328 TEE POPULAR SCIENCE MONTHLY 



road. To-day quite a different procedure is employed. The stone is 

 applied in two courses and in two sizes. The first or lower course con- 

 sists of larger stone than that used for the surface, from an inch and 

 a half to two and a half inches in diameter. It is carefully and uni- 

 formly spread to such a thickness that it has, when compacted, a thick- 

 ness of from three to five inches, depending on the character of the 

 foundation and that of the traffic which the road is to carry. The 

 second or upper course, which forms the surface or crust of the road, 

 is composed of finer stone, one half to one and a half inches in diameter, 

 to a thickness of at least three inches when compacted. Great advances 

 in highway construction have taken place since Macadam's day, in that 

 steam rollers have been available for some decades for compressing and 

 putting in place in a proper manner, the broken stone after it is applied 

 to the foundation. Each course is rolled separately until it ceases to 

 move under the roller. 



After the compression is completed a binder or filler of much finer 

 stone is spread over the surface, all of it passing -an opening three 

 eighths of an inch in diameter and a considerable portion being fine 

 dust, for the purpose of filling the voids in the upper course of stone, 

 closing up the surface and preventing infiltration of water through it. 

 This is accomplished by the use of water, applied with a watering cart, 

 which washes the fine material into the interstices in the stone. The 

 road is then again rolled with the steam roller, to aid in forcing the 

 filler into the surface, and to render it compact and waterproof. Skill 

 is required in the manipulation of the roller to produce a surface of 

 proper conformation and uniform density. 



The method of constructing a water-bound broken-stone road, which 

 has been described in a very general way, is still in use for work of this 

 type, and was the only one employed up to the beginning of the present 

 century, in building roads to meet the most trying conditions then exist- 

 ing. It was a very satisfactory form of construction and still is, under 

 certain conditions and environment. The execution of such work was 

 an art involving great skill and experience, but science contributed but 

 little to perfecting it and placing it on a rational foundation, with the 

 exception of methods of examining the character and availability, from 

 this point of view, of the various rocks which are used in building roads, 

 which were developed in France, and on a much more elaborate scale 

 by the Massachusetts Highway Commission, and in the Office of Public 

 Eoads of the United States Department of Agriculture by Mr. Logan 

 "Waller Page, the present director of that office, who, with his assistants, 

 has contributed largely to the application of science to the improve- 

 ment of highway construction. These methods are of so much interest 

 that they are worthy of description. 



In determining the suitability of a rock for the construction of a 



