[ "3 ] 



with re/pect to the axis of vifion, or feem 

 to move backwards; and the Sun will move 

 quicker than the axis of vifion in the back- 

 obfervation, or will feem to get before it. When 

 the motion of the Sun in this arch is parallel 

 to the horizon, the plane of the quadrant coin- 

 cides with the vertical circle palling through the 

 Sun, and confequently the quadrant is in a pro- 

 per pofition for taking the Sun's altitude. But 

 if the quadrant be held a little deviating from 

 the perpendicular pofition to the horizon, and 

 turned about upon an axis, either vertical or only 

 nearly fo, the arch defcribed. by the Sun ap- 

 parently will cut the horizon, but will never 

 move parallel to it, and confequently the quadrant 

 will not be brought into a proper pofition for 

 obferving the Sun's altitude. 

 III. If the quadrant be turned on the line going 

 to the Sun as an axis, the reflected Sun will be 

 kept constantly in the axis of vifion, and will 

 defcribe an arch of a parallel circle about the 

 real Sun, with a velocity which is to the angular 

 motion of the quadrant, as the fine of the Sun's 

 altitude is to the radius ; and when the motion 

 of the reflected Sun is parallel to the horizon, 

 the quadrant is vertical. 



Hence naturally arife the three methods of taking 

 an altitude, which have been mentioned before. In 

 the firft, the axis of vifion is fuppofed always directed 

 to one and the fame part of the horizon, namely 

 that which is in the Sun's vertical. In the fecond, 

 the obferver is required to hold the quadrant truly 

 vertical, and to turn himfelf upon a vertical axis; 



Vol. LXII, Q but 



