ON THE ERECTION OF ALEXANDER III. BRIDGE IN PARIS. 473 
eontractors, Messrs. Letellier and Boutrinquieu, in spite of the intelli- 
gence displayed by the inspectors, who had great experience of compressed 
air work (especially in connection with the late Pont Mirabeau), the engi- 
neers were somewhat anxious about the success of the operation of sinking 
the caissons, never attempted before in such conditions and on such a scale.! 
Great precautions were taken to insure regularity. Among the other 
difficulties encountered were those due to the variable resistance of the 
soil and to the presence in it of stones and piles belonging to the old 
demolished suspension: bridge and to the foundations of the quay. 
Frequent observations on the flexure of the metallic frame were 
considered to be the most convenient means to prevent the caisson from 
breaking. For this purpose a general water-level pipe was placed in the 
working space, so fitted that at every instant the inspector could state 
the flexure of walls and main girders. Moreover, daily observations were 
made in the open air to ascertain the true situation of the caisson. 
When the caisson reached the proper depth the surface beneath it— 
consisting of large calcareous flagstones on the right bank and of sand on 
the left one—was cleaned and levelled and the working space was filled 
with concrete. The five rooms of each caisson were successively filled, 
the men retreating from one to another so that electric lighting could be 
maintained to the end. 
The caissons were sunk and filled with concrete, and the backing 
constructed of heavy stone masonry laid in cement. The facings of the 
abutments on the river side were made of courses of hammered ashlar 
of cut granite stones ; lastly, five granite courses were put behind the 
sockets perpendicular to the direction of the thrust, their surface increasing 
in size, so that the ordinary masonry in connection with the last one bears 
a crushing stress only of about 18 kilogrammes per square centimetre (252lb. 
per square inch), while the granite-bearing stones in connection with the 
sockets have to resist about 48 kilogrammes per square centimetre (780lb. 
per square inch). 
Temporary Works Rolling Bridge.-—The moving system designed for 
the erection of the arches consisted really of a steel riveted bridge of 
120 metres in length supported above the level of Alexander III. bridge 
by framework of pyramidal shape, the whole forming a gigantic 
travelling crane rolling on double sets of rails fixed on the upper part of 
the abutments. 
This apparatus was arranged to support the load of centreings and 
arches, and also to carry the centreings to their successive situations and 
to secure a convenient platform for handling the arch pieces. 
As the necessities of navigation did not require a temporary passage 
more than 50 metres (164 feet) in width, the contractors for the metallic 
part of the work were allowed to mount the side parts of the arches near 
the springings on wooden centreings supported by piles in such a manner 
that they could be moved by slipping with the central part of the 
centreings fixed to the rolling bridge. In this way it was possible to 
relieve this temporary bridge of a large part of the load, and also to 
sustain its trusses at two intermediate points by metallic columns resting 
on rows of piles while the arches were being erected. The rolling bridge 
had generally in consequence three spans, two of 33:50 metres (110 feet) 
» The engineers to the Port du Havre, encouraged by this success, have projected 
to sink caissons of more than 22,000 square feet, through the bad soil they have 
to deal with. 
