But within the last thirty years a new treatment of weather problems has been introduced, known as the synoptic method, by which the whole aspect of meteorology has been changed. By this method, a chart of a large area of the earth’s surface is taken, and after marking on the map the height of the barometer at each place, lines are drawn through all stations at which the barometer marks a particular height. Thus a line would be drawn through all places where the pressure was 30.0 inches, another through all where it was 29.8 inches, and so on at any intervals which were considered necessary. These lines are called “isobars,” because they mark out lines of equal pressure. When these charts were first introduced, the estimation of the value of the mean pressure was so great that, instead of drawing lines where pressure was equal at the moment, they were drawn through those places where the pressure was equally distant from the mean of the day for each place. These lines were called “is-abnormals”; that is, equal from the mean. After the isobars have been put in, lines are usually drawn through all places where the temperature is equal at the moment. These are called “isotherms,” or lines of equal temperature. Then arrows to mark the velocity and direction of the wind are inserted; and finally letters, or other symbols, to denote the appearance of the sky, the amount of cloud, or the occurrence of rain or snow. Such a chart is called a “synoptic chart,” because it enables the meteorologist to take a general view, as it were, over a large area. Sometimes they are called “synchronous charts,” because they are compiled from observations taken at the same moment of time. [Illustration: Typical Forms of Clouds 1, Squall Cumulus; 2, Pillar Cumulus; 3, Cirrus; 4, High Stratus and Cumulus] When these came to be examined, the following important generalizations were discovered: 1. That in general the configuration of the isobars assumed one of seven well-defined forms. 2. That, independent of the shape of the isobars, the wind always took a definite direction relative to the trend of those lines, and the position of the nearest area of low pressure. 3. That the velocity of the wind was always nearly proportional to the closeness of the isobars. 4. That the weather--that is to say, the kind of cloud, rain, fog, etc.--at any moment was related to the shape, and not the closeness, of the isobars, some shapes inclosing areas of fine, others of bad, weather. 5. That the regions thus mapped out by isobars were constantly shifting their position, so that changes of weather were caused by the drifting past of these areas of good or bad weather, just as on a small scale rain falls as a squall drives by. The motion of these areas was found to follow certain laws, so that forecasting weather changes in advance became possible.