36 Professor Forbes on the Colours of the Atmosphere. 



air is highly charged with moisture, is to the same purpose. 

 But a remark of Mr. Forster, in his " Researches about 

 Atmospheric Phoenomena*," is even more pointed, and is 

 valuable, because his work is pre-eminently descriptive, rather 

 than theoretical. *' Sometimes the tints in the twilight haze 

 come on so suddenly and are so circumscribed, as to induce a 

 belief that very sudden and partial changes take place in the 

 atmosphere at eventide; which may perhaps he somehow con- 

 nected with the formation of dew." He then records an ob- 

 servation made 2nd November 1822. " Being about four 

 o'clock in the evening, near Croydon in Surrey, I observed 

 a very beautiful western sky, caused by the bright edge and 

 dependent fringes of a light bed of cloud being finely gilded by 

 the setting sun. Some detached cirrocumuli also, which 

 formed the exterior boundaries of the aforesaid cloud, were 

 likewise of a fine golden-yellow, and the same colour appeared 

 in different clouds in other parts of the sky, while the scud- 

 like remains of the nimbus floated along in the west wind 

 below. In the course of about a quarter of an hour, the lofty 

 gilded clouds all assumed a deep red appearance, and the 

 change was effected so suddenly, that while looking at them, 

 I only took my eyes 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 pheenomenon 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 no- 

 thing short of a demonstration of the truth of my theory of 

 Atmospheric Colour, the more interesting, because I was un- 

 acquainted with it until after writing nearly the whole prece- 

 ding 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 abso- 

 lute dampness. The vapours, which, during the reversion of 

 the process, might probably produce colour, are not elevated 

 until the action of the sun upon the earth's surface has conti- 

 nued 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 show, that the slowly progressive 



# Third edit. p. 87. 



