In becoming heated it necessarily expands, and becoming lighter that the stratum immediately above it, ascends, drawing in, to take its place, the cooler air that has been in contact with surfaces of air or water less strongly heated. Now because, as we have seen, water heats and cools more slowly than land surfaces, along a coast there is always a tendency to the production of a sea wind from the cooler sea to the hotter land during the latter part of the day, and inversely of a land breeze from the more rapidly cooled land to the slowly cooling sea in the early morning; and in the Tropics, where the sun’s rays are sufficiently near the vertical to produce a rapid and marked effect, these diurnal land and sea breezes form a characteristic feature of littoral climates, and go far to render life tolerable in them. Hence also it follows that everywhere littoral and marine climates tend to uniformity, not only as to diurnal but also as to annual variations, while continental climates tend to wide variations of temperature, and the seasons differ to a degree never experienced in places on or near the sea. As an example may be contrasted the climates of Madeira and Peshawar, in the former of which the difference between the mean temperatures of the coldest months is under 13° F. (7·2 C), while in the latter the difference amounts to 40° F. (22·2 C), or three times as great in the continental climate. It is further noteworthy that while, in the continental climate, the hottest and coldest months coincide with the summer and winter solstices, in the marine climate they lag a month or two after; the coldest month being February and the hottest August in Madeira, owing to the slowness with which the water surrounding the island gains and parts with its heat. Just as the alternating heat of day and coolness of night produce, in littoral regions, the daily land and sea breezes, so the greater heating of the world’s surface over the Tropics produces, throughout the year, a steady flow of air from the north and south, to take the place of the air that has become rarified, and so floated to a higher level. These winds--“the trades”--are not, however, directly from the north and south respectively, but have also a great deal of easting in their direction, a circumstance which is explained by the fact that the air, coming as it does from latitudes where the circumference of the earth is much smaller than at the Equator, is moving from west to east, only at the comparatively slower pace of the rest of the earth’s surface at that latitude; and as they do not at once acquire the quicker motion of the latitudes to which they have travelled, they lag behind the points of the earth moving beneath, and so give the effect of an easterly breeze, just as would be the case with a vehicle driving rapidly eastward through still air.