The mercury barometer had its origins in Florence around 1640, as part of experiments on the possible existence of a vacuum. It soon reached scientific men in France, who observed that the rise and fall of mercury in the tube was somehow related to changes in the weather. By the 1680s barometers were being offered for sale in London.
These basic instruments reflected the William & Mary style of furniture being made at the time, built on oak carcases with cross-grain walnut mouldings and veneered fronts with inlaid saw-cut walnut marquetry. The lower open end of the glass tube was immersed in an open cistern of mercury which, until a form of sealed cistern was devised, was not really practical. A graduated silvered scale was set alongside the upper three or four inches of the mercury level, enabling the observer to note its height, which at sea level generally ranged between 28 and 31 inches.
Figure 14. Although banjos continued to be made in the popular 5-dial format, by 1845-55 the shape changed to accomodate a long thermometer, the frame being squared at the base with an oblong spirit level. This period also sees the first change of veneers, form mahogany to rosewood. signed R. & E.northern Hull.
The small daily changes in the mercury level were not easily measured on these straight tubes. In the 1680s we read of attempts to enlarge the scale by bending the upper portion of the tube, creating a 'diagonal barometer' (see Antique Collecting, April 2002). Their manufacture was a challenge to the glass-worker and not an ideal solution, though some elegant examples survive. At this time the scientifically-minded Honorable Robert Boyle (1627-1691) experimented by bending a short portion of the open tube into the form of a U; when the mercury moved down from the top level, it moved the same distance up the short limb. The combined measurement of the fall and rise gave the complete pressure change.
Robert Hooke (1635-1703), mechanic to the Royal Society, took this principle a stage further: his illustration, published in 1665, shows how the small vertical motion of the mercury level was transposed into the rotation of a pointer round a large dial (figure 1).
Hooke's barometer did away with the need for a cistern, by returning the open end of the tube to form a short limb of 6 to 8 inches. A weight floating on the mercury in this limb is suspended by a cord which passes round a small pulley and is attached to a counterpoise. As the mercury level rises and falls, the cord drives the pulley, whose axis carries a pointer which rotates round a graduated dial. Depending on the ratio between the
pulley circumference and the radius of the dial, one inch of mercury drives the pointer several inches around the dial. From their appearance, these instruments are known as wheel barometers.
The craftsmen producing these early barometers, which called for skills in handling fine woods, brass and glass, were often also makers of clocks and watches, sundials, navigation and surveying instruments. The cases of their wheel barometers are veneered in the finest woods and similar in style to longcase clocks, with dials in a square frame. The most famous of these craftsmen is probably Thomas Tompion (1639-1713).
His customers were drawn from royalty and the wealthy upper classes, some of whom, like Boyle, were also scientific virtuosi. Tompion made barometers for
Boyle and Hooke in 1675; his barometers can also be seen in Hampton Court and other stately homes. Other makers of high quality square-dial wheel barometers include clockmaker John Hallifax (1694-1750) of Barnsley and in the 18th century the London makers George Adams senior (1709-1772) (figure 2), his youngest son Dudley Adams (1762-1826), whose fine barometer can be seen at Woburn Abbey, and Justin Vulliamy (1712-1797), represented by a barometer at Windsor Castle.
France banned the export of walnut in 1720 but imports rose, both from elsewhere in Europe and especially from British colonies in the Americas where the Naval Stores Act of 1721 allowed timbers in free of duty. This Act also encouraged the importation of mahogany, first from Jamaica, then from Honduras and Hispaniola, with Cuban mahogany arriving in the early 19th century and becoming dominant by mid-century. Other tropical hardwoods such as ebony were imported from the Caribbean.
All wheel barometers made between 1680 and around 1780 were produced by clockmakers and fine instrument makers, having almost without exception square dials and casework influenced by clock cases. Although we may consider these as domestic, only wealthy people could afford them. Their linkage rendered them less precise than stick barometers, which continued to be made for those requiring really accurate measurements.
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