.26 



LECTURE 



with decided leading features, the experienced engineer will often he 

 enahled, after a thorough reconnoissance of this kind, to determine 

 within narrow limits upon the location ; but in an extended and difficult 

 or broken country, it will often be necessary to make a survey of several 

 trial lines before a sufficient amount of information can be collected. 



In the United States, where, except along the sea-coast and in Mas- 

 sachusetts, no regular and reliable general surveys have been made, 

 the maps will be found quite deficient, and in many cases the engineer 

 must prepare one more or less extended for his own use. This will be 

 particularly the case in a rough mountain country, where much time 

 would be lost in making the surveys of trial lines, many of which would 

 turn out to be impracticable when nearly completed. 



Much information can be gained even from a map which has only 

 the streams marked upon it. Since the stream- always runs through 

 the lowest line of the valley, the position of the valleys and the general 

 inclinations of the country will be indicated by them. A very crooked 

 stream, with softly rounded bends, will almost always indicate a smooth, 

 nearly level, alluvial bottom or meadow land through which it flows; 

 while straight streams, with sharp angles, and with branches running 

 abruptly into them at large angles, indicate a rocky, hilly country, 

 with narrow, steep-sided valleys. These indications are, however, so 

 very general that a map, showing the different heights of the various 

 points of the country, is absolutely essential. Such a map is called a 

 topographical map. 



There are two methods in use of delineating upon paper the topo- 

 graphical features of a country — by hachure lines and by contour lines. 

 The first and older system indicates the inclinations by short lines 

 drawn in the direction of the slope of the ground, and the amount of 

 the inclination by the greater or less thickness of the lines, in accord- 

 ance with some arbitrary standard. In the second system, the relative 

 heights of the various points are indicated by continuous lines of equal 

 level, at certain vertical distances apart. The first originated with, 

 and is especially adapted to the wants of, the military engineer, since 

 the inclination of the surface is the matter which most concerns him ; 

 the disadvantage of it, however, is, that it conveys but a faint idea of 

 the true features of the surface, even to the expert. Figs. 1 and 2 show 

 the two methods applied to the same surface. 



■^diiim- 





Fig. 2 



