In the case of one series of such maps--viz., the daily synoptic charts of the North Atlantic, begun by Niels Hoffmeyer, of Copenhagen, and now prepared jointly by the Danish Meteorological Institute and the Deutsche Seewarte, in Hamburg--the charts have actually been published and sold, though they are so costly that the number of sets in libraries throughout the world is probably small. These remarkable charts present daily pictures of the winds and barometric pressure over the North Atlantic Ocean and the adjacent continents from 1873 to 1876, and from 1880 down to a recent date. From what we have already said it will be seen that the marine observers cooperating with the United States Weather Bureau and kindred institutions abroad are all contributing toward the great task of recording the history of the weather over the oceans from day to day and assembling data that can be digested in the form of marine climatic statistics and used as the basis for many scientific investigations. This concerted undertaking does not, however, constitute the whole scope of marine meteorology. Every intelligent mariner finds it necessary to acquaint himself with the laws of the winds, indications of coming storms, means of determining the proximity of icebergs, the systems of storm signals used in different countries, the method of constructing weather maps from wireless bulletins, etc. He ought, in short, to become an accomplished meteorologist. One of the classic problems of the navigator is that of handling his ship in a violent cyclonic storm, especially a tropical hurricane. The reader will recall that a cyclone, besides traveling as a whole at a rate of several hundred miles a day, consists of a system of winds rotating around the center. The result of this double motion is that the winds on one side of the center are not only more violent than those on the other, but they are also so directed as to drive a vessel running before the wind, across the storm track ahead of the advancing center, while those on the other side tend to drive a vessel to the rear of the storm. The two halves of the storm area are accordingly known as the “dangerous” and “navigable” semicircles, respectively. While this simple statement sets forth the fundamental facts involved, the actual problem is complicated by many features, such as the fact that the winds do not blow in circles, but more or less spirally, that the area of the storm cannot be readily determined, that two storms may occur in close proximity to each other, etc. The accompanying diagram, published by the United States Hydrographic Office, represents a cyclonic storm in the northern hemisphere, the circles being isobars, or lines passing through places at which the same barometric pressure prevails (indicated in inches), and the arrows indicating the direction of the winds. The diagram is thus explained: