280 LIGHT SCIENCE FOR LEISURE HOURS. 



metrical observations, or by a combination of both 

 methods. " The barometer," says Sir A. "Waugh, " is 

 more especially useful for determining the level of 

 low spots from which the principal trigonometrical 

 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 

 referable to the original trigonometrical station. The 

 height above the sea-level of all points coming under 

 any of the following heads are especially to be deter- 

 mined, for the purpose of illustrating the physical 

 relief of the country : 



" 1. The peaks and highest points of ranges. 



" 2. All obligatory points required for engineer- 

 ing 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. 



" 3. 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, regu- 

 lar 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 



