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barometers
Barometers don't forecast the weather, they just measure air pressure. In the early 17th Century an Italian, Evangelista Torricelli, first discovered that air pressure could be measured by filling a glass tube with mercury and plunging it upside down into a reservoir of mercury. The level in the inverted tube rose or fell according to the weight of air on the surface of the reservoir.
The first barometers were 'stick' barometers, which repeated the experiment by fixing the glass tube in or onto a decorative piece of wood. A later development to get round the difficulty of reading tiny movements was the wheel or 'banjo' barometer, which still used mercury but the tube was hidden in the back of the case and operated a pointer on a circular dial via a pulley, which rotates as a glass weight floating on the mecury in the tube rises and falls against a counterbalance glass weight.
The trouble with mercury is that, being a liquid metal, it is troublesome to transport and tends to expand when warm so whilst mercury barometers are highly collectable, they're not always accurate. Mercury barometers are also big and the vapour is toxic. In 1843 the first aneroid ('without liquid') barometer appeared. This is the type most commonly found in hallways and it looks like a small wheel barometer but instead of mercury, it conceals a flat circular corrugated sealed 'capsule' in which the two halves are kept apart by an internal spring. Most of the air has been removed from the capsule by a vacuum pump so high pressure presses on it to squeeze it and low pressure allows it to expand under the power of the spring. This tiny movement is then measured by a cable or chain linking the capsule to a wheel with a pointer on the front of the dial.
So how does a barometer help forecast the weather? Well, you need to compare today's reading with yesterdays - there is a brass set hand with a knob to set it daily over the pointer. If next day the pointer shows that the pressure has risen, then the weather is improving. If the pressure has fallen, then the weather is becoming more unsettled. Then factor in wind speed and direction, moisure levels and temperature. Should you tap the glass? Well, a light tap will do no harm and may release the pointer if it's stiff but many a glass is cracked by tapping too vigorously so be careful.
The authority on ' English Barometers' is Nicholas Goodison but his book isout of print so expect to pay around £50 for a copy. Much cheaper are the various alternatives by Bert Bolle or Edwin Banfield. And much cheaper still is Anita McConnell's 32 page booklet in the Shire range.
Recallibrating a mercury barometer involves partly dismantling it so is best left to someone experienced and with the necessary transit jig. If you need to check or recallibrate your aneroid barometer the pointer can be set manually by a small set screw in the back. If you're in Essex, then Shoeburyness is the nearest weather station and you can find today's readings at the Meteorological Office website. They give the pressure in millibars at sea level so if you want to be precise, you'll need to add an inch of pressure for every 1,000 feet above sea level. If you don't know the altitude where you are, try the Get-a-Map service on the Ordnance Survey website - just type in your town and look for the thin orange contour lines shown in metres. But as I said above, if you're just trying to anticipate the weather, it's the change in pressure you should be checking, not the actual pressure.
If your barometer is old and the scale is in inches, multiply the result by 0.0295301 or just use this two-way converter:
Millibars
Inches Hg
(Insert a value in one box and then click outside the box for the result)
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