Fig. 113.
Note.—“A weir is a dam erected across a river to stop and raise the water, as for the purpose of taking fish, of conveying a stream to a mill, of maintaining the water at a level required for navigating it, or for the purposes of irrigation.”
For facilitating the computation of the quantity of water flowing over weirs, Weir Tables, are used, based upon approved formulas, of which “Francis’ Formula” is perhaps the most reliable. These tables are applicable to the subject of water wheels but cannot be printed in this work.
Fig. 114.
[Fig 114] represents an over-shot water wheel (F G H L, with axis at O) in which the water flows upon the top of the wheel at h, in the same direction in which it revolves, therefore the impact of the water is utilized upon the upper buckets H, a, b, after which the weight of the water acts in the buckets c, d, e, F, e´, d´ and c´. At b´ the buckets begin to overflow and empty themselves as shown at a´. It will be seen that the water acts upon almost one-half the circumference of this wheel, thus realizing the greatest mechanical effect with the smallest quantity of water.
The current-wheel is perhaps the first application of the force of water in motion, to drive machinery. In the first century B. C., water-wheels for driving mills were used in Asia Minor and on the Tiber. In the former case we suppose, but in the latter case we know, that these were current-wheels.
Fig. 115.