OF PIERS.
The piers of most of the ancient bridges in England were made of great thickness, in proportion to the span of the arches which they sustained. In London bridge some of them are larger than the original openings of the arches.
These piers consisted of small rubble stone laid in lime-mortar, surrounded merely by a thin casing of squared stones. It is probable that the Roman bridges, whose piers were said to be equal in thickness to one half the span of the arches, were constructed in the same manner. But these piers occupying so great a proportion of the breadth of the river, either raise a head, and tear up the bed, or cause a very great widen ing of the banks. In order to avoid, or as much as pos sible to lessen these inconveniences, the piers have been reduced in thickness, and constructed wholly with squar ed stones, each course being of equal height quite through the body of the pier. In Bewdly, Conon, l)un keld, and Lovat bridges, Mr Telford has made them from about a seventh to less than an eighth part of the span ; and at Ncuilly, Mr Perronet has given only one ninth part of the span to his piers. All these measures are taken at the springing of the arches.
In discussing the principles of bridges, it has been demonstrated, that the thickness ought to be regulated by the span and rise of the arches, combined with the height of the piers ; we have therefore cited the before mentioned examples, merely as being some of the nar rowest already practised with success on a large scale, but upon which engineers ought in future to improve.
In determining to what height piers ought to be car ried, great attention should be paid not only to the high est points to which, from the best evidence, the water appears to have risen, but also to the nature of the coun try where the bridge is situated. If, in the course of the upper part of the river, there are large lakes, or if the adjacent country is very flat, there can be no risk of the flood rising to a very great height ; but if there are no lakes in the course of the river, and if the bridge is placed where the adjacent country on each side is above its level, then the floods being thus confined, may possi bly rise to a still greater height than has been formerly noticed. For example, in the rivers Ness and Lochy in Scotland, which in their upper parts are connected with large lakes, the floods never rise more than four feet above the level of low water ; but in the river Clyde, where there are no lakes, the floods have lately, near to Glasgow, risen 20 feet ; and the river Severn, in Eng land, since it has been embanked in the low lands in Montgomeryshire, has frequently, in some parts of Shropshire, risen 15 feet.
Unless the bed of the river consists of rock, or is com pletely protected by inverted arches or well-constructed causeways, it is advisable to raise the piers as near to the line of ordinary floods as due regard to the arcliea and access will admit of ; and at all events to avoid finishing them under the line of ordinary low water.
The dimensions of the piers having been determined, the next step is to consider their shape. The portion of pier which supports the arch is usually of an oblong form, with its sides right-lined and parallel ; under low water, the pier increases in breadth downwards to the fountain, at the rate of from one inch in the Pont Royal to nine inches in the bridge of Neuilly, for every foot in height, and the platform extending from two to six in ches beyond the masonry. The rate of this increase of breadth must be in some measure regulated by the na ture of the foundations, and the proportions which the body of the pier bears to the span of the arches. In Neuilly, the thickness of the pier being at the springing of the arches only one ninth of the span, it became ne cessary to spread the base, in order to embrace a consi derable extent of ground ; but as in practice it is not easy to get the workmen to make the back part of the beds of the outside stones equally perfect with that which is near the face, these large offsets throw too much of the weight upon the imperfect workmanship of the beds, and should therefore be avoided. An increase at the rate of three inches for foot in height, appears preferable, and has been adopted in several of the Bri tish bridges. Large offsets certainly afford an opportu nity of supporting the centres very advantageously ; but this may be sufficiently obtained by having them three or four inches in the stone-work, and making the wooden platform to project considerably more around the pier. The shape of the points of the piers are, 1st, Acute-angled ; 2d, Right-Angled ; 3d, Semi-circular; and 4th, Having two segments of a circle intersecting each other. The 2d and 4th seem the preferable modes. These projecting points usually diminish from the line of each side of the piers, though we have seen them formed upon a narrower base than the breadth of the pier, leaving a square projection on each side ; but these projections obstruct the current, and cause a head which frequently injures the foundations. In the case of St Maxcnce, Perronet has departed from the right-lined figure, and also divided the body of the pier longitudi nally into two parts, leaving an opening between them equal to their thickness ; but as every water conduit should have its sections equal, and its course as direct as possible, this mode seems very objectionable ; it likewise diminishes the strength of the pier: and as he has made an inverted arch under the openings, to em brace the whole foundation, the saving of masonry,when compared with the whole mass, is very trifling ; and, with due deference to that able engineer, we cannot help observing, that, in point of taste, a work which is to convey an idea of durability, should rather have the appearance of stability towards its base, and diminish as it ascends.