278 LIGHT SCIENCE FOR LEISURE HOURS. 



So soon as the country has been accurately planned, 

 the configuration of the ground has to be sketched up. 

 This process is the end and aim of all the preceding 

 work. 



The first point attended to is the arterial system, 

 or water-drainage, constituting the outfall of the 

 country; whence are deduced the lines of greatest 

 depression of the ground. Next the water-sheds or 

 ridges of hills are traced in, giving the highest level. 

 Lastly, the minor or subordinate features are drawn 

 in with the utmost precision attainable. " The out- 

 lines of table-land should be well defined," says Sir A. 

 Waugh, "arid ranges of hills portrayed with fidelity, 

 carefully representing the water-sheds or divortia 

 aquarum, the spurs, peaks, depressions or saddles, 

 isthmuses or connecting-links of separate ranges, and 

 other ramifications. The depressed points and isth- 

 muses are particularly valuable, as being either the 

 sites of ordinary passes or points which new roads 

 should conform to." 



And here we must draw a distinction between 

 survey and reconnoissance. It is absolutely necessary 

 in making a survey that the outlines of ground as de- 

 fined by ridges, water-courses, and feet of hills, should 

 be rigorously fixed by actual observation and careful 

 measurement. In reconnoitring, more is trusted to 

 the eye. 



The scale of the Indian topographical survey is 



