Who among us has not had a parent or partner who began every day by tapping the barometer and predicting the day's weather, especially if they were farmers or fishermen?
Today we will look at a barometer from around the 1820s, made by London company Ortelli. This barometer was donatedto Whangārei Museum by G. Carruth. Little is known about the Ortelli family; it is presumed they came from Italy to England as immigrants and set up their barometer and clock business in Maccesfield.
They produced elegant fiddle-shaped barometers, usually in mahogany cases that were often inlaid with mother of pearl, like this example. A round dial predicts changes in the weather and a mercury-encased tube above shows changes in air pressure.
On the back of the case is a hinged door that, when opened, reveals glass tubes and weights that operated the results seen on the front.
The mercury barometer is the oldest type of barometer, invented by an Italian physicist, Torrecelli, in 1643. His first experiments were with a barometer using a tube of water.
Water is relatively light in weight, so a very tall tube with a large amount of water had to be used to compensate for the heavier weight of atmospheric pressure.
Torrecelli's glass tube was more than 10 metres high. This odd device, taller than a house, caused suspicion among the neighbours, believing it connected with witchcraft. So to keep his experiments secret, Torrecelli switched to mercury.
The main work of the barometer is to measure the pressure of the atmosphere, which is the layers of the air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth.
The 1820s barometer at Whangārei Museum. Its round dial predicts changes in the weather and a mercury-encased tube above shows changes in air pressure (1986_63_63).
This barometer was donated to Whangārei Museum by G. Carruth.
Weather predictors use this measurement to predict short-term changes in the weather; a rapid drop in pressure means that a low-pressure system is arriving.
Such low pressure cannot push away clouds or storms. Winds rise ahead of such systems as well. A rise in pressure can clear the skies and bring in cool dry air.
Meteorologists today have a raft of scientific equipment, including barometers, to assist with their weather predictions, but, as we all know, sometimes they don't get it right!! Especially when we are planning an outdoor event.
• Alison Sofield is a collections volunteer with Whangārei Museum at Kiwi North.
