Stone Roads. 473 



cart and the watch is a scale by which the size of the pieces 

 may be judged, its diameter being a trifle less than two 

 inches. 



587. Thickness of Layer. The thickness of a layer 

 placed at one time should vary somewhat with the size of 

 the pieces, the depth being greater with the larger frag- 

 ments. With pieces of the size shown in Fig. 224 the layer 

 when packed should not be greater than four inches and 

 three inches will pack more quickly and closely than four 

 inches. A too thick layer tends to form a crust on the sur- 

 face, making it difficult to fill all the voids below com- 

 pletely. 



588. Rolling. The function of rolling is to arrange the 

 fragments in the positions of the greatest stability with ref- 

 erence to the rolling of wheels and the tramping of horses. 

 The first effect of the roller is to bring the pieces nearer 

 together and to reduce the size of the voids. This is clearly 

 brought out by the two photo-engravings, Figs. 224 and 

 225. 



There is one other important thing which rolling should 

 secure and that is to put the several pieces of stone together 

 in the positions of the most stable equilibrium ; that is, "in 

 positions such as to make certain that they shall not tip 

 or turn when the stress of the wagon or team is brought 

 upon them. 



589. Size and Weight of Roller. The diameter of the 

 roller should be large to prevent it from shoving the stone 

 forward as it moves and in order that the thrust may be as 

 nearly directly downward as possible. It will be observed 

 that even the front wheel of a loaded wagon often slides 



. rather than rolls when coming upon the unpacked layer of 

 rock on the road, and such movement cannot do proper 

 packing. 



There appears to be a lack of agreement between prac- 

 tical men regarding the proper weight of the roller, some 



