dark.
The black clouds which are often seen higher up than those which are illuminated by the sun are shaded by other clouds, lying between them and the sun.
Again, the rounded forms of the clouds that face the sun, show their edges dark because they lie against the light background; and to see that this is true, you may look at the top of any cloud that is wholly light because it lies against the blue of the atmosphere, which is darker than the cloud.
[Footnote: A drawing in red chalk from the Windsor collection (see Pl. XXIX), representing a landscape with storm-clouds, may serve to illustrate this section as well as the following one.]
477.
OF CLOUDS, SMOKE AND DUST AND THE FLAMES OF A FURNACE OR OF A BURNING KILN.
The clouds do not show their rounded forms excepting on the sides which face the sun; on the others the roundness is imperceptible because they are