EQUATORIAL. An astronomical strument, contrived for the purpose of directing a telescope .upon any celestial object of which the right ascension and declination are known, and of keeping the object in view for any length of time, notwithstanding the diurnal motion. For these purposes, a principal axis C D, resting on firm supports, is placed parallel to the axis of the earth's rotation, and consequently pointing to the poles of the heavens. On this polar axis there is fixed, near one of its extremities, a graduated circle A B, the plane of which is dicular to the polar axis, and therefore parallel to the earth's tor. This circle is called the equatorial circle, and measures by its area the hour angles, or differences of right ascension. The polar axis is pierced at E F, and penetrated by the axis of a second circle (P II, at right angles to it. The axis of the second circle has consequently no connection with any external support, but is sustained entirely by the polar axis. The plane of the second circle G II, which is called the declination cir cle, and carries the telescope K, is thus in all positions at right angles to the plane of the first or equatorial circle A B. Now it is easy to conceive, from this general description, that when the tele scope is pointed to a star, the angle be tween the direction of the telescope and the polar axis is equal to the polar dis tance of the star ; consequently, when a motion is given to the polar axis without altering the position of the telescope on the declination circle, the point to which the telescocc is directed will always lie in the small circle of the heavens coinci dent with the star's diurnal path ; and hence, if the motion communicated to the polar axis be just equal to the earth's diurnal rotation, the star will remain constantly, and as long as we please, in the field of the telescope, at least while above the horizon. In many observa
tions this is indispensable, and it is an advantage which attaches to no other in strument. The polar axis may be moved by a peculiar kind of clock machinery, adjusted to sidereal time ; and the best and largest equatorials are now furnished with such an apparatus. Besides reliev ing the observer from the fatigue of turning the instrument, the motion thus given is perfectly equable, and all those jerks avoided which, when the instru ment is turned by the hand, often prove fatal to an observation.