Robert Collies Jr., age 13, of Camden, N.J., for his question:

WHAT IS A MACH NUMBER?

Aeronautical engineers and pilots use Mach numbers to describe the speed of planes flying near or above the speed of sound. A plane traveling at twice the speed of sound is flying at Mach 2.

Mach numbers are used because the speed of sound in the air is not always the same. The speed depends on the altitude and temperature of the air. At sea level at 32 degrees Fahrenheit, sound travels at about 740 miles per hour. The speed of sound decreases at higher altitudes. At 40,000 feet, for example, sound normally travels at about 660 miles per hour.

Mach 1 is the speed of sound, or transonic. Mach 0.5 is half the speed of sound, or subsonic. Mach 2, which is twice the speed of sound, is called supersonic.

Mach numbers receive their name from an Austrian  physicist and psychologist named Ernst Mach. He studied the action of bodies moving at high speeds through gases and developed an accurate method for measuring their speeds in terms of the speed of sound.

Mach was born in 1838 and died in 1916, before man could fly at the speed of sound. But he was deeply interested in the historical development of the ideas on which the science of mechanics is based.