878 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 the 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.R., 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, 
