14 BULLETIN 220, U. S. DEPARTMENT OF AGRICULTURE. 



The second or top course should be spread on the bottom course to 

 such depth that it shall have, when completed, the required thickness. 

 Blocks of wood of proper size may be used to gauge the depth of the 

 loose stone. Care must be taken to preserve the grade and crown, 

 also to prevent a wavy surface, and all irregularities and depressions 

 ought to be made up with stone the size of the top-course stone. - 

 After the surface is true to line, grade, and cross-section, and rolled 

 until the stone ceases to wave in front of the roller, it should be 

 covered with a light coating of screenings, spread on dry, rolled, and 

 swept in. The filler for the top course must be of top-course stone 

 screenings in which the percentage of dust is not excessive. The 

 spreading, sweeping, and rolling in of screenings should be continued 

 until no more will go in dry, after which it is best to sprinkle the road 

 until the top course is saturated. The sprinkler is then followed by 

 the roller. More screenings should be added if necessary, and the 

 sweeping, sprinkling, and rolling continued until a grout has been 

 formed of the screenings, stone dust, and water that shall fill all the 

 voids in the top course and shall form a wave before the wheels of 

 the roller. 



When the wave of grout has been produced over the whole section 

 of the road this portion of the road should be left to dry, after which 

 it can be opened to travel. Enough screenings should be spread on 

 top of the macadam to leave a wearing surface about three-eighths 

 of an inch thick. 



A miniature steam roller is shown in Plate I of this bulletin. For 

 further information on the subject of macadam roads, Farmers' Bul- 

 letin 338, * " Macadam Roads," may be of interest. 



BITUMINOUS MACADAM ROADS. 



Since the coming of the automobile, the resulting aggravation of 

 the dust nuisance, and the consequent raveling of the water-bound 

 macadam roads, numerous methods have been devised to secure a 

 lasting road surface reasonably free from dust and at the same time 

 within the financial means available for main-line country roads. 



While the several methods differ greatly as to name and the details 

 of carrying out the work, the fundamental aim of each is the same. 

 This is to secure a smooth, waterproof, and durable surface composed 

 of mineral aggregate, usually broken stone, bonded not only by the 

 mechanical interlocking of the fragments, but by a bituminous mate- 

 rial which coats the fragments and fills the interstices. Refined tars, 

 oil asphalts, and fluxed natural asphalts are the binders more com- 

 monly employed. 



1 Copies of this publication will be sent free to persons applying to the Secretary of Agriculture, Wash- 

 ington, D.C. 



