A barometer is an instrument for measuring atmospheric pressure, and the three barometers listed in this section of the Catalogue use mercury columns. The other common type of barometer is the "aneroid" barometer, which uses a metal vacuum chamber, whose response to atmospheric pressure is conveyed mechanically to a needle or recording pen. It is one of the latter which is present in the "Travelling Set" (376). The fact that the atmospheric pressure changes with the weather made the barometer very popular as a weather predictor, and most affluent homes of the last century would have possessed one. Mercury barometers are still widely used to-day, in spite of the fact that the weather forecast on radio or TV is much more reliable, and that mercury vapour is now known to be toxic. Many of them are very attractive, and prized as antiques.
The most common type of mercury barometer in the eighteenth century was the stick barometer, which has a simple closed glass mercury column with a sealed cistern at the bottom, usually held in a wooden case, which can be ornately decorated, and with the height of the column related on scale plates to changes in the weather. A refinement of this is the wheel or banjo barometer, which uses a J-shaped tube, in which the height of the mercury in the short open limb is translated, via a float and a pulley and weight system, to a needle reading a large circular dial, again giving an indication of changes in the weather anticipated from the height of the mercury column. These also could be highly ornate. For more accurate measurements, a standard barometer was introduced by Frenchman Jean Nicolas Fortin (1750-1831) in about 1800 (Turner 1983,234). In this, the mercury cistern has a glass portion through which the mercury exposed to the atmosphere can be seen, and an ivory needle which was made just to touch its mirror image in the mercury before the reading was taken. This allows for the fact that, when the mercury column in the closed tube falls, the level in the cistern rises, and the difference in height between the two cannot be accurately determined unless the height of the latter is taken into account.
The barometer had a key role in the development of Meteorology. Knowles Middleton (1969,3) records: "The science of meteorology began in the seventeenth century with attempts to find a relation between the weather and the puzzling fluctuations of the height of the mercury column, and the possibility of forecasting was first seriously explored by drawing isobars [i.e. places of equal atmospheric pressure] on synoptic [i.e. weather] maps".
The nature of what is now known to be a vacuum (if mercury vapour and volatile impurities are ignored) at the top of a closed barometer tube containing mercury, and immersed in a bowl or cistern of mercury, was the cause of much controversy historically. The ancient view - as held, for example, by Aristotle (384-322 BC), the Greek philosopher - was that a vacuum was actually and logically impossible. The French philosopher René Descartes (1596-1650) agreed with him. Of Descartes, Knowles Middleton (1969,4) records that he "had an immense influence on seventeenth-century science even though most of his physical ideas have turned out to be wrong"! Descartes reply to a student about why mercury in a tube closed at its upper end did not fall out was unconvincing. The Italian, Evangelista Torricelli (1608-1647), a pupil of Galileo, realised that the reason why the mercury did not fall out was because of the pressure of the atmosphere on the mercury in the bowl in which the open end of the tube was held. He was one of the first to argue that a vacuum (in the modern sense of a space totally void of any matter) was indeed produced in the gap between the mercury and the closed top of the tube. Different liquids rose to different heights in closed tubes, the height depending on the density of the liquid. For example, a tube of water will rise to a height of about 10.4 metres (34 feet). Mercury was most convenient since it is so dense, and the tube can thus be relatively short (about 79 centimetres). Further, if a sphere is blown into the top of the column, the height of the liquid remains the same, so it is not due to any properties of the vacuum or any supposed "force of vacuum" (Gillispie 1981,13,439).
It was found that the atmospheric pressure, and thus the barometer reading, varied with height, and the barometer could thus be used as an altimeter, so that a traveller or mountaineer could determine his or her height above sea level. A barometer was used in this way to establish that the summit of Everest had been reached in the successful Irish expedition in 1993. The aneroid barometer in the "Travelling Set" (376) was used for this purpose, and the same phenomenon was involved in the use of the "Heater to Calibrate Thermometer" (203), when it was used to determine altitude, since the boiling point of water depends on the atmospheric pressure. Used in this way, such a water-boiling instrument is called a hypsometer.

 301 Stick Barometer

302 Banjo Barometer

303 Fortin Barometer