THE TOPOGRAPHICAL SURVEY OF INDIA. 2$$ 



stations are invisible. In using this instrument, how- 

 ever, in combination with the other operations, the 

 relative differences of heights are to be considered the 

 quantities' sought, so that all the results may be refer- 

 able to the original trigonometrical station. The height 

 above the sea-level of all points coming under any of 

 the following heads is especially to be determined, for 

 the purpose of illustrating the physical relief of the 

 country : 



' 1st. The peaks and highest points of ranges. 



( 2nd. All obligatory points required for engineering 

 works, such as roads, drainage, and irrigation, viz. : 

 the highest points or necks of valleys ; the lowest 

 depressions or passes in ranges; the junctions of rivers, 

 and debouchements of rivers from ranges ; the height of 

 inundation -level, at moderate intervals of about three 

 miles apart. 



' 3rd. Principal towns or places of note.' 



Of the various methods employed to indicate the 

 steepness of slope, that of eye-contouring seems alone 

 to merit special comment. In true contouring, regular 

 horizontal lines, at fixed vertical intervals, are traced 

 over a country, and plotted on to the maps. This is an 

 expensive and tedious process, whereas eye-contouring 

 is easy, light, and effective. On this system all that is 

 necessary is that the surveyor should consider what 

 routes persons moving horizontally would pursue. He 

 draws lines on his chart approximating as closely as 

 possible to these imaginary lines. It is evident that 

 when lines are thus drawn for different vertical eleva- 



