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KF5JRV > TECH 30.07.16 13:35l 60 Lines 3198 Bytes #-3552 (0) @ WW
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Subj: Barometers
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Barometers measure air pressure, and until around 1675 they were primarily
used to calculate height above sea level. However, natural philosophers soon
noticed that readings on barometric instruments corresponded to atmospheric
conditions and could therefore also help predict the weather. Early domestic
barometers were luxury objects displayed in semi-public spaces such as the
parlour. They gave a general indication of local weather and used vague
weather-terms such as 'stormy', 'rain', 'change', 'fair', and 'dry'.
Before the modern barometer was invented, simple devices like the 'Donderglas'
(Dutch named 'thunder-glass') were used to anticipate thunderstorms using
changes in air pressure. The Donderglas, sometimes called a 'Dutch water
barometer', was hung vertically and water was introduced through the spout
until the water level inside the vessel was equal to that in the spout.
When atmospheric pressure increased, the level of the water in the spout would
fall. When atmospheric pressure decreased, as in the case of an impending
thunderstorm, the level of the spout water would rise. Glass buttons or
pinches on the vessel acted as a scale.
Unfortunately, this type of weather-glass was susceptible to variations in
temperature, causing water to expand or contract, and therefore it provided
only a rough guide of local atmospheric activity.
The invention of the barometer
Barometers, in the sense in which we use the term now, were first invented to
measure the weight of air or air pressure, and were most closely associated
with studies of the physical properties of air.
Evangelista Torricelli (1608-1647), who may well have encountered Dutch water
barometers, noticed that when an open-ended vertical tube was partially
submerged in a water bath, the level of water in the tube remained elevated
above the surface of bath water. Torricelli attributed the raised water level
in the tube to the weight of air exerted upon the water column, in
contradiction of the Aristotelian assumption that air was weightless.
Blaise Pascal (1623-1662) agreed with Torricelli's theory and suggested that
the weight or pressure of air would decrease with altitude. In 1648 he
convinced his brother-in-law, Florin Périer (1605-1672), to climb Puy-de-Dôme
with a simple mercury-in-tube barometer. If Torricelli and Pascal were correct
then the column of liquid in the barometer tube would increase with altitude.
Périer carefully recorded the barometer as he ascended and noted how the level
increased, while, at the base of the mountain, a local monk monitored a second
barometer in order to calibrate fluctuations in air pressure. Périer's trek up
Puy-de-Dôme thus helped to confirm that the weight of air is heavier at sea
level than at the top of a mountain.
73, Scott kf5jrv
KF5JRV @ KF5JRV.#NWAR.AR.USA.NA
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