The Dome
This circular steel building is capped by a circular plate supported by massive girders on which is placed a curved railroad iron rail turned and adjusted perfectly level and circular. The hemispherical dome turns on this rail on 24 massive wheels mounted on roller bearings and is rotated by an electric motor mechanism operated by reversing switches on each side of the south pier. The framework of this dome, which is 66 feet in external diameter and about 38 feet high, the lower 5 feet being cylindrical, consists essentially of a circular base to which the bearing wheels are attached and two double, very deep and rigid main ribs, one of which can be seen in Fig. 3, 16 feet apart in the clear and extending parallel to each other entirely across the dome. These are united 6 feet beyond the zenith by a cross girder and this opening, which can be revolved to any azimuth and can be closed by double motor operated shutters, enables the telescope to observe at any part of the sky. Auxiliary circular ribs reaching from the base ring up to the main ribs all around the dome except at the shutter opening, form the support for the double steel covering of 12 inches separation. This ventilating space is united through suitable weather guards with the space between the building walls and forms the protection against overheating by the sun in the day time.