IF you have a barometer in your hallway, the chances are it is an "aneroid". It will be a device rather like a clock, with one hand that rotates clockwise with rising pressure, and another "dummy" hand intended as a kind of locum tenens to mark, for future reference, some ephemeral indication of its volatile twin.
The dial provides helpful advice, ranging from dire warnings of "rain" and "storms" to cheerful intimations of "bright" or "sunny" weather. But the atmospheric pressure Is also specified in one or more of four possible units they may be inches, millimetres, millibars or hectopascals.
The predecessor of the aneroid which means "without fluid" was the mercury barometer, invented in the 17th century. In its original form the latter consisted of a glass bulb fitted with a neck "two cubits" long about 40 inches the tube was filled with mercury and inverted into a dish containing more mercury. 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.
It was with this latter quality that early users of barometers were primarily preoccupied rather than with the niceties of how to label atmospheric pressure. It seemed natural, therefore, to capture the relationship between the weather and the mercury in terms of the length of the column either in inches or in millimetres. In due course aneroid barometers measured pressure without the use of any liquid whatsoever, but the unit of equivalent length of mercury was retained.
By the late 19th century, 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. Then in 1986 there was a further change, which brought in the hectopascal it being 100 pascals, the basic unit of pressure in the now official System International.
Obviously, since each of the different units provides a measure of the same quantity at a given instant, it is possible to convert from one to the other. Millibars and hectopascals present no difficulty, since they are numerically equivalent 1,000 mbs is exactly the same as 1,000 hpa. We can convert either of these to inches of mercury by dividing by 33.86 which, if you work it out, means that 1,000 millibars is the same pressure as 29.53 inches. And of course, any pocket dictionary will tell you at the back how to convert inches into millimetres or vice versa.