Big rotating lenses were floated on a bath of mercury from the 1890s onward. The float carries several tonnes with so little friction that a clockwork drive and a falling weight can turn the whole assembly, and the period stays steady enough to time a flash character by. The mercury is also why decommissioning these towers is expensive.
Fl(2) WR 10s unpacks as: two flashes per ten-second cycle,
shown white in some sectors and red in others. The sectors are cut into
the lantern glazing, so the colour tells you which side of a hazard you
are on without any instrument at all.
Many minor lights now do exactly that. The rotating optic survives where range matters: concentrating a fixed lamp into a narrow rotating beam buys far more intensity than flashing an omnidirectional source, for the same power budget.