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Kathy Isreal, age 11, of West Asheville, North Carolina, for her question:

How do they measure the height of a mountain?

There are several ways to measure the height of a mountain and every few decades, somebody invents a better method. In the past, a person needed a good head for figuring. In more modern methods, most of the figuring is done by sensitive instruments. All a person has to know is how to read the number of feet above sea level. However, none of these methods is perfect. We are still waiting for an absolutely accurate, foolproof method to measure the precise altitude of a mountain.

When geographers talk about the height of a mountain, they mean its altitude above sea level. And the basic job of verifying sea level is very tricky. Standard sea level is the midway point between high tide and low tide. It is based on countless measurements and estimates taken at numerous points on the world's oceans. In making maps, generations of careful surveyors have measured the altitudes of much of the land. For the convenience of future generations, they left durable markers to show the altitudes at different elevations.

This tedious surveying of the slopes is done on foot. Among other instruments, the expert surveyor carries an altimeter for measuring up the grades in easy stages. The altimeter measures the barometric pressure of the air and translates it into an estimated number of feet above sea level.

The altimeter is a sophisticated grandchild of the barometer. It measures the weight of the atmosphere pressing down on the ground and translates this into feet above sea level. The standard barometric pressure at sea level is taken to be 14.7 pounds per square inch. This is the weight of a square inch column of air, reaching from sea level to the top of the atmosphere. Measured from greater heights, the column is shorter    and therefore less weighty. What's more, it is well known that the air gets thinner, and less weighty, as we rise above the earth. An altimeter is designed to measure increasing altitude from decreasing air pressure.

Altimeters can miscalculate when weathery air masses cause variations in the barometric pressure. A newer device uses the speed of radio waves to measure for the benefit of airplane pilots flying over tricky terrain. A radio beam, traveling at the dependable speed of 186,000 miles per second, is directed at the ground. The time it takes to get there and bounce back is divided by two to give the plane's altitude above the solid ground. Perhaps someday geographers will find a way to use this or some other precise system to measure the altitudes of mountains.

Many of the world's tall peaks were measured by geographers in patient stages, using various surveying techniques. Then the altimeter was invented. Nowadays, even a surveyor of gentle slopes carries an altimeter to double check his measurements. And mountain climbers always carry altimeters to estimate altitude    right up to the top of the tallest peak.


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