390 * Fron Bridges on the Principle of Tenacity 
of iron that compose the arch, cast on a similar plan, arranged 
in alike order, and Sena ag in the same way upon one another, 
as the blocks of stone in the arches of Waterloo bridge, and re# 
quiring, in consequence of that arrangement, a coresponding bulk 
and strength in the piers and abutments, not only to bear the 
perpendicular pressure or gravity of the materials, but to afford 
an adequate resistance to what is termed the lateral pressure, the 
pressure of the sides of the arch, or bridge, upon the bases on 
which they rest. In the construction, however, of such bridges 
as are proposed to be erected at Hammersmith and Rotherhithe, 
the iron is made use of so, as that its property of tension should 
be most effectively and advantageously employed, and the pieces 
of which the structures are conyposed, are so adjusted with a view 
* to the mutual dependence of the parts and the independence of 
the whole, as to diminish the necessity of bulk, without injury to 
the strength of the fabric; and to promote a proportional light- 
hess in its appearance and effect, at the same time that it almost 
annihilates the occasion of the lateral pressure. An illustration of 
the manner in which the weight or pressure operates in reference 
to such an arch, will enable the reader to perceive the way in 
which these important objects are attained, 
Let the action of an archer’s bow be considered, when the upper 
side of the arch is pressed by the hand, while its ends or points 
are resting ona table. The force applied upon the bow would 
produce a ‘spread, which, in the case of a bridge, would be termed 
its lateral pressure, and which in that case would require a cor- 
responding strength and resistance in the building of the abut- 
ments or piers. If the cord, however, be attached to the bow, 
and the same force as before be applied to press it, the cord would 
seein to pull and counteract the spread to which the bow would 
be disposed, and prevent any lateral pressure being experienced 
beyond its points, In the structure of an arch, if formed as a 
bow of iron, or in that of a bridge composed of a series of such 
arches or bows, the like result must be produced, if every are be 
furnished with its proper chord of iron, and that chord be, as care 
should be taken that it should be, of ‘adequate atréripth.- A fa- 
miliar and accurate idea of such a figure may be conceived, from 
recollecting that of the brass segment which usually composes 
part of a case of mathematical instruments. An iron structure 
of that form, if constituted so as to be made an arch of a bridge, 
would not on any seale require abutments to resist its pressure, 
or the weight that might be laid upon it. It would rest at its 
points upon the upright standards that would be provided to sup- 
port and raise it above the water, and would press or act upon 
them only in a perpendicular direction, and in a way that could 
most easily and economically be resisted. | 
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