[ 110 ] 
quently a parallel circle about the Sun, as a pole j 
inch a tranflation 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 vifion, 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 propefly combining and proportioning two mo- 
tions, one of the quadrant upon the axis of vifion, 
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 vifion. 
If it fhould be afked, why the obferver fhould be 
dire&ed to perform two motions rather than the 
fingle 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 
