396

HISTORY OF ENGINEERING.

Book I.

Sineaton again turned his attention to the formation of a diving-bell, to be used in layingthe foundations of a new advanced pier. Instead of the form of a bell, he used a squareiron chest weighing £ ton ; it was 4 feet 6 inches high and long, and 3 feet in width ; andthere was sufficient room for two men to work under it, who were supplied with a constantinflux of fresh air, by a forcing air-pump placed in a boat upon the water’s surface.

The advanced pier was built in caissoons, twelve of which being fixed, extending 120feet, the masons commenced their work ; this constituted about one-third of the intendedlength.

The timber breakwater, at the external angle of the east pier, being washed away,it was determined that its reconstruction should be of stone, and in the year 1790 this workwas commenced.

In the year 1791, the dry dock built in the basin was tried for the first time, since it hadbeen found necessary to introduce a timber floor, which was constructed in a new andpeculiar manner, on account of the springs in the chalk rising so powerfully under it as toforce up the stone floor, with which it had before been twice tried; the experiment showedthat its construction was complete, and all that could be desired ; and the advanced pierwas very nearly finished.

The harbour now contains between its substantial stone piers an area of 42 acres,the piers extending 1310 feet into the sea. The inner basin is used as a wet dock, andcontains a dry dock, where vessels of from 300 to 400 tons can be repaired.

The entrance to the outer harbour is 200 feet, wide, and opens to the south-west. Theaverage rise of spring-tides at the pier heads is from 13 to 14 feet, and of neap tides 9feet, giving to the entrance 19 feet at high water of spring-tides, and 16 of neaps.

The sluices for scouring the harbour are very powerful, and are constructed through thecross-wall of the inner basin; the water they discharge serves to keep open the channel,and the gullies which extend round the harbour at the foot of the piers, in certain portionsof which, near the entrance, the depth increases to about 6 feet at low water. The mud inthe middle of the harbour serves as grounding banks, ami affords a soft bed, on whichvessels entering can ground with safety. The opening of the gates of communicationbetween the outer and inner harbour is 42 feet. In the outer harbour has been laiddown one of Morton’s patent slips, on which steam-vessels of too great beam to enterthe graving-dock in the inner basin can be hauled up and repaired.

There is no natural breakwater to this tidal harbour, so essential for the purpose ofscouring, nor does the line of cliff offer shelter against any winds but those which blow fromoff the land ; it is, however, at present the best to be found on the south-eastern coast ofEngland, and affords a place of refuge to vessels of considerable draught of water that runfor protection at tide time.

The entire management of this harbour is vested by parliament in trustees.

Broadstairs had a wooden pier in the time of Henry VIII. , erected for the security ofthe fishing boats ; this pier is now about 100 yards in length, and extends from the northernside of a small bay. The entrance faces the south-west, and the harbour is much exposedto the sea, which is driven in by winds from the eastward.

At spring-tides there is about 16 feet water at the pier-head, and 10 feet at neaps; butthe whole harbour is dry at low water, and during spring-tides nearly 100 yards outsidethe pier is left uncovered.

Margate had a pier at a very early period, near which was a small creek ; the land on eachside was, in the course of time, washed away by the sea, when it was necessary to protectthe shores by additional piling and piers.

The harbour is siiuaied in a smali bay, between two extensive flats ot chalk rocks, theNayland on the west, and the Fulsam on the east, both of which are covered before highwater. The artificial harbour is formed by a stone pier, which commences on the easternside of the bay, and extends 800 feet to the westward, in an irregular course, leaving theentrance open to the north-west.

The rise of average spring-tides at the pier-head is about 13 feet, and that of neaptides 8 feet; but spring-tides ebb outside of the pier-head, and leave the harbour dry at lowwater.

A wooden jetty has been run out from the root of the pier, over the Fulsam rocks* tothe distance of 1100 feet, for the convenience of passengers landing from the steam-packetsat low water.

Douglas, in the Isle of Man , has an extensive bay on the eastern side, its width, betweenDouglas Head on the south, and Banks IIow, on the north, being miles. . Between thehead and Quarry Point it is only I mile and 5 furlongs wide, and about 7 furlongs indepth, at right angles with this line, and which may properly be considered the bay. It isbounded by steep and perpendicular rocks of clay-slate, and has a depth of water at lowspring-tides of from 2 to 5 fathoms.

The southern shores of the bay stretch as far as Douglas Head, on which the lighthouse