A somewhat more familiar variety is shown in Plate 8. Little irregular feathers of cirrus, from which long tapering streamers point downwards in graceful curves, or else lag behind in the direction from which the clouds have travelled. If clouds of this type are carefully watched, it will soon be seen that each feathery head is a centre of condensation, and that the tails or streamers are nothing else than falling particles, which dwindle slowly away by evaporation, and which gradually sink below the level of the heads. It is usual, in dealing with cloud-forms like these, to speak of air-currents of different velocities almost as if the winds at different levels were as clearly separated as oil and water, or even air and water. This can hardly be the case, for if such a thing should occur as an air-current of one velocity flowing over another of less speed, or of a current in one direction over another moving in a different course, the two must inevitably mix at their junction, and in a very short time the passage from the lower current to the upper one would be quite gradual. No doubt we can often observe two, three, or more layers of cloud moving in different directions; but if we were to send up a balloon, it would be rare indeed to find its direction of horizontal movement changed in a few metres of ascent. Different and distinct air-currents are often invoked to explain cloud-forms quite unnecessarily. It is far more likely that the differential movements involved in the explanation of the features of these cirrus varieties are due to increased velocity with greater altitude, to progressive change of direction, to irregular eddies, or to the interaction of ascending and descending convection currents. Indeed, it is probable that careful study of the growth and decay of these clouds will, in time, lead to a clearer understanding of atmospheric movements, and so enable us to say more precisely why they are as we see them to be. The variety shown in Plate 8 is rare except in combination with other forms. It might well be termed tailed cirrus or cirrus caudatus. [Illustration: Plate 8. TAILED CIRRUS. (_Cirrus Caudatus._)]