A reciprocating engine is one that has one or more cylinders in which pistons are pushed back and forth by the steam which enters alternately one end of the cylinder and then the other, thus turning the shaft. This piston, running first up and then down, is joined to a “connecting rod” which in turn is connected to a “crank shaft” which is a continuation of the propeller shaft. As the piston moves up and down, one end of the connecting rod moves with it, for it is fastened by a hinge to the lower end of the piston rod which runs out of the bottom of the cylinder. The other end of the connecting rod is attached to the crank shaft which has a section of itself carried out to one side just as the shaft on which a grindstone is mounted is bent at right angles and attached to the handle. As the piston goes up, carrying the connecting rod with it, the off-centre section of the crank shaft is carried up also, as the handle of a grindstone is carried up when the operator begins to turn the wheel. When the piston has reached the top of its stroke the connecting rod has pulled the crank until it is pointing straight up. Then the steam pushes the piston down and the piston pushes the connecting rod, which in turn pushes the crank, so that the shaft is turned, just as you might turn a grindstone by hand, your arm representing the piston rod and connecting rod, and the handle representing the crank shaft. The turbine, however, is a very different machine. There are no pistons and no other parts similar to those of the reciprocating engine. Instead there is a shaft on which is mounted a great steel wheel. Around the edge of this wheel are mounted thousands of little vanes, and the whole wheel works on the same principle as a windmill. A windmill carries a comparatively small number of vanes arranged somewhat as the blades of an electric fan or a propeller are arranged. When the wind blows against these “vanes” the wheel revolves. Now a turbine is a very highly developed example of this same principle, and the steam is led to it through pipes and directed against these vanes, which are small but are very numerous. The result is that this windmill type of engine revolves at a very rapid rate. One can get some idea of these turbines when he learns that the eight turbines of the _Majestic_ contain a total of 900,000 vanes. Turbines, however, have two major failings: First, they cannot be reversed—that is, a turbine can turn in only one direction—and second, they are most efficient when they operate at high speed. In order to use turbines on ships, then, it is necessary to have one turbine on each propeller shaft to drive the propeller ahead, and another turbine with which to drive it astern. Consequently, the _Majestic_ has eight turbines—two to each propeller shaft.