Mach 1

   , MACH NUMBER

   When airplanes first started to fly, their speed was measured like that of a moving vehicle on the ground. As air speeds increased, that method became obsolete, and a new method had to be found.

   Ernst Mach (1838-1916), an Austrian physicist, philosopher, mathematician, and physiologist, has given his name to the measurement of aircraft speeds. Mach was born in Moravia and held successive university chairs, first at Graz, in mathematics; second, Prague, in physics; and then in Vienna, in the philosophy of inductive sciences.

   Mach studied the action of bodies moving at high speeds through gases, and computed their speed in terms of the ratio between their velocity and the speed of sound. This, in air at sea level, is about 764 miles per hour. His experiments were published but remained obscure until the speed of aircraft began to approach the speed of sound. On October 14, 1947, thirty-one years after Mach's death, Captain Charles Yeager broke the sound barrier or, as scientists prefer to call it, "Mach 1."

   Mach's investigation into supersonic sound speeds of projectiles led to the present system for measuring speeds faster than sound—the Mach number. A Mach number under 1 (0.50) indicates a subsonic speed, a number over 1 (1.5, 3.2, etc.) indicates a supersonic speed—one faster than the speed of sound. Mach in addition gave his name to the Mach angle, the angle a shock wave makes with the direction of flight. Mach rejected Newton's idea about absolute space and time, which affected the thinking of Albert Einstein, and which cleared the way to one of the greatest discoveries for mankind—Einstein's theory of relativity.