enerated by the engines for driving the others
while the ship was in port, this having been a success already on a
smaller scale. For a time this plan gave great satisfaction, since it
diminished the amount of coal to be carried and the consequent change
of displacement at sea, and enabled the ship to be worked with a
smaller number of men. The batteries could also, of course, be
distributed along the entire length, and placed where space was least
valuable.

"The construction of such huge vessels called for much governmental
river and harbour dredging, and a ship drawing thirty-five feet can now
enter New York at any state of the tide. For ocean bars, the old
system of taking the material out to sea and discharging it still
survives, though a jet of water from force-pumps directed against the
obstruction is also often employed with quick results. For river work
we have discovered a better method. All the mud is run back, sometimes
over a mile from the river bank, where it is used as a fertilizer, by
means of wire railways strung from poles. These wire cables combine in
themselves the functions of trolley wire and steel rail, and carry the
suspended cars, which empty themselves and return around the loop for
another load. Often the removed material entirely fills small,
saucer-shaped valleys or low places, in which case it cannot wash back.
This improvement has ended the necessity of building jetties.

"The next improvement in sea travelling was the 'marine spider.' As the
name shows, this is built on the principle of an insect. It is well
known that a body can be carried over the water much faster than
through it. With this in mind, builders at first constructed light
framework decks on large water-tight wheels or drums, having paddles on
their circumferences to provide a hold on the water. These they caused
to revolve by means of machinery on the deck, but soon found that the
resistance offered to the barrel wheels themselves was too great. They
therefore made them more like centipeds with large, bell-shaped feet,
connected with a superstructural deck by ankle-jointed pipes, through
which, when necessary, a pressure of air can be forced down upon the
enclosed surface of water. Ordinarily, however, they go at great speed
without this, the weight of the water displaced by the bell feet being
as great as that resting upon them. Thus they swing along like a
pacing horse, except that there are four rows of fe