METHOD OF CONSTRUCTION. 163 



excluded, and with moderate pumping the ship was kept dry 

 enough for access. In a tideless sea, or where there was compara- 

 tively little or no rise and fall, a graving dock was precisely 

 similar, except that the whole of the water had to be pumped out. 

 As ships increased in size, dry docks became important works ; 

 solid masonry was adopted for lining, and the sides constructed 

 with steps technically termed altars for the double purpose of 

 affording convenient access and also for the support of the 

 necessary timbers for shoring. Pumping machinery on a large 

 scale was added for rapidly clearing the water, or for rendering 

 that operation altogether independent of the tides. 



It is worthy of note that the first principles involved in early 

 dock construction still obtain. A first-class dock of the present 

 day, however, differs largely in its dimensions, details of con- 

 struction, and in the elaborate character of its equipment, and 

 necessitates a large amount of engineering skill of the highest 

 order. 



The method of construction to be adopted will depend greatly 

 on the strata, and whether it is likely to be waterlogged on the 

 completion of the works and the cessation of the temporary 

 pumping. 



With such material as mud, silt, and sand, or any pervious 

 strata, it may be necessary to surround the site with one or 

 more rows of continuous sheet-piling for the purpose of cutting 

 off the water. 



The work in the sides of a dock will partake of the character 

 of retaining-walls of a section and weight sufficient to withstand 

 the thrust of the material in a saturated condition. They will 

 also be called upon to counteract, as abutments or piers, the 

 upward hydrostatic pressure on the floor. 



The floor is generally constructed with radial joints so as to 

 preserve the character of an inverted arch, by which the upward 

 hydrostatic pressure is transferred to the sides. In the case 

 of very wide docks, where the floor is formed of horizontal work, 

 special care is required to ensure stability. 



The efficiency of all flat work in the floor of a dock that has 

 not in itself a weight per unit of area exceeding the hydrostatic 

 pressure will depend upon the means and care taken to prevent 

 the water penetrating between the foundation strata and the 

 work, and so lifting, and possibly destroying it ; hence the im- 

 portance of using such precautions as close sheet-piling for the 



