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Steam Turbines 165 have now to examine in greater detail the reaction principle. Suppose the vessel shown in Fig. 100 to be sus- pended by a thread and to be filled with steam which can only escape through the nozzle. Now the ex- pansive force of the steam is exerted equally in all directions, and it will, therefore, press equally over the interior surface of the vessel. But at the nozzle it will escape, Fig. 100.—Diagram to explain reaction due to jet of escaping steam and since, according to Newton’s Third Law of Motion, “ to every action there is an equal and opposite reaction,” the force exerted on the es- caping steam will create an equal and opposite force on the vessel, causing it to swing backwards in the direction shown by the arrow. To give a con- crete example. Suppose the pressure of the steam in the vessel to be 200 lb. on the square inch. Then, with the type of nozzle shown in the diagram, the pressure at the nozzle will be only 116 lb. on