reat distances by the most
gentle breeze. Other plants are fitted for dispersion by means of an
attached wing, as in the case of the fir tree, so that they are caught
up by the wind as they fall from the cone, and are carried to a
distance. Amongst the comparatively small number of plants known to
Linnaeus, no less than 138 genera are enumerated as having winged seeds.

As winds often prevail for days, weeks, or even months together, in the
same direction, these means of transportation may sometimes be without
limits; and even the heavier grains may be borne through considerable
spaces, in a very short time, during ordinary tempests; for strong
gales, which can sweep along grains of sand, often move at the rate of
about forty miles an hour, and if the storm be very violent, at the rate
of fifty-six miles.[852] The hurricanes of tropical regions, which root
up trees and throw down buildings, sweep along at the rate of ninety
miles an hour; so that, for however short a time they prevail, they may
carry even the heavier fruits and seeds over friths and seas of
considerable width, and doubtless are often the means of introducing
into islands the vegetation of adjoining continents. Whirlwinds are also
instrumental in bearing along heavy vegetable substances to considerable
distances. Slight ones may frequently be observed in our fields, in
summer carrying up haycocks into the air, and then letting fall small
tufts of hay far and wide over the country; but they are sometimes so
powerful as to dry up lakes and ponds, and to break off the boughs of
trees, and carry them up in a whirling column of air.

Franklin tells us, in one of his letters, that he saw, in Maryland, a
whirlwind which began by taking up the dust which lay in the road, in
the form of a sugar loaf with the pointed end downwards, and soon after
grew to the height of forty or fifty feet, being twenty or thirty in
diameter. It advanced in a direction contrary to the wind; and although
the rotary motion of the column was surprisingly rapid, its onward
progress was sufficiently slow to allow a man to keep pace with it on
foot. Franklin followed it on horseback, accompanied by his son, for
three quarters of a mile, and saw it enter a wood, where it twisted and
turned round large trees with surprising force. These were carried up in
a spiral line, and were seen flying in the air, together with boughs and
innumerable leaves, which, from their height, appeared reduced t