The ordinary domestic barometer has a dial that provides helpful advice about the weather yet to come. Dire warnings of "rain" and "storms" alternate with cheerful intimations of "bright" and "sunny" weather, as the needle moves backwards and forwards through a range roughly defined in "clock" terms by the region between "ten to" and "ten past" the hour.
But if you look at the barometer more closely, you will see that it also provides a reading of the atmospheric pressure in one or more of four possible units: the inch, the millimetre, the millibar and the hectopascal.
The first two of these obviously relate to length, and have their origins in the traditional mercury barometer invented more than 350 years ago. In its original form, the instrument consisted of a glass bulb fitted with a neck "two cubits" long, or about 40 inches; this bulbous tube was filled with mercury and inverted into a dish containing more of the same liquid.
It was found that in these circumstances the mercury in the tube fell to a certain level, about 30 inches above the surface of the mercury in the dish, but no further. Moreover, the length of the column rose and fell with changes in the weather.
Now it was primarily with this latter quality that early users of barometers became preoccupied, rather than with the niceties of how to label atmospheric pressure. It seemed natural to capture the relationship between the weather and the mercury in terms of the length of the column, in either inches or in millimetres.
In due course aneroid barometers were invented which measured pressure without the use of any liquid whatsoever, but the unit of the equivalent length of mercury was retained.
By the late 19th century, however, this anomaly had become an irritant, and in 1900 meteorologists adopted the millibar, one-thousandth of a bar, the unit in which pressure was then measured. And finally in 1986 there was a further change, which brought the hectopascal into being, it being 100 pascals, the basic unit of pressure in the now official Systeme International.
Naturally, all four units of pressure can be converted into their equivalent in any of the other three. To convert millibars to inches of mercury, for example, it is necessary only to divide by 33.86 which, if you work it out, means that 1,000 millibars is the same pressure as 29.53 inches.
And happily, not entirely by coincidence, a hectopascal is numerically equal to a millibar, the two being merely different names for the same thing: 1,000 millibars is the same as 1,000 hectopascals.