The conditions attending the actual building operations upon the rock, which may be accessible only for an hour or two per day in calm weather, prevent the blocks of granite being shaped and trimmed upon the site. Accordingly, the lighthouse in the first place is erected piecemeal on shore. A horizontal course of stones is laid to see that each dovetail fits tightly and dead true. The next course is laid upon this, and so on for perhaps eight or ten courses, the trimming and finicking being accomplished as the work proceeds. Each projection has to be only just big enough to enter its relative indentation, while the latter must be exactly of the requisite dimensions to receive the projection, and no more. Each stone is then given an identification mark, so that the masons on the rock may perceive at a glance its precise position in a course, and to what ring of stones it belongs. Therefore the mason at the site has no anxiety about a stone fitting accurately; he has merely to set it in position upon its bed of cement. On shore--generally in the quarry yard--when a series of courses have been temporarily built up in this manner and have received the critical approbation of the resident engineer, the topmost course is removed and retained, while the other blocks are despatched to the site. This topmost course forms the bottom ring in the next section of the lighthouse which is built up in the yard, and the topmost course of this section in turn is held to form the bottom course of the succeeding part of the tower, and so on from foundation to lantern parapet. During the past two or three years reinforced concrete has been employed to a certain extent for lighthouse construction, but granite of the finest and hardest quality still remains the material _par excellence_ for towers erected in exposed, sea-swept positions. The Russian lighthouse authorities have adopted the ferro-concrete system in regard to one or two shore lights, especially on the Black Sea, while another fine structure upon this principle was built by the French _Service des Phares_ in 1905 at the entrance to the River Gironde. The system has also been adopted by the Canadian lighthouse authorities; one or two recent notable lights under their jurisdiction have been constructed in this material, although on somewhat different lines from those almost invariably followed, so far as the general design is concerned.