96 THE BOOK OF NATURE STUDY 



and allows the movement of the stones under traffic. This at 

 once introduces friction between adjacent stones with renewed 

 wear and the production of still more dust. At the same time 

 the bonding of the metal breaks down, and it is more easily shifted 

 by water currents. Such a road soon begins to show cross channels 

 running down the camber, which are filled with runnels at every 

 watering and every shower, and are cut gradually deeper until 

 the road surface is destroyed. 



It is in summer that alternate wetting and drying combined 

 with traffic give rise to most disintegration. But in winter frost 

 is more effective, and its results may be easily studied. Water 

 between the grains of the binding material freezes and expands. 

 This either raises the whole surfacing of the road or else pushes 

 up the binding material between the stones. The latter effect is 

 better seen in a garden path. While the frost lasts the ice cements 

 the whole material together into a hard rock-like mass ; but 

 directly the ice thaws and is replaced by water the metal is left 

 in a loose condition and the surfacing is readily ground up by traffic 

 or washed away by water currents. 



A waterproofed road should be studied in contrast with a 

 surface of imperfect macadam. In this case all dust is first 

 removed ; the metal is rolled in with rock-chips, not ground to 

 dust, and the whole is bound together with tar. Water being 

 thus excluded, frost cannot work between the stones, and the 

 binding material cannot so easily work up into mud and dust. 

 Movement of adjacent stones against each other is checked, and 

 wear is reduced to the direct friction of traffic on the surface. 

 Disintegrating agents being thus rendered inoperative, the effect of 

 storm water is very much diminished. Unless traffic breaks through 

 the waterproof crust, which happens when the foundation on 

 which it rests is unstable, such a road will last until the surfacing 

 is absolutely worn away. 



A detailed and comparative study of the two types of roads 

 is capable of furnishing some important conclusions which flow 

 logically and easily from the observations. The process of de- 

 struction is seen to be a twofold one, each part of the process 

 playing into the hands of the other. Disintegration is carried on 

 by water in the liquid state or when freezing, and by the move- 



