4p8 Mr . Pond on the different methods of 
Whoever attentively examines the construction of the 
mural circle at Greenwich, will perceive that its operation is 
entirely limited to the measurement of the meridional distance 
between different stars. It is, in fact, a theodolite placed 
vertically. By an extension of its principle, it can measure 
the distance between a star and its reflected image from a 
mercurial horizon, and thus determine the altitude of a star; 
and, in common with every other circle, it can measure the 
distance of a circumpolar star from the pole by an observation 
of its inferior and superior passage over the meridian. As 
the instrument cannot be reversed, a plumb line could not 
be applied, with any advantage. In circles that turn freely 
in azimuth, a double observation of a star gives the angle 
which the star makes with the axis of the instrument round 
which it turns, and a plumb line is properly applied in 
this case to ensure the verticality of this axis ; but on the 
Greenwich circle, a plumb line could only serve as a point of 
departure ; and this having no reference to the real zenith, 
would, in my opinion, be very injudiciously chosen. 
Let us suppose, by way of example, that twelve stars had 
been observed with the mural circle, and one of these the 
pole star ; moreover, that this star had been observed below 
the pole. Then, as it would be easy to determine the polar 
point, the position of all the remaining stars, with reference 
to this point, might likewise be ascertained. It is however 
evident in this case, that each result would be affected by a 
double error, viz. the error committed in the determination of 
the polar point, and also that due to the observation of each 
star. 
