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38° TECHNOLOGY. 
the arch, the various forms of which exert an important influence upon 
the whole arrangement and appearance. 
It is the fortune of but few architects to be intrusted with the building 
of large bridges, and consequently to have experience on this subject. | 
Perronet in France is amongst the most celebrated of modern bridge- 
builders, and has planned and erected a great number of important bridges, 
amongst others that of Neuilly, which will be referred to hereafter, and 
which has already been mentioned under Architecture, and represented in 
Plates, Div. VIL, pl. 60, jig. 9. The first step towards the erection of a 
bridge is to build the piers and abutments ; for this purpose, when possible, 
the bed of the river where the pier is to rest must be laid bare, and we will 
here exhibit some of the usual methods of accomplishing this and of laying 
the foundations beneath the surface of the water. 
The space which the pier is to occupy is first inclosed with a coffer-dam. 
For this purpose piles are driven into the ground below, inclosing a rectan- 
gular space, and four to six feet within these another series is driven; the 
two walls thus formed are well stayed and strengthened with timbers, and 
the space between the two is rammed full of puddled clay. Much depends 
upon the careful preparation of the coffer-dam, as a slight blunder may lead 
to disastrous consequences, even the entire destruction of the dam. 
Under certain circumstances, the dam may be built cylindrical. 72g. 5 
shows the disposition of the piles and timbers, and jig. 6 is a vertical section 
of the same; @ is the outer wall of piles, f the interior piling, and c ¢ ¢’” 
braces for the greater security of the dam. The dam being completed, the 
next operation must be to empty the inclosed space (jig. 4), for which 
pumping arrangements of some kind are necessary, which are driven by a 
water-wheel placed in the stream at the side of the dam, or by a steam- 
engine. It is very seldom possible to lay the bottom dry; so great is the 
pressure of the water from without, that it is constantly forced up through 
the bottom of the dam, which obliges the pumps to be kept in operation 
until the masonry of the pier is completed to a level with the surface of 
the water. So soon as the bottom of the dam is dry, or as nearly so as 
possible, the foundation of the pier is commenced, preliminary trials having 
been made to ascertain the nature of the ground below. If rock be found, 
the intermediate layer of earth is removed, and the pier is placed imme- 
diately upon the rock. 
In most cases, however, sand, gravel, or clay is found, and then a fram- 
ing of timber becomes necessary, which, if the ground below be firm, is laid 
upon the surface, and covered with a plank floor, upon which the masonry 
is commenced ; generally, however, it becomes necessary to pile the whole 
area to be covered by the foundation. To this end large piles are driven, 
at a distance of 2 feet from each other, to such a depth that they can no 
longer be sunk deeper by repeated blows of the monkey. When the piles 
are driven, they are all cut off upon the same level, and upon the surface 
thus produced the floor is laid which is to receive the masonry, the space 
between the piles and lower timbers of the frame being rammed with clay, 
stones, &c.; the pier is then raised of cut stone, or, if of rubble, the stone 
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