830 SURVEY OJP THE COAST 



tiplying and the motions of the axis, could evidently be exe- 

 cuted in the vertical as well as in the horizontal circle. By 

 making the latter revolve in the manner of a transit, not 

 only all counterpoising weight might be avoided, (and tliere- 

 fore the weight of the instrument very much diminished,) but 

 also its stability in the observation might be secured. 



Upon these considerations 1 founded the construction of 

 an instrument, of which I presented a plan in full size to Mr. 

 Trougliton, who, approving the principles, thought however, 

 at first, that with eighteen inches diameter, as 1 wished it, it 

 would present some difficulties, and therefore executed two 

 of them of twelve inches in diameter, availing himself of the 

 liberty left to him, as a skilful and experienced artist, to alter 

 in various respects their external appearance. 



Plate VIII. fig. 1 is a full perspective view of the instru- 

 ment, and fig. 3 is a vertical section of the horizontal circle 

 through the centre. 



The centre piece aaaa of the stand part is two-thirds of a 

 strong sphere, perforated to receive the axis of the horizontal 

 circle. The three hollow conical arms forming the legs of 

 the instrument are fastened to this piece by large strong 

 screws, which are stopped below by the small screws 6, b. 



These arms terminate in spherical nobs which receive the 

 vertical double screws a', c', in the same manner as in the 

 two-feet theodolite and the repeating circle. They rest 

 upon three truncated cones having lead in the top, and being 

 sufficiently elevated to admit the verification telescope be- 

 tween the instrument and the stand. 



The vertical axis cc of the instrument is fitted in the sphe- 

 ric centre piece by a collar, in the same manner as in the 

 two-feet theodolite, and fastened by six screws from below. 

 The lower part of this, represented by ee, is nearly two 

 inches higli, and forms the axis of the horizontal circle, which 

 revolves upon it by means of the socket dd. The upper 

 part, seven inches long, has its diameter diminislied so as to 

 leave a collar ee of about one-tenth of an inch to rest the 

 upper part of the instrument upon. This socket is in the 



