  Mach’s name is, of course, associated with the speed of sound, where Mach 1 means the speed of sound in a given medium. Since the speed of sound varies with the density of the medium it is traveling through, Mach numbers are not absolute quantities but relational ones. In the late 1860s developments in gun and artillery technology produced projectiles that traveled faster than sound vibrations. This was realized over a period of time as two effects were investigated—a second loud ‘boom’, distinct from the usual one of the explosion, as well as a different type of wound. It was the latter that began Mach’s interest. In 1868 warring European powers had signed The St.  Petersburg Declaration that banned a new type of bullet that exploded on impact, thus causing ‘crater’ type wounds. Soon thereafter, however, similar types of ‘crater’ wounds appeared again. Various theories over their cause were put forward, leading Mach to investigate. By 1885 Mach had worked out the details of supersonic motion, along the way developing high-speed photographic techniques. Most importantly for engineers, Mach Number is the ratio of the speed of the object to the speed of sound in the given medium; his work is essential to modern aerodynamics, and through it the word ‘Mach’ has bizarrely entered into popular culture as an icon for razors, sound systems, fighter pilots, and high speed fuels.