. Elements of technology :. ut the experimentsof Mr Smeaton lead to the conclusion, that in practice there isa hmit of velocity, and that overshot wheels do most workwhen their circumferences move at the rate of about three feetin a second. Undershot Wheel,—An undershot Fig. 5. water wheel, is a wheel furnished witha series of plane surfaces, called floatsor floatboards, projecting from its circum-ference for the purpose of receiving theimpulse of the water, which is deliver-ed by a proper canal, with great velocity,upon the under part of the wheel. Awheel of this kind is represented in Fig. 5
. Elements of technology :. ut the experimentsof Mr Smeaton lead to the conclusion, that in practice there isa hmit of velocity, and that overshot wheels do most workwhen their circumferences move at the rate of about three feetin a second. Undershot Wheel,—An undershot Fig. 5. water wheel, is a wheel furnished witha series of plane surfaces, called floatsor floatboards, projecting from its circum-ference for the purpose of receiving theimpulse of the water, which is deliver-ed by a proper canal, with great velocity,upon the under part of the wheel. Awheel of this kind is represented in Fig. 5. When an undershot wheel is put in motion by a stream ofwater striking against one of its floatboards, in a direction atright angles with the radius, the action of the water will dimin-ish, as the velocity of the wheel increases, till at last the mo-mentum of the water, or of the accelerating force, is just equalto the momentum of the resistance, or of the retarding motion of the wheel will then become 262 OF THE MOVING FORCES USED IN THE ARTS. By calculation it appears that a machine thus driven by theimpulse of a stream produces the greatest effect, or does mostwork in a given time, when the wheel moves with one third ofthe velocity with which the water moves. * But in practicethis rule is liable to some variation, for the water does not es-cape as soon as it has given its impulse, but is confined by thechannel for some time, and acts with a variety of Mr Smeatons experiments, which are cited as authoritiesby most writers since his time, it was found that an undershotwheel when working to the greatest advantage, had a velocitywhich varied from one third to one half the velocity of thestream } and that in great machines it was nearer to the latterof these hmits, than the former. It is advantageous that the size of undershot wheels should beas great as circumstances will permit, and it ought never, saysDr Brewster, to be less than seven time
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