the water in a bucket which contains a little very fine sand oscillate to and fro. We cannot, however, suppose that air oscillates this way backwards and forwards, though one current may easily flow over another in pufifs or gusts. We also often see rising mist dragging along a mountain side assume a very fleecy appearance, apparently owing to the effect of little eddies caused by friction along the ground. Here is a very good example of a rising drifting mist taken by myself in the Himalayas from an altitude of nearly ten thousand feet. You see that the lowest and thinnest part of the mist is decidedly fleecy in structure.
But whatever uncertainty there may be as to some of the conditions under which fleecy clouds are developed, whenever we do see them we do not think about flocks of sheep, or of who shepherds the herd, but of the upper currents of the atmosphere, and of their varying speed and direction, and of what circulation of the atmosphere will produce the woolly structure.
Flat Structure.
I shall pass by with barest notice the flat thin layers or sheets of cloud that are so often found in fine weather, and which are technically known as stratus-clouds. Here is a typical example from London (Fig. 3), and another one nearly from the Antipodes, at Ohinemotu in New Zealand.
Fig. 3.— Flat Cloud, usually known as Stratus. Taken in London.
