Austrian physicist and philosopher (1838-1916) who, as a result of his research into the effects of supersonic velocity, lent his name to the ratio of the speed of a projectile to the speed of sound, aka the Mach number. Though Einstein would later credit him as the forerunner of the Theory of Relativity, Mach ultimately rejected Einstein's theory. ALS in German, signed “E. Mach,” one page on a 5.5 x 3.5 postcard, July 20, 1897. Untranslated letter to R. Innker in Vienna. In fine condition, with two file holes to the top.
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