Mach number which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as "Mach's number", never "Mach 1". Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables. As modeled in the International Standard Atmosphere, dry air at mean sea level, standard temperature of , the speed of sound is . The speed of sound is not a constant; in a gas, it increases as the absolute temperature increases, and since atmospheric
.17 Mach IV .17 Mach IV The .17 Mach IV is a wildcat centerfire rifle cartridge, based on the .221 Remington Fireball case, necked down to fire a .172 bullet. The cartridge was introduced in 1962 by Vern O’Brien. The cartridge offered an easy case conversion and good ballistics, but could not compete against the .17 Remington. The name, Mach IV, comes from the claim that the bullets can reach 4,000 ft/s. Due to the relatively small case capacity, even small variations in powder of can lead to the difference between a safe and dangerously over pressure load. The .17 Mach IV became