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230
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TUNNELS AND SLUICES.
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The opening at the points of divergence was originally made 14 feet wide, and a drop of 1-1/2 inches given from the main sluice to the turn-out sluice, which latter was set on a " swing " of 4 inches to 16 feet.
The
sluices thus constructed were found to run satisfactorily; but on
increasing the swing (as became necessary) to 5 inches the boxes on
either side of the junction choked, only partially discharging the
material, which difficulty could not be obviated by increasing the
grade. On increasing the width of the discharge opening from the main
sluice, which was gradually widened from 14 feet up to 24 feet, the
sluices ran uninterruptedly and no further difficulty was experienced.
The
first box bottom was cut in the form shown in Fig. 66-— that is, from a
point to full width ; the succeeding half-box, of 8 feet, was high on
the outside, set with a slight increase in grade, and given a 4-inch
swing. All the other boxes were set with a swing of 8 inches to the
box, and on the grade of the main sluice for a total distance of 200
feet, after which it was found necessary to straighten the sluice for
some distance to give the water opportunity ' to regain its velocity.
These experiments showed that in a 200-foot swing on a 2 per cent.
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