erected over the Garonne. 



265 



Great as this bridge is in point of magnitude, yet it is nei- 

 ther by the number nor by the size of its arches that it recom- 

 mends itself to the notice of the professional engineer. 



The depth of the river, the rapidity of its currents, and 

 particularly the instability of its bed, were the real difficulties 

 which called forth the talents of the engineer ; and in these 

 respects, the bridge of Bourdeaux will not suffer by a compa- 

 rison with any other work of the same kind. 



The Garonne has a depth of twenty, twenty-five, and in 

 some places of about thirty-five feet, and twice every day the 

 flux and reflux of the sea raise its waters sixteen and even 

 twenty feet high ; and its currents, in both directions, have 

 often a velocity of more than ten feet in a second. This ri- 

 ver flows over a sandy and muddy bottom, which is easily dis- 

 placed, and which collects in banks in different parts of its 

 course. 



In order to found such a building upon a soil of such con- 

 sistency, 250 piles of fir-wood were driven in as the founda- 

 tion of each pier. After they had penetrated the ground from 

 twenty-six to thirty-three feet, they were all cut over on a level, 

 about thirteen feet below the low water of the river. A large 

 boat, or floating caissoon, with a flat bottom, and of a pyra- 

 midal form, received the first row of stones of the pier ; and 

 when this was, as it were, thrown down in its place, the work- 

 men descended in diving-bells, and made the caissoon rest on 

 the piles destined to bear it. A general pavement, consisting 

 of loose stones, covered the bed of the river in the direction of 

 the arches. These stones enveloped and agglutinated by the 

 mud which collects between them, form a bed impenetrable 



