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Steam Turbines 167 this is not so in the reaction machine. Owing to the form of the blades in the latter, the pressure at b is less than that at a, and hence under the fall in pressure the speed over the blades increases as the fluid moves from A to b. In other words, it acquires momentum just as in the case of the suspended vessel in Fig. 100 ; and for this reason the blades are urged in the direction of the arrow exactly as in the case of the suspended vessel by a force which depends upon the difference of pressure between inlet and exit of the nozzle. The result is precisely the same when the blades are free to move, though the angles have to be proportioned suitably to the speeds. A little further thought will show that the be- haviour of the steam in the moving blades of a reaction turbine is similar to its behaviour in the fixed blades of an impulse turbine. And since the fixed and moving blades of a reaction turbine per- form similar purposes they are similar in form. Further, as the fixed blades of both impulse and re- action turbines perform the same office, it is in the moving blades that the differences exist. Ihe essential features of the reaction turbine are (i.) the steam undergoes a fall in pressure, and conse- quently expands as it passes through the moving blades, and (ii.) work is done as a result of the change of momentum suffered by the steam in passing through the moving blades as in the impulse type, and this arises partly from the expansion within the vanes. On account of the fact that there is a