274 LIGHT SCIENCE FOR LEISURE HOURS, 



lines. The former of these methods is applicable to a 

 hilly district, the latter to a flat country. 



When the district to be surveyed has been triangu- 

 lated, the work of filling-in the topographical details 

 is commenced. Each triangle being of moderate ex- 

 tent, with sides from three to five miles in length, and 

 the angular points being determined, as we have seen, 

 with great exactness, it is evident that no considerable 

 error can occur in filling-in the details. Hence, methods 

 can be adopted in the final topographical work which 

 would not be suitable for triangulation. The triangles 

 can either be "measured up," or the observer may 

 traverse from trigonometrical point to point, taking 

 offsets and intersections ; or, lastly, he may make use of 

 the plane-table. The first two methods require little 

 comment ; but the principle of plane-tabling enters so 

 largely into Indian surveying, that our notice would be 

 incomplete without a brief account of this simple and 

 beautiful method. 



The plane-table is a flat board turning on a verti- 

 cal pivot. It bears the chart on which the observer is 

 planning the country. Suppose, now, that two points 

 A and B are determined, and that we require to mark 

 in the position of a third point c : It is clear that if we 

 observed with a theodolite the angles ABC and B A c, 

 we might lay these down on the chart with a protractor, 

 and so the position of c would be determined with an 

 accuracy proportioned to the care with which the ob- 



