These are the huge cable wheels of the Eiffel tower main elevator from the 2nd to the 3rd level. It is the kind of mechanism which could have been easily hidden, but which Eiffel decided to proudly keep in view.

These things are mesmerizing - when you are waiting for the elevator they spin at high speed in absolute silence, the only way to describe them is as something out of a Jules Verne book or H G Wells novel.