Another development which is due mainly to the difficulties of adopting the internal combustion engine for the haulage of heavy loads without shock, is the petrol-electric system. In this system the power of the car engine is used to drive an electric dynamo. This dynamo generates current which is either supplied direct to electric motors or else stored in a battery of accumulators, the former method being the better and more likely to survive. Sometimes one electric motor is used, taking the place of an ordinary gear box, and driving the back wheels through a universally jointed shaft and a differential gear. In other cases, two balanced electric motors are employed in or near the driving wheels. In others again, two motors are used, each driving through shaft and differential gear to one axle of the vehicle, and so providing an electric four-wheel drive. Another arrangement is the provision of four electric motors, one for each wheel. The vehicle is controlled through the medium of a “controller”; that is to say, an apparatus which, by the movement of a handle, varies the electrical connections and so makes the installation suitable for providing either a big torque at low speeds, or a comparatively light torque at high speeds. Electrical machinery is also in a sense self-regulating, and consequently a well-designed petrol-electric transmission is tantamount to the provision of an infinitely variable change speed gear. One of the strongest arguments against the petrol-electric method is that, when the machine is running fairly light and fast, the electrical machinery involves certain unnecessary power losses. Consequently, systems have been devised in which mechanical and electrical drive are combined, the latter only operating the vehicle under conditions equivalent to an increase of load on the engine. Efforts have been made for many years past to evolve a satisfactory internal combustion engine using paraffin or some other heavy and comparatively cheap oil in place of petrol. While these attempts have by no means failed, the practical results are up to the present more or less limited to the use of paraffin fuel in tropical or semi-tropical countries, where the higher temperature facilitates its employment. Among the disadvantages of paraffin are difficulties in starting up, a tendency to soot up the sparking plugs, the need of more frequent cleaning of cylinders, and a certain amount of disagreeable smell, partly due to the creeping of the liquid through every available crevice.