CLASSES OF DOCKS. 3 



Graving or dry docks comprise all structures formed mainly 

 below the water level, and into which vessels may be floated. 

 They may vary in design and methods of construction according 

 to circumstances and local conditions. 



Floating docks are structures of iron or wood which are 

 sunk sufficiently deep in the water to enable a vessel to be 

 placed over them. 



The slip consists of an arrangement of ways constructed 

 on an incline which runs into deep water, and carries a cradle 

 or carriage which can be placed under the vessel. 



The function of the three latter classes is to lay the vessel 

 dry for examination and repair. This is accomplished in the 

 first case by pumping the water out of the dock, or by running 

 it out wholly or in part as the tide recedes, the vessel being 

 securely shored as the water is lowered. In the second case, 

 by lifting the vessel bodily out of the water ; and in the third, 

 by placing her on a carriage, and hauling her up the inclined 

 slipway until she is above the water level. 



Advantages of Docks. The particular advantages attaching 

 to docks or basins are, in tidal basins, i.e. with open entrances 

 affording free ingress and egress to the tides that ships can 

 be accommodated in the smallest possible space, and can be 

 kept constantly afloat, and be at all times easily moved from 

 place to place as may be required. Whereas, in an open harbour or 

 river, when at low water ships take the ground, they run great 

 risk of being strained or otherwise seriously injured ; further, 

 in open anchorages, when the tide is falling, vessels are liable 

 to fall against each other or chafe against the quays; and in 

 harbours at all exposed, when there is a run of sea during 

 stormy weather, or in a river during land floods, there is the 

 risk of the breaking of warps and the consequent fouling of 

 the vessels. 



When the dock entrance is closed by gates, the ship's level 

 is never much altered, so that the cargo when being discharged 

 has not to be lifted to the same height as would be the case 

 in a tidal dock. Moreover, the operations of loading and unload- 

 ing can be carried on regularly and at all times. 



It is, however, obvious that these conditions can hardly be 

 maintained if, when the range of tide is considerable, only one 

 set of gates is employed. In such a case, the period during 

 which vessels could be passed in and out of the dock without 



