288 MASSACHUSETTS AGRICULTURE. 



travel goes there, and consequently the wear is mostly in that 

 part. But it is because the surface is crowning that the travel 

 does go upon the centre, for the carriages can stand upright in 

 no other position. 



The only useful purpose served by raising the centre is to 

 allow the water to run off at the sides. 



If, however, too much inclination is given to the sides, the 

 travel will seek the centre, as before remarked, and that part 

 will soon be worn into ruts and hollows, making conduits and 

 basins for the water there. 



For these reasons, practical engineers generally coincide in 

 the opinion that the centre should be but little higher than the 

 sides, and that the best form of cross-section is formed by two 

 straight lines inclined to the sides, connected at the vertex by 

 an arc of a circle of about ninety feet radius. The inclination 

 of the sides ought not to exceed 1 in 30. 



The common practice with us is to give the surface a cylin- 

 drical form, making the cross-section nearly the arc of a circle, 

 with a short radius, so that the inclination increases from the 

 centre towards the. sides. This is the worst possible form, es- 

 pecially when the road is narrow, or, as is usual, the centre is 

 raised very high ; for, in order that vehicles may retain any- 

 thing near an upright position, they must go upon or near the 

 middle of the way, confining the wear to that part ; but if the 

 amount of travel is sufficiently great to compel carriages fre- 

 quently to turn, or to keep upon the sides, they are exposed to 

 accidents by upsetting, and the tendency to slide down the slope 

 increases the wear of the wheels and of the road. 



Besides, the labor of the horses is increased by this tendency 

 of the wheels of carriages to slide in a direction at right angles 

 to the line of draught, and the chances of breakage are multi- 

 plied by the augmented strain upon their axles. 



Gradients. 



The longitudinal inclination of roads, where necessary, should 

 be as slight as possible, and ought never to exceed the angle of 

 resistance for the materials of which the surface is composed. 



The results of numerous experiments to determine this angle, 

 or the angle of inclination, for different materials, which is just 

 sufficient to cause a carriage standing upon the inclination to 



