240

HISTORY OF ENGINEERING.

Rook I.

The most ingenious part of this arrangement is the formation of an embankment in thebed of the river, into which the piles were driven, and of a second embankment over it, inwhich the caissoons were placed. After the wall was constructed, the whole was thenhacked in by earth brought from the neighbourhood, and the spacious quays thus obtained

Fig. 253.

WALL, ROLEN.

A it

Fig. 254.

NEW QUAY, ROLEN.

were paved throughout. To maintain a level above low water, where some difficultiesoccurred in cutting off the heads of the piles, arches were turned 7 feet span, and thewhole most ingeniously connected.

Cordouan Lighthouse . — Since the time of the ancients there has never been a moreimportant and superb pharos erected than that of Cordouan ; it is situated on a rock formingan island at the mouths of the Garonne and Dordogne , and but for the warning it affords,the vessels entering or leaving would be always in danger of wrecking. It serves as a sea-mark during the day and a lighthouse at night; there are only two passes, the one calledthe pas des anes , between St. Saintonge and the tower of Cordouan , the other betweenthe tower and Medoc, called the pas des granes , both equally dangerous to vessels thatmay be unfortunately surprised by a heavy westerly wind. The tower is in 45° 35' latitude,and 16° 53' longitude, two leagues from Bordeaux . All around it are rocks covered only3 or 4 feet, against which the billows rise to a great height, and break with tremendousviolence, rendering the access to this tower very difficult; vessels of three tons only canapproach it by a single channel, about 100 feet in width ; at 600 feet from the tower there isa sand on which they can run aground at the moment of low water, of which advantagemust be taken, the rest is nothing but unapproachable rocks. This magnificent tower, 169 feethigh from its foundations, was built in the reign of Henry II. by Louis de Foix, who com-