Page:Popular Science Monthly Volume 80.djvu/335

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struction was merely an art, more or less skillfully carried on, based on experience, but purely rule of thumb in its execution and not founded on any rational principles. With the application of scientific methods as a means of determining the character of the stone to be selected the building of a water-bound broken-stone road was placed upon a much more satisfactory and rational basis. Roads of this type, so constructed, especially in Massachusetts and under the supervision of the Office of

PSM V80 D335 Diabase.png

Fig. 1. Diabase (Trap).

Public Roads in other states, were, and are to-day, entirely suitable and satisfactory for carrying horse-drawn traffic. When, however, self-propelled or motor vehicles became an important part of the traffic which these sufaces have to sustain, the latter have been found to be entirely unsuitable for the purpose.

The automobile has introduced an entirely new element into the road problem, and one which can only be solved by the application of the scientific methods. It is in this direction that science has proved itself of the greatest service to the highway engineer,

The destruction of the surface of a water-bound broken-stone road by motor traffic is due, according to experiments conducted by the Office of Public Roads, to the shearing or grinding action of the tires of the rear wheels of cars, which under the impulse of the engine, revolve at a slightly higher rate than that corresponding to the movement necessary to conform to that of the car over the road. It thus acts like a grindstone and loosens up the fine material which is necessary to cement