240 HARBOUR CONSTRUCTION, 



required. The staging used in constructing the Holyhead 

 breakwater is shown in Fig. 60, p. 238. 



In order to comply with special requirements, the number 

 and spacing of the supports, as well as the arrangement of the 

 roads, necessarily varies in almost every stage ; nevertheless, a 

 strong family likeness is noticeable in many of the stages which 

 have been used for constructing sea- works during recent years. 



The principle of construction illustrated in Fig. 61, p. 239, 

 has been extensively adopted, and has in most cases proved 

 satisfactory. The stages used in constructing the breakwaters 

 at Dover, Alderney, Douglas, Ymuiden, and Hartlepool were all 

 more or less after this pattern. At Ymuiden and Hartlepool, 

 the central row of piles was omitted, and the cross-timbers, or 

 sills, connecting the two outer rows were strengthened by 

 trussing ; while at Alderney and Douglas an additional row of 

 piles was introduced. 



The arrangement for discharging the rubble from the waggons 

 at Holyhead was as follows : 



The staging carried five lines of railway, running parallel to 

 each other. These were spaced 30 feet apart, centre and centre, 

 and their gauge was 7 feet. This wide gauge was adopted in 

 order that the rubble might be discharged from the waggons 

 between the longitudinals carrying the rails. The waggons 

 were of peculiar form, and were designed by the late Mr. G. M. 

 Dobson, when resident engineer on the Holyhead harbour works. 

 The sides were fixtures, but the bottom, which was in the form 

 of a huge coal-scuttle, was supported by trunnions so arranged 

 that when the waggon was empty the bottom, being heavier 

 behind than in front of the trunnions, came up and closed of its 

 own accord ; whereas when the waggon was loaded, the weight 

 forward of the trunnions was the greater, and caused the bottom 

 to tip when catches underneath the waggon were released. 



At the point where it was desired to discharge the rubble 

 into the sea, two tumbling blocks were fixed. These disengaged 

 the bottom catches of the waggons as they passed over them, 

 thus causing the contents of the waggons to fall into the sea. 

 As soon as the contents of a waggon were discharged, the bottom 

 of the waggon fell back into its place, and the catches into 

 theirs, the waggon being thus again ready for filling. The whole 

 process was automatic, and worked admirably. 



The rubble forming the mound was carried up to the level 



