ROAD MODELS. 5 



Section C shows the second course, which was of small stones laid 

 and beaten by hand hammers. The finished layer was composed of 

 broken stones about the size of walnuts and was spread with a shovel. 



Section D represents the finished road as consolidated by travel. 

 The crown was made 6 inches, the width 18 feet, and the total thick- 

 ness about 10 inches. 



E shows the curbs, which were composed of rough, flat stones, set 

 on edge. The upper edge was made flush with the road surface. 



MACADAM METHOD OF CONSTRUCTION. 



The macadam method of construction was introduced in England 

 and Scotland by a Scotchman, John Loudon MacAdam (1756-1836). 

 The chief features of MacAdam' s construction were a raised, thoroughly 

 drained, and crowned earth foundation, stone broken to a uniform 

 size not exceeding 1J inches, and no addition of binding material to 

 the broken stone. MacAdam also insisted that the finished road 

 should have a slight crown and that broken stone when spread on 

 the road should be kept raked smooth until thoroughly consolidated 

 by traffic. This form of construction continued practically unchanged 

 until the introduction of the road roller, about 1870. 



During the past 40 years the methods of construction and mainte- 

 nance have been greatly modified, yet the term "macadam" is still 

 applied to broken-stone roads. 



At the present time the macadam road is built in courses, with 

 the coarser stones at the bottom and the finer on top. Stone screen- 

 ings or sand are used for binding. In practically every case the 

 stone is broken by machinery. 



Plate III, figure 2, illustrates a macadam road of the type con- 

 structed during the first period of macadam construction, which 

 began about 1816. 



Section A shows the earth foundation, which was always made 

 higher than the surface of the adjacent ground so as to facilitate 

 the escape of water from the foundation and the surface. 



Section B shows the layer of hand-broken stone, with a depth of 

 10 inches. This stone was broken to sizes weighing about 6 ounces, 

 and no stone was used that exceeded 1J inches in its greatest dimen- 

 sion. The surface of the road was raked regularly during the process 

 of consolidation. No rollers were used, and the stone was compacted 

 by traffic. 



Section C shows the finished road, from 16 to 18 feet wide. The 

 crown was raised from 4 to 6 inches. MacAdam contended that 

 the stones would lock or bond by virtue of their angularity and so 

 make a water-tight crust, and that it was neither necessary nor 

 desirable "to bond a road with earth, clay, chalk, or other material 

 that would imbibe water or be affected by frost." The surface 



