Derrick Allen, age 11, of Staten Island, N.Y., for his question:

WHAT IS A BAROMETER?

A barometer is an instrument that is used to measure the pressure of the air. It is used for forecasting weather and in measuring the heights of mountains.

A mercury barometer has mercury in a reservoir and a glass tube. Air pressure on the reservoir forces the mercury into the tube. Changes in pressure cause the mercury either to rise or fall.

An aneroid barometer has an airtight box containing a partial vacuum.

Air pressure causes the box to expand or contract. The barometer needle is attached to the box by a chain, levers and a spring. The needle moves left when the box expands and right when it contracts. Another needle can be set by hand directly over the barometer needle. Later, when the barometer is read again, the space between the needles will show if the air pressure is falling or rising.