The petrol engine has become so firmly established as the motive power for aircraft, automobiles and small vessels that a short while ago it would have seemed almost impossible that its supremacy could be challenged. It is no exaggeration to say that it was the petrol engine which made aircraft possible, while on the road, despite the fact that the steam engine is fundamentally more suited for traction work, the petrol engine has practically ousted its rival from the field. Even for the heaviest vehicle, where, owing to cheap coal, the lower running cost at one time gave the steam vehicle an advantage, the petrol engine is nearly universal, while at sea, for launches and small craft, the steam engine has practically disappeared.