ter from a
basin acres in extent has been entirely carried away, leaving the
surface, as described by an observer, apparently dry enough to plough.

Fortunately for the interests of man, as well as those of the lower
organic life, the paths of these storms, or at least the portion of
their track where the violence of the air movement makes them very
destructive, often does not exceed five hundred feet in width, and is
rarely as great as half a mile in diameter. In most cases the length
of the journey of an individual tornado does not exceed thirty miles.
It rarely if ever amounts to twice that distance.

In every regard except their small size and their violence these
tornadoes closely resemble hurricanes. There is the same broad disk of
air next the surface spirally revolving toward the ascending centre,
where its motion is rapidly changed from a horizontal to a vertical
direction. The energy of the uprush in both cases is increased by the
energy set free through the condensation of the water, which tends
further to heat and thus to expand the air. The smaller size of the
tornado may be accounted for by the fact that we have in their
originating conditions a relatively thin layer of warm, moist air next
the earth and a relatively very cold layer immediately overlying it.
Thus the tension which serves to start the movement is intense, though
the masses involved are not very great. The short life of a tornado
may be explained by the fact that, though it apparently tends to grow
in width and energy, the central spout is small, and is apt to be
broken by the movements of the atmosphere, which in the front of a
cyclone are in all cases irregular.

On the warmer seas, but often beyond the limits of the tropics,
another class of spinning storms, known as waterspouts, may often be
observed. In general appearance these air whirls resemble tornadoes,
except that they are in all cases smaller than that group of
whirlings. As in the tornadoes, the waterspout begins with a funnel,
which descends from the sky to the surface of the sea. Up the tube
vapours may be seen ascending at great speed, the whole appearing like
a gigantic pillar of swiftly revolving smoke. When the whirl reaches
the water, it is said that the fluid leaps up into the tube in the
form of dense spray, an assertion which, in view of the fact of the
action of a tornado on a lake as before described, may well be
believed. Like the tornadoes and dust whirls, the l