340 HISTORY OF ENGINEERING. Hook I.

mud, that it might be washed out to the mouth, and afterwards carried away by thetides.

John Smeatou, in the year 1769, finding that during spring-tides, at the tide of ebba strong current set past the end of the pier, which was diverted from the harbour mouth,suggested that openings, should be made, by which the water might pass, for the purposeof clearing away the mud accumulated near the middle of the harbour: he recommendedtwo sets of tunnels to lie made, three and three together, 12 feet wide each; these tunnelsto be arched over, so as to form a level platform at top, and the passage for the water tobe made through the rocks, down to the level of low water mark. He also suggestedthat the channel should be turned, and that the small pier already constructed should beremoved, and the rock levelled, that the water in its passage from the tunnels should meetwith no impediment; and then the tidal water would pass with more force, and act moreefficiently in scouring away the deposits.

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Fig. 336. SECTION" OE SLIP LONGITUDINALLY.

Thus, by means of strong currents, and the aid of men, as before, it was calculated thatthe harbour might be kept tolerably clean; and he advised that the sluices should be madelarger, as well as the basin; and that the water should not be let off in small quantities, asthat was not making the best use of it, for a quantity of water, when a certain power wasgiven, would move that very expeditiously, which, applied in a less degree, suffered thematter to remain.

Fig. 337.

SLIP AT DUNDEE.

The water in the basin, discharged at once in a body, would remove obstacles, whichwould remain, if the capacities of the openings were only such as to allow the waters tobe a quarter of an hour running out.

Fig. 338.

SECTION OP SLIP TRANSVERSELY.

Smeaton recommended that the sluices should be made quite water-tight, and that thedoors should not be cut, so as to form a valve to let in the water, but that one door should