G.— ENGINEERING. 127 



This could only have resulti'd from some reduction of the frictional resis- 

 tances due to compressed air or hydraulic action on the under side of the 

 blocks and between them and the pier top where a space or void existed. 

 This upward pressure decreased the friction to such an extent that the 

 force of the sea was enough to slide the blocks horizontally. It has been 

 determined in the north-east coast of Scotland that blows due to sea action 

 sometimes amount to from two to three tons per square foot on small 

 areas and up to two tons per square foot on large areas. It would appear 

 that some sort of ' pressure lubrication ' was produced between the bearing 

 surfaces which in mechanical design has not, I think, been developed as 

 far as it might be to reduce friction between moving parts. 



One has frequently observed, having poured hot water into a glass and 

 turned it upside down on a wet glass shelf, with what small amount of 

 force the glass begins to move along the shelf. This reduction of friction 

 would appear to be due to the air finding its way under the edge of the 

 inverted glass and the movement caused by some small inclination of the 

 glass shelf on which it rests. This, I submit, is an indication of what 

 happened by sea action to the pile of heavy blocks on the unfinished pier 

 at Dover Harbour, 



The only information the civil engineer can rely on at the present 

 consists of historic facts which are referred to in difierent publications on 

 the subject of constructed or destroyed breakwaters. All these point to 

 the ad\'isability of putting the top blocks of any breakwater structure in 

 well-cemented joints, and the leaving of no voids, or joints, near the 

 upper levels of these structures. 



This is my excuse for giving a long description of experiences which 

 have come under my own observation. 



I have said that the effect of voids among rocks deposited for the 

 purpose of the defence of harbours and ports against the sea, and the 

 cost of the same, are important considerations in connection with sea 

 defence works. The rocks are usually deposited free from all quarry 

 rubbish and earth arising from the ' overbearing ' which has been re- 

 moved ; such deposited rocks usually contain within their volume voids 

 to the extent of 42 to 43 per cent. If the whole output of the quarrying 

 operations were deposited without special selection, these voids would 

 be reduced to 30 to 33 per cent. 



As can be easily understood, these facts have an influence on the cost 

 of construction. In this connection, the amount of selection which is 

 involved (if a ' limiting minimum ' size of rock masses is demanded), 

 though not entering into the question of the volume of the voids, materially 

 afiects the cost of the deposited rock embankment as a whole. 



The voids in any mass of irregiilar solid lumps are largely influenced 

 (as I have already said) by the uniformity, or otherwise, of the sizes and 

 shapes of the lumps of rock of which a rubble breakwater bank is com- 

 posed. The cost of a rubble embankment is very much influenced by the 

 necessity arising from specified selections of the limiting sizes of its parts. 

 The rock strata from which the material is quarried also makes a great 

 difierence in the ultimate cost if it cannot be blasted into the categories 

 desired. The effect of voids in the mass, especially near high-water level, 

 or where the influence of the wind on the sea may result in the inclusion 



