A steam engine's jog is to turn a wheel. To do this, coal is burned to turn the water in boiler into steam. This steam escapes into the cylinder and pushes up the piston - which in turn pushes up one side of the beam. When the piston reaches the top of the cylinder, the steam escapes and the piston and beam - are allowed to fall back down. The whole process then begins again and the see-saw action of the steam is used to torn the wheel through the 'sun and planets' gears.