s. Very few people have any idea how general is
the truth that the air, even in continuous winds, tends to move in
more or less individualized masses. This, however, is made very
evident by watching the gusts of a storm or the wandering patches of
wind which disturb the surface of an otherwise smooth sea.

[Illustration: _South shore, Martha's Vineyard, Massachusetts, showing
a characteristic sand beach with long slope and low dunes. Note the
three lines of breakers and the splash flows cutting little bays in
the sand._]

Among the notable local winds are those which from their likeness to
the Foehn of the Swiss valleys receive that name. Foehns are produced
where a body of air blowing against the slope of a continuous mountain
range is lifted to a considerable height, and, on passing over the
crest, falls again to a low position. In its ascent the air is cooled,
rarefied, and to a great extent deprived of its moisture. In
descending it is recondensed, and by the process by which its atoms
are brought together its latent heat is made sensible. There being but
little watery vapour in the mass, this heat is not much called for by
that heat-storing fluid, and so the air is warmed. So far Foehn winds
have only been remarked as conspicuous features in Switzerland and on
the eastern face of the Rocky Mountains. In the region about the head
waters of the Missouri and to the northward their influence in what
are called the Chinook winds is distinctly to ameliorate the severe
winter climate of the country.

In almost all great desert regions, particularly in the typical
Sahara, we find a variety of storm belonging to the whirlwind group,
which, owing to the nature of the country, take on special
characteristics. These desert storms take up from the verdureless
earth great quantities of sand and other fine _debris_, which often so
clouds the air as to bring the darkness of night at midday. Their
whirlings appear in size to be greater than those which produce
tornadoes or waterspouts, but less than hurricanes or cyclones.
Little, however, is known about them. They have not been well
observed by meteorologists. In some ways they are important, for the
reason that they serve to carry the desert sand into regions
previously verdure-clad, and thus to extend the bounds of the desolate
fields in which they originate. Where they blow off to the seaward,
they convey large quantities of dust into the ocean, and thus serve to
wear down