254 LIGHT SCIENCE FOR LEISURE HOURS. 



wooded or jungly country, containing a few inhabitants 

 and little cultivated, or where the climate is so dan- 

 gerous that it is desirable to accelerate the progress of 

 the survey. 



On the scale of one inch per mile the practised 

 draughtsman can survey about five square miles of 

 average country per day. In intricate ground, inter- 

 sected by ravines or covered by hills of irregular 

 formation, the work proceeds much more slowly ; on 

 the other hand, in open and nearly level country, or 

 where the hills have simple outlines, the work will 

 cost less and proceed more rapidly. On the scale of 

 one inch per mile all natural features (such as ravines 

 or watercourses) more than a quarter of a mile in 

 length can be clearly represented. Villages, towns, 

 and cities can be shown, with their principal streets 

 and roads, and the outlines of fortifications. The 

 general figure and extent of cultivated, waste, and 

 forest lands can be delineated with more or less pre- 

 cision, according to their extent. Irrigated rice-lands 

 should be distinctly indicated, since they generally 

 exhibit the contour of the ground. 



The relative heights of hills and depths of valleys 

 should be determined during the course of a topo- 

 graphical survey. These vertical elements of a survey 

 can be ascertained by trigonometrical or by baro- 

 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 



