uds above and about it may be seen to take on a
whirling movement around the centre, and under favourable
circumstances an uprush of vapours may be noted in the centre of the
swaying shaft. As the whirl comes nearer, the roar of the disturbance,
which at a distance is often compared to the sound made by a threshing
machine or to that of distant musketry, increases in loudness until it
becomes overwhelming. When a storm such as this strikes a building, it
is not only likely to be razed by the force of the wind, but it may be
exploded, as by the action of gunpowder fired within its walls,
through the sudden expansion of the air which it contains. In the
centre of the column, although it rarely has a diameter of more than a
few hundred feet, the uprush is so swift that it makes a partial
vacuum. The air, striving to get into the space which it is eager to
occupy, is whirling about at such a rate that the centrifugal motion
which it thus acquires restrains its entrance. In this way there may
be, as the column rapidly moves by, a difference of pressure
amounting probably to what the mercury of a barometer would indicate
by four or five inches of fall. Unless the structure is small and its
walls strong, its roof and sides are apt to be blown apart by this
difference of pressure and the consequent expansion of the contained
air. In some cases where wooden buildings have withstood this curious
action the outer clapboards have been blown off by the expansion of
the small amount of air contained in the interspaces between that
covering and the lath and plaster within (see Fig. 9).

[Illustration: Fig. 9.--Showing effect of expansion of air contained
in a hollow wall during the passage of the storm.]

The blow of the air due to its rotative whirling has in several cases
proved sufficient to throw a heavy locomotive from the track of a
well-constructed railway. In all cases where it is intense it will
overturn the strongest trees. The ascending wind in the centre of the
column may sometimes lift the bodies of men and of animals, as well as
the branches and trunks of trees and the timber of houses, to the
height of hundreds of feet above the surface. One of the most striking
exhibitions of the upsucking action in a tornado is afforded by the
effect which it produces when it crosses a small sheet of water. In
certain cases where, in the Northwestern States of this country, the
path of the storm lay over the pool, the whole of the wa