An English diagonal barometer, signed on the register plate, Io. HURT LONDON, c. 1740. The mahogany-veneered pine case has silvered brass register plates with a scale that runs from 28 to 31 inches and has a scale magnification of around eight times. At the top of the horizontal arm of the tube there is a brass setting hand, whilst there are weather conditions on the lower register plate: ‘Stormy’, ‘Rain’, ‘Variable’, ‘Fair’ en ‘Ver.y Dry’. The glass reservoir is protected by a removable cap. • Diagonal barometers, also called angle or sign-post barometers, are fascinating and because of their rarity a sought-after collector’s item. The diagonal system was invented by Samuel Morland, who published his ideas in 1688 for the first time. The aim was to increase the scale which would result in a more accurate reading, so that small variations in air pressure were made visible. Indeed, the Torricelli tube has a functional scale from 70 to 79 centimetres mercury pressure. In normal weather conditions the variations in pressure remain within a maximum distance of about five centimetres. The principle of diagonal barometers is related to the fact that when a mercury tube is positioned at an angle, the vertical height of the mercury column always remains the same, despite the mercury having to cover a longer distance. This results in a greater column length. The greater the angle of the tube, the longer the mercury column will be. Diagonal barometers only have the functional part of the tube at an angle. The tube is vertical over a length of about 70 centimetres above the mercury level in the reservoir at which point there is a sharp bend to the right, the angled part rising to the required vertical height of a normal tube of about 80 centimetres. The length of the angled part can vary considerably, depending on the angle of inclination and the accompanying scale increase.
The Horological Foundation Desk Diary Project.
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