Barometers Realm

The unique trend pointer is driven by a high-sensitivity mechanical unit linked to the main aneroid pointer. This system registers even the slightest movement—on the order of a single millimeter of mercury or less—and transmits it through a micromechanical geartrain. This transmission triggers the movement of the dual arrow, which reacts to the minimal displacement of the main hand and displays the direction of pressure change.

The round case, measuring 150 mm in diameter, is made of lacquered beechwood and fitted with a brass suspension ring. The dial is protected by a convex mineral glass set in a slim brass bezel. Made of aluminum, the dial bears a dual concentric barometric scale: the outer ring shows pressure in centimeters of mercury, while the inner ring is in millibars. At the top of the dial is a curved aluminum plate shielding the trend pointer, labeled “Tendance” and marked on the left with “au mauvais” (toward worse weather) and on the right with “au beau” (toward fair weather). The two halves of the pointer emerge from beneath this plate in either direction depending on the pressure change.

The main hand is crafted from blued steel, with an elegant form and a finely painted white tip.

At the bottom of the dial appears the full name of the instrument: “BAROMÈTRE INDICATEUR DE LA TENDANCE DU TEMPS,” which translates as “Barometer indicating the weather trend.” “INDIC” is a shortened form of indicateur (indicator). The inscription “Breveté S.G.D.G.” stands for Breveté Sans Garantie Du Gouvernement, meaning “Patented without government guarantee.” In France, patents were historically issued without state responsibility for the invention’s functionality, originality, or utility. This formula was in use from 1844 until the reform of the patent system in 1968.

At the heart of the instrument lies a conventional aneroid movement, featuring a C-spring mounted on the aneroid capsule and a system of levers transmitting the capsule’s motion to the pointer shaft. What sets this barometer apart is the integration of a specialized gear mechanism, amplifying and relaying even the smallest shaft movements to the trend pointer. The final gear in the train engages a sector rack—technically a partial gear segment rather than a full arc. Unlike a classical curved rack with a complete ring of teeth, this element has just three teeth and resembles a small mechanical pawl. This creates an intermittent drive: motion is transmitted only when the teeth engage, typically upon a sufficient shift of the pointer shaft. This mechanism is known as an intermittent gear drive. The pawl itself is fixed to the axis of the trend pointer and causes it to rotate.

Due to the specific layout of the movement, it is installed inverted—rotated 360 degrees compared to standard configurations—so that the C-spring faces downward. As a result, the axis of the trend pointer is located at the top of the dial, while the main aneroid pointer’s axis lies below it.

A mechanism like this cannot help but inspire admiration. The complexity added to the aneroid system through the integration of gears and sector elements further blurs the line between barometers and timekeeping instruments. Not without reason are barometers referred to as “weather clocks” or “atmospheric timepieces.”