PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 391 



off them for a minute to stop down the tobacco in a pipe that I was smoking, and 

 when I looked up at them again, the colour was totally changed. Now, what 

 renders the phenomenon remarkable is, that it happened just about the period of 

 the vapour point. The descending sun had scarcely had time to make any great 

 difference in the angle of reflection, and it seemed therefore, that some sudden 

 change, produced by the first falling dew, was the cause of this simultaneous 

 change of colour in all the clouds then visible." I confess it seems to me that 

 this passage is nothing short of a demonstration of the truth of my theory of 

 Atmospheric Colour, the more interesting, because I was unacquainted with it 

 until after writing nearly the whole preceding part of this paper. 



With regard to the Morning the case is very different. In fine weather the 

 strata near the surface of the earth alone, and in the lowest and most sheltered 

 spots, are in a state of absolute dampness. The vapom-s, which, diu-ing the re- 

 version of the process, might probably produce colom-, are not elevated until the 

 action of the sun upon the earth's surface has continued long enough to impart 

 a sensible warmth, by which time the moment of sunrise is past, and the sun's 

 disc has risen above the horizontal vapours. It would be easy, by a more 

 lengthened discussion, to shew, that the slowly progressive transition of vast 

 masses of air through the temperature of the dew-point, can only occur in serene 

 weather at sunset and not at sunrise. The inflamed appearance of the morning 

 sky, considered indicative of foul weather, is, I have no doubt, owing to such an 

 excess of humidity being present, that clouds are actually being formed by con- 

 densation in the upper regions, contrary to the direct tendency of the rising sun 

 to dissipate them, which must therefore be considered as indicating a speedy pre- 

 cipitation of rain. 



Edinbukgh, ith February 18-39. 



VOL. XIV. PART IL 3 G 



