50 



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 diam- 

 eter 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 thickness of sixteen inches in 

 the centre and the ordinary convexity. 



"As has been already stated, the French Civil Engineers do 

 not regard a jpaved bottoming as essential for broken-stone 

 road-coverings, except in cases of a ve7y heavy traffic, or where 

 the substratum of the road is of a very yielding character. 

 They also give less thickness to the road-covering than the 

 English engineers of Telford's school deem necessary ; allowing 

 not more than six to eight inches to road-coverings for light 

 traffic, and about ten inches only to the heaviest traffic. 



" If the soil upon which the road-covering is to be placed is 

 not dry and firm, they compress it by rolling, which is done 

 by passing over it several times an iron cylinder about six feet 

 in diameter and four feet in length, the weight of which can 

 be increased, by additional weights, from six thousand to about 

 twenty thousand pounds. The road material is placed upon 

 the bed, when well compressed and leveled, in layers of about 

 four inches, each layer being compressed by passing the cylin- 

 der several times over it before a new one is laid on. If the 

 operation of rolling is performed in dry weather, the layer of 

 stone is watered, and some add a thin layer of clean sand, from 

 four to eight tenths of an inch in thickness, over each layer be- 

 fore it is rolled, for the purpose of consolidating the surface of 

 the layer, by filling the voids between the broken-stone frag- 

 ments. After the surface has been well consolidated by rolling, 

 the road is thrown open for travel, and all ruts and other dis- 

 placement of the stone on the surface are carefully repaired, by 

 adding fresh material, and leveling the ridges by ramming. 



" Great importance is attached by the French engineers to 



