Chap. VIII.
3 95
planks, a cavity was made in the sand when the water was discharged from the sluice, 7 feetwide and 6 feet deep ; it was again tried, and the hole, after three discharges, was found tobe 10 feet wide upon the surface of the sand, 6 feet deep to the chalk, and 3 feet wide uponthe bed of the chalk, the channel being full 100 feet in length.
The piles were driven, and part planked, for the cross-wall to enclose the basin, in theyear 1779, and soon after the sluices were completed; when all the men employed aboutthe works applied themselves with handles to start the sluices ; the spindles upon whichthe wheels were fixed broke upon the first attempt; but two of the sluices were raised atlast by means of tackle blocks, when the force and power of the stream was so great, that itnot only forced up the chalk to the depth of 6 or 7 feet, but carried away pieces weighingfrom 3 to 4 cwt. to a distance of 60 or 70 feet, and in its course cleared away the siltdown to the chalk to low water mark, the stream continuing 200 or 300 feet beyond theharbour’s mouth.
They could not raise more than two sluices at this time, but after some alteration thiswas rendered easy; the planking of the sluices was put to draw cross ways of the plank,which operating on a rough groove of stone caused considerable friction; when they werealtered, and the spindles repaired and made of wrought-iron, the water was again pent upin the basin, and the whole discharged together, and it was found to have such power, thatthere was fear lest the cross wall should have been undermined; they therefore wereobliged to construct proper aprons to prevent this.
The committee further reported, that since the cross wall had been built, the sea whichbefore broke and spent itself upon the shore had now become so agitated that vessels
were unsafe; that the sea mostly ranged along the western pier, and that the cross wall
stopping and repelling the swell, it returned, not having any vent or outlet, and that thiswiis the cause of the great disturbance and agitation complained of. To remedy this thecommittee then ordered that 200 or 300 feet of the cross wall should be taken down, andthat from thence a wall should be built up towards the cliffs, and also that 80 or 100 feetof the timber pier should be taken away, beginning at the end of the cross wall, the openingto extend towards Jacob’s Ladder; it was also recommended that another sluice shouldbe made from the angle in the old pier, to scour the upper or northern angle of theeast pier.
In the year 1781 the sixth sluice being completed, a new channel was dug through thesand, and a couple of barges were laid so as to direct the water through it; in a fewminutes the bank was considerably diminished, and the water of this*sluice flowed so highthat it overtopped the conduit wall, and it was the general opinion, that by these means
the harbour might be kept cleansed; in the channel under the east pier there was found
19 feet water at spring tide.
IVIr. Smeaton afterwards gave a design for a new dock, and the first stone was laidJuly 31.. 1784, but the walls were carried up without any timber floor; after this dockwas built, the natural springs which rise in the chalk bed broke not only through thecement, but in many places issued with such violence as to break the paving-stones withwhich it was covered. It was then proposed to take up the whole of the pavement, andto do what had been done at Plymouth on a similar occasion, — lay down large blocks ofstone 3 feet by 4, and 2 feet 6 inches deep, each stone weighing l^tons. After these blocksof Portland stone were all laid, and the dock shut in at the time of high water, the wholepavement was again hoisted, and 100 feet of the north wall lifted with it.
Mr. Smeaton was then requested to examine its state, and he reported to the committee,that on the day he arrived the tide rose 13 feet 4 inches upon the apron of the gates of thedock, but that before it had risen 2 feet it began to spring through several joints of thestone floor, which had been laid with the solid Portland blocks 2 feet 6 inches in thickness,in the form of an arch. As the height of the tide increased upon the apron, the leakagethrough the joints increased, so that when it was high water, there was a depth of 5 feet3 inches water upon the floor.
'Tlie cause of these derangements was owing, he states, to the pressure of the water underthe bottom, endeavouring like a vessel swimming in the water to buoy it upwards; amiwhich, with 8 feet pressure, was calculated to produce that of 1000 tons over the entirearea of the floor ; and he was of opinion that had the wooden floor been introducedaccording to the original plan, it would have been subjected to the upright pressure only,and not to a lateral pressure as in stone arches. It was found that it was produced by thesprings issuing from the area of the chalk on which the dock was founded, and that thiswould not have been the case had the soil been a compact clay or rock that would not havesuffered the water to percolate through its pores.
Mr. Smeaton , who hitherto had been only occasionally consulted, was in the year 1788appointed engineer to the harbour, and Mr. John Gwynn, who had already executed manyworks under him, resident surveyor. They immediately commenced rebuilding the dock,a timber floor was laid throughout, and an additional thickness given to the walls; here