p. 252              HYDRAULIC  MOTORS.
Fig. 126 represents a wheel which, though in form a water wheel, acts in some degree upon the principle of the turbine.  It acts to but slight extent by the direct weight of the water, and it gives its best effect with a peripheral velocity, which is not constant for all diameters like that of the ordinary water-wheel, but bears a definite relation to that due the head.  The water entering the wheel does not impinge upon a flat vane or plunge into a mass of water in a bucket.  It glides up a curved vane, comes to rest, and then glides smoothly back and escapes.  It avoids the loss at the entrance which is inseparable from all the forms thus far considered, and it avoids in part the loss at the discharge, since the water leaves the wheel in a direction partly contrary to that of the wheel's motion, and the quantity of energy carried away in the discharge is not great.  The wheel is shown as made of wood, though such a wheel, if made now, would probably be made of iron.  The shaft, arms, and shrouding are put together as

p. 253                WATER-WHEELS.
already described.  The floats are inserted in grooves cut in the shrouding and are firmly confined by through-rods which draw the two sets of shrouding together and grip the floats.  Each float is composed of several plank, and is stayed at the middle by an iron strap bolted to the back.  The through-rods
cause considerable obstruction to the water, but cannot be avoided in a wooden wheel.  In the iron wheel they can be dispensed with.  The proper position for these rods is close to the back of the float.  Situations sometimes offer in which this wheel may, under existing conditions, be judiciously applied, viz., a head not exceeding 6 feet, and an application not calling for a high velocity, as pumping, grinding, or the work incident to a powder-mill.  The best speed for the outer ends of the floats is about 55 per cent of that with which the water enters the wheel, though it can vary from 50 to 60 without

p. 254              HYDRAULIC MOTORS.
material disadvantage.  The diameter of the wheel is three or four times the total head.  The best efficiency found by the earlier experimenters was not over 6o per cent, but a wheel of this form erected at the powder-mill at Angoulême in France, about 1847, showed an efficiency of 68 to 75 per cent, and this efficiency was not materially diminished by a considerable amount of backwater.
In addition to the losses of effect already pointed out, the disadvantages of water-wheels as compared with turbines are very great.  They occupy a great deal more room than turbines of the same power, and must be enclosed in buildings to prevent obstruction b ice in winter.  Backwater affects them injuriously, not only by diminishing the head, but by partly drowning them and causing them to expend their power uselessly in "wallowing," a source of loss from which turbines are wholly free.  The necessarily low velocity with which they move necessitates cumbrous and expensive gearing to raise the speed to the requirements of industry, which is constantly calling for increased rates of speed.  These numerous disadvantages

have caused the water-wheel to be almost entirely superseded by the turbine.