Note 225 — Chapter XII

“This scale, in which the humidity of the air is expressed, is the simple natural scale in which air at its maximum of humidity (i.e., when it is saturated with vapour) is reckoned as = 100, and air absolutely deprived of moisture as = 0; the intermediate degrees are given by the fraction 100 × actual tension of vapour ÷ tension required for the saturation of the air at its existing temperature. Thus, if the air at any temperature whatsoever contains vapour of half the tension, which it would contain if saturated, the degree is 50; if three-fourths, then 75; and so forth. Air of a higher temperature is capable of containing a greater quantity of vapour than air of less temperature; but it is the proportion of what it does contain, to what it would contain if saturated, which constitutes the measure of its dryness or humidity. The capacity of the air to contain moisture being determined by its temperature, it was to be expected that an intimate connection and dependence would be found to exist between the annual and diurnal variations of the vapour and of the temperature.”—Sabine, On the Meteorology of Toronto; Reports of the British Association, vol. xiii. p. 47. The Temperature Tables: by Prof. W. H. Dove; Reports for 1847 should be consulted.