The success of the automobile depends upon the character of the roads it has to traverse. Wheels provide the best form of transportation over a smooth, hard road, but if the road is soft, the wheel will sink into the surface and will be greatly impeded. In traveling over mud or sand runners are preferable to wheels because they have a broader and longer bearing surface. In the snowy regions of the north and the sandy regions of Africa and Asia Minor, sledges are used in place of wheeled vehicles. Sliding friction is less than rolling friction under such conditions. In fact, only where man has constructed special roads is it possible to use wheeled locomotion. It is because in nature we must contend with all sorts of surface conditions, soft and hard and seldom smooth, that the rolling principle of locomotion is not to be found in any species of animal. The legs of an animal are levers, just as a wheel is a system of levers, but in the former case the levers can be folded or extended to adapt themselves to all the unevennesses of the ground. No animal can begin to run as fast as an automobile on a good road, but on the other hand we have yet to build a machine which will run as fast as a horse on soft and uneven ground. WALKING MACHINES Farm operations must be conducted on loose and broken surfaces for which the ordinary wheeled vehicle is unfitted. In some cases broad wheels are used to keep them from sinking into the soil and they are furnished with cleats so as to give them a good grip on the ground, but long ago it was realized that if the horse was to be completely displaced on the farm a more adaptable form of locomotion than that of wheels must be furnished. Inventors sought to make machines that would walk. One interesting machine of this class which met with a certain degree of success attempted to combine walking and rolling. Large wheels were used which were provided with a series of feet or tread plates connected by knuckle joints to the wheel rims. As the wheels revolved, these treads came successively into contact with the ground and the machine actually walked on its feet. The feet were broad enough to prevent the machine from sinking into the ground and they adapted themselves to inequalities in the surface. Instead of having to roll over a rock that lay in its path the machine would plant its feet on the rock and lift itself over.