INVASION OF THE SEA
THE POSSIBILITY that the wind was the first inanimate power utilized
by man, has already been referred to. There are records of the use of
sailing vessels in Egypt that date as far back as 6,000 years before
Christ. Navigators of that early date, however, could hardly claim to
have mastered the wind. They merely used wind power when the wind was
disposed to help them. If the winds were adverse, they had no recourse
other than to furl their sail, step the mast and depend upon oars to
propel them to the desired port. It was not until thousands of years
later that primitive mariners learned how to tack and pursue a zigzag
course against the wind. When this knowledge was acquired we do not
know, but it is certain that the Phœnicians, who rounded the continent
of Africa 1,200 years before Christ, knew how to make use of the power
of opposing winds. Of course they could not explain how it was that a
breeze could be made to drive a vessel in a direction across and even
opposed to that in which it was blowing. In order to understand this
apparent paradox ourselves, we must go back to the very elements of
mechanics.
ELEMENTARY MECHANICS
The popular conception of force is something that produces motion,
but its true definition is “that which tends to produce _or resist_
motion.” There are forces in existence when there is no motion. When
you hold a weight in your hand there is a force tending to pull the
weight to the earth, but this force is opposed by an equal force
exerted by your muscles in holding up the weight. There is no motion
because the two forces are perfectly balanced. If they were unbalanced,
there would be motion in the direction of the greater force. If the
pull of the arm is greater than that of gravity, the weight will be
lifted, and if the weight is too heavy for the arm to support, it will
go down despite muscular efforts to prevent it. In one case the force
of gravity will endeavor to destroy motion by opposing the lift of the
arm, and in the other case the arm will endeavor to resist motion by
opposing the pull of gravity. A book on a table is motionless and yet
it is acted upon by two forces which are opposed to each other and
hence balanced. The table furnishes a force which resists and balances
the force exerted by gravity. If the book were heavy enough, in other
words, if the force directed downward were great enough, the table
would be crushed.
When two forces are in perfect balance they must be equal and opposite.
Unless the directions of the two forces are exactly opposite, there
will be motion in some new direction. Suppose we use an apparatus such
as shown in Fig. 56 to study the result of three coacting forces. It
consists of a T-shaped frame with a pulley _P_ at each end of the cross
arm. These pulleys turn very freely on their axes, so that we need
not be concerned with any appreciable amount of friction. Two fine
cords running over these pulleys are knotted at _O_ to a third short
cord. Each cord is provided with a hook on which weights may be hung.
Now if we put a pound weight on each cord the two _A_ and _B_ will
raise the weight _C_ until the angles between the cords at _O_ are all
equal. In other words each force of one pound is balanced by two other
forces of one pound each pulling at an angle of 120 degrees to it and
to each other. If we put a 3-pound weight at _A_, a 4-pound weight at
_B_ and a 5-pound weight at _C_, the cords will come to rest in the
position shown in Fig. 57. The weight _B_ being heavier than weight _A_
will pull the knot _O_ to the right until the angle between the cords
running to these weights is a right angle.
[Illustration: FIG. 56.--BALANCED FORCES--EQUAL WEIGHTS]
PARALLELOGRAM OF FORCES
Although a force is something that cannot be pictured it can be
represented graphically by means of a line, letting the direction of
the line represent the direction of the force and the length of the
line the strength of the force. In Figure 58 we may measure off 3
inches from _O_ to _a_ to indicate the 3-pound force and 4 inches
from _O_ to _b_ to represent the 4-pound force and 5 inches from _O_
to _c_ to represent the 5-pound force. Now if from _a_ a line is
drawn parallel to _O b_ and from _b_ a line is drawn parallel to _O
a_ we shall have a parallelogram _a O b d_, and if we extend the line
_O c_ it will bisect the parallelogram, running diagonally from _O_
to _d_, and this diagonal will be found to measure exactly 5 inches
which represents the 5-pound force. This is what is known as the
_parallelogram of forces_. It shows us the resultant of any two forces
that are not directly opposite and it gives us the direction as well
as the strength or magnitude of this resultant. It is only because we
happen to choose the forces 3, 4 and 5 that the angle at _O_ is a right
angle. In Figure 59, where the forces are all equal, our parallelogram
is lozenge-shaped and the line _O d_ is just as long as the line _O a_
and _O b_, showing that its magnitude is the same as that of the two
forces that balanced it.
[Illustration: FIG. 57.--BALANCED FORCES--UNEQUAL WEIGHTS]
The greater the angle between the two lifting forces the less weight
can they lift. If two men are carrying a ten-pound satchel, each will
be lifting five pounds, if the pull is directly upward; but this is a
rather inconvenient way of carrying the bag and usually they pull at a
slight angle from the vertical, and so each must carry more than half
the weight. If they move so far apart that the angle between them is
more than 120 degrees, each will be carrying more than the full weight
of the bag.
[Illustration: FIGS. 58 AND 59.--PARALLELOGRAMS OF FORCES]
SAILING AGAINST THE WIND
Now that we know something about the parallelogram of forces we
may return to the problem of sailing across and against the wind.
In Fig. 60 we are looking down on the deck of a ship and the wind
is represented by the arrow. The dotted line _A B_ represents the
direction in which the boat is traveling and the line _C D_ represents
the plane of a sail. If the line _E F_ represents the magnitude and
direction of the force of the wind at the center of the sail, then
we can tell how much pressure is being exerted directly against
the sail, by drawing the line _g F_ perpendicular to the sail and
completing the parallelogram by drawing from _E_ a line parallel to
the sail intersecting _g F_ at _G_ and another line parallel to _g F_
intersecting the plane of the sail at _H_. Then the length of the line
_G F_ represents the pressure against the sail. If the line _G F_ is
half as long as the line _E F_, then only half of the force of the wind
is exerted in the direction _G F_. In other words, a wind pressure of
one pound per square foot blowing in the direction of _G F_ will do as
much work as two pounds in the direction _E F_. The force of the wind
has been broken up into two “components,” one (_G F_) at right angles
to the sail, and the other (_H F_) edgewise to the plane, and of course
the latter has no effect upon the propulsion of the boat.
[Illustration: FIG. 60.--FORCES THAT MOVE A SAILBOAT]
If there were nothing to prevent it, the boat would sail in the
direction _G F_; but the keel of the boat offers resistance to motion
in this direction, and we must construct another parallelogram around
the force _G F_ to find the magnitude of the force exerted in the
direction _A B_. The line _K F_ is drawn at right angles to _A B_, and
then the parallelogram is completed by drawing a line from _G_ to _K_
parallel to the line _A B_ and another from _G_ to _M_ parallel to _K
F_. We have then resolved to force _G F_ into two components _M F_ and
_K F_. The former tends to push the boat along its course while the
latter tends to make it drift to leeward. The length of the line _M F_
is little more than a quarter of the length of the wind force _E F_
and the leeward acting force _K F_ is actually considerably greater
than the forward acting force _M F_. Even with a deep keel there will
be some drift to leeward. This is corrected by means of the rudder of
the ship which is turned to head the ship further into the wind so that
although the boat does not actually travel in the direction of its axis
it may be made to travel along the course _A B_. Of course the boat
cannot sail directly against the wind, but it can accomplish the same
result by tacking alternately to port and starboard so that eventually
it can reach a port that lies in the direction from which the wind
comes.
THE SPEEDY CLIPPER
Before the advent of the steamship, sailing vessels were developed to
a high degree of efficiency. The speedy clippers of 1816 to 1845 used
to cross the Atlantic at an average speed of 6 to 9 miles per hour and
sometimes even better, which compares favorably with a common steam
freighter of to-day. The largest sailing vessel ever built was the
_Thomas W. Lawson_, a seven-masted schooner. She was launched in 1902,
but foundered in 1907 off the Scilly Islands. This great ship was a
steel vessel 395 feet long, with a displacement of 10,000 tons and a
cargo capacity of 7,500 tons. She had a sail spread of 40,617 feet.
FROM OARS TO PROPELLERS
Although a century has elapsed since the first steam-driven vessel made
its way across the Atlantic Ocean, sails have been as yet by no means
swept off the face of the sea. Nevertheless, even when sailing vessels
had no competitors they did not furnish a perfectly satisfactory means
of transportation. The fickleness of wind power was felt in this
application as well as in that of windmills, and inventors racked their
brains for some more certain means of propelling ships. Naturally, when
the steam engine was a proven success, efforts were made to apply this
newly discovered power to ships. How to make steam drive a ship was a
problem. At first it was proposed to use a system of oars which would
be moved back and forth in imitation of oarsmen and John Fitch’s first
steamboat in 1786 was driven by a set of paddles operated in a manner
similar to that of paddling a canoe. It was a very natural evolution
from oars to paddle wheel, which consists of a series of oars mounted
in a wheel so that they will come into play one after the other. The
propeller, although not invented by Col. John Stevens (as has been
popularly supposed), was first applied by him to steam navigation when
he constructed a small steamboat on the Hudson River in 1804. But the
simplicity of the paddle wheel and its high efficiency, particularly
in quiet harbors and shallow inland waters, gave it preference over
the propeller. In rough seas, however, the paddle wheel was far from
ideal. It was too easily broken by heavy waves and between 1855 and
1865 the propeller displaced it completely for ocean-going vessels.
WATER JET PROPULSION
Another curious form of propulsion, which dates back to the eighteenth
century and is still periodically revived by inventors, is the water
jet. The idea was to have the engine operate a pump which would drive
a stream of water out of the stern of the boat and drive the boat by
reaction. The British Government actually built two jet-propelled
steamers. One of them, called the _Waterwitch_, was a 1,100-ton vessel
and the other, the _Squirt_, was a small torpedo boat. The latter
attained a speed of but twelve knots while a sister ship of the same
steam power driven by a propeller attained a speed of seventeen knots.
The _Waterwitch_ was even less efficient. Some years ago experimental
water-jet vessels were built in New York in which a jet only ⅝ inch
in diameter with a pressure of 2,500 pounds per square inch was used,
but the experiment proved a failure. The propelling force of a jet is
the reaction of the stream of water against the orifice from which
it issues. The action is just like Hero’s reaction steam turbine
referred to on page 143. The propulsion would be the same were the jet
discharged in the open air or in a vacuum or against a solid stone wall.
WATER AND AIR RESISTANCE
It takes very little power to move a boat slowly because the resistance
that has to be overcome is merely the parting of the water at the
bow and closing in of the water at the stern and the skin friction
along the sides of the hull. In addition to this there is a similar
resistance offered by the air. At very low speeds the resistances
of the water and the air are practically negligible. In perfectly
quiet water with no air stirring the pull of a cord will move a ship
weighing hundreds of tons, but the motion will be very slow indeed.
Unfortunately the speed of a ship does not increase directly in
proportion to the power that drives it. Doubling the power does not
double the speed. If it takes ten horsepower to drive a vessel at a
speed of ten knots it will take not 2 but 2³ or 8 times as much power
to drive it at a speed of twenty knots. In other words, the horsepower
goes up as the cube of the speed. This is an average condition for
ordinary speeds. For very high speeds the horsepower may have to be
increased as the 4th and even the 5th power of the speed. The shape
of the bow and the stern is of utmost importance. The parting and
displacement of the water at the bow and the replacement at the stern
produce waves and the forming of these waves represents so much wasted
energy. The swell that is kicked up by a steamer is evidence of power
uselessly expended. Much of this loss can be overcome by careful design
of the ship’s lines. A vessel that kicks up a high bow wave--one that
sails with a “bone in its teeth”--may present a very pleasing spectacle
and may seem to be traveling at high speed, but the best designed
vessel--the one that slips through the water with no fuss--is much more
economical of power. It is easy to understand that the bow must be
carefully designed to cut through the water, but it is not so apparent
that the stern must also be shaped to permit the water to flow in
readily and fill in the void behind the ship. If the stern is not
carefully shaped, there will be a serious drag on the vessel. The skin
friction of the vessel is greatly increased by fouling of the hull with
marine growths. At high speeds the wind pressure on the superstructure
is considerable. Every spar and line adds its quota. A boat that is
traveling in still air at a speed of twenty-two knots or twenty-five
miles per hour is encountering the equivalent of a twenty-five mile
wind which will exert a pressure of over three pounds per square foot
of frontage.
FLYING ON WATER
Instead of cutting through the water modern speed boats are designed
to ride over it. The boats have flat bottoms which are arranged as a
series of flat planes known as hydroplanes. These planes form steps
and are set at such an angle as to make the boat rise up on the water
in the same way that a kite rises in the air. The higher the speed the
higher the boat rises so that at full speed it skims on the surface.
Hence there is comparatively little power wasted in displacing the
water. Some of these boats are driven by air propellers so that water
resistance to the propeller gear is avoided. These hydroplanes (they
must not be confused with hydroaeroplanes) almost fly over the water.
FIRST OCEAN STEAMER
It was in 1807 that Robert Fulton built the _Clermont_ and established
steam navigation by running a regular service between New York and
Albany, and it was twelve years later that the Atlantic Ocean was
first crossed by a steam-driven vessel. It was an American vessel,
the _Savannah_, that made the trip. She was a 380-ton ship equipped
with steam power to help her along when the wind failed. Seventy-five
tons of coal and twenty-five cords of kindling wood were taken aboard
to feed her furnace. This was thought to be ample for the voyage, but
before the trip was completed the fuel was all gone. The log of the
_Savannah_ bears this entry the night before sighting the Irish coast:
“2 A. M. Calm. No cole to get up steam.” However the captain did raise
steam just before reaching Kinsale, Ireland, by burning wood. Watchers
ashore beholding the smoke issuing from her stack were convinced that
the vessel was afire and boats were dispatched to the rescue. The
_Savannah_ made the trip from Savannah to Kinsale in 23 days and was
under steam propulsion for only 80 hours of this time.
Regular trans-Atlantic steam service was not inaugurated until 1838,
but for many decades steamers were equipped with sails to assist them
when the wind was favorable.
The most notable of early steamships was the _Great Eastern_, a
combined screw and paddle-wheel ship, 692 feet long, built in 1858.
She held the record for size until 1899 when the _Oceanic_, 704 feet
long, was put into service. At present the _Leviathan_, formerly
the _Vaterland_, holds the record with a length of 920 feet. It is
difficult to judge of the size of a vessel out on the open water. If
the _Leviathan_ were placed in Broadway, New York, she would span
nearly four blocks. Because of her 100-foot beam she would be too wide
to be wedged in between the skyscrapers that border lower Broadway. If
she were set up on end she would tower 158 feet above the pinnacle of
the Woolworth Building. Her power plant consists of four turbines which
total 90,000 horsepower and the huge vessel is driven at a speed of
25.8 knots or nearly thirty miles per hour.
The wonder of these huge floating structures lies not merely in their
gigantic proportions but in the fact that they are able to weather
the terrific wrenching strains of heavy ocean storms. A skyscraper
is built to withstand only the steady and direct pull of gravity and
the variable thrust of the wind which, except in western cyclones,
seldom amounts to thirty pounds per square foot. Bridge building is
more difficult because of the leverage of the parts overhanging the
foundations. Wind pressures must be calculated and also the live load
of objects moving over the structure. In naval architecture enter
the problems of building construction combined with those of bridge
building, complicated by the fact that there is no fixed foundation
for the structure to rest upon. At one moment a ship may be spanning a
trough in the seas and at the next it may be seesawing over the crest
of the wave. Of course the bottom of the boat is seldom if ever out
of the water and a certain amount of support is provided throughout
the length of the vessel, but the ship is subjected to the strains
of a cantilever bridge when she is passing over a wave, and to the
strains of a truss bridge when spanning a wave trough. These strains
are increased by the fact that the structure is in constant motion. A
certain degree of flexibility is demanded of the materials which go
into the structure and of the joints between the frame members.
BOATS OF ARTIFICIAL STONE
Originally wood was used for the hulls of ships; then between 1845 and
1855 iron supplanted wood, Between 1875 and 1885 steel supplanted iron
and to-day efforts are being made to supplant steel with concrete. The
advantages offered by concrete are cheapness and speed of construction.
The first large vessel built of this material was the _Faith_, an
8,000-ton ship. This boat stood up very well in heavy weather despite
the rigidity of her structure. It is doubtful, however, that a large
boat comparable in size to the _Leviathan_ could weather a severe ocean
storm.
The proposal to build ships of cement created almost as much of a
popular sensation as did the first iron boat. Although the public
had accepted iron and then steel as a perfectly proper material for
shipbuilding, concrete seemed too much like stone and it did not seem
possible that artificial stone could be made to float. They did not
realize that a cubic foot of steel weighs four times as much as the
same volume of concrete. Of course concrete does not begin to have the
tensile strength of steel and consequently the walls of a concrete ship
must be made relatively thick. For this reason a concrete vessel is
heavier than a steel vessel. She draws more water and requires a larger
power plant, and because of her greater mass she is not so readily
maneuvered.
SUBMARINE NAVIGATION
As was explained in Chapter VI, a body will float only so long as it is
lighter than the volume of water it displaces. It is almost impossible
to keep a body suspended in water unless some portion of it is exposed
above the surface. If it starts sinking it will keep on going down
until it reaches the bottom of the sea. There is a popular notion that
at great depths water becomes dense enough to float solid iron, but
water is practically incompressible and its density at a depth of five
miles is only slightly greater than that at the surface. An object must
therefore either float on the surface or sink to the bottom, unless its
weight is exactly equal to the difference between the upward pressure
of the water under it and the downward pressure of the water above it.
Such an ideal balance it is practically impossible to obtain unless the
object itself is compressible.
How then can a submarine navigate under water without sinking to the
bottom?
A SUBMARINE OF THE SEVENTEENTH CENTURY
The first solution to this problem dates back to the seventeenth
century. Doctor Cornelius Van Drebel, a Netherlander, who was a guest
at the court of King James I of England, built three submarines between
1620 and 1624. These were rowboats covered over with a water-tight
deck and propelled by twelve oarsmen. It is recorded that Van Drebel
discovered a means of holding the boats submerged by observing some
fishermen towing baskets full of fish up the Thames. The barks to which
the baskets were attached by cables were weighted down by the load they
were towing, but when the cables slackened the boats rose a little bit.
Van Drebel’s method of applying this principle was evidently to attach
a weight to the boat which trailed along the bottom. When the oarsmen
propelled the boat, she was pulled down under the surface by the drag,
but when the rowing ceased the boat would float up to the surface. King
James himself is said to have made a journey of several hours’ duration
in one these boats, which was kept at a depth of twelve to fifteen
feet below the surface. Progress must have been very slow because the
range of the submarine was given as five or six miles.
During the Revolutionary War David Bushnell built a submarine with
which attempts were made to sink a British frigate lying in the Hudson
River. This submarine was driven by a hand-operated screw propeller.
The boat was provided with water ballast tanks, and by pressing a valve
with one foot he could let in water enough to submerge the boat while
with the other foot he could operate a pump to empty the tanks and
bring the boat to the surface. When the boat was ballasted so that she
would barely float, a vertical screw propeller was operated to raise
or lower her as much as desired. A 200-pound lead weight was attached
to a long cable which passed up through the bottom of the boat, and by
letting out this cable the submarine could be made to rise instantly in
case of an accident.
FULTON’S HAND-PROPELLED SUBMARINE
To Robert Fulton, however, belongs the credit of building the first
submarine operating on the principle that is now universally used.
His boat was also driven by a hand-operated screw propeller and was
furnished with water tanks which could be filled or pumped out at will,
but after the submarine was weighted until only the conning tower
showed above water, she was submerged or raised by means of horizontal
rudders or hydroplanes which could be tilted to any angle desired. Of
course these rudders, like any other rudders, would not operate unless
the boat were in motion. Such is the case with modern submarines. Like
bicycles, they must keep on going or they will fall. If they are
heavy, they will fall to the bottom, and if light they will “fall”
to the surface. When in motion the hydroplanes will either hold them
down or lift them up according to the angle to which these horizontal
rudders are tipped.
Robert Fulton’s _Nautilus_ had a fish-shaped hull of copper plating on
iron ribs and was twenty-one feet three inches long by six feet five
inches at her greatest diameter. The screw propeller was operated by
two men. When the boat was on the surface a sail was raised to assist
in driving the boat. This sail could be folded up like a fan when it
was desired to submerge.
Fulton deserves full credit for anticipating so many of the essential
features of the modern submarine, but of course the _Nautilus_ was
a mere toy compared to the marvelous machines which swim under the
surface of the sea to-day.
The German U-boats at the outbreak of the war were 150 feet long and
could make only nine knots submerged and twelve knots on the surface,
but later they grew to a length of 300 feet with a submerged speed of
twelve knots and a surface speed of eighteen knots. The British in
the meantime developed a submarine that was 340 feet long and had a
displacement submerged of 2,700 tons as against 800 for the largest
German U-boats. The speed of this big British boat is twenty-four knots
on the surface and ten knots submerged.
SUBMARINES AS SURFACE BOATS
Despite their name, submarines are really surface boats. Only when
necessity demands are they submerged. During the war even the U-boats
did 90 per cent of their sailing on the surface. Origially submarines
were built primarily for submerged travel and consequently they were
given the form of a fish or of a fat cigar, but such a shape was not
adapted for surface sailing. Water piled up on the nose of the boat and
tended to bear her down. To overcome this, submarines are now shaped
more like a boat with a bow high enough to part the waves without
burrowing into them.
POWER PLANT OF A SUBMARINE
The gasoline engines that were formerly used for propelling the boats
have now given way to heavy oil or Diesel engines, because of the
greater efficiency of these engines and the greater safety of handling
heavy oil. Some submarines have been equipped with steam power plants.
Such is the power used on the big British submarines above referred to.
Of course any power that involves the burning of fuel can only be used
on the surface. As has already been explained, an internal combustion
engine burns seven to nine times as much air as oil and it would be
impossible to store enough air on board to keep the engines going very
long; hence they can be operated only while the hatches are open to the
atmosphere.
Coupled to the shafts of the engines are dynamos which generate
electricity and feed it to storage batteries. A dynamo will serve as
a generator when turned by some mechanical power and on the other
hand when current is fed into it, it serves as a motor, so that when
traveling submerged and fuel can no longer be used, the generators are
disconnected from the engines but remain connected to the propeller
shafts and driven by the very current they previously stored in the
batteries.
THE COLLAPSIBLE “EYE” OF THE SUBMARINE
Amidships there is a bridge from which the vessel can be navigated when
on the surface and a conning tower from which she can be navigated
when running awash. Most of the submerged travel is maintained at a
comparatively shallow depth so that the submarine can keep an “eye” on
the surface. The eye is the periscope of which there are two so that
in case of damage to one the other may be used. Periscopes date back
to the “Fifties” when they were used on some experimental European
submarines. In the Civil War, when the river monitor _Osage_ ran
aground in the Red River, her captain, now Rear-Admiral Thomas O.
Selfridge, rigged up a periscope with which he could look over the high
banks of the river and direct the fire of his guns upon a Confederate
force that was attacking him. This periscope consisted merely of a
three-inch pipe with a hole at each end cut in opposite sides of the
pipe. Small mirrors were set in the pipe so that the light coming in
through the upper hole was reflected down through the pipe and out of
the lower hole. This crude periscope is the same in principle as the
modern submarine periscopes except that the latter are provided with
lenses to gather and focus all the light possible on the eyepiece so
that the operator will have a perfectly clear view. Periscopes extend
fifteen to twenty feet above the roof of the conning tower. While
periscopes are insignificant objects on the broad seas when a submarine
is moving very slowly, they are made conspicuous by the wake of foam
that follows them when the boat is traveling even at a moderate speed.
For this reason during the war the Germans developed a telescoping
periscope which could be shot up to the surface whenever desired in
order to give the commander a glimpse of his surroundings.
The submarine is submerged by letting water into the ballast tanks and
then turning the hydroplanes to a diving angle. There is an after as
well as a forward pair of diving rudders.
When running completely submerged the submarine is blind and solely
dependent upon the chart and compass. It is impossible to see far under
water. Searchlights are of no value at all. They will not make visible
an object a hundred feet away. The submarine commander cannot see even
the bow of the boat he is piloting. The ordinary magnetic compass will
not operate when entirely incased in steel as it is in the hull of the
submarine and so a gyroscope compass has to be used instead.
While the compass serves as a guide for travel in the horizontal plane
the depth gauge must be watched to see that the boat does not dive too
deeply. The pressure of the sea increases at the rate of 64 pounds per
foot of depth. Two hundred feet below the surface the pressure amounts
to about six tons on every square foot of the surface of the submarine.
Few submarines can stand a greater pressure than that without being
crushed or at least springing serious leaks. When coming to the surface
the hydroplanes are used and if the boat is to remain on the surface
water is blown out of the ballast tanks.
THE “EARS” OF A SUBMARINE
Although submarines may be blind under water they are not deaf. Sound
detectors are used which enable them to locate other vessels by the
throbbing of their engines or the beat of their propellers and so they
can avoid collisions when coming up to the surface.
Despite the perfection of the submarine the sea is still a great
mystery to us. We know only its surface and its shallows. We have
sounded a few deep holes and brought up samples of deep-sea life, but
we have not been able to penetrate in person its profound depths and
explore with our eyes and hands the world that is buried beneath its
waves. Three quarters of the globe is covered by water and if we are to
claim complete mastery of the earth we must find some way of descending
into the heart of the ocean and exploring its deepest valleys.