378 Transactions. — Miscellaneous. 



found the air full of dust not local in origin. So Piazzi Smyth also found 

 on Teneriffe at the height of a mile ; and on Mount Whitney, in South 

 California, 15,000 feet high, Langley found a sea of dust 6,000 feet deep. 

 Tyndall says, " What mainly holds the light in our atmosphere after the 

 sun has retired behind the earth is, I imagine, the suspended matter which 

 produces the blue of the sky, and the morning and evening red. Through 

 the reverberation of the rays from particle to particle, there may be at the 

 very noon of night a certain amount of illumination. Twilight must con- 

 tinue with varying degrees of intensity all night long, and the visibility of 

 the nocturnal firmanent itself may be due, not as my excellent friend Dove 

 seems to assume to the light of the stars, but in great part to the light of 

 the sun scattered in all directions through the atmosphere by the almost 

 infinitely attenuated matter held there in suspense " (" Hours of Exercise 

 in the Alps "). Incidentally, I must remark that this probably gives us 

 the true cause of the remarkable light that has been observed at midnight 

 in different places during the period of the most intense glows. Given ex- 

 ceptionally large quantities of dust in the air, and assuming the correctness 

 of Tyndall's theory, exceptional luminosity at night-time is just what we 

 should expect. 



4. In the next place, notice particularly Mr. Lockyer's argument that 

 the order of the first appearance of the sun-glows in different parts of the 

 earth is, upon the whole, in proportion to the distance of those places from 

 Krakatoa, and therefore such as we should expect if dust were the reflecting 

 or refracting medium of which we are in search. Generally speaking the 

 tropics first witnessed tbe displays, and first of all those parts of the 

 tropics nearest to Krakatoa. The temperate zones were reached at a later 

 time, and more irregularly — the irregularity of winds in the temperate 

 zones accounting for this naturally enough. I believe, if observations had 

 been carefully made at all the different centres of population, the steady 

 onward progress of the upper-air dust, as it radiated outwards from Kraka- 

 toa, would be even more apparent and convincing than it is now, — but to 

 understand that progress thoroughly we ought to know more than we do as 

 to the upper currents of wind in the earth's atmosphere. 



5. Dust in the upper air is sufficient to account for sun-glows, coloured 

 suns, and all the other phenomena. In the Loes district of China, where 

 the air is often laden with yellow dust, blue suns are constantly seen. 

 F.A.R.E., a writer in "Nature," 12th June, says that the weather in the 

 upper air must have been unusual, for ordinarily whatever matter may be 

 there assists the blue rays of light and scatters them, whereas lately the 

 blue rays have been absorbed. Now, a stratum of larger particles than 

 ordinary, 20 to 40 miles high and descending at the rate of 1,000 feet a day, 



