HIGHWAY CONSTRUCTION 27 



occasioned bv ihr iticliiiution of the road; such is not, however, 

 practically the fact with animal power, for while it is necessary in 

 the ascending journey to have either a less or a greater number of 

 horses than would he requisite if the road were entirely level, no 

 corresponding reduction can he made in the descending journey. 

 On inclines which are more steep than the angle of repose, the load 

 presses on the horses during their descent, so as to impede their 

 action, and their power is expended in checking the descent of the 

 load; or if this effect he prevented by the use of any form of drag or 

 brake, then the power expended on such a drag or brake corresponds 

 to an equal quantity of mechanical power expended in the ascent, 

 for which no equivalent is obtained in the descent. 



The maximum grade for a given road will depend (1) upon the 

 class, of traffic that will use it, whether fast and light, slow and heavy, 

 or mixed, consisting of both light and heavy; (2) upon the character 

 of the pavement adopted; and (3) upon the question of cost of con- 

 struction. Economy of motive power and low cost of construction are 

 antagonistic to each other, and the engineer will have to weigh the 

 two in the balance. 



For fast and light traffic the grades should not exceed 2 per 

 cent; for mixed traffic 3 per cent may be adopted; while for slow 

 traffic combined with economy 5 per cent should not be exceeded. 

 This grade is practicable but not convenient. 



Minimum Grade. From the previous considerations it would 

 appear that an absolutely level road was the one to be sought for, but 

 this is not so; there is a minimum or least allowable grade which the 

 road must not fall short of, as well as a maximum one which it must 

 not exceed. If the road was perfectly level in its longitudinal direc- 

 tion, its surface could not be kept free from water without giving it 

 so great a rise in its middle as would expose vehicles to the danger of 

 overturning. The minimum grade commonly used is 1 per cent. 



Undulating Grades. From the fact that the power required 

 to move a load at a given velocity on a level road is decreased on a 

 descending grade to the same extent it is increased in ascending the 

 same grade, it must not be inferred that the animal force expended 

 in passing alternately each way over a rising and falling road will 

 gain as much in descending the several inclines as it will lose in ascend- 



