The principal peculiarity of the dome, the means by which it is rotated, remains to be described. Usually in such structures rollers or cannon balls are placed at intervals under the edge, and by means of rack work, a motion of revolution is slowly accomplished. Here, on the contrary, the whole dome _b b′ b″_ (Fig. 40) is supported on an arch _h h′ h″_, carrying an axis a at its centre, around which a slight direct force, a pull with a single finger, will cause movement, and by a sudden push even a quarter of an entire revolution may be accomplished. It is desirable, however, to let it rest on the edge _b b″_, when not in use. At _c_ there is an iron catch on the arch, by which the lever _e_, that raises the dome, is held down. The fulcrum is at _d_. The lever is hinged near _c_, so that when by being depressed it should have come in the way of the telescope below, the lower half _g_ can be pushed up, the part from _c_ toward _d_ still holding the dome supported. [Illustration: Fig. 40. The Dome Arch.] The arch can be set across the observatory in any direction, north and south, east and west, or at any intermediate position, because the abutments where the ends rest, are formed by a ring _l l′ l″_, fastened round the circular aperture, through the stationary part of the roof. When the telescope is not in use, and the dome is let down, so that there is no longer an interval of a quarter of an inch between it and the rest of the roof, it is confined inside by four clamps and wedges. Otherwise, owing to its lightness, it would be liable to be blown away. These clamps _a_, Fig. 41, are three sides of a square, made of iron one inch square. They catch above by a point in the wooden basis-circle of the dome _b_, and below are tightened by the wedge _c_. [Illustration: Fig. 41. A Dome Clamp.] When the dome is raised it is prevented from moving laterally and sliding off by three rollers, one of which is seen at _f_, Fig. 40. These catch against its inner edge, and only allow slight play. At first it was thought necessary to have a subsidiary half arch at right angles to the other to hold it up, but that is now removed. All the parts work very satisfactorily, and owing to the care taken to get the roof-circle and basis-circle flat and level, no leakage takes place at the joint, and even snow driven by high winds is unable to enter. c. _The Observer’s Chair._ This is not a chair in the common acceptation of the word, but is rather a movable platform three feet square, capable of carrying two or more persons round the observatory, and maintaining them in an invariable position with regard to the telescope eyepiece. [Illustration: Fig. 42. The Observer’s Chair.]