area of low barometric
pressure--an area where the air has become lighter than the air of
surrounding regions. Under influence of gravitation the air seeks its
level just as water does; so the heavy air comes flowing in from
all sides towards the low-pressure area, which thus becomes a
"storm-centre." But the inrushing currents never come straight to their
mark. In accordance with Ferrel's law, they are deflected to the right,
and the result, as will readily be seen, must be a vortex current, which
whirls always in one direction--namely, from left to right, or in the
direction opposite to that of the hands of a watch held with its face
upward. The velocity of the cyclonic currents will depend largely upon
the difference in barometric pressure between the storm-centre and the
confines of the cyclone system. And the velocity of the currents will
determine to some extent the degree of deflection, and hence the exact
path of the descending spiral in which the wind approaches the centre.
But in every case and in every part of the cyclone system it is true, as
Buys Ballot's famous rule first pointed out, that a person standing with
his back to the wind has the storm-centre at his left.

The primary cause of the low barometric pressure which marks the
storm-centre and establishes the cyclone is expansion of the air through
excess of temperature. The heated air, rising into cold upper regions,
has a portion of its vapor condensed into clouds, and now a new dynamic
factor is added, for each particle of vapor, in condensing, gives up its
modicum of latent heat. Each pound of vapor thus liberates, according
to Professor Tyndall's estimate, enough heat to melt five pounds of cast
iron; so the amount given out where large masses of cloud are forming
must enormously add to the convection currents of the air, and hence to
the storm-developing power of the forming cyclone. Indeed, one school
of meteorologists, of whom Professor Espy was the leader, has held that,
without such added increment of energy constantly augmenting the dynamic
effects, no storm could long continue in violent action. And it is
doubted whether any storm could ever attain, much less continue, the
terrific force of that most dreaded of winds of temperate zones, the
tornado--a storm which obeys all the laws of cyclones, but differs from
ordinary cyclones in having a vortex core only a few feet or yards in
diameter--without the aid of those great masses of condensing