Hints how to u~e a Grindstone. ~8~ wier of given dimensions be made in it, or a bridge be thrown acrossit ; or how much its waters will be raised by turning another stream into it, or sunk by taking off a branch to turn a mill? * *~ • * * * * * Yet these aremost important questions. The causes of our ignorance are the want or uncertainty of our principles; the falsity of our theory which is. belied by experience ; and the small number of proper 6bservations or experiments, and the difficulty of

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... making such as shall be serviceable." Dupuit says :* " The comparison of velocities, observed in larg~ bodies of running water is far from confirming the preceding formula ;" and he also says that M. Menard states in his '~ Gears de Canstruction," that he found the actual outflow at a certain point in the river Meuse to be 33"4 m., while Prony's formula, applied to different. sections, gave 53 m., 56 m., 74 m., 69 m., 34 m. and 29 m. At~ another point the computation gave 28 m., 11 m., 30 m., 39 m. and: 4m., while the actual discharge was 25 m., 5." Etudes th~oriques et pratiques sur le mouvemeat des Aaux, l~r L Dupuit~ Paris, 1863, page 55.