Construction, Cost, Repair, and Management. 393 
' bad foundation, and soon leave the superstructure full of cavities 
and inequalities. Burnt clay, although a dry and porous 
material, if not sufficiently covered with gravel, becomes in wet 
weather spongy, and forces its way to the surface, rendering 
the road loose and dirty. Clay burnt to the consistency of 
bricks, in lumps of about the same size, and then broken up, 
or old brick-bats free from mortar, form an excellent founda- 
tion. The guide in the selection of materials must be the geo- 
logical formation of the neighbourhood, care being taken to 
select the hardest and most durable materials to be procured. 
One very important matter for consideration in the formation 
of a road is the transverse form that is to be given to the section, 
which, while it should be sufficient to void the rain that falls on 
it, should not be so convex as to throw the vehicles rolling over 
the road too much out of the vertical position when moving 
along the sides. Roads of too convex a form always wear badly. 
The tendency of the wheels passing along them is to slide in a 
lateral direction ; and consequently, in addition to the wear and 
tear due to gravity, and to the friction of the wheel in a longi- 
tudinal direction, there is a much greater wear from the grinding 
of the wheel sideways, which disturbs the crust of the road by 
pushing the materials out of their place. 
Convex roads compel the whole of the traffic to pass along 
one track in the centre, because it is there only that the vehicle 
stands upright ; the consequence is that three furrows are made, 
in which the water always stands. 
The section adopted by Telford was a fall of 6 inches from 
the centre to the water-table in a road of 30 feet wide. The 
contour of the surface was not the segment of a circle, but fell 
half an inch at 4 feet from the centre, 2 inches at 9 feet, and 
6 inches at 15 feet. 
Mr. Macadam considered that in a road 18 feet wide, the 
centre should be only 3 inches higher than the sides. 
The chief object in the construction of a good road is to 
facilitate the locomotion of vehicles by reducing the traction. 
With this object it becomes necessary, in carrying out a line 
, across a country, to counteract as far as possible the labour 
! due to gravity in raising the vehicles to the tops of hills, and 
in lowering them again to the valleys. It is obvious that in 
drawing a vehicle over a hill, the horses have to perform work, 
j in addition to the ordinary friction, equivalent to lifting the 
[ whole weight of the vehicle and its contents a height equal 
to the vertical rise of the hill, and in descending their work 
I is equivalent to counteracting or holding back this weight from 
' the same height. 
In Law's ' Treatise on Roads,' a very elaborate Table is given. 
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