Mercury and lighthouses (A Heavy Metal question)

OK: here’s my problem. The National Historic Site I work at has the opportunity of receiving from the Canadian Coast Guard a complete lighthouse lantern housing and a first-order Fresnel lens.

This monster is huge; the lens is a good 12 feet tall, weight several tons, and the lantern housing must be pushing 26 feet tall. It’s fully working, with a surprisingly small electric motor rotating the lens around its 500-watt bulb light source.

Here’s the rub: these 19th-century wonders are only able to rotate by floating the lens assembly on a giant tub of mercury. This was one reason that lighthouse keepers (and hatters!) used to go a bit funny, as inhaling heavy metal fumes tends to have a bad effect on your central nervous system.

Parks Canada will not consider allowing us to accept an artifact with such a ticking time bomb of dangerous material as a component essential to operation.

We could remove the mercury and display the light static, but it would be a much more effective interpretive experience to have the light rotatting (besides, it would be really cool, and the other National Historic Sites would be jealous). Changing the mechanism to giant rollers or bearings is out, as our Cultural Resource Management ethics do not allow us to intrude unduly in the historic fabric of an artifact.

So, Dopers: is there a non-toxic substance with the same density as mercury that might be a substitute?

Do you really need the same density as mercury, or just dense enough to float glass? I doubt that there is anything else that dense and still liquid, but there are probably liquids that could float glass. It would change the fluid levels, and the lens would float lower, but if these changes aren’t objectionable you might have a chance. I don’t think there is a direct substitute though.

“If ignorance were corn flakes, you’d be General Mills.”
Cecil Adams
The Straight Dope

I have no idea what to suggest, but I would just like to say that you, Rodd, have the coolest job that I’ve read about in a long time. I can’t remember the last time I had to fix the light bulb in a light house! :slight_smile:

La franchise ne consiste pas à dire tout ce que l’on pense, mais à penser tout ce que l’on dit.
H. de Livry

Wow, a first order. The largest I’ve ever seen in the flesh was a third order. You win the mine is bigger than yours contest. :slight_smile:

There is probably more than the glass to float, isn’t there Rodd ? A metal framework maybe ? Anyhow couldn’t you simply use water with a platform of high density foam or plastic that is filled with air or maybe a lighter than air gas?

Wouldn’t a static display be intrusive in its own right? Perhaps a combination of a bearing in a lubricating bath that mimics the mercury would work aesthetically. This setup could still use a motor of a reasonable size and would give off less of a rumble than it would with the bearing supporting the whole weight of the lens. Pointing out that the original setup could not be used would be educational too.

FWIW, Whatever you use, I’d confirm that the material will not damage the quartz, just to be safe (the material(s) may react to become acidic.) You might want to call a lensmaker about this.

I don’t get it.

“It is lucky for rulers that men do not think.” — Adolf Hitler

Thanks for your thoughts so far!

The artifact can be thought of as being in two parts:

  1. the first-order Fresnel lens (which is composed of a large brass framework holding several hundred separately cut prisms, and four ‘bullseye’ circular lenses) and its supporting stand, which includes the giant tub of mercury and rolling gear; it is similar, but by no means identical, to this one:

  2. the lantern structure, a red metal cupola with huge glass panes 360 degrees around, which stood at the top of the lighthouse tower. Here’s the abandoned tower today, at the northern tip of Vancouver Island:

To complicate matters, the lens/lantern combo on offer is from two different lightstations: the lens is from Estevan Point,
while the lantern structure is from Triangle Island (above).

We are looking for a tech solution to the matter of finding something to replace the mercury with that will allow us to take the whole structure and keep it rotating (although we have to keep curtains drawn in the windows for most daylight hours, as there is a serious risk of starting fires!).

Our second-best option is to put just the lens on display, and mount it on some form of heavy-duty turntable to give it rotation. This would mean putting the lens inside a building.

And cornflakes is quite correct, this is an intrusive method, however it would allow us to retain the rotation–which is really a primary function of the artifact, and necessary to understanding its operation.

Wow, that’s beautiful.

Judging from the fresnel picture, the mercury tub is a narrow, deep well. Could you extend the well and the submerged section of the lens assembly and place the added sections below the display floor? I’m sure that this would not be acceptable if the tub had to be cut, but it might work if the bottom was a bolted on flange. The lens extension might be fitted and clamped onto the original piece.

Wow, that IS a cool job. If you don’t mind me asking, what does your average workday entail?

On a different note…Its funny remembering when this kid in my elementary school class brought in mercury for show n’ tell and we all played with it in our bare hands. These days that would be like bringing in a chunk of asbestos or a piece of radioactive material or something.

That’s an interesting and inventive idea, cornflakes…we hope to put the lens on display in a new visitor centre (not even designed yet, which means it is easier to get something like you mentioned added!). I suspect that the spectre of rising fumes from the mercury would still have to be addressed, as well as some kind of earthquake proofing, as we are very close to a faultline here and have little rumblers all the time.

Thanks, voltaire! I really enjoy my work; I am very thankful that I have found a career in my field of interest (mostly military history, but I’ve come to love the lighthouse as well).

I’ve been at this historic site since 1980, when I started as a summer student. I feel like I’ve become one of the permanent exhibits, since kids I gave tours to in 80-81 are now coming back with their kids!

After 18 years of being a interpreter (guide), I now look after our collection of artifacts, ranging from small brass buttons from the British Royal Marines to gun barrels weighing 12,000 pounds, and of course, the lighthouse.

Since there are only 8 staff to look after 40 acres and about 60 historic structures, we all do a little of everything. Last week I helped scrape and paint three huge barrack-rooms, helped cut up tree limbs downed in a windstorm, located a supplier of WW2 military tires in Tennessee for our mobile 1942 AA gun, and attended a meeting with the Department of National Defence, who want to tear down some beautiful historic buildings in the next-door navy yard (we’re trying to convince them otherwise). Three weeks ago I was in a War of 1812 fort in Niagara, learning how to fire muskets and smoothbore artillery. In other words, a little of everything: but it ain’t boring!

Two things I’d look into (not being an engineer, so I don’t know that these are impractical):

  • Since it is an inside display, suspend it from a beam above the lens instead of floating it. If the top is not strong enough to support the weight, you might be able to design a fairly unobtrusive web or sling with nearly invisible wires running along the eight dividing columns at the four corners.

  • I have no idea how far development has come, but you might see who (if anyone) is producing buckyballs (buckminsterfullerene) commercially. If buckyballs are still not in commercial production, see if any universities or science companies are interested in trying to design something to help you. (I have no idea if this is remotely applicable, but buckyballs were originally touted as supreme lubricants.)


Could you replace the mercury trough with a solid running surface of Teflon, and maybe soup up the motor that rotates the lens to account for more drag? Probably some sort of metal edging around the bottom of the lens would be needed to do the actual riding on the Teflon. Dunno how fast the Teflon would wear down, but this scheme probably could be made so as not to change the appearance of gismo significantly.

If that doesn’t work, how about maglev? :wink: OK, so I guess that couldn’t be hidden very well and might exceed your budget by more than a bit.

Ray (Don’t ask me; I was an EE, not an ME.)

I’ve used solvents which are about 2g/cm^3, but mercury is over 13 g/cm^3! Even if you found something around 6 g/cm^3, you’d still have to double the volume of the float.

In any case, why is it less intrusive to replace the fluid (and possibly increase the float volume) than to add a set of rollers?

** EUREKA! ** (I know, I don’t-a smell too good myself!)
I’m surprised that a Houston doper hasn’t mentioned this one!

Outside, on the south side, of the Houston Museum of Natural Science (or whatever they’re calling it now that there are butterflies in the planetarium), is a globe. This globe is a sphere of granite, approximately 1.3-1.5m in diameter, with the earth’s features lightly engraved into its surface. The cool thing is that this globe is placed on a concave mount that covers the lower 30-75cm of the sphere and that has an inside radius that’s maybe a millimeter larger than the sphere. Together, the mount and the globe make a ball and socket joint. Water is pumped through the mount to a point directly below the globe, and it escapes by flowing between the globe and the mount to the top of the mount, forming a lubricated plain bearing. The water pressure lifts the 3/4 ton plus globe, without any significant friction. Kids have a blast spinning this big, wet rock.

If the gap between the lens and the tub is of a tight enough tolerance, or if they could unobtrusively be machined/shimmed to an accurate tolerance, you could just pump fluid into the tub at the bottom and recover it at the top. The lens assembly’s center of gravity is probably below the “waterline” (mercuryline?); if this is so, then side loadings on the bearing would be minimal. Pressure inside the tub would be similar to when the tub was full of mercury, so the original tub might be able to be used if it is still sound and if it could be used to get the the bearing gap. The fluid recovery system would have to be designed so that the lubricant doesn’t shoot out and over the exhibit.

I don’t think the fluid choice would be critical. Oil would reduce wear; maybe salt water could be used for its higher density (then again, it may rot the brass.)

Sorry this post is so long, but I think the idea holds promise.

FWIW, in my previous post, the idea of adding to the submerged mass was to reduce the net weight (undisplaced mass) of the lens, not to bury the mercury under the exhibit. I get the feeling that this was lost in the telling. In time, I’m pretty sure that the mercury would either leak or leach out, turning the site into the Canadian equivalent of an EPA Superfund site.

I’m a chemical engineer, and I hate to tell you, but mercury is about the most dense liquid you’re gonna find. There might be some specially created fluids with similar densities, but those are almost certain to be way, way beyond your budget. I’d have to suggest building a turntable, and perhaps including a picture of how the lens was originally mounted.

“I had a feeling that in Hell there would be mushrooms.” -The Secret of Monkey Island

I couldn’t find a picture of the globe on the web. I may have a picture somewhere. I’ll look.
The planetaruim is still there; I just missed it last time. The globe is outside at the butterfly center end of the lobby. Has anyone else seen it?

I’m going way out on a limb here, and this kind of jibes with the buckyballs theory in that I don’t know how far this technology has progressed or if it’s economically feasable, but could you float the lens using some form of a “Super Conductor”. I know the Japanese were experimenting with super conductors and their bullet trains. If a super conductor could make a train “float”, I’m quite sure it could support the weight of your lens.

Dammit, Jim! I’m an historian, not an engineer!

Well, we discussed the possibility of acquiring the lens/lantern at our staff meeting this a.m.

Our budget constraints are such that it looks like our best option is to take only the lens itself, not the lantern housing.

Since the lantern housing and the lens actually came from two different locations, and were not historically part of the same light station, just put together by the coast guard for display purposes, we have no qualms about taking just the lens.

The lens came from Estevan Point lighthouse, which, by the way, was the only Canadian soil attacked by an enemy in WW2: a Japanese I-class sub shot at it with a 5.5 inch deck gun in June of 1942.

Our preferred option right now is to store the lens (and do a little restoration) until our new interpretive centre building is funded, designed and built (which, being government, will probably be three years or so). At least we will be able to design part of the building around this magnificent artifact!

What will most likely be done is the lens will be displayed on a mechanically rotating base, with a light source inside the lens. This will preserve its function, while allowing for close inspection (but no touching!) by the public, and also gives an opportunity for close access by sight and mobility impaired visitors who can’t get out to Fisgard lighthouse. Interpretive panels will explain the optic principle, and also give a history of the lens, and explain our decision to display it in this manner. (Much as Diceman suggested).

The tub o mercury will be thoroughly cleansed, and a thin epoxy coating applied to capture any residue left in micro-cracks, and displayed with the lens, with supporting text contrasting historic and modern sensibilities regarding dangerous materials like mercury.

This is a compromise, sure, but it does not entail any cutting into historic fabric (since the lens is a physically separate piece, and just sits in the mercury tub).

Cornflakes, that Houston Science display sounds incredible: a real fusion of art and interpretation. I wish our funds would run to something as spectacular and effective!

While I wouldn’t recommend anyone inhaling mercury, the connection with hatters is spurious.