Model, (scale 1:10), of Poncelet's Undershot water-wheel in masonry pit with rack and pinion operated sluice, on base board. Original by Jean Victor Poncelet, 1824, model by Regnard Frères, 59 rue Bayen, Paris, France, 1891.

Smeaton improved the undershot wheel, but it was Gen. J. V. Poncelet who, in 1824, brought it to the high state of efficiency illustrated by this model. The characteristic features of his design are the backwardly curved floats and the absence of a sole plate. The water is projected on to the wheel buckets tangentially, at the periphery, through a sluice inclined at an angle varying between 40° and 60° to the horizontal, according to the diameter of the wheel and to the effective head of water. The sluice gate is operated by a hand-wheel, spur gear and rack. The buckets are proportioned so that the water enters without shock, with the velocity due to the head, and flows up the vanes, until brought to rest by gravity, when it falls, exerting downward pressure. The water is discharged from the floats with but little remaining forward velocity. In wheels of this type however the power produced by the weight of the water is negligible compared with that due to its momentum. The floats are inclined on the upstream side about 30°, and the radius of the bucket is usually one-eighth the diameter of the wheel. Owing to its slow speed and cumbersome gearing the Poncelet wheel has been displaced by the low-fall turbine. The effective head of water should not exceed 5.5 ft.; the maximum efficiency is 65 per cent.