[ II0 ] 



quently a parallel circle about the Sun, as a pole $ 

 iuch a tranflatlon of the Sun's image can only be pro- 

 duced by the quadrant being turned about upon a 

 line drawn from the eye to the Sun, as an axis ; a 

 motion of rotation upon this line may be refolved into 

 two, one upon the axis of viiion, and the other upon 

 a line on the quadrant perpendicular to the axis of 

 vifion ; and confequently a proper combination of 

 thefe two motions will keep the Image of the Sun 

 conftantly in the axis of vifion, and caufe both joint- 

 ly to run over a parallel circle about the Sun in the 

 heavens ; but when the quadrant is vertical a line 

 thereon perpendicular to the axis of vifion becomes 

 a vertical axis; and, as a fmall motion of the quadrant 

 is all that is wanted, it will never differ much in 

 practice from a vertical axis ; therefore the obferver, 

 by properly combining and proportioning two mo- 

 tions, one of the quadrant upon the axis of viiion, 

 and the other of himfelf upon his heel, keeping him- 

 felf upright (which gives the quadrant a motion, 

 upon a vertical axis) will caufe the image of the 

 Sun to defcribe a fmall arch of a parallel circle 

 about the Sun in the heavens, without departing 

 confiderably from the axis of viiion. 



If it mould be afked, why the obferver mould be 

 dire&ed to perform two motions rather than the 

 lingle one equivalent to them on a line drawn from 

 the eye to the Sun as an axis ? I anfwer, that we 

 are not capable, while looking towards the horizon, 

 of judging how to turn the quadrant about upon the 

 elevated line going to the Sun as an axis, by any 

 other means than by combining the two motions 

 above-mentioned, fo as to keep the Sun's image al- 

 ways 



