more in going the distance of a mile in
the dry sand dune than in travelling for hundreds along the wet shore.

If the rock matter in the state of sand wore as rapidly under the
heating of the waves as it does in the state of pebbles, the
continents would doubtless be much smaller than they are. Those coasts
which have no other protection than is afforded by a low sand beach
are often better guarded against the inroads of the sea than the
rock-girt parts of the continents. It is on account of this remarkable
endurance of sand of the action of the waves that the stratified rocks
which make up the crust of the earth are so thick and are to such an
extent composed of sand grains.

The tendency of the _debris_-making influences along the coast line is
to fill in the irregularities which normally exist there; to batter
off the headlands, close up the bays and harbours, and generally to
reduce the shores to straight lines. Where the tide has access to
these inlets, it is constantly at work in dragging out the detritus
which the waves make and thrust into the recesses. These two actions
contend with each other, and determine the conditions of the coast
line, whether they afford ports for commerce or are sealed in by sand
bars, as are many coast lines which are not tide-swept, as that of
northern Africa, which faces the Mediterranean, a nearly tideless sea.
The same is the case with the fresh-water lakes; even the greater of
them are often singularly destitute of shelters which can serve the
use of ships, and this because there are no tides to keep the bays and
harbours open.

THE OCEAN CURRENTS.

The system of ocean currents, though it exhibits much complication in
detail, is in the main and primarily dependent on the action of the
constant air streams known as the trade winds. With the breath from
the lips over a basin of water we can readily make an experiment which
shows in a general way the method in which the winds operate in
producing the circulation of the sea. Blowing upon the surface of the
water in the basin, we find that even this slight impulse at once sets
the upper part in motion, the movement being of two kinds--pulsating
movements or waves are produced, and at the same time the friction of
the air on the surface causes its upper part to slide over the under.
With little floats we can shortly note that the stream which forms
passes to the farther side of the vessel, there divides, a