In these latter machines there are a number of cylinders with closed ends and with very smooth interiors, in each of which slides a disc-like object called a piston. The steam enters a cylinder first at one end and then at the other, thus pushing the piston to and fro. The movement of the piston is communicated to the outside by means of a rod which passes through a hole in the cover at one end of the cylinder, the to and fro motion being converted into a round and round motion by a connecting-rod and crank just as the up and down motion of a cyclist's knees is converted into a round and round motion by the lower leg and the crank. The lower part of a cyclist's leg is, indeed, a very accurate illustration of what the connecting-rod of a steam-engine is. As is evident to the hastiest observer, some arrangement has to be made whereby the steam shall be led first into one end and then into the other end of the cylinder: also that provision shall be made for letting the steam out again when it has done its work. Moreover, such arrangements must be automatic. Hence, every reciprocating engine has special valves for this purpose and such valves need rods and cranks (or something equivalent) to operate them. Further, to get the best results the steam must not simply be passed through one cylinder but through several in succession. Engines where the steam goes through only one cylinder are called "simple," where it goes through two they are "compound," where three "triple-expansion," where four "quadruple-expansion." Generally speaking, each cylinder has its own connecting-rod and crank, also its own set of rods, etc., for working its valves. Hence, a high-class marine reciprocating engine is of necessity a complicated mass of cylinders, rods, cranks and other moving parts continually swinging round or to and fro at considerable speeds, all needing oiling and attention and all liable at times to give trouble. And now compare that with the turbine, which has TWO parts, only one of which moves. That part, moreover, is tightly shut up inside the other one, being thereby protected from any chance of damage from outside and likewise rendered unable to inflict any damage upon those in attendance upon it. At first sight it seems very strange that the turbine should be the newer of the two, for it is simply an improved form of the old time-honoured picturesque windmill which used to top every hill and grind the corn for every village and hamlet.