Let us imagine that we can see through the casing of a turbine at work and that looking down upon it from above we can trace the progress of a particle of steam. It rushes in from the inlet and at once makes straight for the outlet at the further end. Suddenly, however, it encounters one of the guide blades (those on the case) and by it is deflected to one side, we will suppose the left. That causes it to rush straight at one of the blades upon the rotor against which it strikes violently, giving that blade a distinct and definite push to the left. Rebounding, it then comes back towards the right but quickly is caught by another guide blade and by it hurled back upon a second rotor blade, giving it a leftward push just as it did to the first. Thus it goes zigzagging from one set of blades to the other until, tired out, so to speak, it finally flows away forceless and feeble through the outlet, having given up all its energy to the blades of the rotor against which it has struck in its course. That, then, is the journey of one single particle. Multiply that by an unknown number of millions and you have a description of what takes place in the interior of a steam turbine. The blades are so proportioned, so arranged and so placed that it is very difficult indeed for a particle of steam to creep past without doing its share of work. Practically every one is made use of and while, of course, the action of a single particle of steam would have but a negligible effect, the vast number engaged cause the rotor to be powerfully blown round. The reason why the casing and rotor are made larger and larger as one proceeds from the inlet towards the exhaust or outlet is that the steam must, if all its energy is to be extracted, expand as it goes and the enlargement provides room for this expansion. One of the great advantages of the turbine is that the steam is always entering at the same end. In the cylinder of a reciprocating engine the steam enters alternately. It comes in hot but as it does its work and finally goes out it becomes very much cooler: the next lot of steam which enters, therefore, is chilled by the cool walls of the cylinder which have just been cooled by the departure of the previous lot of steam: so heat is wasted. Wasted heat means fuel lost, and as any given ship can only carry a limited quantity of fuel, wasted heat means less range and more frequent returns to the base to coal or to "oil."