OF SMOKE AND DUST.

If the sun is in the East the smoke of cities will not be visible in
the West, because on that side it is not seen penetrated by the
solar rays, nor on a dark background; since the roofs of the houses
turn the same side to the eye as they turn towards the sun, and on
this light background the smoke is not very visible.

But dust, under the same aspect, will look darker than smoke being
of denser material than smoke which is moist.

The effect of wind on trees (470-473).