The circles _c_ and _d_ serve to indicate, by means of verniers revolving with the axes, the motion of the telescope in right ascension and declination, respectively.

The weight _w_ serves to counterpoise the telescope, and the screws _s_, _s_, _s_, _s_, serve to adjust the instrument so that the polar axis shall be in its proper position.

The advantage gained by the equatorial method of mounting is that only one motion is required to follow a star.

Owing to the diurnal rotation of the earth, the stars appear to move uniformly in circles parallel to the celestial equator; and it is clear that a star so moving will be kept in the field of view, if the telescope, once directed to the star, be made to revolve uniformly and at a proper rate round the polar axis.
[Illustration: _Fig.

9._] The equatorial can be directed by means of the circles _c_ and _d_ to any celestial object whose right ascension and declination are known.