MERCURIAL BAROMETER (Fortin type)
Until recently all British and American barometers were read in inches and all others in millimeters. Since atmospheric pressure is a force, the practice of measuring it in units of length is rather like measuring time in bushels or potatoes in hours. The inconsistency is serious from a scientific point of view, because it divorces barometric measurements from other physical measurements, in which pressures are measured in units that have nothing to do with length; viz., dynes per square centimeter. Accordingly, some of the leading meteorological services of the world have lately adopted a new unit of barometric pressure, known as the bar, which is equivalent to 1,000,000 dynes per square centimeter. It is subdivided according to the ordinary metric notation, and its most commonly used subdivision is the millibar, equivalent to 0.03 inch on the old-fashioned barometer scale, under standard conditions.
ANEROID BAROMETER, GRADUATED IN MILLIBARS AND INCHES
For the benefit of sailors a curve is shown indicating the mean annual pressure in different latitudes along the meridian of 30° W. (Courtesy of the British Meteorological Office.)
The mercurial barometer is so delicate and cumbersome that for many practical purposes it is replaced by the more convenient though less accurate aneroid barometer. A self-recording barometer (usually an aneroid) is called a barograph. In its ordinary form, this instrument carries a pen, which traces a continuous record of the barometric pressure on a strip of paper wound around a cylinder turned by clockwork. Generally the instrument runs for a week before the paper has to be changed. The barograph is a very instructive instrument, because it shows, not only the pressure, but also the changes of pressure—i. e., just how fast the barometer is rising or falling, or, as meteorologists say, the “barometric tendency.” The way in which barometric changes are related to weather will appear in a later part of this book.
The mercurial barometer consists of a glass tube, sealed at its upper end and having at its lower end a “cistern,” which is open to the air. The tube is filled with mercury at its open end, and then inverted over the cistern, and the mercury descends until the weight of the portion standing above the level of the mercury in the cistern just balances the pressure of the air on an area equal to the cross section of the tube. The height of the mercurial column is read from a graduated scale attached to the tube. Certain corrections are applied to the reading, in order to eliminate variations due to temperature, etc., and, if to be entered on a weather map, the reading is reduced to sea-level value. In the aneroid barometer, a thin-walled metal box, exhausted of air, undergoes changes of shape in response to changes in atmospheric pressure. The movements of the box are communicated by levers to a pointer moving around a dial (or to the recording pen, in the barograph).
Since the pressure of the atmosphere diminishes with increasing altitude at a fairly definite rate, the barometer is used for measuring heights. Sometimes it is graduated directly, for this purpose, in feet or meters, and it is then called an altimeter.