ROADS AND BRIDGES 



335 



layer, and it in turn is thoroughly rolled. When 

 sprinkled on the rock during the rolling process 

 helps to slide the surfaces into firm, locked positions. 

 When this second layer has been rolled many times, the 

 third layer is applied and the fine binding material is 

 gradually added as the roller, by repeated application 

 to the particles of stone, crushes and works them into 

 position. After adding the fine binding materials, the 

 application of water to assist the roller in hardening the 

 surface is of special importance. The choice of rock 

 for the surface stone is 

 very important. It 

 should be both hard and 

 capable of cementing. In 

 some cases, some tougher 



Figure 221. A, crown of macadam road; B, 



and traO rOCkS outer edge of stone surface; C-K, level on straight 

 d-iiu Liop U^JYO edge ln pogition to compare A with leve i on guide 



\\(* rmVprl with limP- stake; O-M, straight edge in position to test 



mxeu wiin me level at outer edge of stone surface . 

 stone, so that the latter 



may help cement the tougher stones which better en- 

 dure the wear of travel. 



To secure the proper depth of each of the three layers, 

 stakes are placed at either side of the line of the road, on 



which are marked the 

 height of the crown and 

 also the height of each of 

 the layers. By occasionally 

 measuring across from 

 these heights, the desired 

 height can be secured at all 

 points. Figure 221 shows 

 how these measures may 

 be taken by means of a straight edge and mason's level. 

 The telescope leveling instrument with measuring rod 

 may also be used where great accuracy is desired. 



Macadam roadways must be built sufficiently thick so 

 that the wheels will not break down the stone or punch 



Figure 222. Cross-section of a rock crusher 

 showing the stationary jaw, X, and the oscil- 

 lating jaw, K, with rock between. 



