11. _The Great Holyhead Road._
} From Islington, by and through Barnet.
12. _The Great North Road._
As to the amount of resistance to traction offered by different kinds
of pavement, or the same pavement under different circumstances, the
following are the general results of the experiments made by M. Morin,
at the expense of the French Government:--
1st. The traction is directly proportional to the load, and inversely
proportional to the diameter of the wheel.
2nd. Upon a paved, or hard macadamized road, the resistance is
independent of the width of the tire, when it exceeds from three to
four inches.
3rd. At a walking pace the traction is the same, under the same
circumstances, for carriages with springs and without them.
4th. Upon hard macadamized, and upon paved roads, the traction
increases with the velocity: the increments of traction being directly
proportional to the increments of velocity above the velocity 3·28
feet per second, or about 2-1/4 miles per hour. The equal increment of
traction thus due to each equal increment of velocity is less as the
road is more smooth, and the carriage less rigid or better hung.
5th. Upon soft roads of earth, or sand, or turf, or roads fresh and
thickly gravelled, the traction is independent of the velocity.
6th. Upon a well-made and compact pavement of hewn stones, the traction
at a walking pace is not more than three-fourths of that upon the best
macadamized roads under similar circumstances; at a trotting pace it is
equal to it.
7th. The destruction of the road is in all cases greater, as the
diameters of the wheels are less, and it is greater in carriages
without than with springs.
In Sir H. Parnell’s book on roads, p. 73, we are told that Sir John
Macneill, by means of an instrument invented by himself for measuring
the tractive force required on different kinds of road, obtained the
following general results as to the power requisite to move a ton
weight under ordinary circumstances, at a very low velocity.
-------------------------------------------+------------------
| Force, in
Description of Road. | pounds, required
| to move a ton.
-------------------------------------------+------------------
On a well-made pavement | 33
|
On a road made with six inches of } |
broken stone of great hardness, } |
laid either on a foundation of large } | 46
stones, set in the form of a pavement, } |
or upon a bottoming of concrete } |
|
On an old flint road, or a road made } |
with a thick coating of broken } | 65
stone, laid on earth } |
|
On a road made with a thick coating } |
of gravel, laid on earth } | 147
-------------------------------------------+------------------
In the same work the relative degrees of resistance to traction on the
several kinds of roads are thus expressed:--
On a timber surface 2
On a paved road 2
On a well-made broken stone road, in a
dry clean state 5
On a well-made broken stone road,
covered with dust 8
On a well-made broken stone road, wet
and muddy 10
On a gravel or flint road, in a dry
clean state 13
On a gravel or flint road, in a wet
muddy state 32