ON ROADS AND BRIDGES. 



127 



The method of contour lines will he readily understood from the fol- 

 lowing explanation. Let us suppose an island situated in a lake: the 

 water will wash the hase and form a water line, all the points of which 

 will he in the same horizontal plane — that is, on a level with the sur- 

 face of the lake. Now, if we suppose the water to rise one foot, another 

 water line will he made, all the points of which will he in a horizontal 

 plane one foot ahove the first plane, all the points of the surface of the 

 island between these two lines will he less than one foot ahove the level 

 of the lake. By successive stages of the water we shall get a succes- 

 sion of lines, until the island is entirely submerged. Now, suppose 

 we place ourselves in a balloon above the island, and look down upon 

 it as upon a map, we shall see all these horizontal curves projected 

 upon the level surface, as in Fig. 3. 

 And if we make a map of the island, 

 with these lines upon it, our topograph- 

 ical information regarding it will be 

 complete. Knowing the vertical dis- 

 tance between the lines, by measuring 

 the horizontal distance we can determ- 

 ine the inclination. The elevation of 

 any point may be determined by simple 

 inspection. With a map of this kind 

 carefully prepared, the engineer can 

 locate his line in the office, and often 

 to greater advantage than in the field 

 — since he can see the whole country 

 at a glance. Having thus a general 

 map of the country, he will be guided 

 by a few simple principles. If a ridge exists between the points to be 

 connected, it is usually desirable to cross it at its lowest point. A 

 stream commonly starts from such a point, and by following it up, 

 the summit can be reached by a comparatively easy ascent along the 

 valley. The most difficult countries are those which have no leading 

 streams or valleys, but which are broken up by rounded hills and 

 disconnected hollows — since a line which appears practicable for a con- 

 siderable distance will sometimes end in an impracticable spot. In 

 such regions, a carefully-constructed topographical map is indis- 

 pensable to prevent the expenditure of a great deal of time in wild 

 explorations. 



_ It should be distinctly borne in mind that a reconnoissance suffi- 

 ciently accurate for the purpose, can be made in a comparatively short 

 time, by an experienced topographer, with a very small party and 

 portable instruments ; while the running of trial lines is a much more 

 serious matter. In an ordinarily level country, the attention of the 

 engineer will be turned to the selection of the best route, without his 

 ingenuity being taxed to surmount great obstacles; and lie will there- 

 fore aim at making the road as direct as possible, while avoiding any 

 great ascents or descents. 



In an extremely mountainous country like Switzerland, it will some- 

 times be difficult not only to obtain the best line, but to find any line 

 which will be practicable, owing to the great difference of level of the 



Fig. 3. 



