[Illustration: A GROUP OF MOTORS IN THE SERVICE OF THE FLYING CORPS AT A RECENT REVIEW IN FRANCE.]
Throughout its history, it has been greatly helped by the prior existence of the steam traction engine. From these heavy and slow machines, suitable only for limited use in particular spheres, have been developed two very useful classes of lighter steam-propelled machines coming under the provisions of the Motor Car Acts. The first is the steam tractor, which is merely a small edition of the traction engine, able, on account of its lighter weight, to travel at considerably higher speeds. The other is the steam lorry, which is an extremely valuable machine for the carriage of anything up to about six tons of goods at speeds of about five miles per hour. From the five-ton steam lorry there has more recently developed a lighter type of steam vehicle in the shape of the three-ton lorry, generally running on rubber tyres, and so entitled legally to travel at much higher speeds. The great economy of steam motors made it absolutely necessary for the makers of internal combustion industrial vehicles to study every possibility of reducing operating costs. They had on their side advantages as regards higher speed possibilities and more complete independence of fuel supplies. The steam motor of ordinary type cannot be conveniently designed to carry with it fuel and water supplies adequate for very long journeys. On the other hand, the steamer has the great advantage of being able to exert tremendous power at low road speeds. The steam engine is more flexible and more capable of standing a heavy overload than the internal combustion engine. Even if it is brought almost to a standstill, it can go on applying the full steam pressure behind its piston during every stroke. Given adequate supplies of fuel and water, it is an admirable and very economical machine for all sorts of rough and heavy work. Curiously enough, the steam lorry and the steam tractor have been essentially British developments, and as such they have done much to bring the British industrial petrol vehicle up to its present high standard of perfection.
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.