01 May 2014

Why did lighthouse keepers go mad?

It wasn't the solitude. The Old Salt Blog explains:
In the 19th century, lighthouse keepers had a high frequency of madness and suicide.  Many assumed that they went mad from solitude and the demands of the job. It turns out it was something simpler and more sinister.

Fresnel lenses were the great lighthouse innovation of the 19th century.  The lenses developed by French physicist Augustin-Jean Fresnel greatly increased the intensity and range of the lighthouse beacon. For rotating lights, just as importance as the strength of the light, however, was maintaining a specific speed of rotation, so that if the chart said that the light flashed every twenty seconds, the light, in fact, rotated so that the light was visible every twenty seconds.  The best near zero-friction bearing of the day was created by floating the light and the lens on a circular track of liquid mercury.  When dust, dirt or other impurities built up in the mercury, part of the light house keeper’s job was to strain the mercury through a fine cloth.
For an exhaustive academic study of the subject, see Lighthouse Keeper's Madness: Folk Legend, or Something more Toxic? in the Proceedings of the 11th Annual History of Medicine Days at the University of Calgary.

The last operating lighthouse Fresnel lens in the United States is in the Split Rock Lighthouse near Two Harbors along Minnesota's famed North Shore of Lake Superior (top photo).

Their blog entry from 2009 describes the extreme precautions taken nowadays when the lens needs to be serviced:
Nearly two gallons of mercury was drained, and the mercury bowl and float cleaned, and the mercury replaced.  The very small surface area of mercury that is exposed to the air when the float is closed was covered with mineral oil to stop any mercury exposure to the air.


  1. I know of at least one allegedly "mad" former lighthouse keeper, but I think it was either tourists or the government overseers who drove that person crazy, and not either the no seeums or the mercury. It could be just a legend, however..

  2. Extended solitude is also hard on the normal psyche: solitary confinement is judged to be torture, while confinement with murderers is not. People who choose solitary work as lighthouse keepers might already be selected for being unhinged.

  3. Mercury wasn't used in lighthouses until the last decade of the 19th century. There is a lot of 'heresay' passed on as fact regarding this.

  4. For a while, I understand the president appointed lighthouse keepers due to wartime reasons. I doubt therefore if someone would be "selected" as a lighthouse keeper based on an opinion that they were unhinged. Some had a wife and children living with them. In the south, some died due to malaria. If they were in an isolated assignment, maybe alcoholism was a factor due to boredom. Lots of speculation for sure.

  5. This article has many inaccuracies. First, no US lighthouses had mercury floats until about the time of WW-I. Any 19th Century madness of lighthouse keepers in the US could not have been due to mercury poisoning from mercury floats. The mercury float was the best bearing for the heavy fresnel lenses, but prior to implementing them, various bearings of wheels or roller bearings were used. Even once mercury floats were in use in the 20th century, filtering the mercury was an uncommon task. What was more common was to top off the mercury to replace what had evaporated over a long period of time, presenting risks of spills in handling or storing mercury. Since the lighthouse lanterns were ventilated for letting out the fumes of non-incandescent light sources, there should not have been any significant buildup of mercury vapors in the lantern or tower. Second, there are a great many original fresnel lenses still in use today in lighthouses in the United States, not just in museums. What is unique at Split Rock is the mercury float. All of the other mercury float bearings were drained in the 1960's and either replaced with other types of bearings, or obsoleted altogether as newer lighting technology replaced fresnel lenses.

    1. You make some good points, but just to be clear, the article was not addressing the madness of lighthouse keepers as an American problem. The research was performed at the University of Calgary and published in their proceedings. The first Fresnel lenses were put into use in the 1820s, and mercury flotation was instituted at Cape Le Heve, France in 1893. I don't dispute any of your points, but I don't see any inaccuracies when the History of Medicine proceedings are read carefully and not extrapolated to modern times.


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