402

HISTORY OF ENGINEERING.

Book I.

St Aubin's Harbour is also in the Island of Jersey, and upon it Mr. Smeaton also drewup a valuable report when lie visited the last mentioned harbour. lie observes : —“ Ex-perience having shown that the new pier of St. Aubin, called the Upper Pier, intended tobring up ships and vessels close to the town, for the convenience of fitting them out,loading and unloading, is, from its situation, liable to fill, from the great quantity of gravelwhich the sea washes in from the back of it, and that the depth of water is, frora that

Fig. 400.

action of the sea, diminishing more and more.” And he further states, “that where asloping shore is interrupted by the erection of a wall, as a wall has not that tendency tospend and destroy the waves of the sea that a sloping shore constantly has; wherever wallsare erected in the confines of a harbour, it is rendered, in a degree, less tranquil by thismeans.”

The reason of the gravel accumulating at this upper pier seems to have arisen from thewest end of the Island of Jersey lying so exposed to the Atlantic, without any land toshelter it; the south-westerly winds, therefore, drove forward the gravel brought coast-wise, towards the bottom of St. Aubin’s Bay, where it met the upper pier.

Smeaton suggested that a pier should be carried out in a north-easterly direction, which,flanking the current with a considerable obliquity, might prevent this accumulation.

Having now enumerated most of the harbours of Great Britain, upon which vast sumshave been expended, it is a subject of regret that few answer the purpose of shelteringvessels at all times from the heavy gales our 2000 miles of coast are subject to.

Milford Haven, Portsmouth, Plymouth, Cork, and a few others, may be entered at alltimes of the tide, whilst the harbours on the eastern shores in particular arc nearly allchoked up, and incapable of receiving large vessels, excepting at the highest state of thetide. A good harbour requires a d^pth of water which will permit the largest vessels atall times to enter, and that it should be easy of access, with quay and piers, at which shipscould load and unload their cargoes without inconvenience.

Most of our harbours being formed by piers, not sufficiently carried out from the mainland, are consequently dry at low water, have bars at their entrances, and do not afiord anyshelter to ships of the largest class.

Mere tidal harbours are serviceable only to coasters, as their draught of water is usuallyvery inefficient.

It has been very properly observed by the parliamentary commissioners, that deep waterharbours can only be formed in the sea by means of breakwaters detached from the mainland. Dover Bay affords an excellent site for such a harbour of refuge, there being at adistance of 400 fathoms from the shore a depth of 2 fathoms at low water of spring tides,and but 6 fathoms at 1100 yards, which affords sufficient space for the construction of acapacious deep water harbour, without getting into such a depth for the site of the piers orbreakwaters as would add greatly to the expense. A breakwater at a distance of about1000 yards from the shore, with piers projected from the land towards the eastern andwestern ends, having four entrances, according to the plan there given, would form a mosteffectual harbour. It is also suggested that the piers should be built with hard chalk,