HIGHWAY CONSTRUCTION 



afforded by the road surface. The tangent of the angle of inclination 

 should not be greater than the coefficient of tractional resistance; 

 therefore it is evident that the smoother the road surface, the easier 

 should be the grade. The smoother the surface the less the foothold, 

 and consequently the load. 



The loss of tractive power on inclines is greater than any inves- 

 tigation will show; for, besides the increase of draught caused by 

 gravity, the power of the horse is much diminished by fatigue upon 

 a long ascent, and even in greater ratio than man, owing to its anatom- 

 ical formation and great weight. Though a horse on a level is as 

 strong as five men, on a grade of 15 per cent, it is less strong than 

 three; for three men carrying each 100 pounds will ascend such a 

 grade faster and with less fatigue than a horse with 300 pounds. 



A horse can exert for a short time twice the average tractive 

 pull which he can exert continuously throughout the day's work; 

 hence, so long as the resistance on the incline is not more than double 

 the resistance on the level, the horse will be able to take up the full 

 load which he is capable of drawing. 



Steep grades are thus seen to be objectionable, and particularly 

 so when a single one occurs on an otherwise comparatively level road, 

 in which case the load carried over the less inclined portions must 

 be reduced to what can be hauled up the steeper portion. 



The bad effects of steep grades are especially felt in winter, 

 when ice covers the roads, for the slippery condition of the surface 

 causes danger in descending, as well as increased labor in ascending; 

 the water of rains also runs down the road and gulleys it out, destroy- 

 ing its surface, thus causing a constant expense for repairs. The 

 inclined portions are' subject to greater wear from horses ascending, 

 thus requiring thicker covering than the more level portions, and 

 hence increasing the cost of construction. 



It will rarely be possible, except in a flat or comparatively level 

 country, to combine easy grades with the best and most direct route. 

 These two requirements will often conflict. In such a case, increase 

 the length. The proportion of this increase will depend upon the 

 friction of the covering adopted. But no general rule can be given 

 to meet all cases as respects the length which may thus be added, 

 for the comparative time occupied in making the journey forms an 



