Lectures in Navigation.

by Ernest Gallaudet 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 cla.s.ses, might learn the elements of Navigation and thus a.s.sume, without delay, their responsibilities as Junior Officers of the deck, Navigators and a.s.sistant 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, ent.i.tled "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 Compa.s.s 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 grat.i.tude 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 Inst.i.tute, 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 a.s.sistance in eliminating errors from the text.

E. G. D.

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, ill.u.s.trating 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 s.e.xtant, 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_

WEEK I--PILOTING

TUESDAY LECTURE

THE COMPa.s.s

Everyone is supposed to know what a compa.s.s looks like. It is marked in two ways--the old way and the new way. Put in your Note-Book this diagram:

[Ill.u.s.tration]

The new way marked on the outside of the diagram, starts at North with 0, increases toward the right through East at 90, South at 180, West at 270 and back to North again at 360 or 0.

The old way, marked on the inside of the diagram, starts at North with 0, goes to the right to 90 at East and to the left to 90 at West. It also starts at South with 0, goes to the right to East at 90 and to the left to West at 90.

A Compa.s.s Course can be named in degrees, according to either the new or old way. For instance, the new way is just 45. The old way for the same course is N 45 E. New way--100. Old way for same course--S 80 E.

There is another way to name a compa.s.s course. It is by using the name of the point toward which the ship is heading. On every ship the compa.s.s is placed with the lubber line (a vertical black line on the compa.s.s 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 compa.s.s card nearest the lubber line will be the point toward which the ship is heading.

The compa.s.s card of 360 is divided into 32 points. Each point, therefore, represents 11-1/4. The four princ.i.p.al points are called _cardinal_ points. They are--North, East, South, West. Each cardinal point is 90 from the one immediately adjacent to it. It is also 8 points from the one adjacent to it, as 90 is 8 points, i.e., 11-1/4 (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 or 4 points from the nearest cardinal point. Midway between each cardinal and inter-cardinal point--at an angular distance of 22-1/2 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, 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 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 compa.s.s 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 compa.s.s card.

In changing a point course into a degree course, for either new or old compa.s.s, 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 compa.s.s and its angular measure in the old compa.s.s. 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 . 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 10' 30".

Although this guide just given you is given as an aid to quickly transfer a point course into a new or old compa.s.s 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 | N 45 E | NE 90 | 90 | N 90 E | EAST SE E | 123 45' | S 56 E | SE E S 20 E | 160 | S 20 E | S x E 3/4 E S 2 pts. E | 157 30' | S 22 30' E| SSE NW 3/4 W | 306 34' | N 53 W | NW 3/4 W 289 41' | 289 41' | N 70 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, a.s.sign for reading _in the cla.s.s 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 compa.s.s, if correct, would have its needle point directly to the real or _true_ North. But practically no compa.s.s 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 compa.s.s 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 W, 15 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 compa.s.s-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 compa.s.s card will be some such inscription as this: "Variation 9 West in 1914. Increasing 6' per year."

Now let us see how we apply this Variation so that although our compa.s.s needle does not point to true North, we can make a correction which will give us our true course in spite of the compa.s.s reading. Note these diagrams:

[Ill.u.s.tration]

The outer circle represents the sea horizon with the long arrow pointing to true North. The inner circle represents the compa.s.s card. In the diagram to the left, the compa.s.s needle is pointing three whole points to the left or West of True North. In other words, if your compa.s.s said you were heading NE x N, you would not actually be heading NE x N. You would be heading true North.

[Ill.u.s.tration]

In other words, standing in the center of the compa.s.s and looking toward the circ.u.mference, you would find that every true course you sailed would be three points to the _left_ of the compa.s.s course. That is called Westerly Variation.

Now look at the diagram to the right. The compa.s.s needle is pointing three whole points to the right or East of True North. In other words, standing in the center of the compa.s.s and looking toward the circ.u.mference, you would find that every true course you sailed would be three points to the _right_ of the compa.s.s course. That is called Easterly Variation.