As a general rule it may he taken that the last of these methods is the best and most convenient for the public; that is to say, that if the physical character of the country does not determine the course of the road, it will generally be found best not to adopt a perfectly straight line, but to vary the line so as to pass through all the prin cipal towns near its general course.
fountain Roads. The location of roads in mountainous countries presents greater difficulties than in an ordinary undulating country; the same latitude in adopting undulating grades and choice of position is not permissible, for the maximum must be kept before the eye perpetually. A mountain road has to be constructed on the maximum grade or at grades closely approximating it, and but one fixed point can be obtained before commencing the survey, and that is the lowest pass in the mountain range; from this point the survey must be commenced. The reason for this is that the lower slopes of the mountain are flatter than those at their summit; they cover a larger area, and merge into the valley in diverse undulations. So that a road at a foot of a mountain may be carried at will in the desired direction by more than one route, while at the top of a moun tain range any deviation from the lowest pass involves increased length of line. The engineer having less command of the ground, owing to the reduced area he has to deal with and the greater abrupt ness of the slopes, is liable to be frustrated in his attempt to get his line carried in the desired direction.
It is a common practice to run a mountain survey up hill, but this should be avoided. Whenever an acute-angled zigzag is met with on a mountain road near the summit, the inference to be drawn is that the line being carried up hill on reaching the summit was too low and the zigzag was necessary to reach the desired pass. The only remedy in such a case is by a resurvey beginning at the summit and running down hill. This method requires a reversal of that usually adopted. The grade line is first staked out and its horizontal location surveyed afterwards. The most appropriate instrument for this work is a transit with a vertical circle on which the telescope may be set to the angle of the maximum grade.
Loss of Height. Loss of height is to be carefully avoided in a mountain road. By loss of height is meant an intermediate rise in a descending grade. If a descending grade is interrupted by the intro
duction of an unnecessary ascent, the length of the road will be in creased over that due to the continuous grade by the length of the portion of the road intervening between the summit of the rise and the point in the road on a level with that rise—a length which is double that due on the gradient to the height of the rise. For example, if a road descending a mountain rises at some intermediate point to cross over a ridge or spur, and the height ascended amounts to 110 feet before the descent is continued, such a road would be just one mile longer than if the descent had been uninterrupted; for 110 feet is the rise clue to a half-mile length at 1: 24.
Water on Mountain Roads. Water is needed by the work men and during the construction of the road; it is also very necessary for the traffic, especially during hot weather; and if the road exceeds 5 miles in length, provision should be made to have it either close to or within easy reach of the road. With a little ingenuity the water from springs above the road, if such exist, can be led down to drinking fountains for men, and to troughs for animals.
In a tropical country it would be a matter for serious consider ation if the best line for a mountain road 10 miles in length or up wards, but without water, should not he abandoned in favor of a worse line with a water supply available.
Halting Places. On long lines of mountain roads halting places should be provided at frequent intervals.
Alignment. No rule can be laid down for the alignment of a road; it will depend both upon the character of the traffic on it and upon the "lay of the land." To promote economy of transportation it should be straight; but if straightness is obtained at the expense of easy grades that might have been obtained by deflections and increase in length, it will prove very expensive to the community that uses it.
Where curves are ,necessary, employ the greatest radius possible and never less than fifty feet. They may be circular or parabolic. The parabolic will be found exceedingly useful for joining tangents of unequal length, and for following contour lines; its curvature being least at its beginning and• ending, makes the deviations from a straight line less strongly marked than by a circular arc.