86 Fai'm Roads on Strong Soils. 



cross culvert or drain shall be no higher than the bottom of the 

 under-drains, thus — 



Crosi 



Culvert. 



^ndkr-drain. ^'*^^^^^^^^^^^^^^^**'^ Unckr-drain. 



:,-i.(f.J 



At the ends of each cross culvert there should be an iron grating 

 to prevent the passage of vermin. Tlie vmder-drains should be 

 carefully filled to the height of the metal bed with the earth taken 

 out of the drains. 



6th. The surface of the road, for its entire breadth of 18 feet or 

 more from C to C, should be raised above the ground surface in 

 a convex shape. 



7th. The height of the centre of the road above the ground 

 surface should be as near as possible the thickness of the me- 

 talling, whatever it be. It is necessary, however, that the 

 ground or base upon which the metalling is placed should be 

 perfectly solid, and, if possible, unbroken. It should be convex 

 and accord in form with the intended convex shape of the surface 

 of the road itself. 



8th. The metalling may consist of two or more layers or 

 strata, which, together, should be 9 inches deep at least. This 

 depth will suffice where the materials are of the best sort, but 

 they should be proportionately deeper in cases where the ma- 

 terials are softer and less durable. 



9th. The metalling should be of uniform depth for its entire 

 width. 



10th. The under stratum or foundation of metalling may con- 

 sist of any durable porous or non-absorbent material, of the 

 depth of 4i inches at least. It may be judiciously increased 

 where the material required for the upper stratum or covering is 

 very costly, when the depth of the covering may be reduced 

 accordingly. It is not essential that the material for the founda- 

 tion shall be of uniform size as long as it lies compact, is well 

 bonded, and its surface is even and regular to receive the 

 upper stratum. 



11th. The upper stratum or covering should be of the best 

 description of clean stone, or sifted gravel, or pebbles, the country 

 will afford. If gravel, it should be screened to get rid of sand and 

 dirt, and the larger stones should be broken, so that no particle 

 shall be left on the road exceeding 2^ inches in the longest 

 diagonal line. The depth should be 4| inches, and should, if 

 possible, l)e placed on the road in two courses. 



12th. The side spaces (4 feet 6 inches each at least), i. e. from 

 B to C, should be carefully finished off in close conformity with 



