The equatorial and the building to contain it have now been described, but there is another piece of apparatus which is required as much as any adjunct to the equatorial, and that is the chair or rest for the observer. Since the telescope may be sometimes horizontal, and at other times vertical, the observer must be at one time in an upright position, and at another lying down and looking straight up. A rest is required which will carry the observer in these or in intermediate positions. A convenient form of rest for small telescopes consists of a seat like that of a chair, with a support moving on hinges at the back of the seat; a rack motion fixes this at any inclination, so that the observer’s back can be sustained in any position, between upright and nearly horizontal. The seat with its back slides on two straight bars of wood, sloping upwards from near the ground at an angle of about 30°, and about 8ft. long; these are supported at their upper ends by uprights of wood, and at their lower ends in the same manner by shorter pieces. These four uprights are firmly braced together, and have castors at the bottom. A rack is cut on one of the inclined slides, and a catch falls into it, so as to fix the seat at any height to which it is placed. In larger observatories a more elaborate arrangement is adopted, the rails, on which the seat moves, are curved to form part of a circle, having the centre of motion of the telescope for its centre; as the seat with its back is moved up or down on the curved slides, its inclination is changed, so that the observer is always in a favourable position for observing. The seat on its frame runs on circular rails round the pier of the telescope, so that the eyepiece can be followed round as the telescope moves in following a star. A winch by the side of the observer, acting on teeth on one of the rails, enables him to move the chair along, and a similar arrangement enables him to raise or lower the seat on the slides without removing from his place. A steady mounting for the telescope, and a comfortable seat for the observer, are the two things without which a telescope is almost useless. The observing chair is well seen in the engravings of Mr. Newall’s and the Cambridge telescopes. The eyepieces and micrometer can be carried on the rest, close to the observer, when much trouble is saved in moving about for things in the dark; and for the same reason there should be a place for everything in the observatory, and everything in its place. [Illustration: FIG. 155.—Section of Main Building—United States Naval Observatory, showing support of Equatorial. ]