142 Mr Thomas Stevenson's Description of 



The excavation of the tide-bason, which formed the land- 

 ward part of the work, was effected by means of a series 

 of dams, consisting of walls, built of Pozzolano rubble. 

 These were found to be quite water-tight, and to answer 

 remarkably well in every respect; but they required, for 

 their protection against the waves, a considerable bulwark 

 or breakwater of Pierres perdues to shelter them from the 

 waves. 



In the excavation, however, which had to be undertaken 

 seaward of the breakwater of Pierres perdues, any attempt 

 to exclude the water dm*ing the whole of the tide, was what 

 I never considered practicable. A trial was accordingly made 

 to effect the excavation by means of a low wall, composed 

 of clay-rubble, resembling in its object those low dams 

 consisting of logs of wood bedded in clay, which are often 

 adopted in harbour-works, and which are only intended 

 to keep out the tide during the first part of the flood, and 

 to be pumped dry before the operations of the next tide 

 are begun. But after many attempts with this clay-wall, 

 it became quite evident that it would not be possible, wiih 

 its assistance, to carry the excavations to near the level 

 of low-water springs, which was due principally to two 

 causes. First, because sand and shingle were, during almost 

 every tide, washed in large quantities over the top of the 

 wall into our excavation pit ; and, secondly, because the waves 

 washed out the clay from among the stones, so as to render 

 the barrier no longer water-tight. 



Being now compelled to set about some other way of car- 

 rying on the woi'k, I had recourse to the simple method 

 shortly to be explained, and which more than realised ray 

 expectations. Before giving a description of this method, 

 however, it will be interesting, as well as still farther expla- 

 natory of the required objects, to quote a few lines relating 

 to somewhat similar difficulties, from a Report upon the Har- 

 bour of Peterhead, which was drawn up in the year 1806 by 

 the late Mr John Rennie : — " The next material object of 

 consideration," says the Report, " is that of deepening the 

 hai'bour, which at present cannot well accommodate vessels 

 drawing more than 12 feet of water in the spring-tides, but 



