OF THE UNITED STATES. 333 



To the angle ol' this piece, which is turned downwards, is 

 attached a piece c/, whicii bears a steel pin litiiiii!; in a vertical 

 slit in the bar olthc triangle, which prevents all angular mo- 

 tion of this j)iece around the colunin. 



In the middle, between the two verniers of the bar of the 

 horizontal triangle, is the centre socket p, within which the 

 axis of the telescope revolves. This socket, and that of the 

 telescope, are held to the axis by the ring //, which is screwed 

 to the shoulder of the bell metal part of the axis of the te- 

 lescope foiming the axis to these two motions. 



The two magniliers, whicli serve for all readings, revolve 

 with their arms, round the socket of the triangle. They are 

 fixed to a ring that turns round this socket, and wjuch is held 

 in its place by the projection of the same piece u. 



In observing vertical angles, the circle is clamped al- 

 ternately to the alhidadc of the telescope or to the triangular 

 piece, by the alternate use of the clamps at X or at Z, and 

 the two series of motions give two separate series of angles, 

 each with two diametrically opposite readings. 



All the verniers of both circles are double in this instru- 

 ment, as in the repeating circle, having one half vernier on 

 eacli side. 



The adjustment of the axis of the telescope is made by 

 two strong screws under the pieces y, //, showing a capstan 

 head at ;• in the uppermost plate of the column, and pressing 

 these pieces upwards by their spring around the screws fi. 

 l)y which they are held last to the columns. 



A detached level is placed upon the ends of the axis, pass- 

 ing between the radii of the circle, for the final accurate 

 levelling of the instrument. When the instrument is adjusted, 

 this level is removed and placed uj)on an arm /, at the top 

 of the column opposite the circle, in a position paiallel to 

 the same, and there adjusted by a screw, below one end of 

 the arm, to serve as a constant test ol" the sfal)ility of the 

 level of the instrutnent during the observation. 



By bending the lower angular part of the triangle above 



VOL. ir. — X 2 



