of the Equatorial Instrument. toy 
at right angles to this axis ; and thirdly, it is adjusted to the 
meridian. 
Now, let the error of collimation in right ascension, in the 
same manner, be observed with any star out of the equator, by 
a circumpolar star, (the nearer the pole the better) suppose the 
pole star. If any difference should be noticed in its passage, 
with the circle east or west, halve that difference,* and it will 
be equal to the angle that the plane of the declination circle 
makes with the polar axis, if the observed star were actually 
in the pole ; if not, divide it by the sine of its declination, 
and the true angle of the plane of this circle (or of the line 
of collimation) with the polar axis, will be had. Again, if 
this operation be repeated with any other stars, and the error 
so found be divided by the sine of their declination, the error 
of the plane of the declination circle at the pole, viz. its 
greatest error, or angle with the polar axis, will be had. And 
note, if these observations are made with stars on each side 
of the equator, these quantities will be had in opposite direc- 
tions. Finally, the same may be done by two land objects, 
one to the north, and the other to the south ; the north and 
south meridian marks, for instance, proper consideration be- 
ing had to their declination ; by this means the error will be 
thrown in contrary senses, or doubled, and, from a variety of 
such results, a very correct mean quantity may ultimately be 
* By difference is here meant, the difference taken in minutes and seconds of a great 
circle passing through the star, and which can only be directly measured by a micro- 
meter ; but if, as is most convenient, this quantity should be observed by time, or by 
the divisions on the equatorial circle, (15 and 16) this quantity must be diminished in 
the proportion of the radius to the sine of the polar distance, viz. multiplied by the 
co-sine of the declination ; hence it is, that this method is capable of great preci- 
P 2 
sion. 
