﻿3$6 ox EXTENDING A GEOGRAPHICAL SURVEY 



ciilar instrument. The method which I used was as 

 fellows : 



I FIRST laid out the direction of the hypothenuse, 

 by a boning rod, placed at a distance, to be seen ' 

 with the small telescope of the sextant. -Another 

 boning rod was then placed at a convenient distance, 

 so that the cross vane might be bro.uglit to corre- 

 spond with the cross wires of the levelling telescope, 

 after it had been carefully adjusted to the horizontal 

 dire61ion by the spirit level. Then, upon the same 

 boning rod was placed another cross vane, and tlie te- 

 lescope elevated, or depressed, b}' the linger screw, 

 until the cross wire3 were brougiit into the direction 

 of the hypothenuse by the vane on the distant boning 

 rod. —In taking the angle witli the sextant, I placed 

 the axis of motion close to tlie Y of ^le levelling 

 telescope, at the opposite end, with the finger screw; 

 50 that the two vanes, on the distant and near bon- 

 ing rods, appeared to correspond in the retlector of 

 the sextant, and then the angle was taken. 



In this manner all the smaiier angles of elevation 

 and depression were taken, and though not exactly 

 in the way I could have wished, yet 1 have no doubt 

 of their being nearly correct, perhaps as much so as 

 any direction can be measured. 



Hence -the line was determined, which passed 

 through the axis of the levelling telescope, and was 

 parallel to the h3'pothenuse. In order to place the 

 pickets for receiving the coffers, a piece of wood 

 ■uas contrived for being placed upon the head of 

 each, with a cross vane to slide up and down. Then, 

 a picket was driven, at any given distance in the 

 allignement, and the above piece applied to its top. 

 When the cross piece corresponded with the mark, 

 the picket remained in that state, and the rest of 

 them were driven down in the same manner, and the 

 piece applied to their respective heads ; and being 

 all adjusted by that means, their tops were conse- 

 quently parallel to the line of direction. 



The 



