REPEATING CIRCLE.] 



ASTRONOMY. 



995 



in pierced with a rectangular opening, traversed in 

 its middle by the screw 6, which is formed to turn 



Fig. ITS. 



upon itself in the collars fixed at the two extremities 

 of the opening. Screw c acts upon 6, and is besides 

 Fig. 176. attached to the clamp 



d, which clasps the 

 edge of the limb 

 above and below. The 

 screw e is to regulate 

 the place of the clamp 

 on the opening, press- 

 ing the edge of the 

 limb between its jaws, 

 and thus fixing the 

 screw, c, to the limb. 

 When it is required to 

 displace the telescope 

 rapidly, it is necessary 

 to press the screw c, 

 5sai which throws it loose, 

 so as to move round 

 the circle under the 

 impulse of the hand. 

 When the glass has 

 nearly attained its pro- 

 per position, it is 

 trained more exactly 

 The vice d and the screw 



c are acted on and fixed again to the limb, and by 

 turning the screw 6, a slow movement is communicated 

 to the screw a, which trains the telescope to the 

 exact spot required. 



Another disposition has been adopted for pro- 

 ducing, in an analogous manner, the movement of 

 the circle round its centre. The axis, after having 

 traversed the socket B B (Fig. 175), and the cylin- 

 drical counterpoise D slightly inclined from below, 

 carries a toothed wheel of the same diameter as 

 the counter-weights. An endless screw is connected 

 with this wheel, and is carried by a collar fixed to 

 the counterpoise D, as is shown in Fig. 175. If the 

 endless screw a 6 be moved by turning one of the 

 two milled heads, the toothed wheel with which it 

 is connected will receive a rotatory motion, in 



Fig. 17T. 



Fig. ITS. 



by turning the screw e. 



which the circle to which it is fixed by the same axis 



will necessarily participate. 



But the screw a b may be V A' 



withdrawn from the toothed 



wheel in such a manner as to 



stop all communication be- rf 



tween them. To do this, it * 



is only necessary to turn the g 1 



finger a of the screw when ' 



the two are disengaged, as in 



Fig. 178. Having thus ex- 

 plained the mechanical details 



of the repeating circle, its ap- 

 plication to measuring the 



angle is as follows : 



Let A and B (Fig. 179) be g 



the two distant points, which ". 



are the sides of the angle to 15" 



be measured. Having placed 



the upper glass at zero of the 



graduated scale, and fixed it 



to the circle in that position, 



the circle is now placed in the 



plane of the angle and made 



to turn -in this plane, so that j 

 the telescope of the face is directed ~ 

 towards the point A, the other .2> 

 telescope being directed to the "** 

 point B, the circle remaining in 

 the position Fig. 179. The circle 

 is now to be turned until the 

 lower instrument is directed to- 

 wards the point A, as in Fig. 180. 

 The circle being again fixed, the 

 upper telescope is detached and S 

 trained so as to be directed to the ^ > 

 point B, as in Fig. 181. The upper H 

 instrument is thus made, to de- 

 scribe an angle exactly double 

 that sought for, and the index 

 travels over the limb the exact 

 measure of this double angle, 

 which it represents in degrees, 

 minutes, and seconds ; dividing = 

 this number by two, the exact ". 

 value of the angle is obtained. ? 

 Supposing the operation to require 

 confirmation, the same process 

 maybe repeated, turning the whole 

 instrument round the axis of the circle, so that the 

 upper telescope is again directed towards the point 



