Meeson. — On the recent Sun-ylows, 885 



8, Again it may be asked, if dust were the cause of the sun-glows, why 

 were they not seen in the daytime ? Whatever foreign material in the 

 upper air caused the glows, this difficulty would remain the same. Prob- 

 ably the greater perpendicularity of the sun and therefore the greater 

 intensity of his light during the daytime account for the glows then being 

 absent. Towards night and morning, also, his rays would traverse a greater 

 length of the dust-laden upper air. 



9. Lastly, if dust caused the green and blue suns and the air is always 

 dust-laden, why do we not have such phenomena more frequently ? Ordin- 

 arily the dust in the air is not very high, and the particles near the earth 

 are large, and aqueous and gaseous vapour counteract the effect of dust by 

 absorbing more light at the blue end of the visible spectrum than at the 

 red. 



In conclusion, there is no reason, as far as I know, why we should con- 

 fine ourselves to any one theory. Perhaps many causes have co-operated 

 accidentally to produce one phenomenal result. If we must have only one 

 theory, I incline to the Krakatoa dust theory, because I think the balance of 

 evidence is in its favour. In any case, whatever the upper-air medium 

 may have been, it certainly seems to have emanated from the Straits of 

 Sunda during the memorable eruption of 27th August. However, when we 

 come to speculate upon the nature of the substance thrown into the upper 

 currents, difficulties and objections and doubts meet every suggestion or 

 supposition. We shall, therefore, if we are wise, guard ourselves against 

 all dogmatism on the subject, particularly as fresh facts and fresh theories 

 are coming forward daily. 



Even as I write there appears, in the notes on popular science by 

 Dr. Taylor in the pages of the Australasian, a modification of the aqueous 

 vapour theory which is well deserving attention. It appears that Dr. 

 Prince, a well-known meteorologist, thinks the phenomenal sunsets are 

 due to the crystallization of saline particles from masses of sea- 

 water ejected in the form of vapour into the upper air by the Krakatoa 

 eruption, and he argues that the greatest displays having occurred in the 

 coldest weather, can only be accounted for on the theory that the crystal- 

 lization of saline products is a great factor in their production. So long ago 

 as last January, M. Beyerinck, of Wageningen, collected from rain which fell 

 during a storm succeeding a fine after-glow a remarkable quantity of com- 

 mon salt. Again, I find in "Nature" of 3rd July, that M. Gay, in a paper 

 read before the Paris Academy of Sciences on the 23rd June, not only con- 

 nects the recent sun-glows with the Krakatoa eruption, but thinks that the 

 persistent rains of this year are owing to the same cause, directing attention 

 to the fact that after the volcanic eruptions of 1783, 1831, 1856, and 1862, 

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