This dome, although it has 250 square feet of surface, only weighs 250 pounds. That at the Cambridge (Massachusetts) Observatory, 29 1/2 feet in diameter, weighs 28,000 pounds.
The slit or opening is much shorter than usual, only extending half way from the base towards the summit. It is in reality an inclined window, 2 1/2 feet wide at the bottom, 1 1/2 wide at the top, and 4 feet long. It is closed by a single shutter, as seen in Fig. [37], and this when opened is sustained in position by an iron rod furnished with a hinge at one end and a hook at the other.
Fig. 40.
The Dome Arch.
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.
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.
Fig. 41.
A Dome Clamp.