Science Museum | Heavy Weather | The barometer



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exhiblets

 
introduction

 
the
barometer

 
admiral
fitzroy and the fitzroy barometer

 
the
tay bridge disaster

 
storms

 
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heavy weather

The barometer
The mercury barometer measures atmospheric pressure. It works this way.
Completely fill a long glass tube with mercury. Turn it upside down, and
place the top below the surface of more mercury in an open basin. Rather
than pouring out again, the mercury in the tube will only fall until the
height of the column is about a meter. This is because the pressure of the
air on the mercury in the basin is equal to the pressure of the mercury in
the column. The gap at the top of the tube is not air but a vacuum. The
difference in height between the top of the column and the top surface of
the mercury in the basin is a measure of the weight of the air, which
changes as the weather changes. The air pressure is then measured in units
called millibars (mb) or hectoPascals (hP), both of which have the same
scale.
Because atmospheric pressure is the result of the weight of the
atmosphere, pressure varies according to how high or low you are. The
higher you are the lower the air pressure is. In Britain, at sea level the
pressure is 1013 mb, but as you climb higher this decreases by 1 mb per 10
metres. Pressure also varies horizontally, though changes here are much
more gradual: 1mb per 100km.





[enlarge]

A diagram of a mercury barometer.
Diagram by Ben
Ashforth
Changes in air pressure also reflect potential changes in weather. If
there is low pressure then a depression can develop which may cause stormy
weather. High pressure is usually associated with dry and sunny weather.

There are several different types of barometers within the Science
Museumłs collection. These include marine, coastal, mountain and portable
barometers. Perhaps one of the most interesting is the Admiral FitzRoy
barometer, which was widely used at the time of its development and is
still famous today.





[enlarge]

Quare's portable barometer ca.1700
© Science Museum/Science & Society Picture
Library



 
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FitzRoy and the FitzRoy
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