. Appleton's dictionary de machines, mécanique, Moteur-travail, et de l'ingénierie. , Bien sûr, compter sur le diamètre de la roue, andthis est presque arbitraire. Toutefois, nous allons nous efforcer d'indiquer brièvement le principe général whichought d'être pris en considération pour fixer le diamètre, sans entrer à toute enquête stricte de thequestioa les roues à l'eau. 873 comme une conséquence de la théorie générale a déjà été expliqué, il s'ensuit que l'effet de lavis de la dynamique dépend de la relation qui la vitesse de la flotte par rapport à celle de l'eau ; butthis rapport est manifestement independ

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. Appleton's dictionary of machines, mechanics, engine-work, and engineering. , of course, depend upon the diameter of the wheel, andthis is almost arbitrary. We will, however, endeavor briefly to indicate the general principle whichought to be kept in view in fixing the diameter, without entering upon any strict investigation of thequestioa WATER-WHEELS. 873 As a consequence of the general theory already explained, it follows that the dynamical effect of thewheel is dependent upon the relation which the velocity of the floats bears to that of the water; butthis relation is manifestly independent of the diameter. The velocity due to the current of water to beused is always an ascertainable quantity, and may therefore be regarded as known. Another deter-minate element of the calculation is the number of revolutions which it is desirable the wheel shouldmake in a certain unit of time, as a minute, in order that the effect may be transmitted to the workingpoints, with a rate of velocity the most advantageous for the particular purposes intended, and obtained 3811.. in the simplest manner po.-sible. and with the least quantity of intermediate geering. It is also important that the wheel have a velocity and dimensions rendering it capable of fulfilling the purpose of afly at the rate of motion which it is intended to maintain. If we put « to denote the velocity ofthe extremities of the floats, and N the number of turns desired in a minute, the diameter will beexpressed by 60 m u -^=- = 19-1 — And to obtain an effect approaching the maximum, we may assume u ?diameter expressed in terms of the velocity and height of fall will be 24 </ H; and therefore the 191 X 2-4 v H 46 *J H very nearly. N N Thus, supposing the fall to be 6 feet, and the number of turns per minute required to be 10 = N;then the diameter will be |£ X y/ 6 = 46 X 2-45= 11^ feet nearly. This is nearly the minimumdiameter of wheel which would under any circumstances be employed; 12 feet to 25 feet may

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