23. Routes placed upon the immediate bank of a large stream, are generally crossed by a great number of deep gorges, which serve to drain the side lands.
24. Routes placed upon sloping land, when the axis of the road and the natural descent are at right angles to each other, are more subject to slides than when placed upon plateaus or “bottoms.”
25. Lines crossing the dividing ridges of separate waters, rise and fall a great deal; thus rendering necessary a strong motive power to work the road. Such roads are the Western of Massachusetts, passing from the valley of the Connecticut at Springfield, to the Hudson River valley at Greenbush. Also those roads crossing the Alleghanies. And such will be the Pacific road, crossing first the Rocky Mountains to the Great Basin, and second, the Sierra Nevada into the Sacramento valley.
CHAPTER I.
RECONNOISSANCE.
26. The object of the reconnoitre is to find approximately the place for the road, (i. e. within half of a mile,) to find the general form of the country, and to choose that part which with reference to the expected traffic, shall give the best gradients; to determine the elevations of summits upon competing routes; and, in fine, to prepare the way for the survey.
GENERAL TOPOGRAPHY.
27. The general topography of a country may be ascertained by reference to State maps, where such exist, and when not, by riding over the district. The direction and size of watercourses, will show at once the position of summits.
Fig. 1.
28. Water flowing as in fig. 1, indicates a fall from B to E; and also traverse slopes from a a and c c to d d.