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Much of this was of course ground to impalpable dust by
the violence of the discharge, and was carried up to a height of many
miles. Here it was caught by the return currents of air continually
flowing northward and southward above the equatorial zone; and since,
when these currents reach the temperate zone, where the surface rotation
of the earth is less rapid, they continually flow eastward, the fine
dust was thus carried at a great altitude completely around the earth.
Its effects were traced some months after the eruption in the appearance
of brilliant sunset glows of an exceptional character, often flushing
with crimson the whole western half of the visible sky. These glows
continued in diminishing splendour for about three years; they were seen
all over the temperate zone; and it was calculated that, before they
finally disappeared, some of this fine dust must have travelled three
times round the globe.
The same principle is thought to explain the exquisite blue colour of
the deep seas and oceans and of many lakes and springs.


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