1514
THEORY AND PRACTICE OF ENGINEERING. Book II.
means of this arrangement the masses of ice which strike the pier tend to mount a littlealong this edge; their weight then acts on each side so as to break them, when each part iseasily carried away by the current.
The form of the starlings down stream is not so important as those above : there areeven a great number of ancient bridges in which they are omitted; in this case thespace which they would have occupied is filled by stagnant water or a whirlpool, whichit is necessary to prevent: it is therefore more advantageous to place them on both sides.When a river has been narrowed by an obstacle, such as a pier, the effect of which is todiminish the width of its bed, the two currents formed along its faces unite, describinga species of curvilinear triangle, and are then prolonged on each side the faces ofthe starling below bridge; hence result underminings, which are scarcely less frequentbelow than above; this should induce builders never to omit the starlings below bridge,but even to give them greater length than is customary. When the waters rise muchabove the springing of the arches, the form of the starlings has little influence on thenature of the contraction, which then depends principally on that of the arch, and themanner in which the intrados of the vault is united with the face of the starlings : one ofthe best methods of diminishing the contraction in this case is that adopted at the bridgeof Neuilly : to effect it, the two faces of the starlings may be terminated by a curvedsurface agreeing with that of the vault; examples may be found at the bridge of Gignac,and at that of Navilly on the Doubs. In a design’projected by Gauthey for a bridge atAuxonne , the starlings had the form of the prow of a vessel; when they are destined todivide the waters and break the foe, they should rise to the height of the greatest floods,or at least to that where the breaking-up of the ice takes place. We must observe thatin semicircular arches the starlings diminish the contraction but little when the watersrise much above tbe springing, because the tympanum of the arch then presents a greatsurface, which is directly opposed to the current; and the starlings serve only to receivethe first shock of the ice. In the last century much attention was paid to their decoration,which is not so important as the solidity of their construction: the crowning easilydecomposes, and plants grow in the joints, which increase this effect, which might beprevented by forming them with great stones.
In fig. 2762. is the elevation and plan of one of the piers of the bridge ofMoulins; both starlings above and below are triangular, with the angle projecting androunded, which gives greater solidity, a precaution from whence no inconvenience canresult, and which it is proper always to take; it is preferable to the use of a bar ofiron in the masonry, which latter method should not be resorted tounless the bad quality of the stone obliges its use: the crowning hasthe form of a triangular pyramid; in the stones of which it is con-structed, a vertical part has been preserved, which should have from 8to 10 centimetres in height: starlings of a triangular base have some-
PIEK OP MOULINS’ BHIDGE.
Fig. 2763. PONT au change.
times been crowned with a cap formed by the prolonging of their faces cut by an inclinedplane coming forward, as in the Pont au Change at Paris , fig. 2763.
Figs. 2764. and 2765. represent one of the piers of the bridgeat Orleans. The base of the starling is terminated by two arcsequal to the sixth of the circumference; that of down stream is a halfcircle; the projecting angle of the upper starling is not sufficientlyacute to require anyrounding, and the formof the pier is that per-haps which best answersall the conditions re-quired. The crowning ofboth the terminationsof the starlings has a
height which might be Fig. 2764. pier of Orleans’ bridge.
diminished ; they are formed by a conical surface.
Figs. 2766. and 2767. represent one of the piers of the bridge of St. Maxence : in someforeign works are examples of piers divided into two parts, the interval of which is covered