THE year of the Torricellian experiment was 1643. It comprised, in essence, the invention by Evangelista Torricelli, a close associate of Galileo, of the barometer as an instrument to measure atmospheric pressure.
His apparatus consisted of a glass bulb fitted with a neck "two cub its" (or about 40 inches) high and filled with mercury. When the tube was inverted into a dish containing more mercury, it was found that the liquid in the tube fell to a certain level - about 30 inches above the surface of the dish - but no further.
The inference was that it was the external pressure exerted by the atmosphere that kept the mercury inside the tube in suspended animation, and even more interestingly, it was observed that the height of the mercury column varied with changes in the weather.
Others found that different liquids could be used. Blaise Pascal, with Gallic elan, panache and savoir faire, used burgundy to fill his French barometer, and found it worked quite well except that it needed a tube some 30 ft in length to achieve the desired effect: the reason was that wine has only one thirteenth the weight of mercury, so a greater "head" was created when the system was in balance.
Then Otto von Guericke mayor of the German town of Magdeburg 1652, used this extra length to amusing and practical effect. The tube of the water barometer in his garden was so long that it extended above the roof of the house. He placed a little man like doll on a cork to float on the water surface at the top of the column.
The Wettermannchen, or little weather man, disappeared from sight when poor weather was in prospect, atmospheric pressure low, and the level of the water dropped.
When the pressure rose again, however, and good weather might be on the way, the Wettermannchen popped into sight again above the level of the roof, much to the delight of the citizens of Magdeburg.
Nowadays we are more likely to consult an aneroid barometer for an indication of our future weather. The heart of an aneroid barometer consists of thin hollow capsules of corrugated metal - each one in size and shape being rather like a Marietta biscuit.
When the air has been removed from inside these capsules, they expand and contract in response to changes in the pressure of the atmosphere outside.
This movement is linked mechanically to a needle, which indicates the pressure value on a dial, or even more helpfully, provides a concise prediction in terms such as "Rain", "Set Fair" or "Very Dry".