"During the second decade of the twentieth century the engineers
decided to try the plan of running half of a transatlantic
liner's screws by electricity generated 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.'