1510

THEORY AND PRACTICE OF ENGINEERING.

Book II.

to one or the other condition, that form must be selected which is most proper to producethe result required: the following experiments will enable engineers to make this choicewith a sufficient knowledge of the subject.

These were made by means of a rectangular canal between boards of 50 centi-metres in length, in which were placed models of piers, 15 centimetres in thickness,and where the water flowed about 40 millimetres in height. By means of a fall, velocitywas given to the fluid of about 3 metres 9 centimetres per second, which is about that ofgreat rivers in floods: care was taken to observe all the circumstances of the contraction,and the eddies and currents formed were accurately measured. The first experi-ment (fig. 2752.) was made on a pier, thestarling of which was rectangular; theeddy formed before the pier presented aspecies of band, A, nearly circular, which wasraised 34 millimetres above the level of thewater, falling at the angle of the shoulderalmost vertical. The length of the faces ofthe sides of the pier exhibited two othercurrents, B, C, uniformly inclined, and whichwere only real at the surface; below, thewater, from the rapidity due to the fall ofthe eddy, formed a stream both rapid anddangerous : when two abutments were sub-stituted for the pier, two similar bandswere produced, which met each other in thecentre of the arch, where they reciprocallycrossed: below the figure the profile ofthe water taken in different parts is traced,which the letters indicate.

In the second experiment, fig. 2753., thestarling of the pier was a triangle, the pro-jecting angle being a right one; it did notform so great an obstacle as the rectangle,nevertheless the eddy rose as high: the two %

currents established against the flanks of the pier were much less strong, but the fall formedat the side of the shoulder was as deep, and more dangerous; the current before the eddybecame a species of sheet where the water fell, returning on itself, as seen by the section.

The third experiment, fig. 2754., was made on a pier the starling of which was a half-circle : the eddy rose nearly the same height as in the preceding, but it was not sowide; it formed two currents, the first, A, followed in gentle inclination along the facesof the pier; the second, B, appeared to be produced by the opposition of the first, and

a - p neither of the two formed a fall against

the pier; the water rose before the pierabove bridge, but in the middle of the

Fig. 2752. RECTANGULAR STARLING.

r . c .

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m

..

Fig. 2753. TRIANGULAR PIER,

Fig. 2754. SEMICIRCULAR STARLING.

arch it evidently preserved its level, and only increased in rapidity in proportion as theprincipal current was narrowed by the two currents resulting from the eddy. v