234

HISTORY OF ENGINEERING.

Book I.

Fig. 245.

TKEPOUT.

The mouth cr entrance is exposed to the winds between west-north-west and south-south-east. This position renders it easy for the access of vessels, as when they miss theport, they can run out again without being exposed to the danger they incur at Dieppe .

When vessels bound to Havre, Fecamp , or Dieppe , entering the channel by winds blowingbetween west-south-west and north-north-west, miss the entrance of these ports, they in-variably run aground at the mouth of the Saone ; this they would avoid by making for

ELEVATION OF SCOURING SLUICE.

Fig. 246.

Treport; its position is therefore most important as a refuge harbour. Du Cessart im-proved this port in the year 1778, and constructed the scouring sluice by which the harbouris kept constantly cleansed ; it has two passages for the water, each 21 feet in width,separated by a pier 8 feet thick, terminated at the two sides by walls 10 feot in thickness,with wing walls and returns.

The platform fills the space between the walls, both above and below; below the apronis a row of sheet piling, which keeps the earth of the foundation from washing away.Above these are two rows of sheet piles, which prevent the work from being undermined.

Each of the two passages is closed by two gates, 12 feet in height; a wooden bridge,13 feet in width, affords a communication across the sluice. The upper platform is con-structed of timber, so that a reparation can be effectually made, without the walls sustainingany injury. The pivots and sockets on which the gates move are of cast-iron, each weigh-ing 193 pounds : the lower rail of the gates is kept 2 inches above the level of the platform,a sill of the same height fills the space, and prevents the loss of water.

Turning gates made in the ordinary way lose a great quantity of water in the uprightjoints, as well as through the space between them and the facing of the piers; to remedythis De Cessart made a circular indent in the side walls, radiating from the centre of theturning post, and by this means effectually closed the joint at the side. There is also anobjection in their being centred at two-thirds of their width, the one side being double theother. The gates open with great velocity, which strains them as well as injures the plat-form : when the sea is violent the waves striking the gates at low water forces them open,and they remain so until the water they retain is in equilibrio with the power applied to it.The wave then suddenly returning, the gate is closed with great force against the shuttingpost, which materially injures not only the post but the work. To remedy this incon-venience De Cessart centred these gates in such a manner that the difference of their sides