INTRODUCTION 95 



the invariable roughness of roads with high gradients, dust binding 

 being quite insufficient to hold the road-metal together. 



Material swept off the road will be found deposited, unless 

 the natural or artificial drainage to a stream has been unchecked, 

 where the gradient slackens, and if such a spot can be found the 

 deposit should be studied. It will show at the surface sticks, 

 straws, leaves, organic debris, and anything which could be 

 floated by the water. Under this will be heavier material too 

 dense to remain in the water unless it is in swift motion ; this 

 is said to have been carried in suspension. The upper part of 

 this deposit will be made of the finest grained material, that which 

 would take longest to settle through the water. Lower down 

 the size and weight of the grains will increase ; and lowest of 

 all, there may be recognisable fragments of the stones used in 

 mending or making the road, at first tiny, but becoming coarser 

 and coarser towards the bottom of the deposit. 



Opportunity should be taken to observe the action of normal 

 and gentle showers in contrast to torrential rains ; it is instructive 

 also to note the action of the same rain on slight and on steep 

 slopes ; and if the condition of the road has been noted before the 

 rain it will be valuable to study the effect in relation to the amount 

 of disintegration which the surface has undergone. The effects 

 may be co-ordinated with such conditions as the dryness or wetness 

 of the road, the occurrence of frost or thaw, the breaking up by 

 heavy traffic, the smoothness of the surface, and perfection of in- 

 corporation by rolling, the nature of the road metal, and the 

 composition and amount of binding material between the stones. 

 These observations will help to bring home the influence of original 

 consolidation and of prior disintegration as factors in surface 

 destruction. 



It is now known as the result of experiments that wet stone 

 wears more rapidly than dry stone, but there is compensation 

 in that a moderate amount of moisture keeps the binding material 

 together and checks the movement of stones under traffic. Thus 

 slight watering of the road is on the whole beneficial ; but the 

 heavy watering usually practised, particularly with intervals of 

 drying in between, not only makes the stone wear and break up 

 more easily, but liquefies the binding material, washes it loose, 



