390

HISTORY OF ENGINEERING.

Rook T.

iron bolts and tics. 'The whole of the space between was filled in with large stones, someweighing 20 tons, which were freighted on rafts, supported by empty casks, from theneighbourhood of Folkestone ; at this time more than 80,000/. were expended. Soon afterthe formation of this pier, a vast bar or shelf was formed across the harbour, by an immense

Fig. 394.

DOVKK HARBOUR.

quantity of beach being thrown up, which totally impeded the passage, there being only asmall outlet left for the current of the water of the river. Every exertion was made toimprove the entrance into this port, and it appears from a survey made about 1652, that ithad 22 feet water at spring tides. Since that time various acts of parliament have beenpassed, and enormous sums of money expended ; jetties have been erected towards the east,to prevent encroachments of the sea, and though the south-west winds still throw up largequantities of beach at the mouth of the harbour, they arc partly dissipated by sluicing, thedepth of water at springtides varying from 18 to 20 feet, and at neaps about 14 feet.

This harbour has undergone many changes, the mouth at present being in a very differentposition to what it was formerly, which seems to have been occasioned by the constantmotion of the beach or shingle, which is driven coastwise from west to east: the BritishChannel opens towards the west, and contracts eastward, so that the seas are much moreviolent and heavy from the south-western than from the south-eastern quarter. The violentstorms, however, at south-east move the shingle westward, though the general prevalenceis the other way, from west to east.

This shingle or beach is composed of flints produced by the destruction of the chalkcliffs, which when undermined are* precipitated in large quantities into the cea ; the chalkis dissolved, and the flints, rounded by attrition, form a constant succession of beach, animmense quantity of which is in continual motion along the coast, from west to east, partof which lodges and fills up every recess where it can be deposited and lie quiet.

This beach was the cause of the destruction of the old harbour, and it seems that themouth has been shut up more than once, and has remained so for years; the mouth of thepresent harbour is nothing more than a cut through the beach to allow the land waters,pent up on the inside of the harbour, to have a passage, which in time was improvedand defended by two piers composed of wooden piles, filled in with rough and heavy stones ;after passing these, vessels arrive in a capacious harbour, defended from all winds, buthaving an open communication with the sea, the water flows and ebbs, and at low waterspring tides the harbour is dry; it is divided by a dam or cross wall, in which, whenSmeaton made his survey in 1769, was an opening 38 feet wide at top, and about 36 feetat bottom ; in this was placed a large pair of gates pointing to landward, through whichvessels at high water might pass from the outer to the inner harbour or basin, and be keptafloat.

Besides these great gates, there were two other openings in the cross wall, 12 feet wide,furnished with a pair of draw-gates.

The interior harbour was again divided by a second cross-wall with an opening of