The difficulty so far has been that the wheel demands a road and destroys a road, and that, whilst it is easy, though frequently very costly, to make a road which will suit a wheel, it is most difficult to make a wheel which will not damage a road; for failing a cheap and simple form of Pedrail wheel, a system of multi-wheels has to be resorted to, and this system leads directly to the tracked machine, which not only can dispense with roads, but, what is equally important, can make its own track, just as the feet of a man form a path by frequently crossing the same piece of ground. This is not the place to examine in detail the technicalities of roadless vehicles; but to-day there are two main types of these vehicles; an all-tracked machine of the tank type, and a half-tracked machine which has wheels in front and tracks in rear. The first is more suitable for heavy loads, and the second for light. In the manufacture of these vehicles three main problems must be solved: (1) The vehicle must be able to use roads without damaging them; nor must it damage the surface of the ground it travels over. (2) It must be able to move across country without damaging itself. (3) The cost per ton-mile must be equal or lower than that of existing vehicles. It may seem a paradox to lay down that the first requirement of a roadless vehicle is that it can negotiate roads, but, in fact, it is not so; for it stands to reason that, when prepared tracks do exist, it is only wasting time and energy to travel across country. Further, if the tracks of the vehicle are so constructed that they do not damage roads, they will not damage the surface of the ground, and, consequently, by continually travelling over the same ground, they will compact and consolidate its surface and rapidly form a road of their own which will require no metalling. This advantage is one of the great secrets of its success. As movement across country entails traversing rough ground, the tracks of a roadless vehicle must permit of the absorption of obstacles. This absorption is attained by springing the tracks. In an unsprung machine, obstacles are either crushed into the ground or the vehicle has to lift itself over them. In both cases the result is injury to the machine, and loss of power and discomfort. It stands to reason that the vehicle must be durable, simple and easy to maintain; also that the ton-mile cost must be low. As regards this latter requirement, experimental machines have so far proved that this is a possibility. A one-ton roadless Guy Lorry recently travelled from London to Aldershot, and its ton mileage was fifty-two to the gallon. It has also been worked out that the cost per ton-mile of the Sentinel tractor, “including overhead charges, depreciation, interest on capital and all running charges, and allowing for a 20-tons net load for a reasonable number of working days in the year,” will be slightly under twopence per ton-mile. [Illustration: SENTINEL TRACTOR