solid mass of masonry,--the blocks of stone being held together by means of stone joggles, dovetailed joints, and oaken tree-nails. All the floors of the edifice are arched; to counteract the possible outburst of which, Smeaton bound the courses of his stone work together by belts of iron chain, which, being set in grooves while in a heated state, by the application of hot lead, on cooling, of course, tightened their clasp on the tower. Throughout the whole work the greatest ingenuity is displayed in obtaining the greatest amount of resistance, and combining the two great principles of strength and weight,--technically speaking, cohesion and inertia. On the 16th October 1759, the warning light once more, after an interval of four years, shone forth over the troubled waters from the dangerous rock; but it was but a feeble illumination at the best, for it came from only a group of tallow candles. It was better than nothing, certainly; but the exhibition of a few glimmering candles was but a paltry conclusion to so stupendous an undertaking. For many years, however, no stronger light gleamed from the tower, till, in 1807, when it passed from the hands of private proprietors into the charge of the Trinity House, the mutton dips were supplanted by Argand burners, with silvered copper reflectors.