In very many cases, perhaps in a large majority of them, the predictions made by the weather bureau substantially come to pass. It has been stated in former chapters that the changes of weather accompany the movements of what are called cyclones and anti-cyclones, the cyclone being accompanied by low barometric pressure and the anti-cyclone by a higher one. The winds of the cyclone move spirally around the center of lowest depression with an upward trend, the motions being in a direction reversed to that of the hands of a clock. In the centers of high pressure the current is downward instead of upward and the direction of the wind around it is opposite to that around the low-pressure area. The fundamental factor in predicting the weather is the direction of movement of these areas of low pressure. In almost all cases the direction of movement is from the west to the east, but not always in a straight line. These movements, however, are classified so that after the direction has become established one can predict with considerable accuracy as to whether it will move in a curved or a straight line. By movement we do not refer to the direction of the wind at any particular point, but the onward movement of the whole cyclonic system, which is usually from twenty-five to thirty miles an hour, but in some cases the speed is much greater. Not only does the upward movement of the whole system vary, but the velocity of the wind around any given cyclonic center varies. There are about eleven classes of cyclones that appear in the United States, each class having its own path of movement and origin. A large number of these appear to originate north of the Dakotas, and move directly east to the Gulf of St. Lawrence. Three other classes originate on about the same line, a little west,--say, north of Montana,--moving first in a southeasterly direction, passing over the center of Lake Michigan and bending northerly through Lake Ontario and finally landing in the Gulf of St. Lawrence. Two other classes start at the same point, one of them going as far south as Cincinnati, and the other as far south as Montgomery, Ala., and both turning at these points northeasterly to the Gulf of St. Lawrence. Two other classes originate in Colorado, one moving in a northeasterly direction slightly curved, and the other directly east. Still others have their origin farther south in the Gulf of Mexico, and move in a northeasterly direction. Very rarely they originate in the Atlantic east of Savannah, moving first in a northwesterly direction, but finally bending to the northeast.