For Great Britain, oil fuel has the disadvantage that it has to be imported whereas the finest steam coal in the world is found in abundance in South Wales, but the difficulty may eventually be overcome by distilling from native coal an oil which will serve as well as that which is now imported. So much for the turbine, the engine of the big ships: now for the Diesel oil-engine which drives the submarines. It belongs to that family of engines called "internal-combustion" since in them the fuel is burnt actually inside the cylinder and not under a separate contrivance such as a boiler. There have been oil-engines, so called, for many years, but they were really gas-engines since the oil was first heated till it turned into vapour and then that vapour was used as a gas. The Diesel engine, however, actually burns oil in its liquid state. To understand how it works let me ask you to conjure up this little picture before your mind's eye. A hollow iron cylinder is fixed in a vertical position: its upper end is closed but its lower end is open: inside it is a piston, free to slide up and down: by means of a connecting-rod hinged to it and passing downwards through the open lower end the piston is connected to a crank and flywheel. At the upper end of the cylinder are certain openings which can be covered and uncovered in succession by the action of suitable valves. Now let us assume that that engine is at work, the piston going rapidly up and down in the cylinder. As it goes down it draws in a quantity of air through a valve which opens to admit the air at just the right moment. The moment the piston reverses its movement and starts to go up again that valve closes and the air is entrapped. The piston continues to rise, however, with the result that the air becomes compressed in the upper part of the cylinder. Now it is necessary to remind you at this point that compressing air or indeed any gas, raises its temperature. This air, therefore, which was drawn in at the temperature of the outer atmosphere, by the time the piston has reached the top of its stroke has attained a temperature well above the ignition point of the oil fuel. The piston, having arrived at the top of its stroke, the upper part of the cylinder is filled with hot compressed air: the next moment the piston commences its descent, but at precisely that same moment a valve opens and there is projected into the cylinder a spray of oil. Instantly it bursts into flame, heating the air still more, so that as the piston descends the air, expanding with the heat, pushes strongly and steadily upon it. The amount of that push can be varied by varying the duration of the jet. The longer the jet is injected the more heat is generated and the more sustained is the push. On the other hand, if the jet is cut off very quickly the push is only a gentle one. The power of the engine can thus be adjusted to suit varying circumstances by a slight variation in the valve which controls the jet.