The fact just described has an interesting practical result. If after a day or night of storm and rain, the temperature falls, we know that the depression causing the storm has passed us, and that we are feeling the effects of the colder winds in its rear. If the thermometer suddenly rises again, then a new depression is approaching, and we are feeling its warm breath before its winds reach us. The clearness and chilliness of the air after a stormy or windy period gives us one of our commonest meteorological sensations, and produces a marked psychical effect, reflected in much of our literature. One other reason for the eastward motion of the cyclones with us is that they seem to prefer damp air, and so tend to follow the North Sea and pass towards the Baltic, where they often die away. In the British area, though the depressions move faster in winter than in summer, they have only a mean speed of about 16 miles an hour, while in the United States their mean speed is 25 miles per hour, and their effects are often disastrous except when discounted by the warnings of the Weather Bureau. In the case of the British Isles cyclones are most frequent and best marked in winter, and they are of great importance in producing our mild and windy winters. In summer they travel further northwards, and as a rule affect our climate less. When, however, from causes still inadequately known, they are better marked in summer than usual, we have a "bad" summer, that is, one which is wet and relatively windy. The fact that the English Channel is one of the favourite tracks of cyclones has been an important element in protecting the British Islands from foreign invasion, while we all know that it is also a factor in diminishing free intercourse with the Continent. [Illustration: Fig. 11.--Diagram showing the changes in temperature, pressure and wind due to a cyclone passing to the north of a point of observation A. The passage of the cyclone figured occupied a period of six days. It will be noted that as it approaches A the wind is southerly and light (arrows with single barbs) and the temperature high. As it passes the winds become violent (arrows with double barbs), and shift to the south-west, and the barometer falls rapidly. As it disappears the pressure rises, the temperature falls, and the wind veers to the north-west, while remaining violent. The fall of the wind and its shifting to a south-westerly direction mark the return to the normal condition of affairs, the influence of the cyclone being past.]