Even with all the earth's irregularities we do know a good deal with certainty about the earth's prevailing winds: the trades; the anti-trades; the south-west monsoon, which sets in so regularly in India that year by year its advent hardly varies by more than a day; and, in the descending scale of regularity, the east winds that usually sweep England in March, and the prevailing south-westerly to westerly winds which bend most of the young trees of the country a little to the north-east. Besides these regularly or irregularly defined winds, there are certain paths along the earth's surface where the winds always move like a trout stream with eddies in it. These eddies of the air we call cyclones, and they are continually travelling in one direction. No doubt they arise from the air in one place becoming hotter or moister than in the surrounding regions. As the air grows hotter it becomes lighter and ascends, while the heavier air round it pours in. These eddies always travel eastwards and incline in the northern hemisphere towards the north. They usually originate somewhere on the North American continent, and move across the Atlantic about the pace of a slow railway train, winds whirling round them all the time at a much greater pace. Usually the centres of these eddies bear northward past the north coast of Scotland to the north-west of Norway. Sometimes, however, they take a more southerly course, keeping to the south of the British Isles and passing over Central Europe on to Siberia, where they appear to die away.