[225] “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. [226] Sir David Brewster’s _Optics_, and Memoirs in the Philosophical Transactions. Sir John Herschel’s Treatise on _Light_, Encyclopædia Metropolitana. [227] _On the Colour of Steam under certain circumstances_: by Professor Forbes; Philosophical Magazine, vol. xiv. p. 121, vol. xv. p. 25. In the first paper the following remarks occur:--“I cannot doubt that the colour of watery vapour under certain circumstances is the principal or only cause of the red colour observed in clouds. The very fact that that colour chiefly appears in the presence of clouds is a sufficient refutation of the only explanation of the phenomena of sunset and sunrise, having the least plausibility, given by optical writers. If the red light of the horizontal sky were simply complementary to the blue of a pure atmosphere, the sun ought to set red in the clearest weather, and then most of all; but experience shows that a lurid sunrise or sunset is _always_ accompanied by clouds or diffused vapours, and in a great majority of cases occurs when the changing state of previously transparent and colourless vapour may be inferred from the succeeding rain. In like manner, terrestrial lights seen at a distance grow red and dim when the atmosphere is filled with vapour soon to be precipitated. Analogy applied to the preceding observations would certainly conduct to a solution of such appearances; for I have remarked that the existence of vapour of high tension is by no means essential to the production of colour, though of course a proportionally greater thickness of the medium must be employed to produce a similar effect when the elasticity is small.”