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 “Mach,” one page on a 4.75 x 3.25 postcard, September 25, 1875. Untranslated note sent to a gentleman in the Penzing borough of Vienna. Mild show-through from postmark stamp and slight surface loss affecting the recipient’s name in the address, otherwise fine condition. RR Auction COA.
