in our summer and southward in our winter, its centre following the
vertical position of the sun. Near the centre of the course which marks
the meeting of the northern with the southern winds is a "Belt of
Calms" where the air draws upward in a strong draught. The colder air of
the trade winds is pushing up the columns of light, heated air. This
strip is known by sailors as "the Doldrums," or "the region of
equatorial calms." Though never wider than two or three hundred miles,
this is a region dreaded by captains of sailing-vessels, for they often
lie becalmed for weeks in an effort to reach the friendly trade winds
that help them to their desired ports. Vessels becalmed are at the mercy
of sudden tempests which come suddenly like thunder-storms, and
sometimes do great damage to vessels because they take the sailors
unawares and allow no time to shorten sail.

Until late years the routes of vessels were charted so that sailors
could take advantage of the trade winds in their long voyages. It was
necessary in the days of sailing-vessels for the captain to understand
the movements of winds which furnished the motive power that carried his
vessel. Fortunate it was for him that there were steady winds in the
temperate zones that he could take advantage of in latitudes north of
the Tropic of Cancer and south of the Tropic of Capricorn. What becomes
of the hot air that rises in a constant stream above the "Doldrums,"
pushed up by the cooler trade winds that blow in from north and south?
Naturally this air cannot ascend very high, for it soon reaches an
altitude in which its heat is rapidly lost, and it would sink if it
were not constantly being pushed by the rising column of warm air under
it. So it turns and flows north and south at a level above the trade
winds. Not far north of the Tropic of Cancer it sinks to the level of
sea and land, and forms a belt of winds that blows ships in a
northeasterly direction. Between trades and anti-trades is another zone
of calms,--near the Tropics of Cancer and of Capricorn.

The land masses of the continents with their high mountain ranges
interfere with these winds, especially in the northern hemisphere, but
in the Southern Pacific and on the opposite side of the globe the
"Roaring Forties," as these prevailing westerly winds are known by the
sailors, have an almost unbroken waste of seas over which they blow. In
the long voyages between England and Australia, and in the Indian trade,