ospheric mass of each
hemisphere is seen, when viewed as a whole, to be carried in a great
whirl about the pole of that hemisphere, so the local disturbances
within this great tide are found always to take the form of whirls about
a local storm-centre--which storm-centre, meantime, is carried along
in the major current, as one often sees a little whirlpool in the water
swept along with the main current of the stream. Sometimes, indeed, the
local eddy, caught as it were in an ancillary current of the great
polar stream, is deflected from its normal course and may seem to travel
against the stream; but such deviations are departures from the rule. In
the great majority of cases, for example, in the north temperate zone, a
storm-centre (with its attendant local whirl) travels to the northeast,
along the main current of the anti-trade-wind, of which it is a part;
and though exceptionally its course may be to the southeast instead, it
almost never departs so widely from the main channel as to progress to
the westward. Thus it is that storms sweeping over the United States can
be announced, as a rule, at the seaboard in advance of their coming by
telegraphic communication from the interior, while similar storms
come to Europe off the ocean unannounced. Hence the more practical
availability of the forecasts of weather bureaus in the former country.

But these local whirls, it must be understood, are local only in a very
general sense of the word, inasmuch as a single one may be more than
a thousand miles in diameter, and a small one is two or three hundred
miles across. But quite without regard to the size of the whirl, the air
composing it conducts itself always in one of two ways. It never whirls
in concentric circles; it always either rushes in towards the centre in
a descending spiral, in which case it is called a cyclone, or it spreads
out from the centre in a widening spiral, in which case it is called an
anti-cyclone. The word cyclone is associated in popular phraseology with
a terrific storm, but it has no such restriction in technical usage. A
gentle zephyr flowing towards a "storm-centre" is just as much a
cyclone to the meteorologist as is the whirl constituting a West-Indian
hurricane. Indeed, it is not properly the wind itself that is called the
cyclone in either case, but the entire system of whirls--including the
storm-centre itself, where there may be no wind at all.

What, then, is this storm-centre? Merely an