A cyclone owes its origin to some local excess of heat, such as might arise from a heavy rainfall, the heat causing an upward movement in the air. The inrush of cool air which then follows begins a circular or whirling motion. The moist air in front of the cyclone gives up its moisture with the fall in temperature, causing the rains that are invariably found in front of such a movement. The air after the rainfall is dry and warmer, and its ascent keeps up a partial vacuum or area of low pressure, which is the centre or vent of the cyclone. It is really the rainfall in front of the cyclonic system that causes its forward movement, assisted by the rotation of the earth. Each space relieved of its moisture forms in its turn the new centre. A coast-line, or an anti-cyclonic movement of the air in front of a cyclone, will alter its course. When one reaches the shores of Europe, it soon spends itself for want of the moisture-laden winds in front to keep up the system. In the northern hemisphere the direction of rotation of a cyclone is opposite to the movement of the hands of a watch; in the southern hemisphere it is in the same direction as the movement of the hands of a watch. This is the effect of the rotation of the earth, as will be clear after a little thought on the matter. In the North Atlantic the forward motion of a cyclone is always from the westward to the eastward; hence the “storm warnings” which reach us from the United States. [Illustration: _Diagram of a typical Atlantic cyclone._] Our islands lie in the most common track of those Atlantic cyclones, and the centre of the whirl often passes over or near the Orkneys. Now if you will look at the chart or diagram of a typical cyclone as given here, and suppose it to be moving slowly from south-east to north-west, or suppose yourself to be moving through it in the opposite direction while it remains still, you will see how the changes of wind and weather which we have described must result from this movement.