CH. X EFFECT OF GRADES I 69 



These figures are for a heavy four-wheeled vehi- 

 cle, on hard macadam (Morin, pp. 45 and 127). 

 As the grade increases, its importance in using up 

 the power is manifest, until at a grade of 1 in 55 the 

 percentages of rolling resistance and grade resist- 

 ance are equal, and at 1 in 45 the grade resistance 

 is one-half of the whole. At 1 in 20, which may 

 be considered a permissible grade on a mountain 

 road, the grade resistance is nearly three-quarters 

 of the whole, and at 1 in 10, which should be the 

 utmost limit of mountain roads, it is 84 per cent. 



Grades of 1 in 8, to 1 in 6, now and then met with, 

 can be ascended with an ordinary load only by the 

 use of extra horses. 



Macneill, in 1838, recommended 1 in 40 as the 

 maximum grade on a main road of the best class, 

 but on a large proportion of the great European 

 roads, in mountainous countries, 1 in 25 is constantly 

 used, and there are grades of that steepness on the 

 great Holyhead coach road in England and Wales. 

 In all these cases, the surface is nearly perfect. 



A grade of 1 in 20 is the steepest up which a 

 fresh team, with a moderately loaded coach, should 

 trot, and then only when the distance is short, not 

 more than 300 yards. 



All experiments agree in putting the resistance 

 to rolling, on a well-paved stone street, at about two- 

 thirds of that on a good, macadam-surfaced road, 

 and every attentive coachman will feel the difference 

 in the action of the horses and the movement of the 



