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Object Overview
Before us is a rare Baromètre absolu — an absolute barometer of French manufacture, made around 1880 to the invention of Hans and Hermary. This is neither a mercury nor an aneroid barometer: atmospheric pressure is determined here by comparing the readings of two thermometers, an ordinary one and an air thermometer. The instrument belongs to a very rare class of meteorological devices that never went into general production and are known today almost entirely from printed sources.
Léon Paul Hans and Hippolyte Alexandre Hyacinthe Hermary were French artillery officers and former students of the École Polytechnique. Hans held the rank of . It was he who obtained patent no. 95809 for the absolute barometer, filed on 2 July 1872. The invention went through the full course of scientific recognition available at the time:
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glass, mercury, paper, steel, wood
All four references — the Académie, the Association, the Exhibition and La Nature — are present on the instrument itself: the first three on the front dial, the last in the text of the instructions on the back cover.
Later the constructor Lespérut completely reworked the form of the instrument, making it circular, with cursors on a concealed mechanism and control knobs around the edge of the frame. The present example belongs to the original rectangular form of Hans and Hermary, preceding Lespérut's revision: here the cursors are moved by hand by means of small tabs, and the scale is linear.
An ordinary thermometer responds only to temperature. A thermometer whose reservoir holds a quantity of air responds both to temperature and to the pressure to which that air is subjected. Comparing the readings of the two makes it possible to separate one influence from the other.
The air thermometer of the absolute barometer is a glass tube, bent twice, with a reservoir at one end. It contains two liquid columns in two U-shaped bends:
Sulphuric acid was not chosen by chance: across the range of temperatures found in a European climate it retains the required fluidity, gives off no appreciable vapour, has no action on the air of the reservoir, and produces perfectly constant capillary effects. Over a century and a half the indigo in the acid darkens, and in surviving examples the column appears almost black.
The essential difference from an ordinary thermometer: the capillary is not sealed. The far end of the tube carries an open aperture connecting the system with the outside atmosphere. It is through this that the pressure of the air is transmitted to the liquid columns, compressing or expanding the gas confined in the reservoir. A sealed thermometer would be insensitive to pressure; an open one becomes a manometer, and in that lies the whole idea of the instrument.
The U-shaped form of the bends was chosen not only for compactness: it allows the maker to use a rapid whirling motion, as with a sling, to drive out air bubbles that might break the liquid columns.
All the elements of the instrument are mounted on a wooden frame entirely covered in leather. The leather is of a fine, close grain and a warm reddish-brown tone, ranging from a light cognac on the flat surfaces to a deep burgundy on the tooled bands, where the dye has been compacted by the pressure of the die.
The outer contour of the frame is stepped: the broad surfaces are broken by symmetrical setbacks on all four sides, with projecting stepped corners. This crenellated silhouette is characteristic of French decorative taste of the early 1880s.
The ornament is executed in blind tooling: several concentric borders of fine lines following the stepped contour; small roulette tracks of dotted beading run along the outer bands; and in the four corners of the inner field are square cartouches with rosettes. The whole design depends on differences of depth and compaction in the material alone.
At the back, the thermometers and their reservoirs are protected by a metal cover painted black, screwed to the wooden ground. Pasted to the cover is a large printed instruction sheet for using the instrument, executed in engraved cursive script, together with a separate narrow slip carrying an additional note on setting the instrument for the altitude of the place. At the top of the cover are two brass hanging loops.
The entire front panel is occupied by a paper dial laid down on the ground. Over it lies a flat mineral glass, set flush with the field of the frame.
In front of the glass, on the outside, are the moving and sighting parts:
The thermometer scale. In the upper part of the dial, "THERMOMÈTRE CENTIGRADE", the centigrade scale of the ordinary thermometer, running from 0 to 45 °C, divided in millimetres and numbered every five degrees. The zero mark is labelled "Glace" — ice, that is, the melting point of ice.
The barometric scale. In the lower part, "Hauteurs Barométriques", barometric heights calibrated in centimetres of mercury, from 79 to 69 (that is, 690–790 mm). The scale is divided in millimetres and numbered in centimetres, and gives the height to which the column would rise in a good mercury barometer at a temperature of 0 °C.
Attention should be drawn to the reversed direction of this scale: the larger values stand at the left, the smaller at the right. This direction follows from the inverse response of the air thermometer, in which a rise in external pressure compresses the confined gas and shifts the liquid column in the direction opposite to the familiar one. The divisions of the scale come out very nearly equidistant.
The weather legend. Below the barometric scale runs the verbal indication: TRÈS SEC (very dry), B.au FIXE (set fair), B.au TEMPS (fine weather), VARIABLE (changeable), PLUIE ou V.t (rain or wind), G.de PLUIE (heavy rain), TEMPÊTE (storm). The order of these terms is likewise reversed with respect to the traditional arrangement on European barometers, where "storm" stands at the left and "very dry" at the right.
The legend is carried on a separate sliding block running in the lower part of the case. On its edge is an altitude scale in metres above sea level: 0, 100, 200, 300, 400, 500, with intermediate strokes every fifty; on the fixed frame beneath it is a tiny brass pointer. By moving the block until the pointer coincides with the altitude of his own locality, the observer brings the whole weather legend into agreement with local conditions. The barometer is regarded as set for a given place when the pointer indicates on the scale the height of that place above sea level.
Other inscriptions. In the middle of the dial, in large letters: BAROMÈTRE ABSOLU. Below it, BREVETÉ S.G.D.G., the standard French formula for "patented without government guarantee", indicating a patent in force. Then: Médaille d'argent à l'Exposition Universelle de Paris — 1878 and Présenté à l'Académie des Sciences (Juillet 1873).
This line is flanked by two shield-shaped cartouches: at the left H.H, the monogram of the inventors, Hans–Hermary; at the right C.C, whose attribution has so far not been established.
The geometrical basis of the instrument can be stated in two sentences. When temperature alone varies, the elongations of the two thermometric columns are proportional to one another; consequently, if the stems of the thermometers are set parallel, the straight line joining the ends of the columns will always pass through one and the same point. When the pressure varies, this point shifts and describes a locus which is precisely a straight line. To each position of the point on that line there corresponds a definite pressure.
The straight line joining the ends of the columns is made material by the stretched thread. The instrument is thus a nomogram made physical: the calculation of the temperature correction has been replaced by a construction which the observer carries out with his hands in a few seconds. The correction factor is embodied in the distances between the horizontal elements, so that the relative heights of the tubes and rods constitute the calibration of the instrument and must not be altered.
Taking a reading:
The need for this manipulation seems a drawback only at first sight. The operation is performed with the greatest ease, and since the setting of the cursors remains until the next observation, the instrument retains the reading of the previous measurement — and so makes it possible each time to judge not only the magnitude but also the direction of the barometric change, which for meteorological observation matters more than the absolute figure.
The authors laid particular stress on the fact that the new arrangements in the air thermometer made the instrument insensitive to transport and its working life practically unlimited. In an enlarged version, fitted with a simple device to prevent the weight from swinging, the barometer was intended for use aboard ship as well: by the trials of the day, the motion of the sea did not impair the regularity of its indications.
Translation of the text pasted to the back cover.
The absolute barometer consists essentially of an air thermometer and an ordinary thermometer.
The air thermometer is affected at once by temperature and by external pressure, but the readings of the ordinary thermometer would allow the effect of temperature to be corrected by calculation. A comparison of these two instruments can therefore lead to the determination of atmospheric pressure.
In order to render this system practical, the use of calculation has been avoided and replaced by a very simple geometrical construction arising from the manner of consulting the barometer. This construction is effected by means of the accessories which complete the instrument, namely: two cursors sliding on rods, and a thread tensioned by a weight.
As regards the theory of these instruments and the construction of the air thermometer, see the Comptes rendus of the Académie des Sciences (session of 14 July 1873) and of the French Association for the Advancement of Sciences (Paris Congress 1878), the journal (no. 318, 1879), etc.
The air thermometer registers the volume of the gas confined in the reservoir. The colder it becomes, the smaller that volume, and at −273 °C it would fall to nothing. The place on the tube where this would occur is unattainable, yet perfectly definite — and, decisively, it does not depend on pressure: nothing remains nothing, however hard the gas is squeezed.
The ordinary thermometer has a corresponding mark of its own: extend its scale downwards, beyond the engraved divisions, to the same −273°, and you obtain the place where the liquid column would end.
Two imaginary points on two tubes — that is the whole foundation of the design.
Now join, in the mind, the ends of the two columns, the liquid and the gaseous, by a straight line.
Let the pressure hold steady while the temperature varies. Both columns lengthen in step, each measuring its own distance from its own absolute zero. The line joining them does not wander at random: it pivots about a single fixed point. Temperature has no influence whatever on the position of that point; it is determined by pressure alone.
Let the pressure change, and the point shifts. But not just anywhere: all its possible positions lie on one straight line, and that line passes through both of the imaginary absolute-zero marks.
Hans calls it the line of absolute zeros. It is the pressure scale. To graduate the instrument means to set out the marks along that line; to take a reading means to find where the stretched thread crosses it.
Hence the name. The barometer is absolute because its scale rests physically upon the absolute zero of temperature.
There is also a second, plainer sense, set out on the dial itself: hauteur barométrique absolue — the height that a good mercury barometer would give at 0 °C. That is, the reading is already reduced to zero degrees and requires none of the temperature correction obligatory for a mercury instrument.
Hans's patent gives the formula by which the scale was laid out:
H = 760 × (273 + t) / (273 + t′)
where H is the pressure in millimetres of mercury, t the temperature by the ordinary thermometer, and t′ the number of degrees the air thermometer would show if it had been graduated as an ordinary thermometer at the normal pressure of 760.
The air thermometer by itself cannot distinguish temperature from pressure: it measures their ratio and nothing else. The ordinary thermometer says what the length of the gas column ought to be. The air thermometer shows what it actually is. The whole difference is charged to the atmosphere.
The formula is most easily read as a measure of the disagreement between the two thermometers:
And, above all: the owner of the instrument has nothing to calculate. The formula served the maker in laying out the scale. The observer moves two cursors and reads off a figure — the calculation was performed once and for all by whoever drew the line of absolute zeros.
The absolute barometer of Hans and Hermary was a genuine engineering achievement of its age. It brought together the ideas of thermometry, gas physics and geometry, offering an alternative to the familiar mercury and aneroid instruments. Its compactness, its legibility and the elegance of its principle made it a singular instrument for laboratory and meteorological observation.
It never came into wide use — the market stayed with the aneroids, cheap to produce and demanding no manipulation from their owners. All the more valuable, then, is a surviving example: it remains a vivid witness to the search, in the nineteenth century, for new ways of measuring atmospheric pressure, striving for greater accuracy, greater simplicity and independence from complicated calculation.
Manner of consulting the barometer
1° Set the upper cursor so that the point of the needle comes level with the end of the column of the ordinary thermometer, placed towards the top of the frame.
2° Slide the lower cursor until the thread passes through the end of the column of the air thermometer, which is arranged horizontally between the two rods carrying the cursors.
3° Read, against the vertical part of the thread, the barometric height or the weather forecast.
Note: the instrument must be suspended vertically. Each cursor should be taken by its small tab and slid while holding that tab perpendicular to the glass.
Important observations
The barometric scale is divided in millimetres and numbered in centimetres. It gives the height to which the column would rise in a good mercury barometer at a temperature of 0°.
The legend bearing the words "Changeable", "Fine weather", etc., must be suitably placed with respect to the altitude of the locality; to that end the small rule placed beneath the scale below must be slid in accordance with the indications of that scale.
Slip below: The barometer is set for a given place when the pointer corresponds on the scale to the height of that place above sea level.