Gustave Eiffel also designs the dome for…

Gustave Eiffel also designs the dome for the Astronomical Observatory in Nice in 1886.

This is the most important building in a complex designed by Charles Garnier, who will later be among the most prominent critics of the Eiffel Tower.

The dome, with a diameter of twenty-two point four meters (seventy-three feet) is the largest in the world when built and uses an ingenious bearing device: rather than running on wheels or rollers, it is supported by a ring-shaped hollow girder floating in a circular trough containing a solution of magnesium chloride in water.

This had been patented by Eiffel in 1881.