316 SURVEY OF THE COAST 



same observations, by adding only one observation more at 

 the end, and if accident should occasion the loss of one se- 

 ries, the other may be preserved. 



Plate VII. fig. 1, is a general view of the instrument from 

 behind, where the additions made to it are all visible. Fig. S 

 represents a section of the limb and two readings, in the di- 

 rection of a radius. 



The limb of the circle consists of two circular rings fixed 

 to each other, as seen in the second figure. A section of this 

 limb or of the wings which form it resembles letter T. To the 

 lower ring the radii r of the frame of the circles are fastened, 

 the upperonebeing the divided limb itself, which is sufficiently 

 elevated above the radii to give passage to the clamping piece 

 D of the front telescope. The verniers of the front telescope, 

 which are four as usual, reach to the division of the limb from 

 the inside. To the back telescope and level, a frame, exactly 

 similar to that of the front telescope, is adapted, with four 

 verniers W, X, Y, R, which are fastened to the part of this 

 frame extending beyond the circle by two pillars, bringing 

 them round the limb outside, to reach to the division of the 

 limb ; so that there remains a sufficient space of the division 

 free between two verniers to read them accurately, when 

 two such verniers stand opposite each other. This space 

 is about half the length of the strokes. The verniers then 

 pass by each other freely, and are read upon the same divi- 

 sion. The two magnifiers, which revolve upon the centre 

 on arms, in the front of the circle, as seen in b, fig. l, serve 

 equally for both series of readings. As to the divisions, tlie 

 strokes denoting degrees and half degrees are drawn out on 

 both sides equally, so that tliey show equally for both the 

 inner and the outer verniers. The degrees have points in 

 the middle of the strokes, wliich will show when two ver- 

 niers are opposite, and will serve to count them from the 

 number seen on the left beyond the alliidade. Every ten 

 degrees being engraved outside, and every five inside of the 

 division, there is always a clear and equal reading for both 

 positions of the verniers. 



