164 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



purpose of cutting off springs, and removing all doubtful material 

 before putting in the permanent work. 



When the excavations are in rock, it may be necessary to 

 adopt a lining only. In such a case, it will be of the utmost 

 importance to key the work in such a manner as to get a good 

 grip on the foundation strata, and at the same time to break up 

 all continuous joints between the lining and the natural rock. 



It is customary to give the floor of a dock a rise of a foot or 

 more towards the head, so as to drain the water to the entrance. 

 In the earlier docks, with a moderate depth of water over the 

 sill, and when the water was drained into the river or sea at low 

 tide, there was an advantage in this ; but in modern docks, with 

 the greater draught of water over the sill, bringing it as a rule 

 some depth below low-water mark, there is no advantage in 

 forming the floor out of level longitudinally ; it is more con- 

 venient and certainly not more costly to construct the floor 

 level throughout, giving a sufficient fall transversely from the 

 centre either way, and providing for any longitudinal drainage 

 that may be necessary in the side gutters. 



The dimensions of a dry dock must be determined by the 

 class of vessel it is intended to accommodate, and the charac- 

 teristics of the site. As regards the cross section, the desideratum 

 is to have ample room around all parts of the ship, so as to 

 admit of efficient examination and repair, and the admission of 

 light and air to the bottom of the vessel. An abnormal width 

 involves additional cost of timber and labour for shoring, and 

 the additional cost of pumping ; excessively long shores are in- 

 convenient to manipulate, especially in removing from the dock 

 after use, and where the width of the wharf on the dock side is 

 contracted, they are a decided disadvantage. To meet some of 

 these objections, Mr. Abernethy, M.I.C.E., in constructing docks 

 at Birkenhead and Falmouth, adopted at certain portions of the 

 dock upright piers in place of the altars. 1 In a later dock, 

 however, constructed by the same engineer at Falmouth, these 

 upright piers were omitted, presumably on account of the ob- 

 struction they offered to the light and air reaching the bottom 

 of the ship. 



At the head, the plan should follow as closely as possible, or as 

 circumstances will permit, the bow lines of the largest vessel to 

 be accommodated ; by doing so, less expense is incurred in con- 

 1 M.PJ.C.E., vol. xxv. p. 321. 



