GREENWICH OBSERVATORY. 
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observations, and having regard to the impossibility of seeing 
the moon on the meridian during that critical part of her 
orbit when she is near the sun, and also to the frequent loss of 
meridional observations from temporary cloudiness of the sky, 
Sir Gleorge Airy deemed it highly desirable to devise means of 
securing observations off the meridian comparable in accuracy 
with those made on the meridian. The only form of instrument 
competent for this purpose was an altazimuth of massive con- 
struction. He accordingly designed and mounted (in 1847) the 
instrument here figured. It consists of a telescope (4 inches 
aperture) solidly fixed into a broad-rimmed vertical or altitude 
circle which has a divided silver limb read by four microscopes. 
This circle turns between two semi-cylinders of cast-iron con- 
nected by top and bottom plates to form a frame which turns 
on vertical pivots, the lower one borne by a stone pier and the 
upper one by a triangular framework of iron bars. The ver- 
tical frame carries on one side the four microscopes which read 
the altitude circle, and around its base other four which read a 
horizontal circle fixed to the stone pier for azimuthal measures. 
There are levels for showing the inclinations of both axes; and 
they, and the microscopes, are carried by supports which are cast 
upon (not screwed to) the parts they spring from. The nightly 
observations with this instrument consist of an altitude and 
azimuth of the moon, azimuths of stars for zero of the azimuth 
circle and collimation error of the telescope, and observations 
of a distant terrestrial mark for zenith-point error of the vertical 
circle. All observations are made in two (reversed) positions 
of the telescope ; and all times of exact position-determinations 
are registered by the chronograph. The azimuths and altitudes 
are in effect reduced to Right Ascension and Polar Distances by 
after-calculations, which are very laborious, but are, in common 
with every computation in the observatory, facilitated by 
skeleton forms of which about two hundred, simple and elabo- 
rate, are in constant use. 
The Altazimuth secures about two hundred observations of 
the moon in a year, the Transit-Circle only one hundred ; and a 
large percentage of the former are of the highest value as afford- 
ing tests of the lunar tables for parts of the orbit when the 
moon is near conjunction with the sun, and cannot be touched 
by a meridian instrument. 
Another auxiliary instrument of the exact class is the Reflex 
Zenith Telescope, for measuring with great precision the small 
zenith distance of the star 7 Draconis, which passes near the 
zenith of Greenwich, and is favourably situated for determina- 
tion of the amount of stellar aberration. It consists of a hori- 
zontal object-glass, capable of semi-rotation about a vertical 
axis, with a trough of mercury at half its focal distance below. 
