We have, most of us, had the experience from time to time of seeing the rain pour down outside when "the glass" in the hall mendaciously declares "Set Fair". Nonetheless, in the absence of a formal weather forecast, the glass - or to give it its proper name, the barometer - remains the best guide available to tomorrow's weather. But it must be properly adjusted.
The barometer, in the strict sense, gives you a reading of atmospheric pressure in "hec topascals", "millibars" or "inches of mercury". In addition, the ordinary domestic instrument has a dial that provides helpful advice, which ranges from dire warnings of "rain" and "storms" to cheery intimations of "bright" and "sunny" weather.
If the barometer is working properly, the needle will move backwards and forwards during a spell of unsettled but relatively "normal" weather through a range roughly defined in "clock" terms by the region between "ten-to" and "ten-past"; "twenty-past" suggests a heatwave is upon us, while at "twenty-to" - watch out!
A barometer, however, operates on the assumption that the needle indicates the sea-level pressure. A new barometer, in all probability, will be factory set, and if your hallway happens to be at a significantly higher or lower level than that factory - wherever it may be - its readings will be spurious. Pressure decreases rapidly with height at a rate of about one millibar, or hectopascal, for every 30 ft; a difference of only 300 ft, therefore, between the level of the factory and chez vous can introduce an error of 10 millibars - enough perhaps to tilt the balance between "Fine" and "Change able".
The solution is simple. A knob at the back of the instrument allows you to change the current position of the needle to indicate the local sea-level pressure.
Dublin readers of The Irish Times can obtain the correct setting by, for example, checking their barometer at noon today, noting when tomorrow's paper comes what pressure it ought to have read at that time, and then making the appropriate adjustment. Alternatively, and especially if you live outside Dublin, your nearest meteorological station will be glad to oblige with the correct setting for your locality.
Even when it is correctly set, however, the barometer's advice must be treated circumspectly. There will be times when you may feel like Louis MacNiece:
The glass is falling hour by hour, the glass will fall forever;
But if you break the bloody glass you won't hold up the weather.