ROADS AND BRIDGES 



331 



laid broken stone not more than I inch in diameter, and 

 into it should be worked the fine particles and dust from 

 the crusher. Sand or broken limestone may also serve 

 as binding materials and be worked into the surface. 



After placing the first layer of coarse rock it should 

 be thoroughly rolled with a heavy roller. A steam roller 

 is preferred. Rather than roll the road with too heavy 

 a roller, it should be gone over many times with a 



Figure 220. Laying lower course of a telford road. 



machine of medium weight. The object is not so much 

 to embed the stones into the underlying clay, as to 

 work them about so that each stone finds the smallest 

 place into which it will fit. Rock which, when crushed 

 between the jaws of a rock crusher, breaks into angular 

 forms, nearly cubical, may thus be kneaded together 

 into a harder crust than rocks which crumble up or do 

 not have sharp, hard edges, and rough surfaces. 



