252 LIGHT SCIENCE FOR LEISURE HOURS. 



progressing, it has not always been found practicable 

 to check by regularly chained lines. There are, how- 

 ever, other modes of testing plane-table surveys, and 

 as these entail less labour and expense in hilly and 

 jungly tracts, and are quite as effective if thoroughly 

 carried out, they .have been adopted generally, while 

 the measured routes or check-lines have only been 

 pursued under more favourable conditions. Colonel 

 Thuillier states that * the inspection of the work of 

 every detailed surveyor in the field has been rigor- 

 ously enforced, and the work of the field season is not 

 considered satisfactory or complete unless this duty has 

 been attended to.' 



The rules laid down to insure accuracy in the 

 survey are first, that the greatest possible number 

 of fixed points should be determined by regular tri- 

 angulation ; secondly, that the greatest possible num- 

 ber of plane-table fixings should be made use of 

 within each triangle ; and lastly, that eye-sketching 

 should be reduced to a minimum. If these rules are 

 well attended to, the surveyor can always rely on the 

 value of the work performed by his subordinates. 

 But all these conditions cannot be secured in many 

 parts of the ground allotted to the several topographical 

 parties owing to the quantity of forest land and the 

 extremely rugged nature of the country. Hence arises 

 the necessity for test-lines to verify the details, or 

 for some vigorous system of check; and this is 

 more especially the case where native assistants are 

 employed. 



