ALL who have the faculties proper to man must have been to some extent students of cloud form. Go where we will, do what we will, we cannot easily escape from the sky, or avoid noticing some of its features and coupling them with the varying conditions of weather. We all sometimes want to know if it is likely to rain, or whether some other change is probable; and experience soon shows us that the clouds give the simplest and most obvious indication of what we may expect. It is almost impossible to avoid noticing that certain types of cloud, or the simultaneous appearance of certain types, is the usual accompaniment of definite kinds of weather or of particular changes. Thus it is that most people acquire some small measure of weather wisdom before their schooldays are over. Generation after generation, through all human history, the same causes must have led to the same conclusions; and the study of clouds must, therefore, be one of the oldest of all branches of scientific inquiry. Yet, old as it is, it is still in its infancy, having made very little advance indeed towards the precision of an exact science. There are many reasons for this want of growth, and so far as the theoretical aspects of the subject are concerned it is easy enough to understand. Clouds are among the most inaccessible of terrestrial objects. Except by balloon ascents, by sending up kites bearing recording instruments, or by making observations among the mountain-tops, we have no means of getting at them to study the conditions under which they exist. Temperature, pressure, humidity, have generally to be guessed at, those guesses being based on the scanty data which have been laboriously obtained by one or another of these cumbrous methods. Moreover, many clouds have such vast dimensions that it is very difficult to grasp all that goes on in such a space. Besides the difficulty of attacking the problems presented by cloud formation, it is probable that even if we could have got among the clouds at will, we should have understood little more than we do, from a want of sufficient certainty on many of the purely physical questions involved. It is not many years since Mr. J. Aitken discovered the necessity for material nuclei as a first step in the formation of cloud particles, and not many months have elapsed since Mr. C. T. R. Wilson showed that those particles can be formed by the action of radiation on the air itself. There is nothing surprising, therefore, in the fact that our theoretical knowledge of the why and wherefore of the facts revealed by a study of clouds is limited to general principles, and quite fails to say exactly why each special form should be assumed. The matter for surprise is quite different.