The study of the movements of the atmosphere constitutes a rather formidable branch of science known as _dynamic meteorology_. This subject has engaged the attention of a number of able physicists--though far too few--and has begun to assume the character of an exact science, but is still fruitful of unverified hypotheses. We shall have only a little to say here about the theories and hypotheses relating to atmospheric circulation. They are at present, to a notable degree, in process of revision. Important modifications in them have resulted from the revelations of upper-air research, as well as from progress in other fields of inquiry. There are, however, a few fundamental matters that we must not ignore. We shall start with the solar heat that keeps the atmospheric machinery in motion. Of the heat that comes to us from the sun, it is estimated that more than one-third is reflected by clouds and the earth, or scattered by dust and air molecules, and thus passes back into space without having had any effect in heating the atmosphere. Part of the remainder heats the atmosphere directly, and the rest indirectly, after first heating the underlying land and water. In both cases, certain atmospheric gases--notably water vapor--absorb a great deal more heat than others. The first step in the production of a wind is a difference in temperature between two parts of the earth’s surface, and hence of the overlying air. Such contrasts of temperature always exist, both locally and on a large scale. The high sun of the equatorial regions heats the earth much more strongly than the low sun of high latitudes; a water surface has a more equable temperature than an adjacent land surface; a stretch of bare earth is warmer by day and colder by night than a neighboring tract covered with vegetation; and so on. Differences in atmospheric temperature produce differences in pressure, which gravity tends to adjust by setting up a circulation. The exact manner in which this circulation is begun and maintained is not yet perfectly clear, and current ideas on the subject are difficult to put into brief language. Meteorological writers now lay less stress than formerly upon the lateral spreading, at high levels, of air that has been heated and expanded at the earth’s surface, and the inward flowing of the lower air toward the heated area. There is, we know, an initial impulse that tends to drive air from a region of high pressure toward a region of low pressure; but the actual movement of the air is another matter. The “life history” of an air current is found to be a very devious affair. The important fact, for practical purposes, is that air does not flow in a straight line from the place where the pressure is high to that where it is low. As soon as it begins to flow it curves from the straight path, in accordance with Ferrel’s Law, which is thus stated: