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Steam Turbines 153 32.2, the gravitational constant. Engineers make this alteration at once by expressing momentum as 32'2 Suppose now that m lbs. of steam flow past the vanes per second, and that each particle in passing has its velocity measured in the direction taken by the moving vanes changed from v to z/2. At the begin- ning of a second m lbs. of steam have a momentum m x Vi ^2'2 ’ 'while at the end the momentum is only ----—2. The force f lb. acting upon the vanes, 32 2 tending to drive them round, will be, therefore, m x vx _ m x v2 or F w (vx — v2) 32 2 32'2 ’ ’ 32'2 This formula is of primary importance in turbine design. In de Laval’s turbine the vanes, or blades, as we shall call them in future, are fixed radially round the edge of a disc, as in Fig. 89 on Plate 10, and one or more nozzles (Fig. 87) are fixed so that they strike the ring of blades at an angle. Owing to the motion of the blades and their shape the jets are deflected. Suppose they could be arranged so that the steam impinging upon a blade with the wheel at rest was turned completely back upon its path. It would then exert its greatest force upon them, but no work would be done on the wheel so long as it did not move. Next suppose that the wheel moved at such a rate that the blades had the same speed as the steam. The steam would pass through the blades