132 DESCRIPTION OF THE GRAPHOMETER. 



X. 



Defcription of ike Graphometer, or Infirument ^/Cit.Carak* 

 geau, for meqfuring the Angles of Cn/fiak. — From the Mi- 

 ?ieralogie of C i t . H a u y * . 



Stru&ureof the AN order to determine the mutual inclination of the faces 



Two concentric °* a cr J^ a ^ or i* 8 prominent angles, an infirument is era- 



equal quadrantal ployed, for the invention of which we are indebted to Citizen 



a rc S joinedbyan Carai aa> This i nftruroent wn i Gn confiderably refcmblcs 



hinge j to which ° J 



are applied com- the graphometer, confifts of a femicircle M T N, (PI. VIII. 



pafs legs or radii, fl g# 1# ) f brafs or filver, divided into degrees, to which two 



fhortened to ap- indexes, A B and F G, are applied, the one of which,. F G, 



ply to fmall cryf- J s hollowed out from u to K, from face to face, except at the 



place K, where a fmall portion is left for the purpofe of giving 



greater ftrength to the inilrument. This index is attached at 



K and c loa brafc piece or rule iituated behind, and forming 



the fame piece with the limb. The connection of the index 



with this rule is effected by means of two fmall milled-head- 



fcrews, inferted in the groove. The other index, A B, is 



hollowed out in the fame manner from x to c, where it is 



attached above the former, by means of the fcrevv at the center, 



which travc ifes the two grooves. By loofening the fcrews, 



we may ihorten at pleaiure the parts c G and c B of the two 



indexes, accordingly as circumflances may require. 



The index A B having only one point of attachment at c, 

 at the center of the circle, moves round this circle, whilft the 

 index G F remains conftantly in the direction of the diameter, 

 which pafles through the points zero and 180 degrees. 



It may be proper to remark, that the upper part of the in- 

 dex A B ought to be Hoped to an edge, on its fide * z, the 

 direction of which edge, if prolonged, would pafs through 

 the center c of the infirument. The reafon is, that this fide 

 is the fiducial edge ; that is to fay, it indicates upon the gra- 

 duated circumference the dimenfions of the angle we with to 

 meafure. 

 Mod? of opera- Let us now fuppofe that we are defirous of meafuring in 

 any cryftal, the angle which two contiguous planes form witfy 



* This ingenious inftrument hns been iri ufe a confiderable time, 

 and deferves to be more generally known. W. N. 



each 



\*n 



