Chap. XXV.
ON STONE BRIDGES.
1513
body; this effect is analogous to'that just described, and, though it may appear extraordinary*necessarily takes place in all cases, but it is only evident where the rapidity is great.
Fig. 2761.
Fig. 2760.
■H
[Biiiii
The second experiment made with a velocity of 4 metres 87 centimetres per second, ona pier of an elliptical base. Fig. 2761. presented the same effects as that where thecurrent had a velocity of 3 metres 9 centimetres, but they were more marked: hence wemay conclude that the elliptical piers have the property of occasioning the least contraction.
The following table contains the principal dimensions of the currents observed in the pre-ceding experiments.
Form of the Base of the Starlings.
Height of the Water.
Distance of the SideCurrent.
Starlings.
Middle ofthe Pier.
Oppositethe Angleof theShoulder.
Oppositethe Middleof the Pier.
Rapidity 3*9 Metres per Second.
Metres.
Metres.
Metres.
Metres.
1.
Rectangle -
-
0-041
0018
0-099
0-203
2.
Triangle rectangle
-
0036
0 014
0081
0-126
3.
- equilateral
-
0034
0 016
0036
0-072
4.
Half circle -
-
0038
0 023
0 023
0095
5.
Triangle mixtilinear
-
0036
0-016
0027
0 077
6.
Ellipsis ...
-
0032
OOll
0018
0061
7.
Triangle mixtilinear concave • -
-
0036
0009
0036
0-104
8.
-
-
0045
0009
0-041
0-189
4-*7 Metres per Second.
1.
Triangle mixtilinear
.
0072
0032
0090
0-131
2.
Ellipse -
-
0059
0045
0045
0 081
These experiments and observations will enable us to appreciate the advantages and in-conveniences of the different kinds of starlings, with relation to the size of the contractionwhich they cause the current to undergo ; but we must not forget that the starlings arealso useful in breaking the ice, and preventing any floating mass from stopping against thepiers of a bridge, and diminishing the section, which would necessarily tend to produce anundermining.
Ice in places where the temperature is the same is more dangerous, as the course of theriver is slower; in this case the rivers freeze more quickly, and the ice, which has time toacquire a considerable thickness, detaches itself in larger masses; it is essential then, insuch cases, to take precautions in relation to the effect they may produce, and to makethe projecting angle of the starlings more acute ; if any fears are entertained for its solidity,it may be protected by bands of iron, or with a bar of iron placed in the masonry.
One of the best methods proposed for remedying the inconveniences resulting from icein rivers is that which Perronet had adopted for the bridge projected over the Nevaat St. Petersburg!!; it consists in inclining forward the projecting edge of the starling; by