The wind moves, with varying velocity. We have a gentle breeze when the motion of the air is about five or six miles an hour, a good breeze at twenty-five miles an hour, a high wind at thirty-five, and a gale at fifty. Hurricanes travel at sixty and seventy miles an hour, and do enormous damage. Near the Equator we do not find much wind, and this fact has caused the name of the Region of Calms, or “The Doldrums” of sailors, to be bestowed upon that portion of the globe, but this belt of calm has no fixed position. It follows the sun’s course, and is the region of greatest heat, and, as it were, the centre of a concentric circle of currents. The hot air rises and goes away; air rushes in north and south, and causes what are called the North-East and South-East Trades, or Trade Winds, owing to their being so useful in commerce for ships, or to the old meaning of the word trade, a “regular course.” The calms of the Tropic of Cancer are called the “Horse Latitudes.”

Readers of the life of Columbus will remember how his crew were affrighted at the persistency of the wind which bore him across, for no sail requires shifting, nor is a sheet altered while the vessel is making way with the “Trades.” Were the earth covered with water, we should find the trade-winds blowing equally over the surface, but the varying temperature of the land diverts them. The rarefaction of the air in the Sahara causes a westerly wind to prevail, which blows towards the land, instead of the trade wind we might expect to find.

The Monsoons, again, are caused in like manner, for the ordinary “trade” from the south-east is changed by the elevation of the heated air in Central Asia into a south-west wind, and so in the south, in consequence of the heated air from Australia, the north-west trade appears as a north-east monsoon, but is altered to a north-west wind. Nearly all the year round, therefore, we find the two winds, which are modifications of the “trades,” blowing in different directions and from different quarters. From November to March there is a north-east wind north of the Equator, and a north-west wind blows south of the Equator. From April to September a south-west wind blows at the north, and a south-east wind at the south of the line. The term monsoon signifies a “season,” and the changes of these winds give rise to tremendous storms causing great havoc.

Sea and Land breezes are really little monsoons; they are caused by the heat of the sun in just the same way, but with miniature results. We all know the sea-breeze which comes in as the land gets hot during the day, for the land warms more quickly than the sea under equally existing circumstances. So again, in the evening, the land loses its heat more quickly, and then the cool air flows out again to take the place of the warmer sea air which is continuing to ascend. The intensity and regularity varies when the degrees of heat are most different between land and sea and in tropical regions; and the varied coast formation will of course affect the wind, but as a rule the fact may be accepted as plainly explained, sea-breeze in the morning, land-breeze at night, and amateur sailors in boats at our watering-places will do well to bear this in mind.

There are a great number of local winds deriving their names from their direction or influence. We may mention them briefly. The special terms for winds are—

Fig. 715.—On a lee shore.

The Mistral, or Maestrale, is well known at Nice as the north wind, while at Toulon it is a north-east wind. The other winds, such as the Sirocco, which in some places is a warm, damp wind, in Madeira is a hot wind, and likewise in Sicily, where it is equally warm and damp like steam. It has different names in various countries, such as Samiel in Turkey, and sometimes as Föhn in Switzerland, where it may, however, be a north wind—which, as all travellers know, is a dry and a hazy-weather breeze, yet sometimes moist. The Simoon is a very hot wind raising sand-storms in the deserts, and experience has shown it to be very prejudicial to life in consequence of the fine sand and the tremendous heat it carries with it. Egypt is subject to another hot wind, called the Khamsin, and the west coast of Africa is subject to the Harmattan, a dry, easterly wind. The cold, dry wind of the Himalaya is known as the Tereno. In South America there is the same wind, the Pampero blowing east and south-east. The Euroclydon, mentioned by St. Paul, is the modern bora over the Adriatic. Malta rejoices (or laments) in the Gregale, a north-east wind. There are several other terms, such as the Puna of Peru, a very drying wind; the Purgas in Labrador, the Tourmente in France, and Guxen in Switzerland. Then we have the Hurricane, from “Ouracan” of the Caribs; the Typhone, or Tae-fun of China, so called from the dreaded god Typhon of Egypt; and the Tornado—all very violent winds, and circling round, causing, so to speak, whirlwinds, by which trees are uprooted, and houses destroyed.

The measure of the velocity of wind is performed by anemometers, which record the velocity in feet per second, and the amount of pressure. The anemometer is a well-known apparatus, with its four arms terminating in “cups” and a “tablet” anemometer, which is more or less disturbed or deflected from the vertical line by each gust of wind, and thus the score of degrees is marked by an indicator, which is moved as the tablet is deflected. We annex a table of wind pressure and velocity—