276 LIGHT SCIENCE FOR LEISURE HOURS. 



applied as a method of observation subsidiary to the 

 ordinary trigonometrical processes. 



" A hilly country," says Sir A. Waugh, " offers the 

 fairest field for the practice of plane-table surveys, 

 and the more rugged the surface the greater will be 

 the relative advantages and facilities this system pos- 

 sesses over the methods of actual measurement. On 

 the other hand, in flat lands the plane-table works at 

 a disadvantage, while the traverse system is facilitated. 

 Consequently, in such tracts, the relative economy of 

 the two systems does not offer so great a contrast 

 as in the former. In closely-wooded or jungly tracts, 

 all kinds of survey operations are prosecuted at a 

 disadvantage ; but in such localities, the commanding 

 points must be previously cleared for trigonometrical 

 operations, which facilitates the use of the table." 



In whatever way the topographical details have 

 been filled-in, a rigorous system of check must be 

 applied to the work. The system adopted is that of 

 running lines across ground that has been surveyed. 



This is done by the head of the party or by the 

 chief assistant-surveyor. A sufficient number of points 

 are obtained in this way for comparison with the work 

 of the detail surveyors ; and when the discrepancies 

 exceed certain limits, the work in which they appear is 

 rejected. Owing to the extremely unhealthy, jungly, 

 and rugged nature o the ground in which nearly all 

 the Indian surveys have been progressing, it has not 



