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Shockingly, however, spending most of your time in close proximity to 500 pounds of liquid mercury is Not Great For One’s Health and tons of lighthouse keepers started to go crazy from the whole. Mercury poisoning thing. Hence why there are a lot of “haunted” lighthouses or wickies that lose it and maybe do a bit of manslaughter.

In a letter written on April 19, 1825, Augustin Fresnel proposed the use of mercury to reduce the friction in revolving lenses. His statement follows: “I propose to float our rotating devices, of the first order, in a bath of mercury, instead of placing them on rollers. This project won't present many difficulties; nevertheless, as I have not put it into execution, I won't require you to adopt it for your first lighthouse.”

Fresnel’s plan for mercury flotation was not put into practice until 1890 when Monsieur Leon Bourdelles, Chief Engineer of the French Lighthouse Service, designed and built a workable mercury flotation system. The mercury bath allowed the lens to operate in an almost frictionless environment and, additionally, allowed the speed of rotation to be dramatically increased.

It mattered for optics reasons.

Under less-than-ideal conditions, you can only see the beam when it’s pointed more or less directly at you. In-between beams you would not be able to see anything. One solution to this was to create multiple beams, and the lenses Mr Fresnel designed usually created 8 beams. But, even still, duration between flashes could be as long as one minute in the old mechanical roller systems.

The nearly frictionless operation of the Mercury suspension system allowed the lenses (large pieces of precisely ground glass weighing several hundred pounds in some cases) to rotate fast enough that they could be redesigned to create fewer (usually 3) beams. Fewer beams from a similar light source will be proportionally brighter, and the gains in speed were sufficient that duration between flashes could still be reduced to as little as 10 seconds.

This was a big upgrade. It didn’t just make the lighthouse signal faster, it allowed them to completely overhaul the lens and derive more visibility from a light source.

And me.