[ III ] 



ways in the proper part of the horizon-glafs. Wherr 

 the Sun is near the horizon, the line going from the 

 eye to the Sun will not be far removed from the axis 

 of vifion j and confequently the principal motion of 

 the quadrant will be performed on the axis of vifion, 

 and the part of the motion made on the vertical axis 

 will be but fmall. On the contrary, when the Sua 

 is near the zenith, the line going to the Sun is not 

 far removed from a vertical line, and confequently 

 the principal motion of the quadrant will be per- 

 formed on a vertical axis, by the obferver's turning 

 himfeif about, and the part of the motion made on 

 the axis of vifion will be but fmall. In intermediate 

 altitudes of the Sun, the motions of the quadrant 

 on the axis of virion and on a vertical axis will be 

 more equally divided. Hence appears the reafon 

 of the method ufed by the beft obfervers in taking. 

 the Sun's altitude when near the zenith by holding 

 the quadrant vertical and turning about upon the 

 heel, and the defects of the rules that have been 

 commonly given for obferving altitudes in other cafes.. 



As it may conduce to the fetting this matter in? 

 a flill clearer light, I mail here defcribe in order 

 the feveral motions that, will be given to the reflected: 

 image, by turning the quadrant about upon the- 

 axis of virion, a vertical axis, or the line drawn, from.. 

 the eye to the Sun, fucceffively.. 



I. If the quadrant is turned about upon the 

 axis of vifion, the fame being directed to the 

 point of the horizon exactly beneath or oppo- 

 site the Sun, the image of the Sun will move: 

 from right to left, or from left-, to rights acrofs 



the.- 



