fcUOTECTIVE WORK8. 237 



ous portions of the lull side, especially the gullies and ravines 

 should be cut into terraces, the width of which will be in inverse 

 proportion to the steepness of the ruling gradients. The terraces 

 should be given a decided slope outwards and their faces should be 

 protected by revetment walls. As these walls have only a tempor- 

 arv utility, that is to say, are required to hold the toil together only 

 until the covering of tree.s with their dense network of binding 

 roots has been established, dry masonry will be the rule, pakka the 

 exception, and then too only confined to the most dangerous points 

 and to cases of absolute necessity. The revetment masonry 

 between two terraces will often have to be continued for a few f( et 

 both upwards and downwards along their floor (Fig. 79, a and li) in 

 order to protect it against erosion. Indeed, it may sometimes be 

 necessary to continue the pavement across the entire width of the 

 terraces, in which case long blocks of wood may be laid between tho 

 masonry to tie it together (Fig. 79 c). But instead of paving the 

 entire floor it will often suftice to put down paving stones in linei 

 forming a sort of check pattern (Fig. 79 d). In forming this 

 pattern blocks of wood may be substituted for the rows of paving 

 stones, in which case the pattern will resemble Fig. 79 c without 

 the paving stones. In dangerous places numerous long headers 

 ought to be used in the revetment walls, better still blocks of 

 wood as shown in Fig. 80. 



B. To accomplish the second object the channels of the water- 

 courses must be regulated. Contractions have to be widened, 

 sharp bends gently rounded off, the angle at which one water- 

 course meets another altered, and so on, all these various works 

 having for their common object to secure as much as possible an 

 even flow. But the most important point of all is the graduation 

 of the beds of the channels in the direction of their length ; all 

 hollows must be filled up, obstructions remove 1, and the fall so 

 regulated that it gradually diminishes downwards in order to 

 counteract the constant acceleration of velocity of the current due 

 to the force of gravitation. 



Owing to the usually ste3p gradients where such work has to be 

 done, the main end can be secured only by forming the beds into 

 a succession of terraces. The water falls from terrace to terrace, 

 erosion being prevented by means of masonry or other revetments, 

 and flows smoothly down each terrace. 



When the fall permits of it, tlu simplest plan to follow in 

 forming the terraces is to thro-.v dams across the channels at well 

 judged intervals, and of the right height, and strong and tight 



