THE REVERSING ENGINE
All marine engines should be fitted with some apparatus which enables the engineer to reverse them from full speed ahead to full speed astern in a few seconds. The effort required to perform the operation of shifting over the valves is such as to necessitate the help of steam. Therefore you will find a special device in the engine-room which, when the engineer moves a small lever either way from the normal position, lets steam into a cylinder and moves rods reversing the main engine. By a link action (which could not be explained without a special diagram) the valves of the auxiliary are closed automatically as soon as the task has been performed; so that there is no constant pressure on the one or the other side of its piston. To prevent the reversal being too sudden, the auxiliary's piston-rod is prolonged, and fitted to a second piston working in a second cylinder full of glycerine or oil. This piston is pierced with a small hole, through which the incompressible liquid passes as the piston moves. Since its passage is gradual, the engines are reversed deliberately enough to protect their valves from any severe strains. These reversing engines can, if the steam serving them fails, be worked by hand.