d in the other on
floating objects. Each little dust raft with its cargo of condensed
water tends, of course, to fall downward toward the earth's surface,
and, except for the winds which may blow upward, does so fall, though
with exceeding slowness. Its rate of descent may be only a few feet a
day. It was falling before it took on the load of water; it will fall
a little more rapidly with the added burden, but even in a still air
it might be months or years before it would come to the ground. The
reason for this slow descent may not at first sight be plain, though a
little consideration will make it so.

If we take a shot of small size and a feather of the same weight, we
readily note that their rate of falling through the air may vary in
the proportion of ten to one or more. It is easy to conceive that this
difference is due to the very much less friction which the smaller
body encounters in its motion by the particles of air. With this point
in mind, the student should observe that the surface presented by
solid bodies in relation to their solid contents is the greater the
smaller the diameter. A rough, though not very satisfactory, instance
of this principle may be had by comparing the surface and interior
contents of two boxes, one ten feet square and the other one foot
square. The larger has six hundred feet of surface to one thousand
cubic feet of interior, or about half a square foot of outer surface
to the cubic foot of contents; while the smaller box has six feet of
surface for the single cubic foot of interior, or about ten times the
proportion of exterior to contents. The result is that the smaller
particles encounter more friction in moving toward the earth, until,
in the case of finely divided matter, such as the particles of carbon
in the smoke from an ordinary fire, the rate of down-falling may be so
small as to have little effect in the turbulent conditions of
atmospheric motion.

[Illustration: _Pocket Creek, Cape Ann, Massachusetts. Note the
relatively even size of the pebbles, and the splash wave which sets
them in motion._]

The little drops of water which gather round dust motes, falling but
slowly toward the earth, are free to obey the attractions which they
exercise upon each other--impulses which are partly gravitative and
partly electrical. We have no precise knowledge concerning these
movements, further than that they serve to aggregate the myriad little
floats into cloud forms, in which th