394

IIISTOItY OF ENGINEERING.

Rook I.

It was also observed that the sea-weed which drove in came from the westward ; andthat from the east there was a drift of large shingle, which it was thought would be ofadvantage to the new piers. When vessels broke loose from their anchors in the Downs, itwas usually from three-quarter flood to one-quarter ebb, when the current of the tide is tothe north and north-east, which carried them right into the harbour at Ramsgate . TheGoodwin Sands constitute the Downs as a roadway, and at low water these sands form abreakwater to all the easterly winds, and even at high water they are too shallow to admitthe great seas to pass, without being broken and dispersed, and it is not till the tide turnsto the north, which is at about three-quarter flood, that the combined force of wind andtide causes the ships to break from their moorings. The most advisable bearing for theentrance to the new harbour was then determined to be south-south-west; for if placedfull south, the tide near high water would run so across it, that it would be difficult forvessels to get in; and if at south-west, there would be too great an indraught of sullage.

At the commencement of the works, Mr. Etheridge, who had been employed as foremanunder Mr. King, the carpenter at Westminster llridge, held the appointment of residentengineer; he laid the foundations of the piers in cases or caissoons, and showed the methodof excavating a trench under water, and levelling it, which, being attended with certaintyand dispatch, is the practice still followed; and the whole progressed under this engineer’ssuperintendence until the works were ordered to be stopped; in the year 1755 a staircasecalled Jacob’s Ladder was made from the top to the bottom of the cliff, which was thelast work executed by Mr. Etheridge.

The east pier was, at this time, carried out 757 feet from the shore, and the west 849feet.

In the year 1774, the works being incomplete, Mr. John Smeaton was called in to make areport, when he found that a large mass of silt, consisting partly of mud and chiefly of very finesand, had been brought into the harbour by the tide; the tide water upon this part of thecoast being charged with these matters whenever agitated by the wind, and accompaniedby a quick flowing tide. This silty water, finding repose in the harbour, deposited itsheavy matter, and the water only was taken back at the ebb tide; this is the tendencyof all harbours, unless artificial methods are found to prevent it.

The most natural means to disperse it is by a fresh-water river, which continually tendstowards the sea, and in time of floods carries with it whatever forms an obstruction to itscourse; where this means does not exist, such a harbour as that at Ramsgate must incourse of time become dry land : when Smeaton made his report, there was found to be1268,700 cube yards of silt in the harbour; that the two barges with ten men each took outabout 70 tons of silt per day ; supposing 1 ton of silt to be 1 cube yard, it was calculatedthat they would be 12 years in clearing it out, even if there were no fresh accumulations.

The whole harbour contained 46 acres, and the external harbour, where the greatestquantity of silt was deposited, was about 30.} acres ; and supposing that the whole wascovered to the depth of ^ inch per day, there would be 410 cube yards, which, at the rateof clearing 70 tons per day, would require a week. It was calculated, that all which hadbeen deposited was the result of 12} years, which was the time when the curves enclosingthe harbour were raised to the level of half tide ; the increase of silt was thought to be atleast } inch per week.

Smeaton , therefore, proposed to make use of an artificial backwater and sluices, andconstructed a basin to take in the sea water, the tide having considerable rise and fall.

To prevent the basiu itself from silting up, he advised that it should be divided intotwo parts, by a partition with a sluice or sluices capable of retaining the water in eitherwhile the other was empty, so that they might be used for alternately cleansing each other.

The harbour at Ramsgate was admirably suited for the execution of this plan ; thebottom was composed of a hard chalk, and declined with an even fall towards the sea.The set of the tide runs across the harbour’s mouth, so that when the sand was washed outby the artificial current, the natural current of the tides would carry it away, and effectuallyprevent any bar being formed.

Two basins of 4 acres each were proposed, and four draw-gates were to be made in thewesternmost and five in the easternmost basin, the whole pointed in three different directions ;two towards the curve of the western pier, four towards the harbour’s mouth, and threetowards the curve of the eastern piers. To give these sluices their full effect, it was sug-gested to construct a caissoon, shaped like the pier of a bridge, which being floated to itsplace, and there sunk in a proper direction, might be used to divert the current to the rightor left, as was required.

Before this plan was put into execution, the committee of management ordered alighter of 50 tons to be scuttled, 17 inches deep, and 14 inches broad on the starboard bar.and when placed at the end of the cross-wall to be filled with water. When the sluice wasopened at low water, it ran out in a few minutes, and made a cavity in the sand some feetin depth and width; and afterwards, the water being confined in a channel guarded by