Chap. XXV.
ON STONE BRIDGES.
1511
The fourth experiment, fig. 2755., was made on a starling, the base of which was anequilateral triangle; the same effects were remarked as in the two first, but they were notso evident; the height of the eddy was not so much, and the lateral currents divergedless; the fall, although less, still subsisted at the shoulder; the small letters on the severaldiagrams show the situation where the sections below were taken.
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Fig. 2756.
ARCS OF A CIRCLE.
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The two sides of the base of the starling were formed in the fifth experiment, fig. 2756.,by two arcs of a circle equal to a sixth of the circumference, described on the sides of anequilateral triangle; no cataract was formed at the shoulder: along the faces of the pierthe current took an inclination which extended below bridge ; there was produced on eachside, as in the semicircular starling, a second current, which did not commence so soon, norrise at first to so great a height, but which was as considerable on leaving the arch; theeddy did not rise quite so much, but the water equally fell in a sheet.
In the sixth experiment, fig. 2757., the form of the pier was that of an ellipsis, thesmall diameter of which was the fourth of the large one ; the water rose much less beforethe pier than in the preceding experiments, and the lateral currents had a uniforminclination along its faces ; the second current was relatively more considerable, and evenbecame higher than the first, with which it was confounded on leaving the arch : both ex-tended to a less width.
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