“In whatever direction a body moves on the surface of the earth, there is a force arising from the earth’s rotation that deflects it to the right in the northern hemisphere, and to the left in the southern hemisphere.” This law applies to all bodies moving freely over the earth, and not merely to the winds. At the earth’s surface, if the atmospheric pressure is measured simultaneously at various places by means of barometers, we can get a clear picture of the horizontal distribution of pressure by drawing on a map lines, called _isobars_, through places at which the pressure is identical. The isobars reveal the presence of extensive areas over which the pressure is above the average and of others over which it is below the average. If, at the same time, we chart the flow of air by indicating the direction of the winds at various points, we shall notice that the air shows a strong tendency to travel _around_ these areas; and if we could observe its course a thousand feet or so above the earth we should find the tendency even more pronounced at that level. Another important law, springing in part from Ferrel’s Law and describing the movements of the air around areas of high and low pressure, is called Buys Ballot’s Law. One way of stating this law is as follows: “If you stand with your back to the wind, in the northern hemisphere, the barometer will be lower on your left than on your right. The reverse is true in the southern hemisphere.” The reader, whether he lives in the United States or any other civilized country, will have no difficulty in securing documentary evidence of the correctness of Buys Ballot’s Law in the shape of the daily weather maps issued by the various meteorological services. On a weather map showing conditions anywhere in the northern hemisphere it will be found that the winds (which are indicated by little arrows), though subject to a good many local variations, have a general tendency to blow in the direction followed by the hands of a clock (“clockwise”) around an area of high pressure, and in the opposite direction (“counterclockwise”) around an area of low pressure. It will likewise be noticed that, in general, the winds, instead of blowing along the isobars, are strongly inclined inward in the case of a low-pressure area and outward in the case of a high-pressure area. Lastly, if the map indicates the force of the winds at different places, it will be seen that winds are strongest where the isobars are close together and weakest where they are far apart. It is a matter of much interest to aeronauts that the force of the wind generally increases with altitude, and that, in the lower flying levels, the winds are little, if any, inclined to the isobars.