A manual of the principles and practice of road-making: comprising the location, construction, and improvement of roads, (common, macadam, paved, plank, etc.) and rail-roads. By W. M. Gillespie ...

40 WHAT ROADS OUGHT TO BE.
MAXIMUM SLOPE, CONSIDERED AS AN ASCENT.
Suppose that a road is to be carried over a hill, which
rises 100 feet in a horizontal distance of 500 feet, (i. e.,
1 in 5) and which cannot be avoided by any horizontal
circuit within the limits of distance indicated on page 28.
The question which presents itself is, how steep can the
slope of a road up the side of this hill be most advantageously laid out, since, by adopting a zigzag line, the road
may be made as long and therefore as gentle in the ascent
as:may be desired? The shortest line would run straight
up the face of the hill, and this line would give the least
amount of labor; but then this labor for horses would be
impossible: and even if possible, the horses could not
draw up the whole load which they had been drawing on
the other parts of the road, nor could they descend it with
safety. But, on the other hand, the road should approach
this shortest line as nearly as other considerations will
permit, since, if it should zigzag excessively for the purpose of lessening the steepness, it would be so long as to
increase unnecessarily its cost and the time and labor of
travel upon it. A medium and compromise between these
two evils must therefore be found. What shall it be?
Supposing the load of a horse on the level portions of
the road to be as much as he can regularly and constantly
draw, his power of drawing it up an ascent will depend
upon how much extra exertion he is capable of putting
forth. This is not very accurately ascertained or defined,
and depends very much on the length of the ascent, but
may be assumed at double his usual exertion.* Now a
horse drawing a load on a level road of the best character,
e Gayffier, p. 9