BAROMETER

Because air has weight it exerts a pressure. The weight of air at the ‘top’ of the atmosphere presses down on the lower layers of the air. The barometer is an instrument that measures this air pressure. At sea level, air pressure averages about 15 pounds on every square inch.

Barometers are used by weather forecasters to measure air pressure. As a general rule, falling air pressure means stormy weather, and rising pressure means more settled weather. There are two main types of barometers, mercury barometers and aneroid barometers.

The mercury barometer is simply a column of mercury enclosed in a glass tube. It works on the principle that normal air pressure supports a certain amount of mercury in a tube out of which the air has been drawn. This amount (about 30 inches) varies with changes in pressure. When there is a decrease in air pressure, the column of mercury falls. When there is an increase, the column rises.

The mercury barometer works by measuring the height of a column of mercury supported by the pressure of the atmosphere. The aneroid barometer measures the pressure of the atmosphere on a pertly evacuated thin-walled metal box. The domestic barometer shown as the post picture is of this type.

The aneroid barometer is the type often seen in homes. It works on a different principle from the mercury barometer. It contains a small metal box from which most of the air has been drawn. The box has a flexible lid to which a lever is attached. Changes in air pressure move the lid and its lever. The lever is connected to a pointer, which moves across a dial to indicate air pressure.