The reaction-wheels are those in which the water is allowed to escape through apertures upon a vertical shaft, in such a manner as to develope a force in the reverse direction of the outflow, which is cap able of being converted into a movement of rotation. Barker's mill, those designed by de la Cour, and by (Motor, were based upon this principle; but the useful results they have produced have not been such as to lead to their practical application. They are in fact little better than philosophical toys.
It may finally be stated that in the present state of the arts of con struction, the undershot-wheels with straight floats working in a close race are those which are the most adapted to falls of between 4 feet, and 8 feet 6 inches; the undershot-wheels with curved floats are the most advantageous when the fall does not exceed 5 feet. Bucket wheels are desirable when the fall exceeds 10 feet, and is less than 16 feet ; and when the supply is variable, it is preferable to use the breast-wheel rather than the overshot one : the breast-wheels at all times suffer less from backwater than any other form of wheel. Over
shot-wheels are generally used when the fall ranges between 16 and 40 feet ; above the latter fall there are decided advantages in the use of the turbine, which certainly has a remarkable power of adaptation to varying conditions. M. Fourneyron executed one turbine with a head of 9 inches, and another with a head of not less than 354 feet. Mr. G. Rennie states that the coefficients of the useful effects of the various descriptions of wheels are fouud in practice to differ slightly from those quoted above ; and that they cannot with safety be taken at more than, in overshot-wheels, from 0.60 to 0.80 ; in breast-wheels from 0.45 to and in undershot-wheels from to (Smeatou, Ott the Power of Mills ; Banks, Treatise on Milts; Buchanan, Practical Treatise on Mill Work ; papers by Fairbairn, Glynn, &c. ; d'Aubuissen, Traiid cl'Hydrauligue ; Leptis de 316canigue Pratique ; Belidor, Euler, Navier, Bossut, Coriolis, &c.; Repertory of Arts ; Transactions of the Franklin Institution, &c., &c.)