Meeson. — On the recent Sun-glows. 377 



" Knowledge " to have been first advanced by G. T. Symons in a letter to 

 the Times of 1st December. But in these colonies the theory was advanced, 

 I feel pretty sure, long before that date. It was repudiated in fact by Mr. 

 Ellery on the 15th November, before the Eoyal Society of Melbourne, and 

 had been advocated both here and in Victoria some time before that. 



It will be well perhaps if we consider in the first place the various 

 points that are in favour of this view. 



1. We have already by anticipation considered the certainty that there 

 was something unusual projected into the upper air, and that it was thrown 

 there by the Krakatoa eruption, which, as says Professor Eees, was the 

 greatest eruption on record. 



2. The immense quantity of dust ejected from Krakatoa suggests that 

 dust was the thing thrown into the higher atmosphere. As Langley says — 

 " Krakatoa ejected millions of tons which would not soon sink." Thirty 

 miles away from Sunda the ashes fell in such quantities as to make pitch 

 darkness at noon-day. Java was like Holland in its garb of snow. White 

 dust fell on the decks of vessels from 300 to 400 miles away. 



3. Dust in the upper air is not an inherent improbability, because the 

 air at all heights is always more or less charged with quantities of dusty 

 particles, as is easily seen when an isolated ray of light is admitted into a 

 dark room. The motes are always in the sunbeam. The smoke of Chicago 

 was seen, again, on the Pacific Coast, so that dust in the air travels far ; 

 and Nordensfeld found the fissures in Greenland ice full of fine dust. 

 Where did this come from if not from the air ? In Italy from time to 

 time also there occur veritable showers of sand brought over by the sirocco 

 from Africa. Piazzi Smyth, in 1872, noted such an occurrence in Palermo 

 Bay and, if I mistake not, I have read somewhere that the red sand of the 

 Sahara, carried by the Harmattan and other desert winds, finds its way to 

 immense distances across the Atlantic. Dr. Taylor says that the air on the 

 very summit of Monte Mazo, 9,000 feet high, is full of Bacteria, mould, 

 spores, etc. In fact, though ordinary dust is not carried very high and is 

 always denser in the lower strata of the atmosphere, which was demon- 

 strated by Tyndall's experiments on the Bel- Alp, there is always dust in 

 the air in greater or less quantities. This is clearly shown by the pheno- 

 mena of radial polarization of light (i.e. the reflection or refraction of light 

 so that it has new properties — sides as they are called of different intensi- 

 ties), and to some extent it is also shown by the beautiful effects which we 

 call twilight and dawn, though the principal cause of these is the reflection 

 of light from the air itself and the clouds and vapour suspended in it. 

 Professor Langley says that there is a zone of dust to the height of three 

 miles all round the earth. In 1878 he wintered on Mount Etna and he 



