Barometers Realm

The object before us is a Florentine Bottle, one of the earliest types of glass weather instruments, dating from the late eighteenth century. Despite the widespread modern designation weather glass, its operating principle identifies it as a true air barometer, occupying a distinctive place in the history of meteorological instruments as one of the earliest barometers, appearing almost half a century before Torricelli’s invention of the mercury barometer. Its operation is based upon a simple yet remarkably elegant physical principle: a fixed volume of air is sealed within a glass reservoir and communicates with the atmosphere only through an open, curved spout. As atmospheric pressure increases, the trapped air is compressed and the liquid level within the spout falls; when atmospheric pressure decreases, the enclosed air expands, forcing the liquid upward. Although the instrument also responds to temperature changes, it is the fluctuations in atmospheric pressure that establish it as one of the earliest practical forerunners of the modern barometer.

The instrument is also widely known as a Thunder Glass, a name rooted in one of its most dramatic operating characteristics. Before the arrival of a storm, atmospheric pressure often falls rapidly, causing the trapped air inside the reservoir to expand and forcing the liquid upward through the curved spout. During particularly pronounced pressure drops, the liquid may rise high enough to overflow from the open end of the tube. For earlier generations, this striking and easily observable phenomenon seemed to herald approaching thunderstorms, giving rise to the popular name Thunder Glass. Long before the scientific principles of atmospheric pressure were fully understood, such instruments earned a reputation as reliable domestic weather forecasters, particularly among sailors, fishermen, and rural households.

This is an outstanding example of late eighteenth-century European decorative glassmaking, in which a scientific instrument simultaneously serves as a work of decorative art. At first glance it becomes evident that every element has been fashioned entirely by hand. The numerous subtle irregularities should not be regarded as imperfections; on the contrary, they allow the viewer to reconstruct the glassblower’s work almost step by step.

The principal body consists of a large pear-shaped reservoir made from clear glass displaying a delicate natural greenish tint—a characteristic feature of historic glass caused by iron impurities within the sand and by the melting technology of the period. The glass possesses exceptional transparency and a lively play of light while retaining slight variations in wall thickness, an unavoidable consequence of completely hand-blown manufacture.

From the lower portion of the reservoir extends a long, slender spout that first curves gently backwards before sweeping upward and forward. It is here that the hand of the craftsman becomes particularly apparent. If one imagines a vertical centreline passing through the reservoir, the lower attachment of the spout can be seen to be positioned with a slight displacement to the left of the vessel’s central axis. Such asymmetry is never encountered in machine production and serves as characteristic evidence of individual hand assembly. The glassblower first formed the tube separately and then, while the glass remained hot, attached it to the body entirely by eye while continuously rotating the piece before the furnace.

The suspension loop is equally expressive. Rather than forming a geometrically perfect torus, it retains a subtle narrowing where its left-hand end joins the body. This area clearly reveals the point at which the craftsman completed the circle, fused the glass rod to the vessel, and gave the loop its final shape using traditional glassblower’s pincers. Even after more than two centuries, this modest transition remains a kind of craftsman’s signature, preserving visible evidence of the manufacturing process itself.

The principal decorative feature is the celebrated hanekam, or cockscomb, ornament running in two symmetrical rows along the sides of the reservoir. Every individual glass projection was applied separately. The glassblower gathered a small quantity of molten glass, placed it upon the surface of the vessel, and then drew and gently flattened it using specialised pincers. Under magnification, fine parallel striations become clearly visible across the surface of each applied element. These are not marks from subsequent finishing but the direct impressions left by the working jaws of the glassblower’s pincers, preserved permanently as the glass cooled. Through these traces, the decoration quite literally retains the memory of the eighteenth-century tools that created it. No two applied elements are exactly alike: their inclination, thickness, length, and degree of flattening all differ slightly, giving the entire composition an extraordinary sense of vitality.

The upper part of the reservoir terminates in a characteristic annular glass collar, which both reinforces the attachment point of the suspension loop and creates a graceful transition between the body and the hanger. The lower section, by contrast, concludes in a massive, perfectly smooth glass sphere. Beyond its decorative role, this sphere provides a natural visual termination to the composition and functions as a counterweight, balancing the elongated curved spout.

The wooden mounting is no less remarkable. Fashioned from solid oak, it constitutes an accomplished work of cabinetmaking in its own right. The board is carved into an elongated teardrop form that harmoniously echoes the silhouette of the glass reservoir. Particularly expressive are its profiled edges. Here one can clearly distinguish characteristic traces of hand-planing: the moulded profile is not geometrically perfect, while successive passes of the plane occasionally overlap, producing minute steps and barely perceptible irregularities. In several areas along both the upper and lower mouldings, almost mirror-image marks reveal where the cutting tool exited while forming the decorative grooves. Such details cannot be reproduced by industrial manufacture; they arise only when a craftsman guides a moulding plane manually along the entire length of the board, gradually shaping its complex profile. The front surface likewise preserves the warm, living texture of the oak, enhanced by a transparent finish that allows the natural grain to remain fully visible.

The method of mounting is as simple as it is elegant. At the upper end of the board stands a turned wooden peg with a mushroom-shaped head, over which the glass suspension loop fits freely. As a result, the vessel is not rigidly fixed in place but can easily be removed for filling or maintenance while remaining securely supported by its own weight.

It is precisely these seemingly insignificant details that make this Florentine Bottle especially remarkable. Here it is impossible to draw a clear boundary between scientific instrument and work of art. Every element—from the slight asymmetry of the glass spout and the pincer marks preserved upon the cockscomb ornament, to the hand-formed suspension loop and the profiled oak backboard bearing the unmistakable traces of the moulding plane—preserves direct evidence of the labour of late eighteenth-century craftsmen. What we see is not merely an early air barometer, but a rare monument to the craftsmanship of an era when the manufacture of scientific instruments depended entirely upon the skill of human hands, and when technical function and artistic expression existed as a single, inseparable whole.