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this trip。〃
〃It looks all right；〃 commented Mr。 Sharp。 〃Whether it will work is
another question。〃
〃Yes； we can't tell until it's in the water；〃 con ceded Tom。 〃But I hope it
does。 Dad has spent much time and money on it。〃
The Advance was； as her name indicated； much in advance of previous
submarines。 There was not so much difference in outward construction as
there was in the means of propulsion and in the manner in which the
interior and the machinery were arranged。
The submarine planned by Mr。 Swift and Tom jointly； and constructed
by them； with the aid of Mr。 Sharp and Mr。 Jackson； was shaped like a
Cigar； over one hundred feet long and twenty feet in diameter at the
thickest part。 It was divided into many compartments； all water…tight； so
that if one or even three were flooded the ship would still be useable。
Buoyancy was provided for by having several tanks for the
introduction of compressed air； and there was an emergency arrangement
so that a collapsible aluminum container could be distended and filled
with a powerful gas。 This was to be used if； by any means； the ship was
disabled on the bottom of the ocean。 The container could be expanded and
filled； and would send the Advance to the surface。
Another peculiar feature was that the engine…room； dynamos and other
apparatus were all contained amidships。 This gave stability to the craft；
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TOM SWIFT AND HIS SUBMARINE BOAT （or Under the Ocean for Sunken Treasure）
and also enabled the same engine to operate both shafts and propellers； as
well as both the negative forward electrical plates； and the positive rear
ones。
These plates were a new idea in submarine construction； and were the
outcome of an idea of Mr。 Swift； with some suggestions from his son。
The aged inventor did not want to depend on the usual screw
propellers for his craft； nor did he want to use a jet of compressed air；
shooting out from a rear tube； nor yet a jet of water； by means of which the
creature called the squid shoots himself along。 Mr。 Swift planned to send
the Advance along under water by means of electricity。
Certain peculiar plates were built at the forward and aft blunt noses of
the submarine。 Into the forward plate a negative charge of electricity was
sent； and into the one at the rear a positive charge； just as one end of a
horseshoe magnet is positive and will repel the north end of a compass
needle； while the other pole of a magnet is negative and will attract it。 In
electricity like repels like； while negative and positive have a mutual
attraction for each other。
Mr。 Swift figured out that if he could send a powerful current of
negative electricity into the forward plate it would pull the boat along； for
water is a good conductor of electricity； while if a positive charge was sent
into the rear plate it would serve to push the submarine along； and he
would thus get a pulling and pushing motion； just as a forward and aft
propeller works on some ferry boats。
But the inventor did not depend on these plates alone。 There were
auxiliary forward and aft propellers of the regular type； so that if the
electrical plates did not work； or got out of order； the screws would serve
to send the Advance along。
There was much machinery in the submarine There were gasolene
motors； since space was too cramped to allow the carrying of coal for
boilers。 There were dynamos； motors and powerful pumps。 Some of these
were for air； and some for water。 To sink the submarine below the surface
large tanks were filled with water。 To insure a more sudden descent；
deflecting rudders were also used； similar to those on an airship。 There
were also special air pumps； and one for the powerful gas； which was
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TOM SWIFT AND HIS SUBMARINE BOAT （or Under the Ocean for Sunken Treasure）
manufactured on board。
Forward from the engine…room was a cabin； where meals could be
served； and where the travelers could remain in the daytime。 There was
also a small cooking galley； or kitchen； there。 Back of the engine…room
were the sleeping quarters and the storerooms。 The submarine was steered
from the forward compartment； and here were also levers； wheels and
valves that controlled all the machinery； while a number of dials showed
in which direction they were going； how deep they were； and at what
speed they were moving； as well as what the ocean pressure was。
On top； forward； was a small conning； or observation tower； with
auxiliary and steering and controlling apparatus there。 This was to be used
when the ship was moving along on the surface of the ocean； or merely
with the deck awash。 There was a small flat deck surrounding the conning
tower and this was available when the craft was on the surface。
There was provision made for leaving the ship when it was on the bed
of the ocean。 When it was desired to do this the occupants put on diving
suits； which were provided with portable oxygen tanks。 Then they entered
a chamber into which water was admitted until it was equal in pressure to
that outside。 Then a steel door was opened； and they could step out。 To re…
enter the ship the operation was reversed。 This was not a new feature。 In
fact； many submarines to…day use it
At certain places there were thick bull's…eye windows； by means of
which the under…water travelers could look out into the ocean through
which they were moving。 As a defense against the attacks of submarine
monsters there was a steel； pointed ram； like a big harpoon。 There were
also a bow and a stern electrical gun； of which more will be told later。
In addition to ample sleeping accommodations。 there were many
conveniences aboard the Advance。 Plenty of fresh water could be carried；
and there was an apparatus for distilling more from the sea water that
surrounded the travelers。 Compressed air was carried in large tanks； and
oxygen could be made as needed。 In short； nothing that could add to the
com