Produced by Viv, Ted Garvin and the Online Distributed
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[Transcriber's Note:-
Words and phrases surrounded by underscores are italicised.
There are four symbols which are denoted by :-
(.) Circle surrounding a centred dot - Right Ascension of the
Meridian;
(+) Circle surrounding a cross - Earth's Central Progress;
(_) Circle with centre dot and line under - Observed altitude of sun's
lower limb;
-(-)- Circle with line through - True Altitude.
]
LECTURES IN NAVIGATION
Prepared for Use as a Text Book
at the
OFFICERS' MATERIAL SCHOOL
NAVAL AUXILIARY RESERVE
by
Lieutenant ERNEST G. DRAPER, U.S.N.R.F.
Head of the Department of Navigation
Officers' Material School, Naval Auxiliary Reserve
COPYRIGHT
BY
ERNEST G. DRAPER
FOREWORD
These Lectures have been compiled as speedily as possible to meet the
demand for some quick but fairly comprehensive method whereby large
bodies of men, divided into small classes, might learn the elements of
Navigation and thus assume, without delay, their responsibilities as
Junior Officers of the deck, Navigators and Assistant Navigators in the
United States Naval Auxiliary Reserve.
I realize that the haste with which the book has been written is
apparent in many places, and it is hoped that many evidences of this
haste will disappear in case further editions are printed. Besides
acknowledging the help and information which was secured from the list
of navigational works, mentioned on another page, I wish to mention
particularly Prof. Charles Lane Poor's book, entitled "Nautical
Science," from which was secured practically all of the information in
the Lecture on Planets and Stars (Tuesday--Week V); Commander W. C. P.
Muir's book, "Navigation and Compass Deviations," and Lieutenant W. J.
Henderson's book, "Elements of Navigation," the text of which was
followed closely in discussing Variation and Deviation and Traverse
Sailing.
I desire to express my gratitude to Lieutenant Commander R. T. Merrill,
2nd, U. S. N., for suggesting a detailed outline of the whole course; to
Lieutenant Commander B. O. Wills, U. S. N., for his valuable criticisms
and almost daily help during the preparation of these Lectures; to
Lieutenant (j. g.) C. D. Draper, U. S. N. R. F.; Lieutenant (j. g.) R.
Brush, U. S. N. R. F., and Lieutenant (j. g.) P. C. McPherson, U. S. N.
R. F., for many criticisms and suggestions; and to Captain Huntington,
Seamen's Church Institute, for suggesting helpful diagrams, particularly
the one on page 44. This opportunity is also taken for thanking the many
Instructors in the School for their opinions on various questions that
have come up in connection with the course and for assistance in
eliminating errors from the text.
E. G. D.
LIST OF BOOKS CONSULTED
AMERICAN PRACTICAL NAVIGATOR, BOWDITCH
NAVIGATION AND COMPASS DEVIATIONS, MUIR
NAUTICAL SCIENCE, POOR
ELEMENTS OF NAVIGATION, HENDERSON
WRINKLES IN PRACTICAL NAVIGATION, LECKY
WHYS AND WHEREFORES OF NAVIGATION, BRADFORD
EPITOME OF NAVIGATION, NORIE
NAVIGATION, HOSMER
FINDING A SHIP'S POSITION AT SEA, SUMNER
GENERAL ASTRONOMY, YOUNG
PREFACE
TO THOSE TAKING THIS COURSE IN NAVIGATION:
These lectures have been written with the idea of explaining, in as
simple language as possible, the fundamental elements of Navigation as
set forth in Bowditch's American Practical Navigator. They will be given
you during the time at the Training School devoted to this subject. At
present this time includes two morning periods of one and a half hours
each, separated by a recess of fifteen minutes. In general the plan is
to devote the first period to the lecture and the second period to
practical work.
Not many examples for practical work have been included in this book,
but one example, illustrating each new method, has been worked out. If
you understand these examples you should be able to understand others
similar to them.
Toward the end of the course a portion of each second period will be
devoted to handling the sextant, work with charts, taking sights, etc.
In short, every effort will be made to duplicate, as nearly as possible,
navigating conditions on board a modern merchant ship.
DEPARTMENT OF NAVIGATION,
_Officers' Material School_,
_Naval Auxiliary Reserve_
CONTENTS
WEEK I--PILOTING
_Tuesday_--The Compass 1
_Wednesday_--Pelorus; Parallel Rulers; The Lead,
Sounding Machine, Dividers and Log 6
_Thursday_--The Chart 10
_Friday_--The Protractor and Sextant 13
_Saturday_--Fixes, Angles by Bearings and Sextant 16
WEEK II--DEAD RECKONING
_Tuesday_--Latitude and Longitude 20
_Wednesday_--Useful Tables--Plane and Traverse Sailing 23
_Thursday_--Examples on Plane and Traverse Sailing (Continued) 27
_Friday_--Mercator Sailing 28
_Saturday_--Great Circle Sailing--The Chronometer 30
WEEK III--CELESTIAL NAVIGATION
_Tuesday_--Celestial Co-ordinates, Equinoctial System, etc. 34
_Wednesday_--Time by the Sun--Mean Time, Solar Time,
Conversion, etc. 36
_Thursday_--Sidereal Time--Right Ascension 43
_Friday_--The Nautical Almanac 47
_Saturday_--Correction of Observed Altitudes 52
WEEK IV--NAVIGATION
_Tuesday_--The Line of Position 55
_Wednesday_--Latitude by Meridian Altitude 58
_Thursday_--Azimuths of the Sun 61
_Friday_---Marc St. Hilaire Method by a Sun Sight 63
_Saturday_--Examples on Marc St. Hilaire Method by a Sun Sight 66
WEEK V--NAVIGATION
_Tuesday_--A Short Talk on the Planets and Stars--Identification of
Stars--Time of Meridian Passage of a Star 66
_Wednesday_--Latitude by Meridian Altitude of a Star--Latitude by
Polaris 73
_Thursday_--Marc St. Hilaire Method by a Star Sight 74
_Friday_--Examples: Latitude by Meridian Altitude of a Star;
Latitude by Polaris; Marc St. Hilaire Method by a Star Sight 75
_Saturday_--Longitude by Chronometer Sight of the Sun 76
WEEK VI--NAVIGATION
_Tuesday_--Longitude by Chronometer Sight of a Star 79
_Wednesday_--Examples on Longitude by Chronometer Sight of a Star 80
_Thursday_--Latitude by Ex-Meridian Altitude of the Sun 81
_Friday_--Examples: Latitude by Ex-Meridian Altitude of the Sun 83
_Saturday_--Finding the Watch Time of Local Apparent Noon 83
WEEK VII--NAVIGATION
_Tuesday_--Compass Error by an Azimuth 88
_Wednesday_--Correcting Longitude by a Factor 89
_Thursday_--The Navigator's Routine--A Day's Work at Sea 91
_Friday_--Day's Work 105
_Saturday_--Day's Work 105
WEEK VIII--NAVIGATION
_Monday to Thursday_--Day's Work 107-108
_Additional Lecture_--Compass Adjustment 109
WEEK I--PILOTING
TUESDAY LECTURE
THE COMPASS
Everyone is supposed to know what a compass looks like. It is marked in
two ways--the old way and the new way. Put in your Note-Book this
diagram:
[Illustration]
The new way marked on the outside of the diagram, starts at North with
0 deg., increases toward the right through East at 90 deg., South at 180 deg., West
at 270 deg. and back to North again at 360 deg. or 0 deg..
The old way, marked on the inside of the diagram, starts at North with
0 deg., goes to the right to 90 deg. at East and to the left to 90 deg. at West. It
also starts at South with 0 deg., goes to the right to East at 90 deg. and to
the left to West at 90 deg..
A Compass Course can be named in degrees, according to either the new or
old way. For instance, the new way is just 45 deg.. The old way for the same
course is N 45 deg. E. New way--100 deg.. Old way for same course--S 80 deg. E.
There is another way to name a compass course. It is by using the name
of the point toward which the ship is heading. On every ship the compass
is placed with the lubber line (a vertical black line on the compass
bowl) vertical and in the keel line of the ship. The lubber line,
therefore, will always represent the bow of the ship, and the point on
the compass card nearest the lubber line will be the point toward which
the ship is heading.
The compass card of 360 deg. is divided into 32 points. Each point,
therefore, represents 11-1/4 deg.. The four principal points are called
_cardinal_ points. They are--North, East, South, West. Each cardinal
point is 90 deg. from the one immediately adjacent to it. It is also 8
points from the one adjacent to it, as 90 deg. is 8 points, i.e., 11-1/4 deg.
(one point) times 8. Midway between the cardinal points are the
inter-cardinal points. They are--N E, S E, S W, N W, and are 45 deg. or 4
points from the nearest cardinal point. Midway between each cardinal and
inter-cardinal point--at an angular distance of 22-1/2 deg. or 2 points, is
a point named by combining a cardinal point with an inter-cardinal
point. For instance, NNE, ENE, ESE, SSE, SSW, WSW, WNW, NNW. Midway
between the last points named and a cardinal or inter-cardinal point, at
an angular distance of 11-1/4 deg., is a point which bears the name of that
cardinal or inter-cardinal point joined by the word _by_ to that of the
cardinal point nearest to it. As, for instance, N by E, E by N, E by S,
S by E, S by W, W by S, W by N, N by W. Also NE x N, NE x E, SE x E, SE
x S, SW x S, SW x W, NW x W, NW x N. The angular distance between each
and every whole point is divided into 4 parts called half and quarter
points and each representing an angular measure of approximately 2 deg. 49'.
In mentioning fractional points, the U. S. Navy regulations are to name
each point from North and South toward East and West except that
divisions adjacent to a cardinal or inter-cardinal point are always
referred to that point: For instance, N 1/2 E, N x E 1/2 E, NE 1/2 N, NW
1/2 N, NW 1/4 W, NW 3/4 W, NW 1/4 N.
Boxing the compass is naming each point and quarter-point in rotation,
i.e., starting at North and going around to the right back to North
again. Every man should be able to identify and name any point or
quarter-point on the compass card.
In changing a point course into a degree course, for either new or old
compass, a guide is herewith furnished you. This should be pasted into
the front of your Bowditch Epitome. It shows, from left to right, the
name of the point course, its angular measure in the new compass and its
angular measure in the old compass. It also shows at the bottom, the
angular measure of each division of one point. In understanding this
guide, remember that each course is expressed in degrees or degrees and
minutes.
Put in your Note-Book:
In Navigation, each degree is written thus deg.. Each fraction of a degree
is expressed in minutes and written thus '. There are 60' in each
degree. Each fraction of a minute is expressed in seconds and is written
". There are 60" in each minute. Four degrees, ten minutes and thirty
seconds would be written thus: 4 deg. 10' 30".
Although this guide just given you is given as an aid to quickly
transfer a point course into a new or old compass course--or vice
versa--you should learn to do this yourself, after awhile, without the
guide.
Put in your Note-Book:
--------------+-----------+------------+--------------
Ship's Head | New | Old | By Point
--------------+-----------+------------+--------------
NE | 45 deg. | N 45 deg. E | NE
90 deg. | 90 deg. | N 90 deg. E | EAST
SE x E | 123 deg. 45' | S 56 deg. E | SE x E
S 20 deg. E | 160 deg. | S 20 deg. E | S x E 3/4 E
S 2 pts. E | 157 deg. 30' | S 22 deg. 30' E| SSE
NW 3/4 W | 306 deg. 34' | N 53 deg. W | NW 3/4 W
289 deg. 41' | 289 deg. 41' | N 70 deg. W | WNW 1/4 W
--------------+-----------+------------+--------------
I will show you just how each one of these courses is secured from the
guide just given you.
Note to Instructor: After explaining these courses in detail, assign for
reading _in the class room_ the following articles in Bowditch: Arts.
25-26-27-28-29-30-31-32, 74-75-76-77-78-79-80-81-82.
Every compass, if correct, would have its needle point directly to the
real or _true_ North. But practically no compass with which you will
become familiar will be correct. It will have an error in it due to the
magnetism of the earth. This is called Variation. It will also have an
error in it due to the magnetism of the iron in the ship. This is called
Deviation. You are undoubtedly familiar with the fact that the earth is
a huge magnet and that the magnets in a compass are affected thereby. In
other words, the North and South magnetic poles, running through the
center of the earth, do not point true North and South. They point at an
angle either East or West of the North and South. The amount of this
angle in any one spot on the earth is the amount of Variation at that
spot. In navigating a ship you must take into account the amount of this
Variation. The amount of allowance to be made and the direction (i.e.
either East or West) in which it is to be applied are usually indicated
on the chart. On large charts, such as those of the North Atlantic, will
be found irregular lines running over the chart, and having beside them
such notations as 10 deg. W, 15 deg. W, etc. Some lines are marked "No
Variation." In such cases no allowance need be made. On harbor charts or
other small charts, the Variation is shown by the compass-card printed
on the chart. The North point of this card will be found slewed around
from the point marking True North and in the compass card will be some
such inscription as this: "Variation 9 deg. West in 1914. Increasing 6' per
year."
Now let us see how we apply this Variation so that although our compass
needle does not point to true North, we can make a correction which will
give us our true course in spite of the compass reading. Note these
diagrams:
[Illustration]
The outer circle represents the sea horizon with the long arrow pointing
to true North. The inner circle represents the compass card. In the
diagram to the left, the compass needle is pointing three whole points
to the left or West of True North. In other words, if your compass said
you were heading NE x N, you would not actually be heading NE x N. You
would be heading true North.
[Illustration]
In other words, standing in the center of the compass and looking toward
the circumference, you would find that every true course you sailed
would be three points to the _left_ of the compass course. That is
called Westerly Variation.
Now look at the diagram to the right. The compass needle is pointing
three whole points to the right or East of True North. In other words,
standing in the center of the compass and looking toward the
circumference, you would find that every true course you sailed would be
three points to the _right_ of the compass course. That is called
Easterly Variation.
Hence we have these rules, which put in your Note-Book:
_To convert a compass course into a true course_
When the Variation is westerly, the true course will be as many points
to the left of the compass course as there are points or degrees of
Variation. When the Variation is easterly, the true course will be as
many points or degrees to the right of the compass course.
_To convert a true course into a compass course_
The converse of the above rule is true. In other words, Variation
westerly, compass to the right of true course; variation easterly,
compass course to the left.
DEVIATION
As stated before, Deviation causes an error in the Compass due to the
magnetism of the iron in the ship. When a ship turns, the compass card
does not turn, but the relation of the iron's magnetism to the magnets
in the compass is altered. Hence, every change in course causes a new
amount of Deviation which must be allowed for in correcting the compass
reading. It is customary in merchant vessels to have the compasses
adjusted while the ship is in port. The adjuster tries to counteract the
Deviation all he can by magnets, and then gives the master of the ship a
table of the Deviation errors remaining. These tables are not to be
depended upon, as they are only accurate for a short time. Ways will be
taught you to find the Deviation yourself, and those ways are the only
ones you can depend upon.
Put in your Note-Book:
Westerly Deviation is applied exactly as westerly Variation. Easterly
Deviation is applied exactly as easterly Variation.
The amount of Variation plus the amount of Deviation is called the
Compass Error. For instance, a Variation of 10 deg. W plus a Deviation of 5 deg.
W equals a compass error of 15 deg. W, or a Variation of 10 deg. W plus a
Deviation of 5 deg. E leaves a net compass error of 5 deg. W.
LEEWAY
Leeway is not an error of the compass, but it has to be compensated for
in steaming any distance. Hence it is mentioned here. A ship steaming
with a strong wind or current abeam, will slide off to the leeward more
or less. Hence, her course will have to be corrected for Leeway as well
as for Variation and Deviation.
Put in your Note-Book:
Leeway on the starboard tack is the same as westerly Variation. Leeway
on the port tack is the same as easterly Variation. This is apparent
from the following diagram:
[Illustration]
As the wind, blowing from the North, hits the left hand ship, for
instance, on her starboard side, it shoves the ship to the left of her
true course by the number of points or degrees of leeway.
Leave a space and put the following heading in your Note-Book:
I. Complete rule for converting a compass course into a true course:
1. Change the compass course into a new compass reading.
2. Apply Easterly Variation and Deviation +.
3. Apply Westerly Variation and Deviation -.
4. Apply port tack Leeway +.
5. Apply starboard tack Leeway -.
II. Complete rule for converting a true course into a compass course:
1. Reverse the above signs in applying each correction.
I will now correct a few courses, and these are to be put into your
Note-Book:
---------+------+-------------+--------+--------+-------+---------
C Cos | Wind | Leeway | Dev. | Var. | New | Old
---------+------+-------------+--------+--------+-------+---------
N x E | NW | 1/2 pt. | 5 deg. E | 10 deg. W | 12 deg. | N 12 deg. E
S 67 deg. E | S | 1 pt. | 3 deg. W | 5 deg. E | 104 deg. | S 76 deg. E
E x N | SE | 1/2 pt. | 5 deg. W | 10 deg. E | 78 deg. | N 78 deg. E
W x N | NW | 1-1/2 pts. | 1 deg. E | 15 deg. E | 280 deg. | N 80 deg. W
---------+------+-------------+--------+--------+-------+---------
Assign for Night Work the following arts. in Bowditch:
36-8-10-13-14-15-16-17-18-19-20-21-22-23-24.
WEDNESDAY LECTURE
PELORUS, PARALLEL RULERS, THE LEAD, SOUNDING MACHINE, DIVIDERS AND LOG
_I. The Pelorus_
This is an instrument for taking bearings of distant objects, and for
taking bearings of celestial bodies such as the sun, stars, etc. It
consists of a circular, flat metallic ring, mounted on gimbals, upon a
vertical standard. The best point to mount it is in the bow or on the
bridge of the ship, where a clear view for taking bearings can be had.
The center line of the pelorus should also be directly over the keel
line of the ship. The inner edge of the metallic ring is engraved in
degrees--the 0 deg. or 360 deg. and the 180 deg. marks indicating a fore-and-aft
line parallel to the keel of the ship. Within this ring a ground glass
dial is pivoted. This ground glass dial has painted upon it a compass
card divided into points and sub-divisions and into 360 deg.. This dial is
capable of being moved around, but can also be clamped to the outside
ring. Pivoted with the glass dial and flat ring is a horizontal bar
carrying at both of its extremes a sight vane. This sight vane can be
clamped in any position independently of the ground glass dial, which
can be moved freely beneath it. An indicator showing the direction the
sight vane points can be read upon the compass card on the glass dial.
If the glass dial be revolved until the degree of demarcation, which is
coincident with the right ahead marking on the flat ring, is the same as
that which points to the lubber's line of the ship's compass, then all
directions indicated by the glass dial will be parallel to the
corresponding directions of the ship's compass, and all bearings taken
will be compass bearings, i.e., as though taken from the compass itself.
In other words, it is just as though you took the compass out of its
place in the pilot house, or wherever it is regularly situated, put it
down where the pelorus is, and took a bearing from it of any object
desired.
In taking a bearing by pelorus, two facts must be kept in mind. First,
that when the bearing is taken, the exact heading, as shown by the
ship's compass, is the heading shown by the pelorus. In other words, if
the ship is heading NW, the pelorus must be set with the NW point on the
lubber line when the bearing is taken of any object. Second, it must be
remembered that the bearing of any object obtained from the pelorus is
the bearing _by compass_. To get the true bearing of the same object you
must make the proper corrections for Variation and Deviation. This can
be compensated for by setting the glass dial at a point to the right or
left of the compass heading to correspond with the compass error; then
the bearing of any object will be the true bearing. But naturally, you
will not be able to make compensation for these errors unless you have
immediately before found the correct amount of the compass error.
_Parallel Rulers_
The parallel rulers need no explanation except for the way in which they
are used on a chart. Supposing, for instance, you wish to steam from
Pelham Bay to the red buoy off the westerly end of Great Captain's
Island. Take your chart, mark by a pencil point the place left and the
place to go to and draw a straight line intersecting these two points.
Now place the parallel rulers along that line and slide them over until
the nearest edge intersects the center of the compass rose at the bottom
or side of the chart. Look along the ruler's edge to find where it cuts
the circumference of the compass rose. That point on the compass rose
will be the _true_ compass course, and can be expressed in either the
new or old compass, as, for instance, 60 deg. or N 60 deg. E. Remember, however,
that this is the _true_ course. In order to change it into the compass
course of your ship, you must make the proper corrections for the
compass error, i.e., Variation and Deviation and for Leeway, if any.
_The Lead and Sounding Machine_
The lead, as you know, is used to ascertain the depth of the water and,
when necessary, the character of the bottom. There are two kinds of
leads: the hand lead and deep-sea lead. The first weighs from 7 to 14
pounds and has markings to 25 fathoms. The second weighs from 30 to 100
pounds and is used in depths up to and over 100 fathoms. Put in your
Note-Book:
Fathoms which correspond with the depths marked are called _marks_. All
other depths are called _deeps_. The hand lead is marked as follows:
2 fathoms--2 strips of leather.
3 fathoms--3 strips of leather or blue rag.
5 fathoms--A white rag.
7 fathoms--A red rag.
10 fathoms--A piece of leather with one hole in it.
13 fathoms--Same as at 3.
15 fathoms--Same as at 5.
17 fathoms--Same as at 7.
20 fathoms--2 knots or piece of leather with 2 holes.
25 fathoms--1 knot.
30 fathoms--3 knots.
35 fathoms--1 knot.
40 fathoms--4 knots.
And so on up to 100 fathoms.
The large hand leads are hollowed out on the lower end so that an
"arming" of tallow can be put in. This will bring up a specimen of the
bottom, which should be compared with the description found on the
chart.
All up-to-date sea-going ships should be fitted with Sir William
Thompson's Sounding Machine (see picture in B. J. Manual). This machine
consists of a cylinder around which are wound about 300 fathoms of piano
wire. To the end of this is attached a heavy lead. An index on the side
of the instrument records the number of fathoms of wire paid out. Above
the lead is a copper cylindrical case in which is placed a glass tube
open only at the bottom and chemically colored inside. The pressure of
the sea forces water up into this tube, as it goes down, a distance
proportionate to the depth, and the color is removed. When hoisted, the
tube is laid upon a prepared scale, and the height to which the water
has been forced inside shows the depth in fathoms on the scale.
DIVIDERS
The dividers are nothing but an instrument for measuring distances,
etc., on the chart.
THE LOG
There are two kinds of logs--the chip log, used for measuring the speed
of the ship, and the patent log, used for measuring distance run.
The chip log consists of a reel, line, toggle and chip. Usually a second
glass is used for measuring time. The chip is the triangular piece of
wood ballasted with lead to ride point up. The toggle is a little wooden
case into which a peg, joining the ends of the two lower lines of the
bridle, is set in such a way that a jerk on the line will free it,
causing the log to lie flat so that it can be hauled in. The first 10 or
15 fathoms of line from the log-chip are called "stray line," and the
end of this is distinguished by a mark of red bunting. Its purpose is to
let the chip get clear of the vessel's wake. The marks on the line
(called knots) are pieces of fish line running through the strands of
the reel line to the number of two, three, four, etc. A piece of white
bunting marks every two-tenths of a knot. This is because the run of the
ship is recorded in knots and tenths. The knots of fish line are 47 feet
4 inches from each other.
The log glass measures 28 seconds in time. For high rates of speed, a 14
second glass is used. Then the number of knots shown by the log line
must be doubled. The principle of the chip log is that each division of
the log line bears the same ratio to a nautical mile that the log glass
does to the hour. In other words, if 10 knots or divisions of the log
line run out while the 28 second glass empties itself, the ship's speed
is 10 knots per hour. If ten knots or divisions run out while the 14
second glass empties itself, the ship's speed is 20 knots per hour.
The patent or towing log consists of a dial, line and rotator. The large
circle of the dial records the knots and the small circle tenths of
knots. When changing course, read the log and enter it in the log book.
When changing course again, read the log again. The difference between
the two readings will be the distance run.
Both logs are liable to error. A following sea makes them under-rate, a
head sea over-rate. With both logs you must allow for currents. If a
current is against you--and you know its rate--you must deduct its rate
from that recorded in the log and vice versa. The reason for this is
that your log measures your speed through the water. What you must find
is your actual distance made good over the earth's surface.
Put in your Note-Book:
Between Sandy Hook and Fort Hamilton, bound due North, speed by chip-log
was 10 knots, tidal current setting North 2 knots per hour; what did the
ship make per hour? Answer: 12 knots.
At sea in North Sea ship heading S x W, patent log bet. 8 A.M. and 12 M.
registered 32 miles, current running N x E 2 knots per hour; what was
the actual distance made good? Answer: 24 miles.
Directions for allowing for a current setting diagonally across a ship's
course will be given in the proper place.
Assign for Night Work the following articles in Bowditch: Arts.
161-162-163-164-165.
THURSDAY LECTURE
THE CHART
_Aids to Navigation_
A chart is a map of an ocean, bay, sound or other navigable water. It
shows the character of the coast, heights of mountains, depths at low
water, direction and velocity of tidal currents, location, character,
height and radius of visibility of all beacon lights, location of rocks,
shoals, buoys, and nature of the bottom wherever soundings can be
obtained.
The top of the chart is North unless otherwise noted. When in doubt as
to where North is, consult the compass card printed somewhere on the
chart. On sea charts, such as those of the North Atlantic, only the true
compass is printed, with the amount and direction of Variation indicated
by lines on the chart.
Parallels of latitude are shown by straight lines running parallel to
each other across the chart. The degrees and minutes of these parallels
are given on the perpendicular border of the chart. Meridians of
longitude are shown by straight lines running up and down, perpendicular
to the parallels of latitude, and the degrees and minutes of these
meridians are given on the horizontal border of the chart.
Put in your Note-Book:
A minute of latitude is always a mile, because parallels of latitude are
equidistant at all places. A minute of longitude is a mile only on the
equator, for the meridians are coming closer to each other as they
converge toward either pole. They come together at the North and South
poles, and here there is no longitude.
* * * * *
I can explain this very easily by reference to the following
illustration:
[Illustration]
As every parallel of latitude is a circle of 360 deg. the distance from A to
B will be the same number of degrees, minutes and seconds whether
measured upon parallel AA' or EE', but it will not be the same number of
miles as the meridians of longitude are gradually converging toward the
poles. On the other hand, the distances from A to C, C to D, D to E,
etc., must be the same because the lines AA', CC', DD', EE' are all
parallel. That is why the distance is always measured on the latitude
scale (i.e. on the vertical border of the chart), and a minute of
latitude is always a mile on the chart, no matter in what locality your
ship happens to be.
You should be able to understand any kind of information given you on a
chart. For instance, what are the various kinds of buoys and how are
they marked?
Put in your Note-Book:
1. In coming from seaward, red buoys mark the starboard side of the
channel, and black buoys the port side.
2. Dangers and obstructions which may be passed on either hand are
marked by buoys with red and black horizontal stripes.
3. Buoys indicating the fairway are marked with black and white vertical
stripes and should be passed close to.
4. Sunken wrecks are marked by red and black striped buoys described in
No. 2. In foreign countries green buoys are frequently used to mark
sunken wrecks.
5. Quarantine buoys are yellow.
6. As white buoys have no especial significance, they are frequently
used for special purposes not connected with Navigation.
7. Starboard and port buoys are numbered from the seaward end of the
channel, the black bearing the odd and red bearing the even numbers.
8. Perches with balls, cages, etc., will, when placed on buoys, be at
turning points, the color and number indicating on which side they shall
be passed.
9. Soundings in plain white are in fathoms; those on shaded parts are in
feet. On large ocean charts fathom curves, showing the range of
soundings of 10, 20, 30, 40, etc., fathoms are shown.
10. A light is indicated by a red and yellow spot. F. means fixed, Fl.,
flashing; Int., intermittent; Rev., revolving, etc.
11. An arrow indicates a current and its direction. The speed is always
given.
12. Rocks just under water are shown by a cross surrounded by a dotted
circle; rocks above water, by a dotted circle with dots inside it.
Practically all charts you will use will be called Mercator charts. Just
how they are constructed is a difficult mathematical affair but,
roughly, the idea of their construction is based upon the earth being a
cylinder, instead of a sphere. Hence, the meridians of longitude,
instead of converging at the poles, are parallel lines. This compels
the parallels of latitude to be adjusted correspondingly. Although such
a chart in any one locality is out of proportion compared with some
distant part of the earth's surface, it is nevertheless in proportion
for the distance you can travel in a day or possibly a week--and that is
all you desire. The Hydrographic Office publishes blank Mercator charts
for all latitudes in which they can be used for plotting your position.
It makes no difference what longitude you are in for, on a Mercator
chart, meridians of longitude are all marked parallel. It makes a great
difference, however, what latitude you are in, as in each a mile is of
different length on the chart. Hence, it will be impossible for you to
correctly plot your course and distance sailed unless you have a chart
which shows on it the degrees of latitude in which you are. For
instance, if your Mercator chart shows parallels of latitude from 30 deg. to
40 deg. that chart must be used when you are in one of those latitudes. When
you move into 41 deg. or 29 deg., you must be sure to change your plotting chart
accordingly. In very high latitudes and near the North pole, the
Mercator chart is worthless. How can you steer for the North pole when
the meridians of your chart never come together at any pole? For the
same reason, bearings of distant objects may be slightly off when laid
down on this chart in a straight line. On the whole, however, the
Mercator chart answers the mariner's needs so far as all practical
purposes are concerned.
The instruments used in consulting a chart, i.e., parallel rulers,
dividers, etc. have already been described. The only way to lay down a
course and read it is by practice.
The one important thing to remember in laying down a course, is that
what you lay down is a true course. To steam this course yourself, you
must make the proper correction for your compass error.
Assign for Night Work in Bowditch, Arts.
9-239-240-241-243-244-245-246-247-248-249-251-252-253-254-255-256-257-258.
If any time in class room is left, spend it in laying down courses on
the chart and reading them; also in answering such questions as these:
1. I desire to sail a true course of NE. My compass error is 2 points
Westerly Variation and 1 point Easterly Deviation. What compass course
shall I sail?
2. I desire to sail a true course of SW x W. My Variation is 11 deg. W,
Deviation 2 pts. W and Leeway 1 pt. starboard. What compass course shall
I sail?
3. I desire to sail a true course of 235 deg.. My compass error is 4 pts. E
Variation, 27 deg. W Deviation, Leeway 1 pt. port. What compass course shall
I sail?
4. I desire to sail a true course of S 65 deg. W. My compass error is 10 deg. E
Variation, 3 deg. E Deviation, Leeway 1/4 point starboard. What compass
course shall I sail?
FRIDAY LECTURE
THE PROTRACTOR AND SEXTANT
The protractor is an instrument used to shape long courses. There are
many kinds. The simplest and the one most in use is merely a piece of
transparent celluloid with a compass card printed on it and a string
attached to the center of the compass card. To find your course by
protractor, put the protractor down on the chart so that the North and
South line on the compass card of the protractor will be immediately
over a meridian of longitude on the chart, or be exactly parallel to
one, and will intersect the point from which you intend to depart. Then
stretch your string along the course you desire to steam. Where this
string cuts the compass card, will be the direction of your course.
Remember, however, that this will be the _true_ course to sail. In order
to convert this true course into your compass course, allow for
Variation and Deviation according to the rules already given you.
In case you know the exact amount of Variation and Deviation at the time
you lay down the course--and your course is not far--you can get your
compass course in one operation by setting the North point of your
protractor as far East or West of the meridian as the amount of your
compass error is. By then proceeding as before, the course indicated on
the compass card will be the compass course to sail. This method should
not be used where your course in one direction is long or where your
course is short but in two or more directions. The reason for this is
that in both cases, either your Variation or Deviation may change and
throw you off.
Practically all navigation in strange waters in sight of land and in all
waters out of sight of land depends upon the determination of angles.
The angle at which a lighthouse is seen from your ship will give you
much information that may be absolutely necessary for your safety. The
angular altitude of the sun, star or planet does the same. The very
heart of Navigation is based upon dealing with angles of all kinds. The
instrument, therefore, that measures these angles is the most important
of any used in Navigation and you must become thoroughly familiar with
it. It is the sextant or some member of the sextant family--such as the
quadrant, octant, etc. The sextant is the one most in use and so will be
described first.
Put in your Note-Book:
The sextant has the following parts: (Instructor points to each.)
1. Mirror
2. Telescope
3. Horizon Glass
4. Shade Glasses
5. Back Shade Glasses
6. Handle
7. Sliding Limb
8. Reading Glass
9. Tangent Screw
10. Arc
In getting angles of land-marks or buoys, the sextant is held by the
handle No. 6 in a horizontal position. The vernier arrow in the sliding
limb is set on zero. Now, suppose you wish to get the angular distance
between two lighthouses as seen from the bridge of your ship. (Draw
diagram.)
[Illustration]
Look at one lighthouse through the line of sight and true horizon part
of the horizon glass. Now, move the sliding limb along the arc gradually
until you see the other lighthouse in the reflected horizon of the
horizon glass. When one lighthouse in the true horizon is directly on
top of the other lighthouse in the reflected horizon, clamp the sliding
limb. If any additional adjustment must be made, make it with the
tangent screw No. 9.
Now look through the reading glass No. 8. You should see that the arc is
divided into degrees and sixths of degrees in the following manner:
11 deg. 10 deg.
| |
-------------+-----------------|
| |
----|--+--+--+--+--+--|--+--+--+
| | | | | | | | | |
----+--+--+--+--+--+--+--+--+--+
Now, as every degree is divided into sixty minutes, one-sixth of a
degree is 10 minutes. In other words, each of the divisions of a degree
on this arc represents 10 minutes.
Now on the vernier in the sliding limb, directly under the arc, is the
same kind of a division. But these divisions on the vernier represent
minutes and sixths of a minute, or 10 seconds.
To read the angle, the zero point on the vernier is used as a starting
point. If it exactly coincides with one of the lines on the scale of the
arc, that line gives the measurement of the angle. In the following
illustration the angle is 10-1/2 degrees or 10 deg. 30':
10 deg. 9 deg.
----------------+-----------------------+
| |
---------|---+---+---|---+---+---|---+---+---|
| | | | | | | | | |
-+---+---+---+---+---+---+---+---+---+---+---+
| | ^
----+---+---+
0
If however, you find the zero on the vernier has passed a line of the
arc, your angle is more than 10 deg. 30' as in this:
11 deg. 10 deg.
------------+-----------------------+
| |
------------|---+---+---|---+---+---|
| | | | | | |
------------+---+---+---+---+---+---+
| | | ^
------------+---+---+---+
0
You must then look along the vernier to the left until you find the
point where the lines do coincide. Then add the number of minutes and
sixths of a minute shown on the vernier between zero and the point where
the lines coincide to the number of degrees and minutes shown on the arc
at the line which the vernier zero has passed, and the sum will be the
angle measured by the instrument.
Now in measuring the altitude of the sun or other celestial body,
exactly the same process is gone through except that the sextant is held
vertically instead of horizontally. You look through the telescope
toward that part of the sea directly beneath the celestial body to be
observed. You then move the sliding limb until the image of the
celestial body appears in the horizon glass, and is made to "kiss" the
horizon, i.e., its lowest point just touching the horizon. The sliding
limb is then screwed down and the angle read. More about this will be
mentioned when we come to Celestial Navigation.
Every sextant is liable to be in error. To detect this error there are
four adjustments to be made. These adjustments do not need to be learned
by heart, but I will mention them:
1. The mirror must be perpendicular to the plane of the arc. To prove
whether it is or not, set the vernier on about 60 deg., and look slantingly
through the mirror. If the true and reflected images of the arc
coincide, no adjustment is necessary. If not, the glass must be
straightened by turning the screws at the back.
2. The horizon glass must be perpendicular to the plane of the arc. Set
the vernier on zero and look slantingly through the horizon glass. If
the true and reflected horizons show one unbroken line, no adjustment is
necessary. If not, turn the screw at the back until they do.
3. Horizon glass and mirror must be parallel. Set the vernier on zero.
Hold the instrument vertically and look through the line of sight and
horizon glass. If the true and reflected horizons coincide, no
adjustment is necessary. If they do not, adjust the horizon glass.
4. The line of sight (telescope) must be parallel to the plane of the
arc. This adjustment is verified by observing two stars in a certain way
and then performing other operations that are described in Bowditch,
Art. 247.
Do not try to adjust your sextant yourself. Have it adjusted by an
expert on shore. Then, if there is any error, allow for it. An error
after adjustment is called the Index Error.
Put in your Note-Book:
How to find and apply the IE (Index Error):
Set the sliding limb at zero on the arc, hold the instrument
perpendicularly and look at the horizon. Move the sliding limb forward
or backward slowly until the true horizon and reflected horizon form one
unbroken line. Clamp the limb and read the angle. This is the IE. If the
vernier zero is to the left of the zero on the arc, the IE is minus and
it is to be subtracted from any angle you read, to get the correct
angle. If the vernier zero is to the right of the zero on the arc, the
IE is plus and is to be added to any angle you read to get the correct
angle. Index error is expressed thus: IE + 2' 30" or IE - 2' 30".
Quadrants, octants and quintants work on exactly the same principles as
the sextant, except that the divisions on the arc and the vernier differ
in number from the sixth divisions on the arc and vernier of the
sextant.
If any time is left, spend it in marking courses with the protractor and
handling the sextant.
Assign for Night Work the following Arts. in Bowditch:
134-135-136-138-142-144-145-151-152-157-158-159-160-161-162-163.
SATURDAY LECTURE
FIXES, ANGLES BY BEARINGS AND SEXTANT
There are five good ways of fixing your position (obtaining a "fix," as
it is called) providing you are within sight of landmarks which you can
identify or in comparatively shoal water.
Put in your Note-Book:
1. Cross bearings of two known objects.
2. Bearing and distance of a known object, the height of which is known.
3. Two bearings of a known object separated by an interval of time, with
a run during that interval.
4. Sextant angles between three known objects.
5. Using the compass, log and lead in a fog or in unfamiliar waters.
1. Cross bearings of two known objects.
Select two objects marked on the chart, so far apart that each will bear
about 45 deg. off your bow but in opposite directions. These bearings will
be secured in the best way by the use of your pelorus. Correct each
bearing for Variation and Deviation so that it will be a true bearing.
Then with the parallel rulers carry the bearing of one object from the
chart compass card until you can intersect the object itself and draw a
line through it. Do exactly the same with the other object. Where the
two lines intersect, will be the position of the ship at the time the
bearings were taken.
[Illustration]
Now supposing you wish to find the latitude and longitude of that
position of the ship. For the latitude, measure the distance of the
place from the nearest parallel with the dividers. Take the dividers to
the latitude scale at the side of the chart and put one point of them on
the same parallel. Where the other point touches on the latitude scale,
will be the latitude desired. For the longitude, do exactly the same
thing, but use a meridian of longitude instead of a parallel of latitude
and read from the longitude scale at the top or bottom of the chart
instead of from the side.
2. Bearing and distance of a known object, the height of which is known.
Take a bearing of, say, a lighthouse the height of which is known. The
height of all lighthouses on the Atlantic Coast can be found in a book
published by the U.S. Dept. of Commerce. Correct the bearing, as
mentioned in case No. 1. Now read the angle of the height of that light
by using your sextant. Do this by putting the vernier 0 on the arc 0,
sliding the limb slowly forward until the top of the lighthouse in the
reflected horizon just touches the bottom of the lighthouse in the true
horizon. With this angle and the known height of the light, enter Table
33 in Bowditch. At the left of the Table will be found the distance off
in knots. This method can be used with any fairly perpendicular object,
the height of which is known and which is not more than 5 knots away, as
Table 33 is not made out for greater distances.
3. Two bearings of the same object, separated by an interval of time and
with a run during that interval.
Take a compass bearing of some prominent object when it is either 2, 3
or 4 points off the bow. Take another bearing of the same object when it
is either 4, 6 or 8 points off the bow. The distance run by the ship
between the two bearings will be her distance from the observed object
at the second bearing. "The distance run is the distance off."
A diagram will show clearly just why this is so:
[Illustration]
The ship at A finds the light bearing NNW 2 points off her bow. At B,
when the light bears NW and 4 points off, the log registers the distance
from A to B 9 miles. 9 miles, then, will be the distance from the light
itself when the ship is at B. The mathematical reason for this is that
the distance run is one side of an isosceles triangle. Such triangles
have their two sides of equal length. For that reason, the distance run
is the distance off. Now the same fact holds true in running from B,
which is 4 points off the bow, to C, which is 8 points off the bow, or
directly abeam. The log shows the distance run between B and C is 6.3
miles. Hence, the ship is 6.3 miles from the light when directly abeam
of it. This last 4 and 8 point bearing is what is known as the "bow and
beam" bearing, and is the standard method used in coastwise navigation.
Any one of these methods is of great value in fixing your position with
relation to the land, when you are about to go to sea.
4. Sextant angles between three known objects.
This method is the most accurate of all. Because of its precision it is
the one used by the Government in placing buoys, etc. Take three known
objects such as A, B and C which are from 30 deg. to 60 deg. from each other.
[Illustration]
With a sextant, read the angle from A to B and from B to C. Place a
piece of transparent paper over the compass card and draw three lines
from the center of the compass card to the circumference in such a way
that the angles secured by the sextant will be formed by the three lines
drawn. Now take this paper with the angles on it and fit it on the chart
so that the three objects of which angles were taken will be intersected
by the three lines on the paper. Where the point S is (in my diagram)
will be the point of the ship's position at the time of sight. To secure
greater accuracy the two angles should be taken at the same time by two
observers.
5. Using a compass, log and lead when you are in a fog or unfamiliar
waters.
Supposing that you are near land and want to fix your position but have
no landmarks which you can recognize. Here is a method to help you out: