Barometers – English

English Barometers

In an age when life itself depended on successful agriculture, sailing ships and horse-back or foot travel a knowledge of the weather was essential. Weather forecasting primarily relied on observation of the skies until circa 1645 when an Italian mathematician, Evangelista Toricelli (1609 to 1647), discovered the scientific principles that led to the mercurial barometer. By using a long, thin glass tube closed at one end, and with the open end placed in a container of liquid, Toricelli determined that the pressure of the air would force the liquid in the container up the tube. Thus, by measuring how far the liquid advanced up the tube, he could estimate the changes in air pressure. The use of mercury, the heaviest of all liquids, allowed a tube of reasonable length to be used in the instrument, and ultimately permitted the barometer to become a common and very useful scientific instrument.

The earliest English barometers date to about 1685. They were mainly of interest to professional and amateur scientists and were constructed by a few instrument makers, clockmakers and cabinetmakers. By 1710, with growing public interest in science, the barometers, then referred to as baroscopes or weather glasses, were becoming common in fashionable English homes. The dials and scales on antique barometers are engraved or printed with the barometric pressure (or the height of the mercury) ranging from about 27 inches to 31 inches accompanied by familiar weather conditions: usually Dry, Fair, Changeable, Rain, and Stormy. These weather indicators are not the most reliable guides to the weather. It’s the movement of the mercury, not its absolute height, which predicts alterations of the weather. There is usually an external pointer fitted on the dial which can be turned to the current reading, making it a simple task to compare it to the next reading-and avoid a possible unexpected drenching.

Types of Barometers

As England prospered in the 18th century, so did the barometer business. The main suppliers and retailers were the scientific instrument makers, clockmakers and opticians. As the craft of barometer making became an industry, most of these shops used subcontractors to provide the parts which they assembled. In concert with modern business practices, some of the barometer production moved to the large industrial centers away from London. The apparatus of the barometer changed very little from the 17th to the 18th century. It was basically a thin blown glass tube with a closed end at the top and an open end standing in a cistern of mercury. The changes came mainly in the presentation of the barometers and their forms. Barometers mounted on handsome carved or inlaid mahogany or other exotic woods and ivory with finely engraved scales were produced for the gentry and wealthy merchant class. The common and less expensive barometer with a exposed straight tube mounted on a scale with a frame of wood or metal, was the standard.

The angle barometer, produced mainly in the 18th century, had a longer glass tube which bent to the right or left. This longer tube allowed for more detailed readings. They can be found set into the frames of mirrors, serving a dual purpose for the owner. They are now rare and command premium prices in the market.

The “stick” or straight tube barometer became popular at the end of the 18th century, and with its variations in style, demand continued until the end of the 19th century. The name “stick” is an apt description. They are narrow and vertical, with an exposed or partly concealed tube and mounted on a finely crafted wood frame. Mounted on a gimbaled bracket, the stick form was useful in cramped quarters aboard ships. The wheel or banjo form barometer, with its round central dial, was first introduced from France in the 1760′s. It was considered inaccurate at first, but by the 1820′s became the choice of English weather enthusiasts. Many English barometer dials are found carrying Italian names. The early 19th century saw a flood of talented Italian artisans entering England. It was natural that these skilled glassblowers, carvers, gilders and mirror makers would gravitate to making and selling barometers. They almost dominated the trade by the 1850′s.

Always seeking to improve the product, manufacturers added hygrometers to their barometers around the 1760′s. Meant to demonstrate the humidity, they depended for measurements on the “beard” of a wild, oat inserted in the mechanism. Naturally these plant parts disintegrated after a few months rendering the hygrometer ineffective. Alcohol thermometers were mounted on the expensive early barometer frames, but became standard equipment by the 1800′s.

Antique English mercurial barometers are delicate instruments. They are often found to have damaged tubes and to have lost their mercury. Even though their decorative and historical qualities are now the prime consideration for most collectors, finely crafted examples with all the apparatus and woodwork intact are the most desirable.

reference note by p4A Contributing Editor Bob Goldberg.

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