Another effect of speed is noted on the more or less viscous materials of which road surfaces are composed. The pushing of the wheels against the surface causes wrinkles which continue to grow until the wrinkles become waves entirely across the pavement. Such waves may also be produced by expansion and contraction due to changes in temperature, but are probably always accentuated by wheel pressure. Side thrust of wheels often produces longitudinal waves in viscous road crusts. In the classifications given no one seems to have considered the proportion of sprung and unsprung weight in the motor car. There can be no doubt but that the resiliency of the springs relieves the pavement of very much of the shock of impact. This is illustrated by an attempt to drive a nail into a springy board. It can hardly be done because the springiness of the board uses up, absorbs, the work of impact. A mechanical statement is, the work of impact equals the change in kinetic energy, or algebraically stated _Wv_² _Fs_ = ----- 2_g_ when the entire energy has been absorbed. Here _F_ is the acting force and _s_ the distance through which it acts, _Fs_, is the work done by the force _F_. _W_ is the weight of the ram or moving body (vehicle, wheel load), _v_ the velocity of impact and _g_ the acceleration of gravity, a factor that enters the equation in the expressing of mass in terms of weight. Solving this equation for _F_ there results, _Wv_² _F_ = -----, 2_gs_ which shows that the smaller _s_ is the greater the force of impact _F_. When _s_ is made long by means of a spring the force _F_ becomes smaller. This is illustrated by the old method of catching a baseball without gloves--the hands were allowed to go backward so that the work of stopping the ball was spread over a greater distance, the impact force thus becoming so small it did not sting the hands. The effect upon the road, and also the vehicle, is like that of the hammer which hits a nail on the anvil. The nail is flattened, pounded to pieces very soon. But if the nail were not placed upon the solid anvil but upon a slab of springy steel, it might be pounded all day without doing it much harm, the spring at all times absorbing the shock. So with the weight of the vehicle largely sprung the damage to the roadway is comparatively small. Therefore, it would seem, as though a fair classification would take into account the springs of the vehicle. The pneumatic tire, and the cushion tire and wheel, each act as springs and shock absorbers in varying degrees. In some of the censuses, pneumatic or solid tires were noted, and very many of the earlier noted whether rubber or steel tires were used.