The building operations in 1840 were entirely carried on by means of two cranes with moveable jibs, of which one was fixed just beyond the foundation, at the place shewn in Plate III., between the landing quay and the Tower, and was chiefly used in bringing forward the materials; and the other, placed in the centre of the Tower, served for laying the stones, and was raised along with the rise of the building. So perfectly had the stones been dressed in the workyard at Tyree, that no alteration or paring of the beds or joints was required; and such was the facility afforded by the building apparatus, that by working 14 hours, we occasionally _set_, through the activity of Mr Charles Stewart, the foreman builder, so many as 85 blocks in a day. The first course of masonry was laid by means of a wooden trainer; but the place of all the subsequent stones was, as already noticed, regulated by the use of _plumb-templets_, whose inner faces were arcs of the generating hyperbola. By those means we succeeded in _setting_, in a most perfect manner, six courses, which carried the building to the height of 8 feet 2 inches, and contained a mass equal to 10,780 cubic feet. That quantity is not greatly less than the whole materials of the Eddystone Lighthouse Tower, which, according to my computations from the drawings of SMEATON, do not exceed 13,300 cubic feet, and is somewhat more than _one-third_ of the contents of the Bell Rock Tower, which are about 28,500 cubic feet. That frustum was also nearly equal to _one-fifth_ part of the whole mass of the proposed building, which is about 56,000 cubic feet. Of the six courses, the first three are of Hynish gneiss, and the rest are of granite from the Ross of Mull. The comparative merits of those two materials may be stated as follows:--The Hynish stone is harder, and susceptible of finer workmanship, and perhaps its most perfect blocks are more durable; but it requires much more labour in dressing than the Mull granite, which is more homogeneous in its structure and is not intersected by hard veins, like those which occur in the gneiss of Tyree. There is good reason also for concluding that the Mull stone is sufficiently durable, because it contains but a small proportion of micaceous matter, and in its texture closely resembles some of the blocks of St Oran’s chapel in the neighbouring Island of Iona, which have resisted the action of the weather, it is believed, for more than 600 years and still retain the marks left by the tools of the workmen. I had also carefully compared the density of the Hynish and Mull stones, by weighing blocks of known dimensions, and found that it requires 13·16 cubic feet of the former, and 13·66 cubic feet of the latter to weigh one ton, a difference much less than the appearance of the stone would lead one to expect. A Tower of the dimensions of that at Skerryvore, built entirely of Hynish stone, would have weighed about