94 THE BOOK OF NATURE STUDY 



mechanical disintegration, and the disintegrated material is now 

 ready for removal by natural agencies or by artificial means. 



In dry weather the wind picks up the fine particles, sweeping 

 them along for a distance, and dropping them as it dies down. 

 Much of the -dust falls on the road again, some is transported to 

 the road-side, some deposited on the grass, hedges, and trees, and 

 some distributed over the bordering fields. On the whole, much 

 finds its way off the road into the fields, for after a spell of high 

 winds the roads are found to be scoured clean and to become 

 quite dustless. Examination of the surface at this stage will 

 show that the stones project for a fraction of an inch, the dust 

 having been removed to this depth from between them. 



But in wet weather the dust becomes mixed with water, and 

 if enough rain falls the mixture flows off the road, either directly 

 down the slope to the side (the " camber " of the road) or for a 

 time along the wheel tracks, eventually escaping towards the 

 lower ground at the side. If the rain is heavy, or if the gradient 

 of the road, or its camber, is steep, the water gathers in little 

 torrents which flow with considerable velocity. A glass should 

 be filled with the muddy water and allowed to stand. When 

 the water has cleared, a layer of sand and mud will be found to 

 have settled in the glass, and if this is dried and weighed an 

 estimate can be formed of the amount of material transported 

 off the road by the rain. 



The road should be visited as soon as possible after the rain 

 has ceased, and attention should be directed to two principal 

 points ; the course followed by the water, and the bottom of slopes 

 where its flow has been checked and the transported material 

 deposited. The general road surface will be found to have been 

 " washed/' all the mud and dust having been swept off the 

 surface of the bare stones, and the stones themselves will now 

 project slightly, some of the "binding" dust between them 

 having been removed. Besides this, " ruts " will have been cut 

 by small water courses wjiich have run along or across the road. 

 The formation of these will mean not only the removal of dust 

 and mud by the torrents, but also of grit and smaller stones ; while 

 in the case of heavy rains or steep slopes the heavier metal of the 

 road surface may have been cut right through. This explains 



