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).
470.
OF REPRESENTING WIND.
In representing wind, besides the bending of the boughs and the reversing of their leaves towards the quarter whence the wind comes, you should also represent them amid clouds of fine dust mingled with the troubled air.
471.
Describe landscapes with the wind, and the water, and the setting and rising of the sun.