To be sure the first steam engine was invented by Hero, the disciple of Ctesibius, in the second century before Christ, and it was not a water-raising machine. It consisted of a hollow sphere (see Figure 46), mounted to rotate and fed with steam that entered it through the journals. The steam issued from opposite sides of the sphere through two bent tubes. The steam in issuing from the tubes reacted against the tubes, pushing them back and causing the sphere to revolve. Every action is accompanied by an equal and opposite reaction. A bullet fired from a gun kicks the gun back. In order to push the bullet out of the barrel, the powder must have something to push against and the push against the gun is equal to the push against the bullet. Even if there were no bullet to be pushed the discharge of the powder would react against the gun. In order to push its own gases out of the gun, the powder must push back against the gun. [Illustration: FIG. 46.--HERO’S STEAM ENGINE] In Hero’s engine, the steam, in order to push itself out of the sphere, must push against the bent tubes. We have a similar reaction motor in the revolving lawn sprinkler. The water issuing from the bent arms of the sprinkler forces them back and causes them to revolve. Were the arms radial and not bent the reaction would be there just the same, but it would be exerted against the center of the wheel and hence there would be no rotation. Reaction has nothing to do with the pressure of the atmosphere as so many people imagine. A gun would kick just as hard and the lawn sprinkler or Hero’s engine would operate just as well and as fast in a perfect vacuum. It was to demonstrate the principle of reaction that Hero built his steam engine. As far as we know it was never put to useful work and remained merely a scientific toy. NEWCOMEN’S ATMOSPHERIC ENGINE