236

HISTORY OF ENGINEERING.

Book I.

also sheltered by two hills which rise from 200 to 300 feet above the level of the sea,affording to the vast basin a calm water, of great value to all vessels that frequent this coast.

The port of Dieppe being situated at the mouth of a river, a deposit of sand had beenallowed to accumulate from time immemorial, and the utility of it as a refuge for shipswas destroyed ; the rising of the ocean had also produced another evil; the right bankof the valley was covered with shingle, as well as with the debris of the left, whichhad been washed away ; by these obstructions the course of the Argues was changed, anda mouth opened for it at the foot of the east hill. The works at this port, as the construc-tion of the timber jetties, were carried on at various times, as necessity suggested, and withoutany regular plan being laid down ; nevertheless vast sums were expended upon the westjetty, as well as upon the prolongation of the Point; and when this costly work was aban-doned, the gravel accumulated to a height of 12 feet above low water; so that in the year1775, it had become only a retreat for small fishing boats. Another great inconveniencewas the want of fresh water in the harbour.

De Cessart , who was named engineer-in-chief for the improvement of the port, findingthat off the coast the tide rose to so considerable a height, did not venture to adopt the usualpractice of building in cofferdams, where they could only work at certain hours, but usedcaissoons, taking care to surround them with one or more .rows of sheet piles, driven untilthey ceased to answer the ram. In excavating foundations for marine works, he observesthat it is rare to meet with earth whose particles are sufficiently adherent to prevent filtra-tion, the deposits on the shore being so various that the water freely passes through them.

The vessels usually frequenting this port required from 15 to 20 feet of water, andallowing for the plunging of the vessel, the natural ground ought to be at least 22 feetbelow the level of high water. Many cofferdams have been constructed of this height, andthe works constantly carried on ; he mentions one formed by Thumbcrg in the Baltic atCarlscrona , which was 84 toises in circuit, sustained 20 feet of water, and contained aspace of 4000 superficial toises.

The jetties are composed of masonry 50 toises at the base, crowned by a platform 42 feetabove the level of low water, and 7 feet 2 inches below that level, so that the total heightfrom the foundation to the parapet was 49 feet 2 inches; De Cessart only formed theearthwork preparatory to this construction, which was sufficiently ingenious to admit of a

Fig. 249.

MOLE.

description: it was partly executed, but in 1793 it was demolished, and the timber sold. Abase of about 50 feet was laid out by driving piles into a hard bottom; these were capped atthe extremities, and the heads filled in with stones ; on this were placed three timbercaissoons, 36 feet in height, framed and planked over, and the interior filled with earth : theslopes were made to vary.

The mole stood 6 feet above the level of high water ; on the side towards the sea the in-clination was at an angle of 60 degrees. The whole may be considered to consist of three