an astronomical instrument contrived for the purpose of directing a telescope upon any celestial object and of keeping the object in view for any length of time, notwithstanding the daily motion of the earth. For these purposes a principal axis, resting on firm supports, is mounted exactly parallel to the axis of the earth's rotation; it consequently points to the poles of the heavens, being fixed so as to turn on pivots at its extremities. To this there is attached a telescope, moving on an axis of its own, in such a way that it may either be exactly parallel to the other axis or at any angle to it; when at right angles it points to the celestial equator. By this means a star can be followed by one motion from its rising to its setting. In some observatories the equatorials have the necessary motion given them by clockwork.