It has a central shaft meant for use as a zenith telescope and for use in gravity and pendulum experiments that connects to an underground laboratory for observers to work ( accessible from the present-day ticket booth ).
12.
While a zenith telescope has the obvious disadvantage of not being able to look anywhere but at a small spot straight up, its simplified setup permits the use of a mirror consisting of a smoothly spinning pan filled with liquid mercury.
13.
Another difficulty is that a liquid metal mirror can only be used in zenith telescopes, i . e ., that look straight very large telescope already exists at the South Pole, but the North Pole is located in the Arctic Ocean.
