st and lower the sails, and
perform the work of loading and unloading. They are admirable colliers
and grain-carriers.

At the beginning of the twentieth century, about ninety thousand
sailing-craft and thirty-five thousand steam-vessels were required to
carry the world's commerce. Of this number, Great Britain and her
colonies register nearly thirty-five thousand, and the United States
over twenty thousand.

HARBOR SAFEGUARDS.--Excepting the open anchorages formed by angles
in coast-lines, the greater number of harbors consist of small
coves and river-mouths. In these, although there may be a
considerable area of water, there is not apt to be much sailing
room; it is therefore necessary to mark off the navigable channels.
For this purpose buoys of different shapes and colors are used by
day; by night fixed and flashing lights are employed.

The buoys of permanent channels are usually hollow metal cylinders
or cones about two feet in diameter, anchored so that the end of
the cylinder projects about three feet above the water. On entering
a channel from the seaward, red buoys are on the starboard, or
right hand; white buoys are kept on the port, or left side. Buoys
at the end of a channel are usually surmounted each by some device
or other fastened at the upper end of a perch. Thus, at the outer
entrance of Gedney Channel in New York Harbor, a ball surmounts the
perch; at the inner entrance the buoy carries a double square.
Sharp angles in a channel are similarly marked. In many instances
the buoy carries, as a warning signal, a bell that rings as the
buoy is rocked by the waves; in others, a whistle that sounds by
the air which the rocking motion compresses within the cylinder;
still others carry electric or gas lights.

The color of a buoy is an index of its character. Thus, one with
black and red stripes indicates danger; one with black and white
vertical stripes is a channel-marker. Temporary channels are
frequently marked by pieces of spar floating upright. In some cases
it is customary to set untrimmed tree-tops on the port, and trimmed
sticks on the starboard.

Light-houses are built at all exposed points of navigated
coast-waters, and beacons are set at all necessary points within a
harbor for use at night. All lights are kept burning from sunset
until su