[ «« ] 
the horizon-glafs, the fame way as the arch of 
the quadrant is carried, both in the fore and 
back-obfervations, with a velocity which is to the 
angular velocity of the quadrant as the fine of the 
Sun’s altitude to the radius, defcribing an arch 
convex downwards in both cafes ; and when the 
motion of the Sun in this arch is parallel to the 
horizon, the quadrant is held truely perpendicular 
to the horizon, and confequently in a proper 
pofition for taking the Sun’s altitude. But, if 
the axis of vifion be directed to, and turned 
round a point in the horizon befide the vertical 
circle palling through the Sun, the Sun’s image, 
when its motion is parallel to the horizon, will 
be neither in the axis of vifion nor the Sun’s, 
vertical, but between both > at the fame time, 
the plane of the quadrant will not be vertical, 
and the altitude found by bringing the Sun’s 
image to touch the horizon will not be the true 
altitude. 
II. If the quadrant be held perpendicular to the 
horizon, and turned about upon a vertical axis, or 
one nearly fo, the Sun will defcribe an arch con- 
vex downwards in the fore-obfervation, and up- 
wards in the back-obfervation, the motion of the 
Sun being the fame way as the axis of vifion is 
carried in both cafes, and being to the angular 
motion of the quadrant, as the verfe-fine of the 
Sun’s altitude to the radius in the fore-obferva- 
tion, but as the verfe-fine of the fupplement of 
the Sun’s altitude to 1 8o° to the radius in the 
back-obfervation. The Sun therefore will move 
flower than the axis of vifion is the fore-obfer- 
vation, and confequently will be left behind, 
with 
