Mercury in the Bath

August, 2017: the sun is shining on the edge of North America, but the foghorn still cries its warning to the clouded sea. Two paired blasts per 60 seconds; a signature to identify location by sound. In the dark, you need to know which horn you’ve heard.

This is Cape Race lighthouse, which holds one of the last remaining hyper-radial Fresnel lenses ever made. Perched on the southern coast of Newfoundland, it offers the first sighting of land for ships crossing the Atlantic. Oh, and it’s got nearly 800 pounds of liquid mercury at the top.

Part 1: sound

The foghorn sounds again, reaching across miles. There’s been a lighthouse with a proper foghorn here since the 1850s, replacing earlier wooden structures and informal warning bonfires. Cape Race has featured on European maritime maps since the early 1500’s, famous both as a landfall beacon and for its many shipwrecks. What an ending to a perilous voyage: making it all the way across the ocean and promptly crashing on the first coastline you meet.

One day many years ago a man walked along and stood in the sound of the ocean on a cold sunless shore and said, “We need a voice to call across the water, to warn ships; I’ll make one. I’ll make a voice that is like an empty bed beside you all night long, and like an empty house when you open the door, and like the trees in autumn with no leaves. A sound like the birds flying south, crying, and a sound like November wind and the sea on the hard, cold shore. I’ll make a sound that’s so alone that no one can miss it, that whoever hears it will weep in their souls, and hearths will seem warmer, and being inside will seem better to all who hear it in the distant towns. I’ll make me a sound and an apparatus and they’ll call it a Fog Horn and whoever hears it will know the sadness of eternity and the briefness of life.”

Ray Bradbury, The Fog Horn

The sound of a foghorn is eerie, haunting, lonely. It’s no wonder Bradbury imagined it waking an ancient monster, the last of its kind, and the encounter he depicts in The Fog Horn is heartbreaking. His story inspired the movie that in turn inspired Godzilla, though they took a rather different approach from the end-of-a-species tragedy he wrote.

Cape Bonavista foghorn signature: one 3-second blast every 30 seconds. Horn points 355°.

The first foghorn at Cape Race was a steam whistle, replaced in 1907 with a 5-inch diaphonic horn powered by a steam compressor. Developed by the maker of the Wurlitzer organ, the horn was based on a kind of organ stop (one they had also recently invented). It used compressed air to drive a piston and open/close two valves to deliver an exceptionally loud high tone, followed by a short low-toned “grunt” as the valves closed asynchronously. That grunt was audible to sailors at greater distances than the high tone, often even when the main blast blended in to background noise from the ship or storm. In 1963, the Canadian government replaced the steam compressor foghorn at Cape Race with the diesel model still in use today.

Cape Race foghorn signature: two 3-second blasts per minute, with 3 seconds of silence in between. Horn points 135°.

Fog signals of some kind have existed for centuries. The original options were a bell or gong, struck by hand or using a wind-up weight mechanism similar to a grandfather clock. Later, explosive foghorns were common; basically a tiny cannon that the lightkeeper would fire at regular intervals during fog. This method sounds archaic today, but it persisted long after the arrival of steam whistles and horns as some authorities were suspicious of those newfangled devices. They believed the sound from explosive signals carried further, though opponents said the brief duration of the sound made it difficult to accurately identify the direction.

Trinity House (the British lighthouse authority) was one of the holdouts, and kept an explosive signal active at Beachy Head Lighthouse in southern England until 1976. This required the lighthouse keepers to attach two small pieces of nitrocellulose and detonators to the arms of a device, then raise it up with a winch until it made contact with an dynamo-powered firing mechanism inside the lantern at the top, causing the explosion. They did this, by hand, every five minutes anytime it was foggy, for as long as the fog lasted.

Preparing the explosive and winching it into place every few minutes (the exact interval depending on the lighthouse, for a distinct marker), sounds exhausting, but horns and sirens were even more labor intensive. At Cape Race, coal and other supplies were brought in by boat, then hauled up to the lighthouse. Starting the engine was a two-person job, and it had to be continuously fed with coal while it ran—in August 1882 alone, lighthouse keepers shoveled 26 tons of coal to power the fog signal. And when the engine wasn’t running, it had to be cleaned, oiled, and maintained so that it would be ready to go at the next sign of fog. One book describes the less-than-idyllic nature of a lighthouse keeper’s job this way:

Then, there was winter. The persistent lows made the lamp burn poorly, and its thick, black soot readily darkened the glass, the mirrors, and the prisms, every night. Cleaning it all wasn’t easy; often the soot flaked and drifted down the tower and covered the table where you ate and lived. When the drums of coal oil didn’t arrive on time, you sat in darkness by your bedside lamp, giving favour to the all-important revolving beam or to the diesel engines, whose compressors must have air for the foghorn’s lament. If the horns quit their groaning, you hand-worked them in the wet and hoped against hope that all was well out there, fearing to think what an offshore light might mean. As you ran low on coal, gale after gale blew through the chinks and crevasses of your damp little dwelling.

Johnson and Walls, To the Lighthouse

This whole process was also very expensive; that’s a huge amount of coal, not to mention the labor involved! Other options also had costs, though: a fog cannon used at Point Bonita Lighthouse in San Francisco went through 5,560 pounds of gunpowder in its first year of use. In the second year, the cost of gunpowder was three times the salary of the keeper employed to fire it, and it was discontinued. Horses have also been used to power compressed air fog signals (their poor ears), but then you have to feed and maintain a horse or two.

Besides the cost of the fuel for the light and the fog signal, you have maintenance and labor costs to consider. Lighthouse keepers in remote or offshore locations in Ireland worked in trios: three men stationed at the light at a time. They would work in four hour shifts, rotated each Sunday for variety: four hours on, eight hours off, four hours on. That means the same keeper would tend the light from, say, 2am–6am, then again from 2pm–6pm. The wicks had to be trimmed and the clockwork mechanism that controlled the lens rotation had to be wound throughout the night, typically every three hours. In some cases that meant manually hoisting a weight—The one at Cape Race weighed 300kg—which would then slowly tick down the tower to turn the lens.

A key part of the job when on duty was watching for fog, and when it appeared, the shifts were doubled. One keeper would sound the foghorn while the other tended the light. After four hours, the light tender would rotate off duty, the foghorn keeper would switch to the light, and the third keeper would take over the foghorn. During prolonged foggy spells, keepers would work 16 hours a day, sometimes for days at a time, with only four hours to rest and eat between each shift. Oh, and did I mention they often traded off sleep shifts in a single bunk? There isn’t much space at many lighthouses.

Back to the fog signals; remember how I mentioned that the explosion occurred inside the lantern? As you may be aware, explosions and glass aren’t the greatest of friends. One of the major drawbacks of the various explosive fog signals was their worrying tendency to break the glass lenses that made the “light” part of the lighthouse work. There were other downsides, too: the explosives used were somewhat unstable and dangerous to store in the quantities required, and groups like the IRA in Ireland regularly raided lighthouses for their explosives.

So, foghorns: less likely to break the important glass bits, safer, and easier to automate. Why did Trinity House insist on explosive signals instead? Early experimentation showed that the sound from horns and sirens were more sensitive to disruptions caused by ocean waves and by temperature fluctuations in the atmosphere. While the sound of a horn or siren carried beautifully under the right conditions, under the wrong circumstances it was considerably less reliable than that of an explosion.

The propagation of sound is not a constant, especially during periods of fog. Cold and warm layers of air cause sound to deflect, skip, bounce, echo and sometimes stop cold. The sound from a fog signal might be heard at one mile, not at two miles and again at three. A mariner thinks he hears a fog signal from one direction, when in fact it originated some 45 degrees to the right…or 30 degrees to the left. And distances cannot be determined with any accuracy. Not only is it hard to tell the distance one hears a signal, it is impossible to predict that a signal can be heard at a certain range. A signal “rated” for four miles might be heard at only two miles or, given the right atmospherics, 8 miles.

From The History of Fog Signals by Wayne Wheeler

Explosions may be slightly more reliable, but they’re a bit less evocative than foghorns. Just ask Alvin Curran, an avant-garde composer who did a live-mixed foghorn concert for radio in 1982. Sadly, the recording was lost, but I wish I could have heard it:

The live Foghorn concert was broadcast on May 22, 1982. It was directed by the American composer Alvin Curran . Reporters recorded the sound of nine fog horns along the coast of France, Belgium, the Netherlands and Germany, namely in Emden, Calais, Nieuwpoort, Scheveningen, Den Helder, Lelystad, Urk, Marken and Kornwerderzand. Via line connections the sound came into the studio in Hilversum, and Alvin Curran made the live composition “The Fog Bank” of these foghorn sounds.

VPRO radio archive, Netherlands

We can get an idea, at least: two years later Curran recorded a series of ten environmental compositions for radio featuring foghorns, bells, nature and maritime sounds, and contributions from a range of musicians and sound artists including Pauline Oliveros and John Cage. These were later released as a two-disc set titled Maritime Rites (see links at the end).

Not everyone agrees with me and Curran about the haunting, “eternal symphony” sound of foghorns. In 1905, a writer for the New York Herald gave a slightly less enthusiastic description:

…a screech like an army of panthers, weird and prolonged, gradually lowering in note until after a half a minute it becomes the roar of a thousand mad bulls, with intermediate voices suggestive of a wail of a lost soul, the moan of a bottomless pit and the groan of a disabled elevator.

As quoted by Shona Riddell in Guiding Lights

Foghorns are slowly disappearing around the world; the last ones were decommissioned in Ireland in 2011. While ships at sea are required by international law to signal their position in fog with a horn (while moving) or a bell (at anchor), GPS has largely taken over for stationary fog signals. Lighthouses, however, remain, and for now at least, most of them still cast their beams out over the sea.

Part 2: light

I love a lighthouse. Lonely, stormy, windswept; set in remote and by definition dangerous places, a bastion against the darkness and a guardian for ships. I think of cold shipwrecks, all hands lost on distant rocky shores, and I think of sailors off-course in storms, terrified and desperate, and that first glimpse of a welcoming light playing across the waves. On balance, I find lighthouses quite comforting, in all their range of characters.

Every lighthouse is different, but being purpose-built, they have commonalities as well. They’re often round or rounded, to reduce the impact of wind and waves on their structure. They need to be tall so that the light can be seen from a greater distance, but not so tall they get lost in the mists—some lighthouses were initially built on clifftops, and later moved down because they were constantly obscured by clouds. They’re painted brightly so as to be visible from a distance, often mostly white to stand out against a darker coastline. And even though many have been decommissioned as systems like GPS improve, there are still a lot of them:

If you take the road north, go to the outermost point till the land stops and there is nothing but the broad dark horizon. Then go a little further. It doesn’t particularly matter which road or which corner of Scotland you choose. Somewhere out there past the back of beyond will be a neat white wall, a few wind-scoured cottages and a tower. Seen from above, they look like punctuation marks between land and sea, a ragged grammar of full stops marking the end of Britain.”

Bella Bathhurst, The Lighthouse Stevensons

That quote is no exaggeration: the Northern Lighthouse Board (Scotland’s authority) is responsible for a network of over 200 lighthouses. That’s a huge number, and amazingly, most of them were built by three generations of one family of civil engineers, the Stevensons.

If the name sounds familiar, that’s probably because of author Robert Louis Stevenson. He was a less engineering-inclined member of the family, and his book Treasure Island was inspired by time spent playing on islands like Fidra while his father, Thomas, built lighthouses on them.

An astonishing amount of ingenuity was necessary to make a light bright enough to penetrate a stormy night at sea, especially given the time period. Most lighthouses predate electricity, so first you have the problem of a consistent fuel source for a steady, reliable flame. A flame by itself isn’t visible from very far, though (unless you’re building a bonfire every night, which was a solution in ancient times but isn’t the most sustainable option). That means you need a way to focus the light to send it further. Once you have a strong focused beam, that’s nice, but then you’re only sending it in one direction, so next you need a way to rotate it so that it will be visible in all directions.

You also need a way to differentiate lighthouses from one another, particularly on challenging rocky coastlines with multiple lighthouses in relatively close proximity. If a sailor can tell them apart from a distance, that helps them find their position at sea. That’s why lighthouses are painted in a range of bright colors and patterns, called a daymark; to distinguish them quickly at a distance.

Bright colors only work on a clear day, of course, so you need a solution to make the light itself unique as well. Coloring the light is one option, but most color filters degrade the strength of the light, and colors can look different under varying atmospheric conditions. Seeing the light from far away is important, so a better solution is to give each light its own pattern: timed flashes, like the timed blasts of a foghorn.

The thing about flames as opposed to electric lights is that it’s hard to make them turn on and off in a reliable pattern. Better to keep the flame constant, and alternately shade and show the light. Enter the rotating lens: focus a beam through a lens, then rotate the lens (or lenses) around the flame to send out spinning beams of light. A light beam passing a stationary object gives the sense of flashing, and as long as you can control the speed at which the lens rotates (as well as the number and location of lenses on the rotating cylinder), you can control the pattern and rate of the flashes. Then you can publish the details of each lighthouse for mariners to use with their charts.

That brings us back to the Stevensons. The first and most famous of that engineering family was Robert Stevenson (Robert Louis Stevenson’s grandfather), who built 15 lighthouses in the early 1800s. Not only did he design and build lighthouses (and roads and bridges and so on), he also invented systems for intermittent and flashing lights.

The rotating lens design sounds fine when we’re thinking about something the size of an oil lamp or a camping lantern. Steady flame, cut some holes in the outer casing and come up with a way to rotate it, no problem. When we reach the scale of a lighthouse, though, and the weight of the massive glass lenses required to broadcast the relatively low-output light sources available in the 1800s, it becomes rather more complicated. The solution dreamed up by the great minds of the day was effective, but had certain… consequences.

Cape Race in Newfoundland has one of the last hyperradial Fresnel lenses in use in the world. Fresnel (pronounced FRAY-nel) was a French inventor who developed complex lenses to focus small lighthouse flames into vertical sheets of light visible over great distances. They were much more powerful than the lenses that came before, and the design allowed for flatter construction with less glass, making them lighter (though still remarkably heavy). Fresnel lenses have a wide variety of applications even today. Not only are they still used in many lighthouses, but also in everything from traffic lights to stage lights to landing lights on aircraft carriers.

The hyperradial lenses made for lighthouses were huge: the one at Cape Race is so large that in order to clean it, you walk into the lens—and you don’t even have to bend over. It’s 17 feet in diameter and the lens alone is about a dozen feet tall, and it contains over 1,000 prisms to direct and focus the light. The whole assembly including the base weighs around 20 tons; a lot of weight to turn at a steady rate, night after night, for years on end.

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.

Augustin Fresnel, April 1825

It won’t present many difficulties, eh Augustin? Well, to his credit, his untested idea worked—well enough that it’s still in use in lighthouses around the world. A 2008 EU study found active mercury mechanisms at 90 lighthouses in France, 12 in Denmark, 6 in Sweden, and more: it estimated between 200-500 such lighthouses in Europe alone, for a total somewhere between 24-125 tons of liquid mercury. A functional mechanical replacement didn’t exist until about 2015, and the process of draining the mercury and replacing it without damaging the lens or poisoning anyone is difficult and expensive. And the quantities are impressive: seven gallons of liquid mercury at Cape Race, and more than 15 gallons at Créac’h lighthouse in France which has one of the heaviest and most powerful lenses in the world.

Floating a giant lens rotation mechanism on liquid mercury reduces the friction to almost zero, allowing it to move smoothly, more quickly than other options, and with barely any effort or wear. It’s great, if you ignore the fact that mercury evaporates when exposed to air, and the minor detail of its extreme toxicity, which has been implicated in research on insanity and erratic behavior among lighthouse keepers. They were not only required to constantly clean the lens (exposing them to mercury vapor), but also regularly clean the mercury itself, by straining it through a fine cloth to remove dust, dirt and other impurities that could impede the rotation of the lens. Oh, and lighthouses in earthquake- or hurricane-prone regions sometimes spilled gallons of mercury from the assembly when the tower vibrated or swayed, cascading down the stairs and lodging in cracks and crannies. Let’s take a moment to imagine the protective gear they were surely (ha!) wearing around all the mercury back then…. Aside from that, though, hardly any difficulties!

Ok, so we have a flashing light with a particular signature, in a lighthouse that’s painted in a distinctive way, marked on nautical charts. There’s one other thing you need to know as a sailor: where are you when you can see it? You can identify the specific light and the direction it’s coming from, which gives you a straight line: you’re somewhere on that line. But how far along that line?

Lighthouse listings contain not only the characteristics of the lighthouse (description, light signal, fog signal, latitude and longitude), but also information about the elevation and range of the light. If you have the following set of information, you can use formulas to determine your distance from the light source:

  • Light nominal range (distance at which the light is visible under good conditions when meteorological visibility is 10 nautical miles)
  • Current meteorological visibility (obtained from weather reports)
  • Elevation of the light
  • Elevation of the eye of the observer

Math: how incredibly useful!

Lighthouses can do more than just warn of a rocky coast. Range lights are pairs of lighthouses placed in narrow or dangerous channels, and help sailors find the safe course at night by aligning the two separate lights. If they’re out of alignment, you’re off course, and you can tell which way to correct based on which side has the higher light. Some lights also use a specific color range, where the lens does not rotate and different portions are colored differently. These are also used to guide a narrow safe path into a difficult harbor; for example, a green light could mean the ship is too far to port, a red light that it’s too far to starboard, and a white light that it’s on the correct path.

Did you know that most modern lighthouses are not only automated, but can automatically replace their own lightbulbs? Many use small, powerful LEDs on a rotating mechanism. When a bulb burns out, the change in current triggers the mechanism to rotate so that a fresh bulb is connected. This is necessary because offshore lighthouses can be inaccessible for days or weeks due to bad weather, which is exactly when a reliable light is most important.

As discussed earlier, lighthouse keeping was hard work. Modern automation is a vast improvement over shoveling coal, hoisting weights and explosives, winding clockwork, and staying awake tending a lamp. And it wasn’t just the time spent on a shining light and sounding fog signal; even on a beautiful sunny day, it wasn’t an easy job. Keepers covered the lens during the day, to prevent tarnish on the prisms and mirrors. They cleaned and polished the glass inside and out daily, removing the soot that accumulated when the lamp burned each night, and clearing away dead birds (mass bird strikes on the glass were not uncommon). They refilled the lamp oil, often filtering it first, and trimmed the wicks. They oiled and maintained the delicate apparatus that controlled the lens rotation and ensured the correctly timed flashing sequence. They cleaned and oiled and fueled the engines that powered the foghorn. Spare parts had to be maintained and replenished as necessary, so that any repairs could always be carried out immediately with materials on hand. They also kept a log of passing ships, and communicated with them using flag semaphore at first, followed by Morse code and eventually radio.

Twice a week, they dismantled the reflectors and carried them down the stairs for a full polish to remove scratches that held soot and dimmed the light. They were also responsible for cleaning and repainting the lighthouse and other buildings regularly, to keep it bright and visible. And aside from the light, they also kept themselves; growing vegetables where possible, keeping chickens and maybe a cow, and attending to fuel for their cooking and heat (each keeper was responsible for procuring their own food and necessities).

At offshore lighthouses in Ireland, each keeper would work six weeks followed by two weeks of shore leave. That meant a boat would visit the lighthouse once every two weeks to swap out one of the keepers and bring in supplies, but of course it was weather dependent. During periods of bad weather, the relief boat was sometimes delayed by days or even weeks, leading to much longer shifts on the rock. And at Bird Rock Lighthouse, 10 miles of the coast of Nova Scotia, access to the summit was by a 130-foot perpendicular ladder, and resupply boats visited only four times a year.

Even when lighthouses are accessible, most authorities have limited resources. For example, Scotland’s Northern Lighthouse Board is currently responsible for 207 lighthouses and 170 buoys, plus radio aids to navigation, over the entire area of Scotland and the Isle of Man…. and they only have two ships. Do you know how long it takes to get from one side of Scotland around to the other side, by ship, in bad weather? Or how long it takes to fetch, clean, repaint, and re-anchor a buoy, or replace one when it’s lost or damaged? I don’t know either, but I can understand why some buoys are only checked about once a year.

And it’s not only lighthouses that need attention; each country’s authority is also responsible for maintaining and listing other navigational aids such as buoys and floats. These can easily number into the thousands, and need to be regularly checked, repainted, and cleaned (the floating ones attract barnacles, which weigh them down and can eventually pull them underwater). Each buoy has a specific character: the color, pattern, shape, and any light characteristic all have very specific meanings that mark dangers or the exact direction of safe passage for a ship. All that detail, plus weekly updates, and still the light listings have to include huge caveats because buoys have a tendency to move around:

The buoy symbol is used to indicate the approximate position of the buoy body and the sinker which secures the buoy to the seabed. The approximate position is used because of practical limitations in positioning and maintaining buoys and their sinkers in precise geographical locations. These limitations include, but are not limited to, inherent imprecisions in position fixing methods, prevailing atmospheric and sea conditions, the slope of and the material making up the seabed, the fact that buoys are moored to sinkers by varying lengths of chain, and the fact that buoy and/or sinker positions are not under continuous surveillance but are normally checked only during periodic maintenance visits which often occur more than a year apart.

…The mariner is also cautioned that buoys are liable to be carried away, shifted, capsized, sunk, etc. Lighted buoys may be extinguished or sound signals may not function as the result of ice or other natural causes, collisions, or other accidents.

For the foregoing reasons, a prudent mariner must not rely completely upon the position or operation of floating aids to navigation…

National Geospatial-Intelligence Agency, Warning on use of floating aids to navigation to fix a navigational position

The next time you see a lighthouse, or a lantern mounted on a little concrete tower, or a buoy floating in a bay, spare a thought for the people who care for that little guiding light and keep ships safe. Even in these modern GPS-enabled times, it’s a difficult and important job.

Further reading

Part 1: sound

Part 2: light

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