A Bewildering Barometer

I bought an old aneroid barometer at a local estate sale many years ago.  It still works well.
Full barom_0723 (2)The convex glass cover was cracked so I replaced it with a piece of very old window glass, after slowly drilling a hole for the pointer.

The aneroid mechanism became popular in the early 1900s.  It uses a sealed and flexible bellows chamber that changes size with variations in atmospheric pressure, instead of the mercury filled fragile glass tubes of older designs.

I thought the case must have been changed at some time because the face references a thermometer (“Thermomètre selon Réamur”) and yet there is none present.  There are no markings on the wood case other than a cryptic “#61” engraved in the back surface. But the dial is very interesting:Dial_0721 (2)

For many years I wondered, and have repeatedly asked guests, what the scale reading from “28” to “31”, and in divisions of 1/12s, could represent?  It must surely be inches of mercury – sea level atmospheric pressure is about 29.5 inches of mercury – but why would the scale be subdivided into twelfths instead of the usual tenths?  None could explain.

The language on the face is surprisingly all in French, although the fine print says “P. F. Bollenbach” and “Barrington, IL”– not a known US francophone location.

At last Philips’ friends, Geoff and Dave, with a little help from King Google, have cracked the code: Before Napoleon’s time, and France’s great conversion to the metric measurement system, it seems the French used an inch measure, called a “pouce” (not the similarly sounding “puce”. That is a French flea!) which they subdivided into 12 “lignes”.  A ligne began about 1,200 years ago with German button makers and was “…the measurement of a round wick, folded flat…”.  It is still used today by some button and snap makers, and a few French and Swiss watch people, according to Google.  Around the same time England was actually dividing their “inch” into 10 subdivisions.  The English inch was then defined as 3 medium size dry barley corn grains laid to end to end, but that turned out to be about 12.6% longer (depending no doubt on the year’s harvest!) than the French inch.

A recent estate sale (it’s hard work being retired!) yielded a fine 1969 Nicholas Goodison 388 page book, “English Barometers 1680-1860”, for a few dollars.  It shows a 1772 Ramsden mercury barometer with a dual scale of quote “..both English and French inches divided into 1/10in. and 1/12in. (i.e. 12 ’lignes’) respectively..”

Ramsden 1772_0728 (2)

The other scales on it are Fahrenheit and Réamur thermometers and a conversion scale.

The earliest example of 1/12 divisions that I can find is the scale on this beautiful Robert Hooke 1665 wheel barometer. The scale here somewhat mysteriously reads “M, N, O, P” for the main divisions, but each gap between letters is subdivided into sixths and twenty-fourths.  Hooke was not French but he did come from the Isle of Wight so perhaps there was a little vin rouge nearby to help his studies?
Hooke Wheel 1665_0725 (2)
The final evidence comes from eBay where a few hundred $ might get you this very fine 1749 Louis XVI instrument.  It also has 1/12 divisions in the scale.Ebay old Barometre 27-29 12 div_0584 (2)So it seems that very old French barometers used the 1/12 divisions when most of the English ones were using the 1/10 parts of their fine scale.  My, perhaps 50 to 100 year old, Illinois instrument appears to have used French wording and one twelfth divisions to give an antique air to a modern aneroid mechanism.  I note too that the face is simple printed paper rather than the engraved metal of genuine antiques. I shouldn’t complain, Goodison’s book says that for accuracy an old mercury barometer needs periodic maintenance by “boiling” (sic) the mercury to remove absorbed water and oxygen!

Isn’t it ironic that despite being partly decimalized before continental Europe (as shown by the tenths divisions on their old mercury barometers) England stubbornly held on to their colorful, but so confusing to me in my school days, non-decimal: fathoms, firkins, furlongs, fortnights, farthings, etc., etc.  (Did you know there are about 5,600 “scruples” in one “strike”, whatever they may be measuring?).  France dropped it’s “lignes” in Napoleon’s time and went metric, or so they claim.   But they have yet to fully adopt the ‘true’, internationally agreed decimal system, “S.I.” (System International).  Although France does agree with the rest of the world that the current inch is now exactly 25.4 mm, many French people will insist on writing it as “25,4 mm”  This can be very confusing if you want to write a dimension of say 1 meter plus 1/4 millimeter (or metres and millimetres depending on whereabouts you happen to float in the Atlantic ocean) in millimeters (thank you very much David for pointing out the ‘mm’ omission) into an international technical drawing:  in SI it should be written “1 000.25 mm”; in France it is often written “1.000,25 mm”; and here in the US it is typically shown as “1,000.25 mm”.  So no wonder that international space probe crashed into Mars a few years back, while trying to land, because its computer thought the planet’s surface was further away than it really was! When flying above the surface of planet Earth it is very important to know what your barometer is actually measuring because that is the instrument which gives you your height above ground.  On the ill-fated trip to Mars I imagine other types of instruments, than mercury filled barometers, were used but sadly they did not give correct final values!

2 thoughts on “A Bewildering Barometer

  1. Very interesting. Units seem to be more complicated than one might think–a curious mixture of historical chances and fundamental principles. I believe the barleycorn unit is somehow related to shoe sizes to this day.

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