The Constnictioii and Maintenance of Roads. 75^ 



is allowed to remain upoD the surface, every passing vehicle aids the 

 removal of water by forming longitudinal channels which help to carry 

 it off. 



Where too great a convexity is given to the surface of the road, the 

 centre, being the safest part of the course, and the only one in which a 

 vehicle stands quite upright, is chosen by every driver. Consequently, 

 the wear of the surface is uneven, and the cost of maintenance is 

 increased. Besides, upon a surface too much rounded the tendency of 

 the load is to force the carriage down the inclined plane : thus the 

 road and the carriage wheels are more worn, and the labour of the 

 horse considerably increased. 



One great mistake in road-making is laying on too thick a coat of 

 metal at one time. As a rule more than three inches is objectionable, for 

 a greater thickness remains so long loose that its strength is greatly 

 reduced before it settles down and binds. 



The size of wheels is a subject which also demands attention, for 

 while the resistance of traction in a load is in direct proportion to the 

 load itself, it is inversely proportioned to the diameter of the wheel. 

 Upon a well-formed road with smooth surface there is no advantage in 

 increasing the width of tyre beyond o\ inches, as above this point the 

 resistance is independent of width. Much greater damage is caused 

 to roads by small wheels than by large ones, and by carts without 

 springs than by springed vehicles. 



In laying out roads of any kind a contour map of the district will be 

 of the greatest service, as a glance at it will enable the surveyor or 

 road-maker to choose his line of road, so as to avoid extremes of 

 inclination. Such a map may easily be laid down by any one tolerably 

 acquainted with the use of the theodolite. 



Elaborate experiments by M. IMorin and Sir John Macneil have 

 cast much light upon the theory of road-making. Upon macadamized 

 and paved roads traction increases with velocity when the latter is above 

 %\ miles per hour ; while upon roads of turf, sand, loose earth, or 

 newly-laid thick gravel, it is independent of speed. Upon a compact 

 pavement of stones, when the pace is slow, the traction is but \ of 

 what it is upon the best class of macadamized roads, while at a round 

 trot resistances are e(jual. As an illustration of the advantages of a 

 firm bottom, the following data are quoted from Sir John Macneil. 

 Tractive force to draw one ton on the level : — 



1. Well-made pavement, equal to 31 lbs. 



2. Road formed of six inches of broken stone, laid on a solid founda- 

 tion of large stones, or on concrete, equal to 44 lbs. 



o. Flint road, or road of thick broken stone laid on earth, equal 

 to 62 lbs. 



4. Thick coating of gravel on earth, equal to 14U lbs. 



