for cultivation, provided just the needed material; since
irregular stones bind together firmly, and, while also insoluble,
combine considerable bulk with weight. South of Hatteras, where stones
are scarce, the sand dredged from parts of the channel was filled into
the crib, the surface of which has a concave metallic cover, a trough
of still water being often the best barrier against the passage of
waves. This double coast-line has been a great benefit, and propelled
vessels of moderate draught can range in smooth water, carrying very
full loads, from Labrador to the Orinoco. The exits are, of course,
protected by a line of cribbing a few hundred feet to seaward.

"The rocks have been removed from all channels about New York and other
commercial centres, while the shallow places have been dredged to a
uniform depth. This diminishes the dangers of navigation and
considerably decreases the speed with which the tides rush through.
Where the obstructions consisted of reefs surrounded by deep water,
their removal with explosives was easy, the shattered fragments being
allowed to sink to the bottom and remain there beneath the danger line.

"Many other great works have also been completed. The canals at
Nicaragua have been in operation many years, it having been found best
to have several sizes of locks, and to use the large ones only for the
passage of large vessels. The improved Erie and Champlain Canals also
enable ships four hundred feet long to reach New York from the Great
Lakes via the Hudson River.

"For flying, we have an aeroplane that came in when we devised a
suitable motor power. This is obtained from very light paper-cell
batteries that combine some qualities of the primary and secondary
type, since they must first be charged from a dynamo, after which they
can supply full currents for one hundred hours--enough to take them
around the globe--while partly consuming the elements in the cells.
The power is applied through turbine screws, half of which are capable
of propelling the flat deck in its inclined position at sufficient
speed to prevent its falling. The moving parts have ball bearings and
friction rollers, lubrication being secured automatically, when
required, by a supply of vaseline that melts if any part becomes hot.
All the framing is of thin but very durable galvanized aluminum, which
has superseded steel for every purpose in which weight is not an
advantage, as in the permanent way on rail