OPERATIONS IX THE PENINSULA. 30/ 



No further account need therefore be given of the 

 manner of performing this very important and deli- 

 cate part of the work, than that in addition to tiie 

 apparatus used in the former measurement near St. 

 Thomas's Mounts there was a transit telescope, in all 

 respects similar to that mentioned in the account of 

 the trigonometrical survey of England for fixing 

 objects in the aUgnement, and taking the elevations 

 and depressions at the same time. The manner of 

 usino- it Avas as follow^s : 



'O 



When the instrument was placed at a sufficient 

 distance behind the commencement of an hypo- 

 thenuse, so as to see distinctly the mark placed on 

 the head of the drawing post, and the elevation or 

 depression of the hypothenuse finally determined, 

 the instrument beino- covered from the sun by a small 

 cloth pandal, remained in that position, tid four ov 

 sometimes five chains were measured. Previous to 

 removing it, a small hooped picket was placed, by 

 signal from the person at the transit, at a proper dis- 

 tance behind the termination of the last chain. ^ In 

 fixing the spot for this little picket, a common rod, 

 with a sharp point was used, and the telescope of the 

 transit depressed to the place on the ground intenxled 

 to be marked. After the spot was fixed on, and the 

 picket driven down, the instrument was removed, 

 and placed in the alignement, with the plummet 

 hanging over the centre of the little picket, and then 

 a new^ hypothenuse was laid out, or the former one 

 continued. 



When the hypothenuse was terminated, a register 

 picket was driven into the ground, opposite to the 

 arrow of the chain, and in such a manner, that when 

 the brass head was fixed thereon, the slide misbt 



X 2 



