y observing how on ploughed ground a flat stone or a potsherd
comes after a rain to cap a little column. The geologist sometimes
finds in soft sandstones that the same action is repeated in a larger
way where a thin fragment of hard rock has protected a column many
feet in height against the rain work which has shorn down the
surrounding rock.

When water strikes the moistened surface it at once loses the droplike
form which all fluids assume when they fall through the air.[4]

[Footnote 4: This principle of the spheroidal form in falling fluids is
used in making ordinary bird shot. The melted lead drops through
sievelike openings, the resulting spheres of the metal being allowed to
fall into water which chills them. Iron shot, used in cutting stone,
where they are placed between the saw and the surface of the rock, are
also made in the same manner. The descending fluid divides into drops
because it is drawn out by the ever-increasing speed of the falling
particles, which soon make the stream so thin that it can not hold
together.]

When the raindrops coalesce on the surface of the earth, the role of
what we may call land water begins. Thenceforward until the fluid
arrives at the surface of the sea it is continually at work in
effecting a great range of geological changes, only a few of which can
well be traced by the general student. The work of land water is due
to three classes of properties--to the energy with which it is endowed
by virtue of its height above the sea, a power due to the heat of the
sun; to the capacity it has for taking substances into solution; and
to its property of giving some part of its own substance to other
materials with which it comes in contact. The first of these groups of
properties may be called dynamical; the others, chemical.

The dynamic value of water when it falls upon the land is the amount
of energy it can apply in going down the slope which separates it from
the sea. A ton of the fluid, such as may gather in an ordinary rain on
a thousand square feet of ground in the highlands of a country--say at
an elevation of a thousand feet above the sea--expends before it comes
to rest in the great reservoir as much energy as would be required to
lift that weight from the ocean's surface to the same height. The ways
in which this energy may be expended we shall now proceed in a general
way to trace.

As soon as the water has been gathered, from its drop to its sheet
state--a process