[ rl 3 ] 
with refpedt to the axis of vilion, 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 polition for taking the Sun’s altitude. But 
if the quadrant be held a little deviating from 
the perpendicular polition 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 polition 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 conllantly in the axis of vilion, 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 line 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 firlt, the axis of vilion 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 
