_Cyclones._--This name is applied to the great atmospheric disturbances marked by an inflowing of air towards a centre of low barometric pressure from adjacent regions, commonly several hundred miles across, and an escape and overflow aloft. As in whirlwinds and tornadoes, there is a spiral movement established in the inflowing currents, but owing to the large size of the area of low pressure, this seldom reaches destructive violence. Cyclonic storms are of common occurrence, especially in the temperate zone, and bring to that region its characteristic diversity of weather. Most of the rain and snow-storms of the continent are due to the vast swirls of the atmosphere about areas of low atmospheric pressure, which cause air-currents from different directions and with different components of heat and moisture to move over the land. The cyclonic storm of the Mississippi Valley, the Atlantic coast States, and southeastern Canada frequently originate in the Great plateau province, and are carried towards the Atlantic owing to the influence of the eastward-blowing winds of the upper atmosphere. At times these storms are of such magnitude and intensity that they cross the Atlantic and are observed in England and Scandinavia. The courses they follow may be traced from day to day on the weather-maps issued by the United States Weather Bureau, and from the directions they are likely to follow and the atmospheric conditions pertaining to their various parts predictions of surprising accuracy as to the changes which the weather in a given locality will experience can be made one or two days before the changes occur. _Hurricanes._--Cyclonic storms of the general nature of the tornadoes, but of vastly greater extent and intensity, originate occasionally during the latter portion of the summer season over the tropical portion of the north Atlantic, move slowly westward to the vicinity of the Lesser Antilles, where normally their courses bend northward, and then skirt the Atlantic coast of the United States and drift eastward under the influence of the eastward-flowing upper air-currents, and not infrequently make their influence felt in the western portion of Europe. Occasionally, on account of the presence of an area of high barometric pressure to the north of Cuba, the course of one of these tropical hurricanes, as they are termed, is rendered irregular, and it passes over the Atlantic States or is deflected still more and crosses the Gulf of Mexico before reaching the border of the continent, as was the case in September, 1900, when a large part of Galveston was destroyed. The normal paths of the tropical hurricanes as they approach the coast of the United States and the exceptional course of the one which passed over Galveston, are indicated on the map forming Fig. 26.