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Speed of water is most deceptive. For example, cascading white water may be much slower than gliding smooth-topped water. Where there's white water there's been dissipation of energy. Furthermore, water flows at different speeds from top to bottom and that's what affects the angler most. With this variation of speeds from bottom to top, how does the hydraulic expert measure true speed? The answer is, he's worked out a formula. He takes two readings, one one-fifth of the way from the top and the other one-fifth of the way from the bottom. The two, averaged, come very close to giving the exact answer. To observe the variations, let's take an easy piece of water to read such as a U-shaped canal. Most of us have already surmised that in such a canal, the water has a slight drag on the bottom and sides. Slight? My eye! It's fully two-thirds slower! And due to surface tension, the top layer also has a drag. Actually, the fastest water in a canal travels in a tunnel-like tube, pretty well toward the top half of the canal, and it slows up decidedly toward both bottom and sides. For example, when it flows at 3 miles an hour in this center tunnel-like tube, it often flows less than mile an hour near the bottom.
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