We are all aware of the importance to our future well-being of a little clock-like dial which is often somewhat obscurely called "the glass". Our forefathers told us:
When the wind backs and the weather-glass falls,
Then be on guard against gales and squalls.
More than 300 years ago, soon after the invention of the mercury barometer, it was noticed that the height of the mercury in the glass tube varied slightly from hour to hour and very considerably from day to day. It seemed that when the weather was good, the mercury climbed higher, and that conversely, when the weather was wet and stormy, the "glass" was often low.
Tradition has it that Robert Hooke first tried to quantify this relationship. Although best known, perhaps, for Hooke's Law of elasticity, he is also the originator of the standard inscriptions on the domestic "weather-glass".
In 1670 Hooke equipped his mercury barometer with a float, connected by a chain to operate a pointer on a clocklike dial. The corresponding height of the mercury column was engraved on the dial, and the word "Change" inscribed at 29.5 in; "Rain" , "Much Rain", and "Stormy" were inserted at half-inch intervals on the low side; and "Fair", "Set Fair" and "Very Dry" on the high side.
The idea, and indeed the terms themselves, became very popular throughout the whole of Europe, where in due course people spoke of a barometer "set up in the English fashion". The descriptions are still with us on modern aneroid barometers in only slight variations of their original form.
But are these legends any good for telling what the weather may be like? They are, it must be said, not too dependable. Accurate forecasts are difficult enough for meteorologists supplied with up-to-the-minute reports of temperature, pressure, humidity and wind over a large area of the globe; the barometer merely measures the atmospheric pressure at a single point, and it is optimistic to expect consistently accurate predictions from the instrument based on this information on its own.
This said, however, the behaviour of the barometer does offer some clues to the future. In general, the trend of the pressure, whether it is rising or falling, is a more reliable guide than its absolute value - how high or how low it is. And the relationship noted more than 300 years ago is still true in a very general way: that high pressure is associated with quiet, settled weather, and low values often bring changeable, wet and windy conditions.