A steam engine utilises the energy contained in steam under high pressure. The energy that is released when steam expands is made to produce rotary motion which can be used for driving the machinery. The seam form boiler is admitted into the cylinder having a piston , when the steam expands, it causes the piston to move, when the piston has travelled to the end of the cylinder and thus completed the stroke, the expanded steam is allowed to escape form cylinder. At the same time the path of the steam is changed over. i.e. live steam under pressure is admitted to the other side of piston causing the later to travel back past its starting point until it has reached the other end of stroke. A steam engine of this type is called double acting because force of steam is applied alternatively on two sides of piston.