O AN ACCOUNT OF A GONIOMETER. 



legs is attached by the pin F, a movable quadrant, passing 

 through the bar B G at C, and graduated to read towards 

 the side of the shorter leg, de. The handle G H is made 

 to project on the opposite side to that on which the legs 

 move, that it may not interfere witli the use of a brass de- 

 gree divided into minutes, to be attached to the centre A 

 upon longer radii. The leg A B is divided accurately into 

 tenths, and the two nearest the point B into twentieths, and 

 these again into fortieths. The whole instrument is about 

 ^ four inches and a half long. 



Method of Now if the crystal, the angle of which is to be measured, 



using it. |} e detached, it is obvious, that, if an acute one, by applying 



it to the angle BAD the vertical and equal angle E A C 

 will be given on the quadrant: if obtuse, by applying it 

 to the angle E and B we obtain its supplement E A C. — 

 But if the crystal be imbedded in the matrix, so that with 

 the common goniometers having the movable vertex it is 

 difficult to procure a mechanical measurement of the pro- 

 truding angle, we take any distance from the vertex of the 

 crystal measured upon the scale of tenths, &c. ; and placing 

 one point of the instrument, (ex, gr.J on x (fig. 2) and the 

 other on z, so as to make the triangle x y z isoscelles^ we get 

 upon the quadrant the angle which the base xz subtends at 

 the radius one inch ; and we have the side .r y measured. 

 As, therefore, when the object is given, the angle it sub- 

 tends, and — 7-, we have the proportion xy : A B : : angle 

 rau. 



found : angle sought. And as the whole radius is one inch 

 divided into tenths, this proportion is easily made in the 

 head. 



The nhort bar is added for the better carrying of the 

 s"ata ascer- quadrant, and renders the instrument useful in geological 

 tained by it. observations, to ascertain the inclinations of strata. This is 

 done by hanging a small plummet to a hook at A, and hav- 

 in°- made, the angle which it forms at B C a right angle, and 

 moving the outer bars, till their upper line coincides with the. 

 stratum, the angle of inclination will be given on the quad- 

 rant. 



II. 



