a, on the other hand, there's a change of as much as fifty
degrees of temperature in a single day. That is because land absorbs
heat quickly and lets it go equally quickly. The interior of a continent
in summer time heats and expands the air in the same way that the air is
heated over the equator, and, in the same manner, sets in motion another
system of winds, for cold air comes rushing down from all sides and
forces up the rising warm air.

"Take Asia, for example, where the continental mass is large and the
plateaus high. The interior becomes so hot that the air is sent up like
the draught in a big chimney, and cool winds from the sea blow toward
the interior from all sides in the summer time, and away from it, to all
sides, in the winter time. That's what causes the famous Indian
monsoons, which blow steadily to the north-east for the six months of
summer and just as steadily to the south-west for the six months of
winter. The native boats, there, are built on purpose for the monsoon,
so that they can only sail with a fair wind and they make one round trip
a year, going south with the monsoon in winter and returning with the
summer monsoon."

The old farmer scratched his head.

"There's more to this than I thought," he said; "I always supposed that
winds just happened."

"No, indeed," the Forecaster answered, "every place in the world has its
own system of winds, though in some parts there are so many variations
that it isn't always easy to distinguish between the regular and the
irregular currents. In the United States the surface winds are very
irregular, for we live in one of the stormiest regions of the entire
world. Still, that doesn't alter the general rule that all our weather
comes from the west."

"And yet," said the farmer, in a puzzled manner, "I don't see why it
comes from the west."

"I think I can explain it to you," the weather expert replied. "You know
that when water is running down a hole at the bottom of a basin, if it
is in motion it doesn't go down straight but with a circular movement,
finally making a whirlpool?"

"Of course," the farmer said.

"So does air," the Forecaster rejoined. "There is something the same
sort of a whirl at the poles. The prevailing westerly winds of the
United States are due to this circumpolar whirl, though modified and
altered by the changes of the seasons, the differences of heat between
day and night, the radiation from the land, the irregularity of the