1824.] Adelmann's improved Goniometer. 213 



that as this instrument can only be destined (tiepeut etre destine) 

 to establish the primitive form, the type of all those which the 

 observed object can present, and which serves as the basis for 

 calculating all the secondary faces, a single crystal of the sub- 

 stance, although very small, which fulfils all the necessary con- 

 ditions, would be sufficient; still, however, some secondary 

 planes are requisite to determine the dimensions, and, in a great 

 number of substances, especially if they do not admit of a perfect 

 and easy cleavage, such a crystal is yet to be sought for." 



" M. Adelman's goniometer has none of the inconveniences of 

 the instruments I have mentioned, and, moreover, possesses 

 advantages which in them are wanting. 1 consider that I shall 

 do a service to the science by describing it, and the manner of 

 using it." 



The instrument consists of a small mahogany box, (1) see 

 Plate XXX, eleven inches long, six wide, and three high, con- 

 taining a drawer (2). The top of the box (3) is covered with a 

 plate of brass, at least two lines thick, to prevent its springing, 

 and to render the base sufficiently heavy. The rest of the 

 instrument is made of brass. Two pillars (4), nine lines in 

 diameter, and at least four inches and a half high, are fastened 

 to the brass plate, at the distance of three inches from each 

 other, and are united and fixed at the upper end by a plate (5). 

 At the top of each column is a box (6) iu which a steel rule (7), 

 seven lines wide and one line thick, moves horizontally ; the 

 rule is placed edgeways, in order to render its motion more 

 smooth and regular. For the same purpose, each box has a 

 roller at the bottom, on which the rule slides. A semicircle (8) 

 is fixed to the rule by its diameter, the length of which is six 

 inches, four lines, and its broad part (9) seven lines wide. The 

 semicircle is not in contact with the moveable rule, but separated 

 from it by an interval of about three lines. This interval is 

 necessary on this side of the semicircle, in order to adapt to the 

 other extremity of its axis (10) the moveable radius (11), which, 

 at its upper end, carries the nonius (12). On this first semi- 

 circle, which is fixed, is placed a second, divided into degrees, 

 minutes, and half minutes, which, from its size, are perfectly 

 distinct. The second semicircle (13) is fixed by its radius (14) 

 to the centre of the first, and is moveable ; but its motion on the 

 fixed semicircle is perfectly smooth, and free from jirks. The 

 radius of the moveable circle is produced beyond the centre, by 

 a steel arm (15) adapted to it, which may be called the measuring 

 radius. The part (16) of the fixed semicircle is necessary to 

 • liable the moveable nonius to travel round the moveable semi- 

 circle. A screw (17) placed behind the nonius serves to fix it 

 at pleasure; (18) is a button for moving the graduated semi- 

 circle. 



Two detached pieces are added to this instrument; one of 



