282 HARBOUR CONSTRUCTION. 



Figs. 93 and 94, p. 281, show a system of " tabling" often used 

 in lighthouse work. It is suitable either for straight or for round 

 work, and may be advantageously used in pier-heads in exposed 

 positions. The face-stones in several of the courses in the head 

 of Alderney breakwater were notched or " tabled " in this 

 manner, others being merely dowelled, as in Fig. 91, p. 279. 



Dovetailed work (Fig. 95, p. 281) is essentially a lighthouse 

 form of construction. It is seldom used in breakwaters, unless 

 under exceptional conditions, but it was adopted for the ashlar 

 work at the toe of the sea-slope of Plymouth breakwater, where 

 much trouble was experienced in getting any work to stand. 



Any of these fancy-shaped blocks, if cut in stone, are very 

 expensive, but if moulded in concrete, very little extra cost is 

 incurred. 



With the view of allowing the sloping block-work super- 

 structure of the Manora breakwater perfect freedom to settle 

 the blocks were laid without any bond, and the breakwater was 

 divided from end to end, and from top to bottom, by a vertical 

 central joint. In fact, it was composed of two vertical walls, 

 each about 24 feet in height, and 12 feet in thickness, touching 

 each other, but not connected in any way, the blocks in each 

 wall being placed one upon the other, like so many slightly 

 inclined basaltic columns. 



The walls moved and settled independently, as was intended, 

 but they did so to a greater extent than was anticipated, with 

 the result that the central joint opened in some places to a con- 

 siderable extent, and a number of blocks in both walls, but 

 chiefly in the one on the harbour side, were displaced by the sea. 



It is frequently and rightly claimed as one of the advantages 

 pertaining to sloping block- work, that the courses are free to 

 slide upon each other, and thus to follow any settlement which 

 may take place. This is more particularly the case during con- 

 struction, when the end of the work is comparatively free, and 

 when local settlement is specially liable to occur. 



In a properly constructed work, the pressure exerted by the 

 tilted courses leaning against each other, coupled with the 

 dowelling which is almost invariably introduced, should, when 

 the work is finished, be sufficient to prevent this sliding motion ; 

 but in the event of settlement taking place, the joints, being the 

 weakest points, would open to the necessary extent, and allow 

 the work to follow it down. 



