Absolute temperature is measured from the absolute zero which is obtained by assuming that 'it is possible to continue the cooling of a perfect gas until its volume is diminished to nothing, following hereby the law of expansion of heat. The absolute zero is considered to be equal to F. Below the melting point of ice on the air thermo meter, thus corresponding to 32° F. Of the freezing point of the latter. Absolute pressure is measured from a perfect zero of pressure that is lbs. Below the atmospheric pressure, or as is mostly used lbs. Of a metric atmosphere, which was taken for the construction of the diagrams.