COMMON GONIOMETER. 63 



are large or small. In either case, the number of degrees, or the 

 value of the angle, is indicated at the border/n of the moveable 

 arm, which coincides with a line coming directly from the centre of 

 the circle. It is necessary that the arms be applied to the planes 

 whose inclination is required, perpendicularly to the. edge at which 

 they meet. But it frequently happens that the crystal we are wish- 

 ing to measure is engaged along with other crystals in its gangue, 

 so that the extremity s of the semicircle prevents the application of 

 the arms to its planes : in order to remedy this obstacle the semi- 

 circle is cut into two parts at t, and reunited by a hinge. In this way 

 we are able, when it is necessary, to turn the part s f, back upon the 

 other, which we restore again to its original place, when we have 

 adjusted the arms to the planes under examination, in order to read 

 off the degrees of the measured angle. 



Figures 2 and 3 represent the common goniometer, which consists ot 

 two parts; the steel arms Fig. 2, constituting one part and the semi- 

 circle Fig. 3, the other. When we have adjusted the arms to the 

 angle to be measured, they are transferred to the semicircle in or- 

 der to read off the degrees. Cut it is obvious that in using this va- 

 riety of the goniometer, it is indispensable, in adjusting the arms to 

 the semicircle, that we place the centre of them upon the centre ot 

 the semicircle, and one arm upon its diameter. This is effected with 

 ease by the following construction of the instrument. The pin 

 which connects the arms at k is allowed to project a little upon one 

 side, and a hole is made in the cross piece Fig. 3 at k y exactly large 

 enough to receive it; besides, there is a little projection upon the 

 cross piece at y, against which the lower edge of that arm may ro- 

 pose, which requires to be exactly upon the diameter of the circle, 

 when the angles are read off. The method of using it is, after hav- 

 ing applied the edges of the arms to the planes of any crystal, to 

 tighten them by means of a little screw at A;, and transfer them to 

 the cross piece of the semicircle, allowing the projection to drop into 

 the hole k, and one of the arms to rest with its under edge upon y,- 

 the other arm will then indicate upon the reporter, the number of 

 degrees which the measured angle contains. This construction of 

 the common goniometer will be found more convenient in its appli- 

 cation than that first described, and on the whole, less liable to inac- 

 curacies in consequence of the greater steadiness of the semicircle, 

 and of the greater exactness with which the arms may be fitted to 

 crystals. 



In making use of the common goniometer, great care should be ex- 

 ercised in selecting erj'stals with smooth and plane faces; and when 



