The correction of the power of a spark-ignition engine for variations of atmospheric pressure, humidity, and temperature, is based upon the two following assumptions: (1) The power varies directly with the dry-air pressure at the carburetor or throttle entrance. (2) The power varies inversely as the square root of the absolute air temperature at the carburetor or throttle entrance. Power measurements can be compared only when they are reduced in accordance with these assumptions. Prior to 1930, power assumptions were based upon the variations in total air pressure and air temperature without regard to the effects of humidity. This report compiles correction factors which include the effects of humidity in addition to air pressure and temperature.