Scientific Road-Making 103 



Me Adam began by saying that " In all three reports of 

 Committees of the House of Commons on the subject of roads, 

 they seem to have principally in view the construction of 

 wheeled carriages, the weights they were to draw, and the 

 breadth and form of their wheels ; the nature of the roads on 

 which these carriages were to travel had not been so well 

 attended to." Proceeding to give the results of his own 

 investigations, he expressed the view that the bad condition 

 of the roads of the kingdom was owing to the injudicious 

 application of the materials with which they were repaired, 

 and to the defective form of the roads ; and he assured the 

 Committee that the introduction of a better system of making 

 the surface of the roads, and the application of scientific 

 principles which had hitherto never been thought of, would 

 remedy the evil. 



The basis of his system, as denned on this and subsequent 

 occasions, was the covering of the surface of roads with an 

 impermeable crust, cover or coating, so that the water would 

 not penetrate to the soil beneath, which soil, whatever its 

 nature, and provided it was kept dry, would, he argued, 

 then bear any weight likely to be put upon it. 



His method of securing the said impermeable crust was by 

 the use of an 8 in. or 10 in. covering of broken stones, these being 

 not more than about i inches each in size, or more than about 

 six ounces each in weight. Such broken stones, if properly 

 prepared and properly laid on a road, would, he showed, 

 consolidate by reason of their angles, and, under the pressure 

 of the traffic, be transformed into a " firm, compact, im- 

 penetrable body," which " could not be affected by vicissi- 

 tudes of weather or displaced by the action of wheels." The 

 broken stones, with their angular edges, would, in effect, 

 dovetail together into a solid crust under a pressure which, 

 applied to pebbles or flints, would merely cause them to roll 

 aside, in the same way as shingle on the seashore when passed 

 over by a cart or a bathing van. 



The difference between his broken stones and the more or 

 less rounded stones with which the roads were then being 

 repaired was, McAdam declared, the difference between the 

 stones that were thrown down in a stream to form a ford and 

 the shaped stones used to construct the bridge that went over 

 the stream ; while inasmuch as the road-arch, or crust, he 



