338 FARM DEVELOPMENT 



particles fall out through small holes near the end of the 

 revolving screen next to the crusher, and the medium- 

 sized particles fall from a screen with larger holes, while 

 the largest particles (to be used in the bottom of the 

 macadam road) pass the largest openings, and run out 

 from the lower end of the screen. These are called 

 "tailings." Tailings are frequently run through the 

 crusher again. The bin under each portion of the screen 

 catches the material as assorted in the three sizes. From 

 the bins the large, the medium-sized and the finely 

 crushed rock are taken by team, preferably in specially 

 constructed distributing carts, and put upon the road, or 

 if the road is at some distance from the crusher, they are 

 transported in flat cars or in boats to the vicinity of the 

 road. It is here placed in carts or wagons and carted 

 to the roadway and laid as above described. 



Revolving screen and dust jacket. For small, portable plants 

 ordinarily used in country road work, a revolving cylindrical screen 

 is used. It is about 8 to 10 feet long and 24 to 30 inches in diame- 

 ter, and is usually divided into three sections of equal length. 

 When the harder and tougher rocks are to be crushed, the first 

 section is punched with holes about f of an inch in diameter, the 

 second about If inches in diameter, while the third section is 

 punched with holes about 2 inches in diameter. Where lime- 

 stone and softer varieties of rock are used, the screen is punched 

 with holes about as follows: First section, 1 inch in diameter; 

 second section, 2 inches, and third section, 3 inches. The latter 

 screen separates the rocks into sizes about as follows: inch down 

 to dust, from the first section; to 1J inches from the second sec- 

 tion; and 1 to 2\ inches from the third section. For traps and 

 other harder and tougher rocks, the screen, being provided with 

 the smaller holes indicated above, separates the rocks into sizes 

 about as follows: \ inch down to dust from the first section; \ to 

 \\ inches from the second section; and \\ to 2\ inches from the 

 third section. The softer rocks are crushed and separated into 

 the larger sizes for the reason that they will wear better not being 

 so easily broken or crushed by the traffic. The harder and tougher 

 rocks have to be crushed smaller, otherwise they will not bind 

 or form a solid, compact mass. In the higher classes of macadam 

 work, a dust jacket is made so as to cover about three-fourths of 

 the area of the first section of the screen. The purpose of the dust 

 jacket is to separate the stone dust from the screenings in order 

 that it may be placed last on the top course so as to be used as a 

 binder for the screenings. If the dust is not separated from the 



