THE SCIENCE OF NAVIGATION
Navigation, I may be permitted to repeat, is the mathematical science
of finding ships’ positions at sea and of laying down courses to be
followed. For the designation of positions latitude and longitude are
used, latitude being measured north and south from the equator to the
north and south poles, the equator being zero degrees of latitude, the
poles being ninety degrees north and ninety degrees south latitude.
Longitude is measured from zero degrees to 180 degrees east and west
from the meridian running from the North Pole to the South through
Greenwich, England, 180 degrees east longitude marking the same
meridian as 180 degrees west longitude. For instance, Three Kings
Island, the tiny island which is the northernmost land of the New
Zealand group, is located as follows: Latitude 34° South; Longitude
172° East. This means that this island is 34 degrees south of the
equator and 172 degrees east of the meridian of Greenwich. Actually
navigation is a problem in spherical trigonometry and astronomy,
depending principally, nowadays, upon an instrument called a sextant,
which is used to measure the altitude above the horizon of a celestial
body (sun, moon, or stars), and upon a very accurate timepiece, called
a chronometer, which shows the time of a given meridian—generally the
meridian of Greenwich, England.
In practice, however, it is necessary to know no mathematics other
than arithmetic, for the formulas have been simplified and handbooks
have been compiled which eliminate any necessity for the practical
navigator to delve into the intricacies of spherical trigonometry, a
subject that would frighten most sea captains more than all the other
perils of the deep.
There is another but less accurate method, called “dead reckoning,”
which is used in connection with the more accurate science, and is
used by itself when clouds obscure the sky or fogs hide the horizon.
When land is in sight both these methods largely or entirely give way
to “piloting,” which makes possible the accurate finding of a ship’s
position by reference to known objects ashore.
I shall not attempt to explain all the intricacies of navigation,
for even a simplified complete explanation would in itself become a
small book. There are many books on navigation. Nathaniel Bowditch’s
exhaustive treatises have been revised many times and the whole
compilation is kept up to date so that, while Bowditch himself died
in 1838, the book bearing his name, and still referred to almost
universally as “Bowditch,” is accepted as a peerless authority. But
it is a huge tome, and other practical books, such as “Elements of
Navigation,” by W. J. Henderson, are available for the person who
wishes to profit by a simpler, if less exhaustive, explanation. To
these two books, and to a dozen others, I refer the interested reader
anxious to learn what, after all, is beyond the range of this outline.
Up to the 15th Century the science of navigation was unknown. Before
that time mariners occasionally ventured out of sight of land, for
short passages during which, because they had no compasses, they
attempted to guide themselves by reference to the sun or stars. When
clouds obscured the sky, however, they usually lost their direction,
and even when the sky was clear they knew no way of ascertaining
anything more than rough approximations of the cardinal points.
It seems just a bit strange that sailors were so backward in developing
means of determining their positions at sea by reference to the sun and
stars, while even the ancients were fairly accurate in their ability to
locate their positions ashore by such methods. This undoubtedly was as
much due to the lack of general knowledge among sailors as it was to
the unsteadiness of the ships themselves which made it difficult for
careful astronomical observations to be made. But whatever the reason,
the fact remains that it was not until after the introduction of the
compass that navigation began to make its first faltering advances.
[Illustration: USING A CROSS STAFF
_This crude instrument was used in an attempt to work out problems in
latitude. After holding one end of the staff to the eye and sliding
the cross staff along until the observer sighted over one end at the
sun and under the other at the horizon, the instrument was placed on a
circle marked in degrees, and the angle was determined._
]
That this beginning was made during the period in which Portugal
expanded her commerce only goes again to show that the application of
new minds to old problems results, almost invariably, in progress.
Columbus, of course, did not begin the era of discovery. Prince Henry,
the “navigator,” sent out an expedition from Portugal in 1432 which
rediscovered the Azores—an astonishing thing for times so early, for
the Azores lie 830 miles west of Portugal and are farther from a
continental mainland than any other of the islands of the Atlantic.
That the islands were known to the ancients, however, is proved by
numerous Carthaginian coins found on the island of Corvo, but their
location and practically everything else concerning them seems to have
been lost until Henry the Navigator attached them to Portugal.
But the rediscovery of the Azores proves only that the sailors put
great faith in their compasses, and sailed, despite their fears, out to
the west where all of them _knew_ (it was no matter of mere _belief_)
that the sea ended somewhere suddenly, and that their cockleshell ships
would, if they but sailed to the edge, fall down the smooth green
cataract of an awful, endless waterfall, into limitless space, or,
mayhap, to the vast foundations upon which the world was built. To them
it was as if a canoe were being paddled downstream to the brink of a
cataract to which Niagara itself would be but a raindrop falling from
the eaves.
At the time of the rediscovery of the Azores navigation was, with
the exception of the compass, without any of the instruments that
later came into use. Prince Henry, however, realizing the importance
of compiling information useful to mariners, systematized all the
information available and erected an observatory to determine more
accurately the data in reference to the declination of the sun.
Most navigators use the sun far more than any of the other celestial
bodies in order to determine their positions, and the first thing
necessary is to know its declination—that is, its distance north or
south of the equator.
During the course of a year the movement of the earth, with its
axis inclined at an angle to the plane in which it moves about the
sun, brings the sun vertically over every section of the earth from
twenty-three and one half degrees north of the equator to twenty-three
and one half degrees south and back again.
During the year, then, the sun is twice directly over our equator.
Suppose at noon on one of these days a mariner wishes to determine
his latitude, that is, his distance in degrees, minutes, and seconds
north or south of the equator. He measures, with his sextant, the angle
between the sun and the horizon. If he were on the equator that angle
would be ninety degrees, for the sun would be directly over his head.
He would then subtract the angle shown by his sextant from ninety, the
number of degrees between the horizon and the zenith. In this case the
answer would be zero. Therefore his latitude would be 0 degrees, and
that is on the equator. If he were at the North Pole or the South, the
sun would be on the horizon, and his sextant would show an angle of 0
degrees. Subtracting this from ninety he would find his latitude to be
ninety degrees, north or south of the equator, as the case might be.
At any position between the equator and the poles the problem would be
worked in the same manner.
But, except for two days in the year—but for two moments I might almost
say—the sun is never directly over the equator, and declination is its
distance at any given time north or south of the equator, measured
in degrees, minutes, and seconds. This cannot be learned by any
observations from a ship at sea. It is comparatively simple, however,
to learn it by careful studies made at well-equipped observatories, and
the results of these studies are now furnished mariners in carefully
compiled form, so that they have merely to turn to their book in order
to learn what the sun’s declination is at any given time.
It was this work that Prince Henry began, and modern navigation may,
perhaps, be said to have begun with his studies.
But all the tables of declination are of no use without an instrument
with which to measure accurately the angle between the sun and the
horizon, and such an instrument was slow in coming. The first apparatus
used was called a “cross staff.” It was made of two rods, one about
thirty-six inches and the other about twenty-six inches long. The
shorter staff was arranged so that its centre slid along the other
while it remained rigidly at right angles to the longer staff. To work
out one’s latitude with this instrument the observer waited until
noon was almost upon him. He then took his cross staff and, placing
one end of the longer crossbar to his eye and holding the instrument
so that the shorter bar stood in a vertical plane, moved the shorter
bar back and forth until he could sight over the upper end at the sun
and, at the same time, beneath the lower end at the horizon. As the
sun continued to mount to its highest point he pulled the cross staff
slowly toward him, measuring a greater and a greater angle. When the
sun had reached its highest point and the angle between it and the
horizon began to lessen, his “sight” was completed, and carefully
holding the crossbar where it marked the greatest angle he laid it on
a table on which a circle was inscribed. The end that had been at his
eye he placed at the centre of the circle, and the segment marked by
the lines from the centre past the two ends of the crossbar showed the
number of degrees in the angle he had measured between the horizon and
the sun.
But any one who has attempted to sight a gun accurately while
standing on an irregularly moving platform will have some idea of
the difficulty these old sailors had in sighting accurately at the
horizon and the sun at identically the same time from the deck of a
bobbing ship. The glare of the sun, the motion of the ship, and the
inaccuracy of the instrument itself could not be expected to give more
than approximate results, especially as the several corrections on the
angle now known to be necessary (the refraction of the sun’s rays as
they enter our atmosphere is one) were either not recognized or were
inaccurately known.
[Illustration: USING AN ASTROLABE
_This instrument was meant to improve on the cross staff. One
man held it, when it was supposed to hang with the horizon line
horizontal. Another man sighted at the sun or the stars, and a third
read and recorded the angle. Needless to say the instrument was very
inaccurate._
]
Later the “astrolabe,” an instrument almost equally crude, was
introduced. It was made of a heavy tin or bronze plate, circular in
shape, and pivoted to its centre was a bar running across it from side
to side. It was marked in degrees and fractions, and while one man
held it, as steadily as he could, a second sighted along the pivoted
crossbar and a third read the angles. Vasco da Gama used one of these
in 1497 on his voyage around the Cape of Good Hope, but it did not turn
out to be much of an improvement on the cross staff.
But up to this time, and even later, the science of navigation
consisted almost solely of the approximate determination of latitude
and mere guesses, based on the estimated speed and direction of the
ship through the water, for longitude. So hopeless did it seem at that
time for mariners scientifically to determine their longitude that an
old writer on the subject is quoted by the Encyclopædia Britannica
as saying, “Now there be some that are very inquisitive to have a
way to get the longitude, but that is too tedious for seamen, since
it requireth the deep knowledge of astronomy, wherefore I would not
have any man think that the longitude is to be found at sea by any
instrument; so let no seamen trouble themselves with any such rule, but
(according to their accustomed manner) let them keep a perfect account
and reckoning of the way of their ship.”
These early sailors learned, of course, that their latitude could be
worked out by observing the North Star, and they used that method,
crudely, of course, but similarly to the way it is used to-day. For
this a contrivance called a “nocturnal” was adopted. With this they
could determine what position the North Star was in, in reference to
the true pole, for, of course, the North Star does not exactly mark the
pole, but revolves about it in a small circle.
While the voyage of Columbus did not actually begin the era of
discovery, it did greatly increase interest in exploration, and as most
of this exploration necessitated long ocean voyages the interest in
navigation grew apace. One of the earliest writers on navigation was
a man named John Werner. In 1514 he explained the use of the cross
staff, which for many years had been used on shore but had been first
utilized at sea not very many years before Werner wrote. A little
later one R. Gemma Frisius wrote a book which contained a great deal
of information useful to men of the sea. In it he described the sphere
with its parallels of latitude and its meridians of longitude much as
we use them to-day. Up to this time, however, no agreement had been
made upon what meridian to base the measurement of longitude. Nowadays
the meridian of Greenwich is used. Frisius, however, suggested the
meridian of the Azores. Any meridian, of course, would do, provided
that the necessary data be based upon it, but the data available in the
early 16th Century were slight indeed.
The necessity for drawing curved lines on flat charts to represent the
courses of their ships now began to be understood, for ships do not
sail on a flat surface but instead sail on the ever-curving surface
of the sea. To the person accustomed, as most of us are, to looking
at maps printed on flat pages, this truth becomes evident when he
draws a straight line on a flat map, and then transfers the line to a
geographical globe, making it pass through the same points.
Mariners were troubled, too, by the difficulty of accurately and easily
drawing parallel lines on their charts, but this was overcome in 1584
when “parallel rulers” were first used by one Mordente. “Parallel
rulers,” which are nothing more than two rulers hinged together so that
whether they touch each other or are separated they remain parallel,
are a part of every navigator’s equipment to-day.
Tables of the tides began to appear in the latter part of the 16th
Century, but they were woefully inaccurate, and other information,
while increasing, still was liable to be seriously in error.
Even points ashore, where observations could be worked out under the
best possible conditions, were thought to be from a few minutes to
several degrees from what we now know are their positions, and when one
realizes that an error of one minute of latitude means an error of one
mile, it will be seen that an error of fifteen or twenty minutes might
be enough to put a ship in grave danger while her captain thought her
safe, and that a position in which there is an error of several degrees
is little more than worthless, for each degree of latitude represents
60 miles, and three or four degrees mean one hundred and eighty or two
hundred and forty miles. When it is realized, furthermore, that such
errors as these were made ashore, where the observations were much more
accurate than they could be at sea, one understands why seamen trusted
their navigation but little, for they were often faced, no doubt,
with errors of three or four hundred miles. And, if anything, their
methods of determining latitude were less inaccurate than those used in
determining longitude. Truly, navigation in those days left much to be
desired.
Other instruments were invented from time to time in the struggle to
master navigation. The “astronomical ring” was one, but it was little
less crude than the astrolabe.
[Illustration: A SEXTANT IN USE
_Sextants are used to measure the elevation of celestial bodies—the
sun, moon, or stars—in working problems in latitude and longitude._
]
[Illustration: A SHIP’S LOG
_The mechanism at the top is fastened on the ship’s rail, and a line
with the rotator shown below at its end is allowed to trail in the
water astern. The passage of the rotator through the water causes it
to turn, the line is twisted, and the log is made to register the
miles travelled._
]
Now up to the 16th Century navigators were without the one essential
instrument necessary to the accurate determination of longitude. That
instrument was an accurate timepiece that could be carried to sea. It
is not necessary to have a timepiece in order to learn one’s latitude,
but longitude is a more difficult problem, and _time_ is an element
in it. But the watches of the 16th Century were too inaccurate to be
of much service, and, as a matter of fact, it was not until 1607 that
it was realized that a day is not necessarily made up of twenty-four
hours. If one stays in one place it is true that there are twenty-four
complete hours from noon to noon, and clocks were designed to register
the time _at one place_. But suppose, as the sun rises to-morrow
morning, you board a very fast airplane and fly it at its fastest speed
toward the west. Suppose this airplane flies at the rate of 1,000 miles
an hour. In twenty-four hours you have flown around the world, and
wherever you have been during that time the sun has been just rising
behind you. It has been early morning for _you_ all the time. Suppose,
on the other hand, you had flown east at the same rate of speed. If you
started at six o’clock in the morning, in three hours the sun would be
overhead—that is, it would be noon for _you_. In three more it would
be evening. In six more it would be morning again, for you would be
halfway around the world. Six hours later evening would come _to you_,
and in another six hours you would be at your starting point and it
would be morning once more—the _second_ morning you had seen after
you started, but only the _first_ morning after for the people you had
parted from twenty-four hours before.
Ships, of course, do not travel at 1,000 miles an hour. But they do
travel many miles, perhaps several hundred, in twenty-four hours.
Therefore, if you start at Guayaquil, Ecuador (I use that, for it is
very nearly on the equator), and sail west for twenty-four hours,
making 240 miles, your watch will tell you that it is exactly the same
time of day that it was when you left Guayaquil. But that is not true.
It _is_ the same time of day _at Guayaquil_, but you are four degrees
west of Guayaquil, and the sun must still travel past four degrees of
longitude before the time at the spot you have reached will be what
your watch suggests. It will take the sun sixteen minutes to cover that
distance, and therefore your watch is sixteen minutes fast.
Great strides were made during the 16th and 17th centuries and many
books were published. Probably the first book entirely about navigation
ever published was one entitled “Arte de navigar,” by Pedro de Medina.
This appeared in Spain in 1545. The fact, however, that the subject was
not really understood is proved by the acceptance at an even later date
of the theory that the earth did not move and that the sun revolved
about it.
Charts became greatly improved during the latter part of the 16th
Century, owing to the studies of Mercator, after whom the “Mercator
projection” is named. The Mercator projection is used in the type
of map that shows the entire surface of the earth as if it were the
unrolled surface of a cylinder, and is the type which is, perhaps,
despite its errors, in commonest use to-day.
But despite many improvements it was not until the 18th Century that
modern navigation really began. Then, suddenly, both the sextant and
the chronometer were invented in rapid succession—the one in 1731
and the other in 1735. The sextant is the instrument (now greatly
perfected) that is used to measure accurately the angles between the
horizon and the celestial bodies being observed, and the chronometer is
the accurate timepiece (now also greatly perfected) used on practically
all sea-going ships to keep a record of the time of the prime meridian
of longitude—that is, the meridian numbered zero. Usually, nowadays,
that meridian, as I have said, is the meridian of Greenwich, England,
for it is at Greenwich that a British observatory is located, and at
this observatory the vital data for seamen are compiled.
With the introduction of the sextant and the chronometer the
determination of longitude became simple. And latitude, too, because of
the sextant, could more accurately be determined.
It is not my purpose to go into detail in explaining the finding of
one’s longitude, but I shall attempt to explain, simply, the theory.
The sun, during a day of twenty-four hours, covers the 360 degrees of
the circumference of the earth. That is, during every hour it passes
15 degrees. If you have a clock that tells you that it is 9 o’clock
in the morning at Greenwich and you know that, according to the sun,
it is 8 o’clock in the morning where you are, you know that because
of that difference of one hour there is a difference of 15 degrees
of longitude, and that you are 15 degrees west of the meridian of
Greenwich. If you were 15 degrees east, your time would be 10 o’clock.
Now if you have some accurate way of telling what time it is by the
sun where you are, and you have a chronometer telling you the time at
Greenwich, all you have to do is to subtract the earlier time from the
later and work out how many degrees, minutes, and seconds of longitude
are represented by the hours, minutes, and seconds of the difference.
If it is later at Greenwich than where you are, you are west of
Greenwich; if earlier, you are east.
On the morning of March 7, 1916, I took a sight of the sun when the
chronometer showed it was 39 minutes and 1 second past 1. My sextant
showed me, after I had made some corrections which I shall not attempt
to explain, that the altitude of the sun was 24° 58′. From this, and
other data that it is necessary to have, I worked out our time _when
I took the sight_. The answer to my problem showed me that the time
was 13 minutes and 4 seconds past 8 o’clock. Subtracting this time
from the time shown by the chronometer I got 5 hours, 25 minutes,
and 57 seconds. Because a difference of one hour of time represents
a difference of 15 degrees of longitude, a difference of 5 hours, 25
minutes, and 57 seconds in time represents a difference of 81 degrees,
29 minutes, and 15 seconds in longitude. The Greenwich time was later
than ours; therefore, our longitude was 81° 29′ 15″ west of Greenwich.
I have purposely refrained from explaining the working of the problem,
for that can only be done with such a reference book as Bowditch
at hand, in order that the compiled logarithms may be looked up.
Furthermore, the explanation is long, technical, and, to the beginner,
tedious, and is beside the purpose of this book. I have given the
incomplete explanation only to show that to find longitude one must
find one’s “local mean time,” and must have a timepiece showing the
“mean time” at Greenwich.
In the foregoing explanation I have left out of consideration several
factors vital to accuracy in navigation. For instance, I have not
mentioned the fact that the sun is not so accurate in its movements
as an accurate chronometer. Sometimes it is a few minutes ahead and
sometimes it is a little behind time. From this, two expressions for
time have come into use: “apparent time” and “mean time.” “Apparent
time” is the time that is shown by the sun; “mean time” is the time
shown by the clock. Because there is this difference there must be
a correction made for it, and this correction is to be found in the
Nautical Almanac, which is a valuable part of the navigator’s equipment.
Again, the navigator takes the angle of the sun from the bridge or some
other elevated part of his ship. The angle he gets from such a height
is slightly different from the one he would get if he were at the water
level. Therefore he must make a correction for the difference. This
he finds by knowing his elevation above the water and looking up the
correction.
[Illustration: USING A PELORUS
_This apparatus consists of a movable plate marked with compass
bearings, set in a stand. The observer sets the plate to correspond
to the standard compass, and then sights across it in determining the
compass bearings of points ashore from which he wishes to learn his
exact position._
]
There are other corrections still, applying to the sextant angle, to
the sun itself, and to time. All of these are necessary if one wishes
to be accurate, and a navigator should always be as accurate as his
science permits.
But often it is impossible to learn the angle between the horizon and
any of the celestial bodies, for clouds and fog sometimes shut off
the sky and the horizon. Sometimes one is clear while the other is
obscured; sometimes both are hidden. But still it is necessary to know
the position of the ship. As a matter of fact, the heavier the clouds
or the thicker the fog the more desirable it is to know one’s position
accurately. Until recently, however, seamen have had to depend only
upon dead reckoning which often is anything but accurate. But now the
radio direction finder and the method of learning one’s position by
asking radio stations ashore to supply it by plotting the directions
from which one’s radio message reaches two or more of them are coming
into more and more common use.
Dead reckoning however, is still highly important, and is used by
every careful navigator. It requires considerable experience for a
navigator accurately to place his ship by dead reckoning alone. As a
matter of fact, if the voyage is long and the sky has been obscured,
the navigator expects to find himself somewhat wrong in his estimation
of his position and is correspondingly careful. He has had to depend
upon his log, which, as I explained in the last chapter, is a kind of
nautical speedometer. As a check against this he often keeps a record
of the revolutions of his propeller, for he knows, from experience, how
far he will sail in an hour with his propeller running at any given
speed. This is advisable because seaweed may foul the rotator of his
log, or driftwood tear it away or bend it.
In addition to the distance he has sailed he must know accurately the
direction he has sailed, and if he has changed his direction he must
know when and how much. Furthermore, he must study his charts carefully
in order to learn whether or not he is sailing in a part of the
ocean in which there are currents, and if so he must figure out very
carefully what effect the current has on his ship.
Suppose a ship was sailing by dead reckoning across the Gulf Stream
directly east of Cape Hatteras. The Stream, let us say, is 100 miles
wide, and he is ten hours in crossing it. The current flows at the rate
of three miles an hour. Therefore, if he has headed straight across,
the current has carried him thirty miles to the northeast, and unless
he knows how wide the stream is, which direction and how fast it flows,
and how long he has been in it, he cannot possibly know just where he
is. It is as if you tried to cross a river in a rowboat and pointed
its bow at right angles to the shore all the way. The current would
certainly carry you downstream, so that you would not land on the
opposite side directly across from where you started.
When it is necessary, then, for seamen to sail their ships entirely by
“dead reckoning” they are always anxious to check up their positions by
any outside aids that are available. It was for this reason that our
captain, on the imaginary voyage we took from Philadelphia to Havana in
the last chapter, sailed so close to Diamond Shoal Lightship instead of
crossing the Gulf Stream and heading out to sea.
I shall add but one more thing before I end this brief and incomplete
explanation of navigation and its related subjects. Navigation and dead
reckoning we have touched upon. Piloting still remains untouched.
This branch of navigation, if branch it really is, shows the navigator
the position of his ship by reference to objects ashore. Let us
suppose that a ship has crossed the ocean and is approaching a harbour
entrance. While at sea an error of half a mile or so meant little, but
as he approaches shore he wants to know _exactly_ where he is.
On each side of the harbour entrance let us suppose that there is
a lighthouse. The navigator gets out his large-scale chart of the
vicinity and lays it on his chart table. This chart shows the harbour
entrance and shows the positions of the lighthouses. Then he determines
the direction of these two lighthouses according to his compass. Let us
suppose one lies exactly northwest and the other exactly southwest. On
the chart, then, he draws two lines, one through the point marking each
of the lighthouses. From the lighthouse to the northwest he draws a
line extending southeast (the opposite direction) out to sea. From the
lighthouse to the southwest he draws a line to the northeast. These two
lines cross, and he knows that his ship was exactly at the intersection
when he took his bearings. As this can be done in a minute or two the
position is very accurate, unless his ship is sailing very rapidly,
which it probably would not be. This is known as the “cross bearing”
method of learning one’s position, and is one of the simplest problems
in piloting.
Suppose, however, that a ship is sailing along the shore, and but one
prominent object can be seen on the land. The navigator watches until
the object (a lighthouse, perhaps) is “four points off his bow”—that
is, until the angle between his course and the direction of the object
is 45 degrees. From that moment the log is watched carefully, until the
object is directly at right angles to the ship’s course. The distance
sailed during that time is the same as the distance from the ship to
the object ashore at the time the second bearing is secured, and if a
compass bearing is taken when the ninety-degree bearing has been taken,
a line drawn on the chart from the position of the object ashore can be
marked with the distance in miles, and the navigator will know exactly
the position of his ship at that moment. This is known as “bow and beam
bearings.” There are other similar methods of obtaining the desired
result.
In foggy weather when ships are “on soundings”—that is, where the water
is shallow enough to permit of the easy use of a line with a weight
attached for measuring its depth—careful navigators invariably use the
“lead line” constantly.
[Illustration: SOUNDING BY MACHINE
_A glass tube with the upper end closed and the lower end open is
lowered in a special case to the sea bottom, and then brought to the
surface. As the tube descends, the water compresses the air in the
tube, and gradually creeps up inside. The inside of the tube being of
ground glass the water leaves a mark showing how far it has entered
the tube. By laying the tube on a special scale the depth to which the
glass was carried can be gauged. There are other methods not greatly
dissimilar from this._
]
This tells them not only how deep the water is, but by putting tallow
or soap on the bottom of the lead weight they bring up sand or mud or
shells from the bottom. With this and the depth, a line is drawn on
tracing paper on the same scale as the chart. Along this line these
soundings and the kind of mud or sand the lead brings up are marked,
at intervals corresponding to the distance the ship has sailed between
soundings. The chart is printed with the depth of the water in fathoms
and with the kind of bottom that will be found. After the navigator
has compiled his data for a few miles the tracing paper with the
line on it can be moved about over the chart, and if care has been
taken in sounding and watching the speed and direction of the ship,
the navigator will find the place on the chart where his series of
soundings will match the printed soundings. Then he will know very
accurately where he is, even if it be a fog-enshrouded night.
Many, many important aspects of these three vital subjects have been
completely passed over in this short chapter. If, however, I have been
able to explain a little of the subjects, and if, particularly, I have
quickened the interest of any of my readers in them, my purpose has
been served. Going to sea is not so difficult as many people ashore are
prone to think. But becoming a thorough seaman and a thorough navigator
is not so simple, perhaps, as to become adept at much of the work that
occupies men ashore.