The trade winds are regular and constant. These were much followed in the days of old. A vast amount of air in the tropics gets heated and ascends, being lighter, and travels to the colder north. A strong current rushes in from the north to take its place. But the earth rotates round its axis from west to east, and the combined motions make two slant wind directions, which are called the "trade winds," because they were so important in trade navigation. Among the periodical winds are the "land and sea breezes." During the day, the land on the sea coast is warmer than the sea; accordingly, the air over the land becomes heated and ascends, the fine cool breeze from the sea taking its place. Towards evening there is the equilibrium of temperature which produces a temporary calm. Soon the earth chills, and the sea is counterbalancingly warm--as sea-water is steadier as to temperature than is land--the air over the sea becomes warmer, and ascends, the current from the land rushing in to take its place. Hence during the night the wind is reversed, until in the morning again the equilibrium is restored and there is a calm, so far as these are concerned. These are therefore called the "land and sea breezes." Of course, it is within the tropics that these breezes are most marked. By the assistance of other winds, a hurricane will there occasionally destroy towns and bring about much damage and loss of life; but better that hundreds should perish by a hurricane than thousands by the pestilence which, but for the storm, would have done its dire work. In countries where the differences of pressure are more marked than the differences of temperature, in the surrounding regions the strength of the wind thereby occasioned is far stronger than the land and sea breezes. The variable winds are more conflicting. These depend on purely local causes for a time, such as "the nature of the ground, covered with vegetation or bare; the physical configuration of the surface, level or mountainous; the vicinity of the sea or lakes, and the passage of storms." Among these winds are the simoom and sirocco.