SCIENCE IN HIGHWAY CONSTRUCTION 



331 



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. Eoads of this type, so constructed, 

 especially in Massachusetts and under the supervision of the Office of 



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-pro- 

 pelled 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 neces- 

 sary 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 



