The essential differences between a tractor and a lorry should here be noted. The tractor is designed merely to haul a load, while the lorry is primarily intended to carry its load. In the first case, the engine and the load-carrying vehicle are two separate units coupled together; in the second, they form one unit. The latter is the more convenient arrangement so far as manoeuvring in enclosed spaces is concerned, since a good deal of skill is needed to back a tractor train with accuracy. Also, the steam lorry uses its load to increase the adhesive power of its driving wheels. On the other hand, the steam tractor can itself be doing useful work, while some of its load-carrying vehicles or trailers are being loaded or unloaded. By providing two sets of trailers, it can be kept usefully employed and need not waste time at its terminal points. Moreover, if it is required to work under very difficult conditions, it is a great advantage to be able to unhitch the engine from the trailer. If, for example, the bed of a river has to be crossed and the wheels sink into loose sand, the tractor is unhitched and run through without its load until it reaches solid ground. When it is brought to a standstill, its engine is employed through the medium of wire rope gear to drag the loaded trailer slowly but very surely out of its difficulty. Thus, for cross-country work, the tractor has much to recommend it, and it is not surprising that the success of the five-ton steamer has led to systematic endeavours to perfect internal combustion tractors possessing all the same advantages, and also self-contained for long journeys as regards fuel and water supplies. Mention has already been made of the fact that, when a tractor is used, the load does not assist the adhesion of the wheels. This constitutes, as it were, an artificial limit to the tractive power, and has naturally caused some designers to consider methods by which the engine power of a tractor could be applied not only to one pair of wheels but to all the wheels, so that the whole weight of the engine itself can be used to secure adhesion. The four-wheel drive is not common in commercial service, as it has only been found necessary under a limited number of very severe conditions. A good deal has been done, however, in this direction, particularly in France. The resulting vehicle need not be purely a tractor. In fact, we often find heavy lorries employed not only to carry a substantial load, but to haul an additional lighter load in a trailer. As a rule, these trailers have iron-tyred wheels, but for service in which economy of engine power is more important than economy of money, rubber tyres are usually fitted, since they have the effect of reducing the power absorbed in hauling the trailer by about 25 per cent.