which takes place as soon as it falls--the fluid
begins its downward journey. On this way it is at once parted into two
distinct divisions, the surface water and the ground water: the former
courses more or less swiftly, generally at the rate of a mile or more
an hour, in the light of day; the latter enters the interstices of the
earth, slowly descends therein to a greater or less depth, and
finally, journeying perhaps at the rate of a mile a year, rejoins the
surface water, escaping through the springs. The proportion of these
two classes, the surface and the ground water, varies greatly, and an
intermixture of them is continually going on. Thus on the surface of
bare rock or frozen earth all the rain may go away without entering
the ground. On very sandy fields the heaviest rainfall may be taken
up by the porous earth, so that no streams are found. On such surfaces
the present writer has observed that a rainfall amounting to six
inches in depth in two hours produced no streams whatever. We shall
first follow the history of the surface water, afterward considering
the work which the underground movements effect.

If the student will observe what takes place on a level ploughed
field--which, after all, will not be perfectly level, for all fields
are more or less undulating--he will note that, though the surface may
have been smoothed by a roller until it appears like a floor, the
first rain, where the fall takes place rapidly enough to produce
surface streams, will create a series of little channels which grow
larger as they conjoin, the whole appearing to the eye like a very
detailed map, or rather model, of a river system; it is, indeed, such
a system in miniature. If he will watch the process by which these
streamlet beds are carved, he will obtain a tolerably clear idea as to
that most important work which the greater streams do in carving the
face of the lands. The water is no sooner gathered into a sheet than,
guided by the slightest irregularities which it encounters, it begins
to flow. At first the motion is so slow that it does not disturb its
bed, but at some points in the bottom of the sheet the movement soon
becomes swift enough to drag the grains of sand and clay from their
adhesions, bearing them onward. As soon as this beginning of a channel
is formed the water moves more swiftly in the clearer way; it
therefore cuts more rapidly, deepening and enlarging its channel, and
making its motion yet more free. T