Our description of the three cloud-types of cirrus, cumulus and stratus, makes it clear that we have to do with a self-contained symmetrical system of forms, within which the two outer, dynamically regarded, represent the extreme tendencies of expansion and contraction, whilst in the middle forms these are held more or less in balance. By adding Howard's nimbus formation to this system, we destroy its symmetry. Actually, in the nimbus we have cloud in such a condition that it ceases to be an atmospheric phenomenon in any real sense of the word; for it now breaks up into single drops of water, each of which, under the pull of gravity, makes its own independent way to the earth. (The symmetry is restored as soon as we realize that the nimbus, as a frontier stage below the stratus, has a counterpart in a corresponding frontier stage above the cirrus. To provide insight into this upper frontier stage, of which neither Howard nor Goethe was at that time in a position to develop a clear enough conception to deal with it scientifically, is one of the aims of this book.)