other motive powers。 Steamers gradually passed through the five
hundred…； six hundred…； and seven hundred…foot…long class； with
other dimensions in proportion； till their length exceeded one
thousand feet。 These were very fast ships； crossing the Atlantic
in four and a half days； and were almost as steady as houses； in
even the roughest weather。
〃Ships at this period of their development had also passed
through the twin and triple screw stage to the quadruple； all
four together developing one hundred and forty thousand indicated
horse…power； and being driven by steam。 This； of course；
involved sacrificing the best part of the ship to her engines；
and a very heavy idle investment while in port。 Storage
batteries； with plates composed of lead or iron； constantly
increasing in size； had reached a fair state of development by
the close of the nineteenth century。
〃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。'
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
feet instead of two； each foot being taken out of the water as it
is swung forward； the first and fourth and second and third rows
being worked together。 Although； on account of their size； which
covers several acres； they can go in any water； they give the
best results on Mediterraneans and lakes that are free from ocean
rollers； and； under favourable conditions； make better speed than
the nineteenth…century express trains； and； of course； going
straight as the crow flies； and without stopping； they reach a
destination in considerably shorter time。
Some passengers and express packages still cross the Atlantic on
'spiders；' but most of these light cargoes go in a far pleasanter
and more rapid way。 The deep…displacement vessels； for heavy
freight； make little better speed than was made by the same class
a hundred years ago。 But they are also run entirely by
electricity； largely supplied by wind； and by the tide turning
their motors； which become dynamos while at anchor in any stream。
They therefore need no bulky boilers； engines； sails； or
coal…bunkers； and consequently can carry unprecedentedly large
cargoes with comparatively small crews。 The officers on the
bridge and the men in the crow's nestthe way to which is by a
ladder INSIDE the mast； to protect the climber from the
weatherare about all that is needed； while disablement is made
practically impossible； by having four screws； each with its own
set of automatically lubricating motors。
〃This change； like other labour…saving appliances； at first
resulted in laying off a good many men； the least satisfactory
being the first to go； but the increase in business was so great
that the intelligent men were soon reemployed as officers at
higher rates of pay and more interesting work than before； while
they as consumers were benefited as much as any one else by the
decreased cost of production and transportation。
〃With a view to facilitating interchange still further； our
Government has gradually completed the double coast…line that
Nature gave us in part。 This was done by connecting islands
separated from shore by navigable water； and leaving openings for
ingress and exit but a few hundred yards wide。 The breakwaters
required to do this were built with cribbing of incorrodible
metal； affixed to deeply driven metallic piles； and filled with
stones along coasts where they were found in abundance or excess。
This； while clearing many fields and improving them 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
hoursenough to take them around the globewhile 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 railways。
The air ships； whose length varies from fifty to five hundred
feet； have rudders for giving a vertical or a horizontal motion；
and several strengthening keels that prevent leeway when turning。
They are entirely on the principle of birds； maintaining
themselves mechanically； and differing thus from the unwieldy
balloon。 Starting as if on a circular railway； against the wind；
they rise to a considerable height； and then； shutting off the
batteries； coast down the aerial slope at a rate that sometimes
touches five hundred miles an hour。 When near the ground the
helmsman directs the prow upward； and； again turning on full
current； rushes up the slope at a speed that far exceeds the
eagle's； each drop of two miles serving to take the machine
twenty or thirty； though； if the pilot does not wish to soar； or
if there is a fair wind at a given height； he can remain in that
stratum of the atmosphere by moving horizontally。 He can also
maintain his elevation when moving very slowly； and though the
headway be entirely stopped； the descent is gradual on account of
the aeroplane's great spread； the batteries and motors being
secured to the under side of the deck。
〃The motors are so light that they develop two horse power for
every pound of their weight； while； to keep the frames thin； the
necessary power is obtained by terrific speed of the moving
parts； as though a steam engine； to avoid great pressure in its
cylinders； had a long stroke and ran at great piston speed；
which； however； is no disadvantage to the rotary motion of the
electric motor； there being no reciprocating cranks； etc。； that
must be started and stopped at each revolution。
〃To obviate the necessity of gearing to reduce th