ACCOUNT OF A GRAPHOMETER. 375 



divided into two quadrants by an hinge, fo that one part may 



be turned back out of the way of any mineral, which may 



require to be brought up towards the centre for admeafure- 



ment; and the fame arc can afterwards be reftored to its 



place, in order to (hew the degree and fraction of the angle. 



n my improved inftrument I avoid this joint, and obtain Improved gra» 



a much firmer framing by making my arc in the form of a femUclrcle s ' 



protractor, as in Fig. 1. Plate XV. having an hollow centre at entire, and the 



A, and a ftud at B, both lying in the diredion of that C0 . m P affes or r ^ 

 . . i rp, cr callipers are ufed 



diameter which terminates the graduations. The compafles, feparately for 



or radii, or legs, are ftiewn in Fig. 2. fepatate from the arc. meafuring the 

 "n* ^ • . .1 , r r , .. . cryftal, and 



lneir centre C is made like thole or the common proportional applied to the 



compafles, and admits of the legs C D, C F being confider- fcmkircle for 

 ably lengthened or (hortened when the two pieces are applied rea ing ° 

 to each other. D E the fixed leg is reprefented as beneath 

 F G the moveable leg or radius, and the lower end of the 

 centre pin is made to fit the hole A precifely, at the fame 

 time that the ftud at B being admitted into the long perfora- 

 tion towards E, the piece D E becomes fteadily attached to 

 the femi-circle, as is feen in Fig. 3. 



The ufe is obvious. The cryftal mull be meafured by the 

 detached compafles as in Fig. 2, which are much more handy 

 for all defcriptions of minerals than Carangeau's entire in- 

 ftrument; and when thus fet, if fig. 2 be applied to .fig I, 

 "as before directed, the angle will be read of at the fiducial 

 edge of G. 



I hope you and your readers will confider this as the ufeful 

 (Amplification of a valuable inllrumenf, and fhall be happy to 

 receive your fanclion by its appearing in a work fo generally 

 known and efleemed as your Journal. 



I am, Sir, 



Your obedient Servant, >, : . ^ 



* 



ROBERT BANCKS. 



Nov. i, 1805. 



Accounts 



