EXPLANATION OF TERMS USED } 
A graving dock necessitates a sound, natural foundation 
at a reasonable depth and a disposition of site which 
frequently involves the appropriation of a considerable 
area of valuable land. 
A floating dock, on the other hand, requires a sheltered 
position with an adequate depth of water which, if not 
obtainable naturally, will have to be gained by dredging. 
The depth of water to be obtained for a floating dock must 
be sufficient to cover (a) the draught of the ship, (5) the 
depth of the pontoon floor, and (¢) the necessary clearances 
of a foot or two each between the floor of the pontoon (i.e. 
the top of its keel-blocks) and the ship, and between the 
underside of the pontoon and the river, or estuary, bed. In 
cases where there is a tidal variation, the amount of this 
must be added, if the dock is to be available at times of low 
water. In this country, as already stated, the tidal range 
may be anything up to 50 ft., and 15 to 20 ft. is quite 
general. For a modern ship of average size, drawing 
28 to 30 ft. of water, the pontoons would be, at least, 
20 ft. deep; this means that the available depth of the 
floating dock berth below high water level should be about 
#0 ft.—a very considerable depth for dredging. It must 
be admitted, however, that this depth is paralleled in 
graving dock construction and exceeded in at least one case 
(Havre), where, owing to the defective foundation, the 
floor of the graving dock has been made as much as 35 ft. 
in thickness, which, added to the draught of the vessel, 
has meant a total depth below coping level approaching 
go ft. 
The cost of a graving dock is generally considered to be 
greater than that of a floating dock, though the variation 
in particular circumstances renders it difficult to make 
any general dogmatic statement on the point. On the 
other hand, a floating dock can be constructed in about 
half the time required for a graving dock. In the case of a 
graving dock of any importance, a couple of years is 
generally necessary for the purpose of carrying out the 
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