We have now considered the mounting and adjustment of the equatorial, be it reflector or refractor. If of large dimensions it will require a special building to contain it, and this building must be so constructed that, as in the case of the Melbourne and Paris instruments, it can be wheeled away bodily to the north, leaving the instrument out in the open; or the roof must be so arranged that the telescope can point through an aperture in it when moved to any position. This requirement entails (1) the removal of the roof altogether, by having it made nearly flat, and sliding it bodily off the Observatory, or (2) the more usual form of a revolving dome, with a slit down one side, or (3) the Observatory maybe drum-shaped, and may run on rollers near the ground. The last form is adopted for reflectors whose axis of motion is low; but with refractors having their declination axis over six or seven feet from the ground, the walls of the Observatory can be fixed, as the telescope, when horizontal, points over the top. The roof, which may be made of sheet-iron or of wood well braced together to prevent it altering in shape, is built up on a strong ring which runs on wheels placed a few feet apart round the circular wall, or, instead of wheels, cannon balls may be used, rolling in a groove with a corresponding groove resting on them. A small roof, if carefully made, may be pulled round by a rope attached to any part of it, but large ones generally have a toothed circle inside the one on which the roof is built, or this circle itself is toothed, so that a pinion and hand-winch can gear into it and wind it round. If the roof is conical in shape the aperture on one side can be covered by two glazed doors, opening back like folding-doors; but if it is dome-shaped, the shutter is made like a Venetian blind or revolving shop-window shutter, and slides in grooves on either side of the opening. [Illustration: FIG. 151.—Dome. ] [Illustration: FIG. 152.—Drum. ] [Illustration: FIG. 153.—New Cincinnati Observatory—Front elevation, showing exterior of Drum. ] [Illustration: FIG. 154.—Cambridge (U.S.) Equatorial, showing Observing Chair and rails. ]