Mercury Floatation System With No Bearings

It is common practice for lighthouses with large Fresnel lenses to use mercury baths as a low-friction rotation mechanism.

The mercury bath consisted of a donut shaped basin in which a relatively small amount of mercury was placed. It had been discussed since the Early 1800s but wasn't put into practical use until the 1890s apparently. From what l've found, many of these lighthouses seem to be from our lost past.