All About Engines

i5o All About Engines 4,000 feet per second may be acquired. However light the particles of steam may be, they exert, by reason of their number and their high velocity, a considerable force upon any object which stands in their way. The principle was employed, in a very imperfect manner, in the engine invented by Branca in 1619. He caused a jet of steam to strike against a number of vanes fixed on a wheel. As each little vane came under the jet it received an impulse—hence the name impulse turbine—and the succession of im- pulses caused the wheel to spin round. But the force exerted on the vanes depends so much upon the form of the nozzle—a fact that was not known in Branca’s days—that his engine may have given only a fraction of the power of which it was capable. The manner in which the steam exerts a force upon a vane will be understood by reference to Fig. 88. Here a b represents an edge view of a curved plate or vane ; c is a nozzle from which issues a jet of water (the principle is just the same if steam replaces water), which enters upon the plate at a and leaves it at B e. If the surface of the plate is smooth, friction between the water and the plate will be small, and the water will move over the plate with practically no change of speed. Owing, however, to the change of direction which the water under- goes, a force is exerted upon the vane creating a tendency to move in the direction of the arrow. So far the vane has been regarded as fixed. If it moves in the direction of the arrow the water,