Meeson. — On the recent Sun-glows. 367 



been burnt up — unless, indeed, the meteoric dust was in our own orbit and 

 moving with our velocity, in which case the phenomena would have been an 

 old acquaintance. Other objections to the cosmic dust theory are the 

 almost instantaneous appearauce and the intermittent character of the 

 phenomena. Every re-appearance would imply another cosmic cloud 

 moreover ; and the amount of meteoric dust requisite to produce the effect, 

 would have to be enormous and very exceptional. The number of meteor- 

 ites that enter our air daily is computed to be about 10,000,000, weighing 

 not less than 100, nor more than 1,000 tons altogether (Langley, " Scien- 

 tific American"). This amount of matter falling into the air, it is considered, 

 would be far too small to produce the result for which we are seeking the 

 cause. A further argument lies in the fact that particles of dust brought 

 down by rain and snow-water recently, when subjected to microscopic and 

 chemical analysis, are found to resemble volcanic matter rather than mete- 

 oric (Professor Kees, New York Academy of Sciences). So that, on the 

 whole, it may be concluded that, if it is dust in the upper air that causes 

 the sun-glows, that dust is not meteoric but terrestrial. 

 II. — The Aqueous Vapour Theory. 

 The precise cause of the brilliant colouring which so frequently beauti- 

 fies the heavens and accompanies the settmg and rising of the sun is not 

 positively known. It is, however, generally supposed to be owing to an 

 extraordinary amount of aqueous vapour (Lockyer, Times, 18th December, 

 1883), or dust (Hazen, "American Journal of Science") in the higher 

 atmosphere absorbing the blue rays which go to the composition of white 

 light, and diffusing the red. The connection between red skies and rainy 

 weather is recognized in the oldest folk lore. 



" Bed sky at night is the shepherd's delight, 

 Red sky in morning is the sailor's warning." 

 " When it is evening ye say it will be fine weather, for the sky is red. 

 And in the morning, it will be foul weather to-day, for the sky is red and 

 lowering." — Matt., xvi., 2, 3. I presume red in the sky in the evening 

 does not imply wet weather, but rather the reverse, because, it shows us 

 that the aqueous vapour in the air is following and passing away with the 

 setting sun. Just as the colouring of the deep sea is owing to the refrac- 

 tion and decomposition of light passing from one medium into another of 

 different density — shallow water appearing, as a rule, colourless — so aqueous 

 vapour in the atmosphere causes the absorption of certain component rays 

 of white light, and the others being dispersed in the air, impart their 

 colour. The ordinary phenomena of twilight and dawn are partly owing to 

 the same cause ; and it is said that when the sun is seen through the 

 funnel of a steam engine it appears green. It was natural, therefore, that 



