4 S4f S/r W. Herschel's atronomical observations 
venient, he will change his position by going round the globe 
till he comes to the situation in which this angle will appear 
of its true magnitude. 
In illustration of this, let NESW, figure 2, [[PI. XXI.] be 
the circle on which the azimuths of celestial objects are to 
be reckoned, and let the meridional line NS pass through 
the 190th degree of right ascension at S ; then will the 
numbers at the circumference of the circle point out the 
degrees, and the quadrant of the azimuth of the situation in 
which any object is to be seen when referred to the milky 
way. The particular use of this azimuth circle will appear, 
when the construction of the figure which expresses the 
profundity of the clusters of stars, of which I have given the 
observations, has been explained. 
Having fixed upon the plane of the milky way as the 
region of the heavens to which the situation of the clusters of 
stars is to be referred, their right ascension and polar distance, 
which are required for this purpose, must be reduced to this 
plane ; and will appear under the denomination of elevation 
and azimuth. The elevation from the plane of the milky way 
will be either north or south, and the azimuths in either the 
northern or southern hemisphere of it, will be in the north- 
east, south-east, north-west, and south-west quadrants. In 
order to make this reduction, we have the construction of 
the triangle ABC, figure 3, in which A is the pole of the 
heavens ; B the north pole of the milky way, and C the 
situation of the cluster of stars ; and there is given the side 
AB, which is the distance of the two poles ; the side AC, 
which is the polar distance of the cluster, and the angle A, 
which is the difference between the right ascension of the 
