72 



than the centre, by giving a slight convexity to the 

 surface from the centre outAvards. A coating of clean 

 coarse gravel, one inch and a half thick, termed a bind- 

 ing, is spread over the surface, and the road-covering 

 is then ready to be thrown open to traveling. 



" In forming a road-covering of broken stone alone, the 

 bed for the covering is arranged in the same manner as 

 for the paved bottoming ; a layer of the stone, four inches 

 in thickness, is carefully spread over the bed, and the 

 road is thrown open to vehicles, care being taken to 

 fill the ruts and preserve the surface in a uniform state 

 until the layer has become compact ; successive layers 

 are laid on and treated in the same manner as the first, 

 until the covering has received a thickness of about 

 twelve inches in the centre, with the ordinary convexity 

 at the surface. 



"Where good gravel can be procured, the road- 

 covering may be made of this material, which should 

 be well screened, and all pebbles found in it over two 

 and a-half inches in diameter should be broken into 

 fragments of not greater dimensions than these. A 

 firm, level form having been prepared, a layer of gravel, 

 four inches in thickness, is laid on, and, when this has 

 become compact from the travel, successive layers of 

 about three inches in thickness are laid on and treated 

 like the first, until the covering has received a thick- 

 ness of sixteen inches in the centre and the ordinary 

 convexity. 



"As has been already stated, the French Civil En- 

 gineers do not regard a paved bottoming as essential for 



