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respect to these opinions on road-making, nothing but the complete ignorance of the public, upon all matters concerning road-making, could ever have suffered rules, so contrary to every thing like sound principles, to have had a single moment of favourable consideration.”*
3d, The resistance produced by gravity, in checking the progress of a moving body on a road, is little or nothing when a road is horizontal, because as gravity acts in a direction perpendicular to the plane of the horizon, it neither accelerates nor retards the motion. + But when the road is not horizontal, the power of gravity is a great impediment.
A mathematical illustration of the effect of gravity on hills is given in note D.
4th, The resistance arising from the action of the air is very variable ; in some cases, it acts powerfully; but as its influence is the same
* Mr. Wingrove was for several years the surveyor of nearly all the roads in the neighbourhood of Bath. In 1825 the author accompanied him in making an inspection of them, and found the rules which Mr. Telford recommends had been most effectually acted upon throughout the whole of these roads, and that they had been brought to as high a state of improvement as the money which was allowed for them would admit of.
+ Wood's Mechanics, p. 20.
whether the road be a bad or a good one, little need be here said on the subject : it will be sufficient to state, that by experiments detailed in Smeaton's Reports, it was found that the force of the wind on a surface 1 foot square was 1 lb., when the velocity of the wind was 15 miles an hour, or what would be termed a brisk gale; 3 lbs. when the velocity was 25 miles an hour, or what would be termed a very brisk gale ; 6 lbs. when the velocity was 35 miles per hour, or what might be termed a high wind; and 12 lbs. to the square foot, when the velocity was 50 miles an hour, or what might be termed a storm. Supposing, therefore, that the surface of that part of a carriage acted upon by the direct influence of the wind to be 50 superficial feet, the resistance it will meet from a brisk gale of wind acting against it will be about 50 lbs. when the carriage is slowly moved; but if the carriage be supposed to move directly against the wind with a velocity of 10 miles an hour, and the wind to move with a velocity of 15 miles an hour, the resistance against the carriage will amount to 3 lbs. on the square foot, or 150 lbs. on the carriage, which is fully equal to the power which two horses should be required to exert, when moving with a velocity of 10 miles an
hour. From this the difficulty is evident of driving stage coaches at a rapid rate against high winds.*
* The proper angle for fixing the line of direction of traces, in which the power for drawing a carriage should be applied, is described in note F.
FORMING A ROAD
In marking out the line of a road, a great deal of expense in cutting and embanking for forming the bed on which the road materials are to be placed, may be avoided by a judicious selection of the high and low ground which the surface of the country affords.
The chief care, where a road must be carried over a high elevation, is to lay it out so that it shall not have any fall in it from the point from which it departs till it reaches the summit. The lowering of heights, and the filling of hollows, should be so adjusted as to secure gradual and continued ascending inclinations to the highest point to be passed over.
It is a most important part of the business of a skilful engineer to lay out the longitudinal inclinations of a road with the least quantity of cutting and embanking.
He must do this by measuring and calculating the quantity of earth to be removed in
cuttings, and taking care that it shall exactly make the embankments for raising the hollows to the required heights; a proper allowance being made for the subsidence of the soil according to its quality, without leaving an overplus to be carried to spoil.
When it is necessary to make a deep cutting through a hill, the slopes of the banks should never be less, except in passing through stone, than two feet horizontal to one foot perpendicular ; for though several kinds of earth will stand at steeper inclinations, a slope of two to one is necessary for admitting the sun and wind to reach the road. The whole of the green sod and fertile soil on the surface of the land cut through should be carefully collected and reserved, in order to be laid on the slopes immediately after they are formed.
If a sufficient quantity of sods cannot be procured in the space required for the road, the slopes should be covered with three or four inches of the surface mould, and hay seeds should be sown on it; by this plan the slopes will soon be covered with grass, which will be a great means of preventing them from slipping.
When stones can be got the slopes should be
* See Mr. Macneill's Work on Cutting and Embanking. Published by Roake and Varty.