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Full text of "Air navigation for flight officers"

AIR NAVIGATION 

FOR FLIGHT OFFICERS 



AIR NAVIGATION 
FOR FLIGHT OFFICERS 



BY 
LIEUT.-COMMANDER A. E. DIXIE, R.N. 



GIEVES 
PUBLISHING COMPANY, LIMITED 

THE HARD, PORTSMOUTH 



By Special Appointment ffyjBSs|Bfe|ffi to His Majesty the King 




JOHN HOGG 

13 PATERNOSTER Row, LONDON 



[All rights reserved] 



PREFACE 

THIS work has been undertaken in the hope 
that it may prove of assistance to officers in 
the Royal Naval Air Service, as it condenses 
into a small compass all the subjects in 
navigation they are required to know. 

A. E. DIXIE 

NAVIGATION SCHOOL, PORTSMOUTH. 
March, 1917. 



37216 



CONTENTS 

CHAPTER I 

PAGE 

Elementary magnetism The earth's effect on a 
compass needle Definitions Various methods 
of making magnets Alloy used in making mag- 
nets Effect of temperature on magnets Effect 
of magnetism on hard and soft iron Permanent 
magnetism and its effect Sub-permanent mag- 
netism, its cause and effect The effect of a 
magnet ' end on * and ' broadside on ' . . i 

CHAPTER II 

The magnetic compass Liquid used in a compass 
To remove a bubble from a compass Remarks on 
placing a compass Features essential in an aero- 
plane compass Magnet block The effect of 
' banking ' on a compass 23 

CHAPTER III 

The analysis and adjustment of deviation The 
various coefficients Analysis of a table of devia- 
tions ........ 32 

CHAPTER IV 

The practical correction of a compass Methods of 
swinging Marking out a flying ground for 
swinging ....... 43 

vii 



CONTENTS 
CHAPTER V 

FACE 

Correcting courses Naming deviation Rules for 
getting the correct bearing from the bearing 
tables Final notes . . . . . 57 

CHAPTER VI 

Meteorology Types of weather Cloud formation 
Fog formation Synoptic chart Dunboyne's 
weather report . . . . . . 67 

CHAPTER VII 

General weather in the British Islands Storm signals 

Beaufort's system of weather notation . . 80 

CHAPTER VIII 
Forecasting by solitary observer . . ' * . 86 

CHAPTER IX 

Astronomy Sketches of the important constellations 
Notes on time How to find the time of sun- 
rise and sunset, and moonrise and moonset, and 
the true bearing at each Explanation of the 
various tables . . . . 93 

CHAPTER X 

Admiralty charts Theory and construction of the 
gnomonic chart Theory and construction of 
the Mercator's chart How to lay off a course 
To measure distance . . . . .143 
viii 



CONTENTS 
CHAPTER XI 

PAGE 

To construct a scale of longitude for any plan Con- 
ventional markings on a chart Use of celluloid 
protractor. To allow for drift due to wind 
Course necessary to steer to intercept hostile 
aircraft Lights, where to find particulars 
Description of the various kinds . . . 152 

CHAPTER XII 
Fixing positions 177 

CHAPTER XIII 

Ordnance maps Conventional markings Use of 

squared maps 186 

APPENDIX ........ 193 

INDEX . . . . . . . . 213 



For List of Illustrations, 6-c., see following page. 



IX 



ILLUSTRATIONS AND 
DIAGRAMS 

PAGE 

MAGNETISM 3 

COEFFICIENTS . .34 and 193 

MARKING OUT A FLYING GROUND .... 52 
DEVIATION ........ 60 

NOTES ON TRUE BEARINGS 63 

METEOROLOGY ....... 70 

CLOUDS facing 75 

WEATHER 88 

ASTRONOMY . . . . . . . .96 

TIME . . .114 

CHART . . . . . . . . . 143 

CHART SYMBOLS . . . . . . .154 

SCALE OF LONGITUDE 166 

ALLOWING FOR DRIFT DUE TO WIND . . .169 
INTERCEPTING HOSTILE AIRCRAFT . . . 171 

FIXING POSITIONS . . . . . .178 

MARKINGS ON ORDNANCE MAPS . . . .189 

COEFFICIENT E ..... facing 193 

BAROMETER SKETCHES ...... 196 

DEFLECTION OF WIND DUE TO EARTH'S ROTATION. 201 
VARIOUS FORMS OF ISOBARS .... 202-204 

HIGH AND Low PRESSURE AREAS . . . 209 

TRIANGLE OF VELOCITIES . . . . .210 

Example on . . . . . - . .211 



NOTE 

In the diagrams throughout the book, solid black 
indicates the red or north seeking end, and broken lines 
the blue or south seeking end of the magnet. 



Attention is directed to the Catalogue of Standard Naval 

Publications at the end of this book. 

x 



AIR NAVIGATION 

FOR 

FLIGHT OFFICERS 

CHAPTER I 
MAGNETISM 

A KNOWLEDGE of magnetism is absolutely 
essential in order to understand the action of 
the iron and steel used in construction on 
a compass. Also to know what causes the 
error, and why this error is introduced. 

Magnetism is a force existent all over the 
world, whose nature is that it exerts its 
influence on iron and steel, causing them to 
become magnetic. It was first discovered in 
a substance called ' Lodestone/ and after- 
wards in certain other iron ores found in 
various parts of the world. 

These iron ores are known as ' natural 



,**, AIR NAVIGATION FOR FLIGHT OFFICERS 
j * ' 

magnets/ which are never used in compass 
adjustment, one reason being that they vary 
greatly in strength. 

Artificial Magnets. These are pieces of 
iron or steel to which magnetic properties 
have been imparted by various methods. 

They have the same magnetic properties 
as natural magnets, but with increased power, 
depending on the amount of magnetism they 
receive. 

Any part of a magnet contains more or 
less magnetism, but its greatest power is con- 
centrated at two points near each extremity, 
these positions being known as the ' Poles ' 
of the magnet. The earth itself possesses the 
properties of a huge magnet, following the 
same laws that an ordinary magnet does. 
Its poles do not coincide with the geographical 
poles of the earth, bat are some distance from 
either ; one being situated north-w r est of 
Hudson Bay, and the other in South Victoria 
Land. 

They are not points like the geographical 
poles, but are areas of considerable extent. 

The earth being a magnet has certain 
lines of force (see Fig. i) passing through 
it, and if any iron or steel is placed in these 

2 



MAGNETISM 

lines of force they will be affected by it and 
become magnetic themselves. 

The magnetism in any magnet is of equal 
and opposite character at either pole, and it 
has been found by experiment that if the same 
like named poles of two magnets be brought 



\ ', / 



1 I 
I I 



\ v ./ - >v:-~ -:x\\ \< / / / 
^ x -\\ :/ / / //V'-.v^; \\ \ \ \\ \//y ,--' 



FIG. i. 

into each other's field, that they will repel 
one another, but that unlike poles will attract 
each other. Hence the following rule holds 
good, which is known as ' The First Law of 
Magnetism ' : 

Like poles repel. Unlike poles attract. 

This rule should be carefully memorised ; 
by doing so, compass adjustment with regard 

3 



AIR NAVIGATION FOR FLIGHT OFFICERS 

to the placing of the adjusting magnet will 
be quite easy to understand. 

Fig. i shows an ordinary bar magnet with 
the lines of force emanating from it. It 
has been found convenient to imagine the 
lines of force as issuing from the north-seeking 
end, and entering the south-seeking end. 




FIG. 2. 

Fig. 2 will show what would happen to 
a small freely suspended magnet if passed 
along an ordinary bar magnet. 

As the earth is a large magnet the following 
figure (Fig. 3) shows what would happen to a 
freely suspended magnetised needle if carried 
from one pole to another. 

The portions of the magnetic poles visible 
are indicated by two white semicircles. 

From Fig. 3 it will be seen that on the 
line joining the red and blue magnetism of 
the earth the small magnet will assume a 
horizontal position, whilst at the magnetic 
poles it will be vertical, so that in any inter- 

4 



MAGNETISM 

mediate place it will tend to set at different 
angles to the horizontal. 

This angle is known as the ' Dip/ and an 
explanation will be given later. 




FIG. 3. 

A magnet cannot exist without having 
two consequent poles, one at each end, 
consequently if it be divided into two or any 
number of pieces, each of these pieces becomes 
a complete magnet in itself, as shown in 
Fig. 4. 

5 



AIR NAVIGATION FOR FLIGHT OFFICERS 

In connection with the foregoing figures 
it will be noticed that the magnets are 
represented as red and blue : red for the 
north- seeking end, and blue for the south- 
seeking end. 

This is the conventional way that magnets 
are painted, and from now onwards the north- 
seeking or red end of a magnet or compass 




FIG. 4. 

needle will be called the north pole, and the 
south-seeking or blue end the south pole. 

Hence the northern part of the earth 
must be coloured blue, and the southern 
half red, to conform with the law given 
before. 

As the geographical and magnetic poles 
do not coincide, the compass needle cannot, 
except in certain positions, point to the true 
north, but at an angle to it, according to the 
needle's position on the earth's surface. This 
angle, which may have any value up to 180, 

6 



MAGNETISM 

is the angle between the true and magnetic 
meridians, and is known as the ' Variation/ 

It is called easterly if the north end of 
the needle is drawn to the right of the true 
meridian, and westerly if drawn to the left. 

At those places where the true and 
magnetic meridians do coincide the variation 
is nothing. 

The value of the variation has been found 
for practically all over the world, and if 
required it can be taken from the Admiralty 
Variation Chart or Compass Manual. 

The continuous lines on the chart denote 
that the variation is westerly ; the pecked 
lines, that the variation is easterly ; and the 
two side by side show the lines of no variation. 

This variation undergoes an annual change, 
probably due to the magnetic poles shifting. 

This change is given on the variation 
chart and also on Admiralty charts, but for 
ordnance maps it must be taken from the 
former if no Admiralty chart is available. 

The magnetic poles are not points like 
the geographical poles ; that is to say, they are 
areas of considerable extent. 

The following definitions will be found 
useful, and should be committed to memory 
and thoroughly understood. 

7 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Line of Total Force. Is the direction that 
freely suspended magnetic needle will take up 
when under the influence of the earth's forces. 

Magnetic Poles. Are the two places on the 
earth's surface where the total force is vertical, 
and to which the needle points in all adjoining 
regions. 

Magnetic Equator. Is the line separating 
the red and blue magnetism of the earth, and 
along which the line of total force is horizontal. 

It does not coincide with the geographical 
equator, and only intersects it in two places. 

Magnetic Meridian. Is the vertical plane 
passing through the longitudinal axis of a 
freely suspended magnetic needle when resting 
in a line of total force and free from local 
attraction. 

Variation. Is the horizontal angle be- 
tween the true and magnetic meridians. 

Deviation. Is the horizontal angle be- 
tween the magnetic meridian and the vertical 
plane passing through the longitudinal axis 
of a magnetised needle when under the 
influence of local attraction. 

8 



MAGNETISM 

It is called easterly or + when the north 
end of the needle is drawn to the right of the 
magnetic meridian, westerly or if drawn to 
the left of the magnetic meridian. 

Compass Error. Is the algebraical sum of 
the variation and deviation. 

Dip. Is the vertical angle between the 
direction of a freely suspended magnetic 
needle resting in a line of total force and the 
horizontal plane passing through the centre 
of the needle. 

Poles of a Magnet. Are the two points 
of maximum intensity situated about one- 
twelfth of the total length of the magnet from 
eitherjextremity. 

Magnetic Latitude. Is measured north or 
south from the magnetic equator, and is some- 
what similar to terrestrial latitude. 

Lines of equal dip correspond to magnetic 
latitude. 

Horizontal Force. Is the horizontal com- 
ponent of the earth's magnetism. 

9 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Vertical Force. Is the vertical component 
of the earth's magnetism. 

N.B. The size of the angle of dip depends 
on the value of these two. 

The following figure shows the connection 
between horizontal force, vertical force, total 
force, and dip. 



FIG. 5. 

H represents the horizontal force, 
Z represents the vertical force. 
T represents the total force. 
represents the angle of dip. 

2 

Then = Tangent Dip. 

Vert. Force ~ . 

' e ' Hor. Force f. Tangent Dlp " 

10 



MAGNETISM 



H 2 + Z 2 = T 2 
i.e. (Hor. Force) 2 + (Vert. Force) 2 = 

(Total Force). 2 

Z - H Tan 
i.e. Vert. Force = Hor. Force x Tangent Dip. 

H = Z Cot 
i.e. Hor. Force = Vert. Force x Cotangent Dip. 

Z = T Sin 
i.e. Vert. Force = Total Force x Sine Dip. 

H = T Cos 
i.e. Hor. Force = Total Force x Cosine Dip. 

The Methods of Making Magnets. There 
are four different ways of making magnets, as 
follows : 

(1) By Percussion. 

The bar to be magnetised is placed in the 
direction of the lines of force of the earth, and 
one end is smartly tapped with a hammer. 
This induces magnetism in it, the amount 
received depending on the number and force 
of the blows and the coercive force of the metal 
to be magnetised. 

The pole of the magnet which is lowest will 
be of opposite polarity to the hemisphere 
where it was manufactured, 

(2) By Single Touch. 

ii 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The bar to be magnetised is placed on a 
flat surface, and one end of a magnet placed 
on one extremity of the bar and drawn smartly 
along the length of the latter, being lifted off 




Result 
FIG. 6. 

at the end of the stroke and replaced on the 
starting-point. 

This operation ma}? be repeated as often as 
necessary. 

The end of the bar last touched by the 
magnet will be of opposite polarity to the end 
of the inducing magnet touching it. 

(3) By Divided Touch. 

12 



MAGNETISM 

The bar to be magnetised is placed on a 
flat surface, and the opposite ends of two 
magnets placed on its centre and drawn 
smartly outwards towards their respective 
ends. This operation is repeated as often as 




Result 
FIG. 7. 

necessary. The ends of the bar last touched 
by the magnets will have opposite polarity 
to the ends of the respective magnets used. 

(4) By Electro Magnet. 

The bar to be magnetised is placed across 
the poles of an electric magnet and kept 
there as long as necessary. The ends of 
the bar will acquire opposite polarity to the 
poles of the electro magnet. 

13 



AIR NAVIGATION FOR FLIGHT OFFICERS 

This method is always employed in the 
manufacture of magnets used in compass work, 
as by its means they can be made stronger and 
more uniform in power. 

Alloy Used in Making Magnets. Magnets 




Result 
FIG. 8. 



are made of hard steel with a mixture of 
5 per cent, of tungsten. 

This has been found to increase its coercive 
force. By ' coercive force ' is meant the 
property by which iron or steel not only 
retains its magnetism after it has been 
imparted to it, but also the resistance it puts 
up against being magnetised. 

14 



MAGNETISM 

Compass magnets, if properly stowed, i.e. 
(unlike poles together) and well looked after 
retain their magnetism without appreciable 
loss for years. 

Effect of Temperature on Magnets. Ordin- 
ary atmospheric changes of temperature have 
practically no effect on a permanent magnet, 
such as those used for compass adjustments. 

If, however, it be placed in a very strong 
magnetic field of opposite power, or if heated 
to a dull red heat, i.e. between 1300 and 1500 
Fahrenheit, it becomes de-magnetised. 

On the other hand, soft iron 'increases its 
capacity for receiving magnetism on being 
heated, this increases up to a temperature of 
1427 Fahr., but after this there is a rapid 
decrease, and at 1445 the iron becomes non- 
magnetic. 

Effect of Magnetism on Hard and Soft Iron. 

The iron or steel used in construction varies 
in its magnetic character. 

This necessitates a little explanation. 

Iron or steel may be classed under two 
headings : ' Hard ' and ' Soft/ 

Hard iron, on account of its coercive force, 
does not pick up or part with its magnetism 
freely. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

It acquires magnetic properties during its 
manufacture on account of the hammering 
and violence it has been subjected to. After 
manufacture it loses some of this magnetism, 
but soon settles down, and the residue may be 
regarded as permanent. 

Soft iron has little or no coercive force, 
and picks up and loses its magnetism freely, 
so that for every direction of the machine's 
head a different amount of magnetism is 
induced. 

Soft iron is seldom absolutely pure, conse- 
quently it nearly always retains a certain 
amount of magnetism, not due to the lines of 
force of the earth. 

The deviations caused by hard iron are 
called semicircular, because they only change 
their sign once in the whole circle. 

They are corrected by horizontal magnets 
placed longitudinally and transversely. 

Those caused by soft iron are termed 
' quadrantal/ because they change their sign 
in each quadrant. 

They are corrected by soft iron balls or 
spheres placed on each side of the compass. 

Sub-Permanent Magnetism, its Cause and 
Effect. This is caused by iron which does 

16 



MAGNETISM 

not come under the category of hard or soft, 
but lying between the two. After being on 
one course for some time it acquires a mag- 




FIG. 9. 



netic character due to the lines of force of the 
earth, and this is accentuated by vibration 
of engines, gunfire, etc. 

On alteration of course this magnetism 
does not immediately disappear as in the case 

17 c 



AIR NAVIGATION FOR FLIGHT OFFICERS 

of soft iron, but only does so gradually, the 
time taken depending on the length of time 
on the course and the coercive force of the 
metal. It cannot be corrected, and its amount 
can only be ascertained by actual observation. 

Its effect, if not allowed for, is always to 
place the machine's head towards the direction 
of the old course, as shown in sketch (p. 17). 

The variation, dip, horizontal and vertical 
force are all given in Admiralty publications. 

Reference to these will show that in the 
south of England the dip is approximately 67. 

It will also be noticed that as the latitude 
gets higher the dip increases, and therefore 
the vertical force in big latitudes is greater 
than the horizontal force. Hence it is neces- 
sary that the compass should be kept the 
greatest distance possible from vertical or 
nearly vertical iron, especially the ends, in 
these latitudes. 

The effect of a magnet ' end on ' to a single 
pole of a compass is much greater than that 
of a magnet ' broadside on/ 

The proof is here given for anyone who 
may be interested in it. 

Proof. AB is a magnet and N is an 
isolated north pole of strengths M and m 
respectively. 

18 



MAGNETISM 

First consider the ' end-on ' position, where 
d is the distance from the centre of the magnet 
to the isolated pole, and L is the length of the 
magnet. 

Then the force acting on N due to the south 
pole of the magnet is : 

Mm 



FIG. 10. 



And the force acting on N due to the north 
pole of the magnet is : 
Mm 



+ 



As these two forces act always along the 
same straight line their total is : 

Mm Mm 



Mmd* -f MwrfL + Mm' - Mmd* + Umdl - Mm 1 ^ 
4 4 



MmdL 



AIR NAVIGATION FOR FLIGHT OFFICERS 



Now in the ' broadside on ' position : 
Force due to south pole of magnet on N : 

M.m 



( 




FIG. n. 

And force due to north pole of magnet 

on N : 

Mm 



And their resultant is evidently equal to the 
line CN, as in sketch following, and which 
equals 2 Sine . 



20 



MAGNETISM 

Hence the total force acting on N due to 
the magnet broadside on is : 



x 2 Sine a 




That is : 

Mm 



FIG. 12. 



MwL 



Analysing these results we find that ' end 
on': 

2 MwL 



Total force = 



(T 
d2 ~~ 4 



21 



AIR NAVIGATION FOR FLIGHT OFFICERS 

And for ' broadside on ' : 

MwL 



Total force = 



( 



* - 



If d is large compared with L we can 
neglect L, and the equations become : 

'End on' ' 



d* d 3 

Mm 



' Broadside on ' 



Showing that force ' end on ' is twice that 
' broadside on/ 

In conclusion it should be understood that 
the magnetic effect exerted by any object 
cannot be screened off from any object liable 
to be influenced by magnetism, if the latter 
falls within the magnetic field of the former. 



22 



CHAPTER IT 
THE MAGNETIC COMPASS 

THIS is an instrument constructed to give 
the direction of the magnetic north, and by 
means of a graduated card fixed to it to give 
any other direction with relation to it. 

A freely suspended magnetic needle would 
of course point to the magnetic north, and if a 
card were attached to it it might seem at first 
sight that this would fulfil all requirements ; 
but it must be remembered that this form of 
suspension would be affected by the varying 
angle of dip, and would therefore only be actu- 
ally horizontal when on the magnetic equator. 
In any other place it would have a varying 
angle of tilt which would make reading awk- 
ward, whilst in high latitudes the card would 
come up against the glass cover of the compass 
bowl and prevent the card from working. 

Various methods, including the lowering of 
the centre of gravity, have been devised to 
overcome this, and the compass card as now 

23 



AIR NAVIGATION FOR FLIGHT OFFICERS 

constructed will remain horizontal in any part 
of the world. 

As the card remains horizontal, the only 
force we need consider as acting on the 
compass card is the horizontal component of 
the earth's magnetism. 

The general system of pivoting a compass 
card is as follows. 

The magnets and card are fixed together, 
and are fitted with a cap in their centre which 
is inverted and fitted with a ruby or other hard 
stone to take the wear and also to reduce 
friction to as little as possible. This is then 
placed on to a metal spike which is given an 
iridium point, the latter being an extremely 
hard metal. (Sapphire or ruby points will 
probably be used in future, owing to the 
deterioration in quality of the iridium now 
being mined.) 

In all the later patterns of aero compasses 
the above arrangement is reversed, the pivot 
being fixed to the card. 

Owing to a sticky deposit which is liable 
to form in the cap this would at first sight 
seem to be a disadvantage, but the fact that 
it gives greater steadiness, coupled with the 
greater angle of clearance between card and 
covering glass, negatives this disadvantage. 

24 



THE MAGNETIC COMPASS 

The card, cap and pivot are enclosed in a 
non-magnetic bowl and covered with a glass 
cover. 

In the earlier pattern compasses the card 
used to work in air, but owing to the great 
vibration encountered in aeroplanes, this kind 
of compass was found to be totally unsuitable, 
so the liquid type had to be introduced instead. 

Its advantages over the compass card 
working in air are as follows : 

The card is steadier, it takes less time to 
settle down if disturbed, and a heavier card 
may be used, as the total weight resting on 
the pivot may be made to any amount re- 
quired by varying the size of the float. 

The size of the bowl is such, that a clearance 
of about one quarter of its diameter is allowed 
for between its inner edge and the edge of the 
card, otherwise when turning rapidly a rotary 
motion is set up in the liquid which is com- 
municated to the card. This makes it liable 
to become unsteady or to lag behind. 

The Liquid used in a Compass. This is a 
mixture of two parts of distilled water to 
one part of pure alcohol, the object of the 
alcohol being to prevent freezing. 

This mixture is quite efficient up to 2 
25 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Fahrenheit. It has been found that a 
slightly higher percentage of alcohol gives 
better results in very low temperatures, and 
all the later pattern compasses are now filled 
with a mixture of three parts of distilled water 
to two parts of alcohol. 

Distilled water only must be used, otherwise 
the impurities in ordinary water would clog 
up the cap and render the compass sluggish. 

To Remove a Bubble from a Compass. The 

fact of an air bubble having formed in a 
compass can always be seen. It should never 
be allowed to remain, as it makes steering 
difficult and also tends to make the compass 
sluggish. 

The following procedure should be carried 
out. 

The bowl should be removed from its 
outer containing case and laid on its side with 
the filling screw uppermost. Remove the 
screw plug and drop in distilled water with a 
pipette or clean fountain-pen filler. 

Rock the bowl gently from side to side 
to make sure the bubble is underneath the 
filling plug. 

As soon as the water overflows replace the 
screw plug and take care that the leather 

26 



THE MAGNETIC COMPASS 

washer is in place. If on examination it is 
found that all the air is not yet out the opera- 
tion must be repeated. 

The bowl should be as cool as possible so 
as to enable the maximum amount of water 
to be introduced. 

Remarks on Placing a Compass. The 

placing of a compass in a good position is of 
great importance, as should a bad position 
be chosen, even the best of compasses will 
be unsatisfactory in their behaviour. The 
points to be attended to are as follows : 

(1) It should be placed in a position where 
the pilot has a clear view of it, and if possible 
in the centre of the longitudinal axis of the 
machine. This tends to make the errors 
more symmetrical and therefore more easily 
adjusted. 

The pilot should also be directly behind 
the compass, to avoid errors in reading due 
to parallax. 

(2) The maximum distance possible from 
magneto, engine, or anything magnetic liable 
to occasional movement. 

(3) If possible, all metal within at least 
2 feet from the compass should be made of 
some non-magnetic material. 

27 



AIR NAVIGATION FOR FLIGHT OFFICERS 

(4) The greatest distance possible from the 
ends of vertical iron rods, struts, etc. 

Essential Features in an Aeroplane Compass. 

(1) Steadiness under all conditions. 

(2) Good expansion arrangements. 

(3) Satisfactory system of lighting. 

(4) Sufficient allowance between card and 
covering glass to prevent their touching each 
other in the event of the machine climbing, 
planing, or banking. 

(5) Good marking of the card and reduc- 
tion of eye strain to a minimum. 

These requirements have been attained as 
follows. 

(i) Steadiness. After numerous experi- 
ments it was found that the most efficient way 
to damp the existent vibrations was to place 
the compass bowl in a bed of horse-hair. This 
effectually deadened shocks. 

The horse-hair is placed in light outer 
containing case, and the bowl rests lightly 
on it. 

This method was found generally to be 
much more preferable to the old gimballed 
type of suspension. 

This applies to the earlier pattern com- 
28 



THE MAGNETIC COMPASS 

passes ; for Pattern 255 and later types the 
reader should refer to the book ' Magnetic 
Compass in Aircraft/ by Captain F. Creagh 
Osborne, R.N. 

(2) Expansion and Contraction. Due pro- 
vision has been made for this by fitting what 
is known as an ' expansion chamber ' in the 
bowl. 

(3) Lighting. In the earlier types this 
was arranged for by a small dry cell battery 
and electric lamp fitted on the front side of 
the bowl. 

In the later types this method is of second- 
ary importance, as the card markings are 
treated with a radium compound enabling it 
to be easily read in the dark. 

(4) Allowance for Heeling. This is 
arranged for by the method of pivoting, 
which allows of a heel of 15 in the earlier 
types ; and by altering the pivoting in the 
later types this has been increased to about 
30. These are the angles that the machine 
has to heel over to before the card touches 
the covering glass. 

(5) Making of the Card. This has been 

29 



AIR NAVIGATION FOR FLIGHT OFFICERS 

done in what is known as the ' New Style/ 
i.e. the card is graduated from o to 360, 
running with the hands of a watch. 

North is thus represented by o or 360, 
North-east by 45, East by 90, South-east by 
135, South by 180, South-west by 225, West 
by 270, North-west by 315. 

North, South, East, and West are called the 
'Cardinal Points/ North-east, South-east, 
South-west, and North-west are called the 
' Quadrant al Points/ Small aeroplane com- 
passes are only marked every 5 to prevent 
overcrowding, owing to the small size of the 
card. The number is given opposite every 
tenth degree. 

Airship compasses are graduated to every 
degree, and large compasses for big aeroplanes 
every two degrees. 

Prisms and Reflectors. These are intro- 
duced to do away with eye strain as much as 
possible, the card being so small. 

Broken Pivots. This causes the card to 
work jerkily, and the compass should be taken 
apart and the pivot examined to see whether 
it is bent or damaged. If it cannot be repaired 
the compass should be returned to store, and 
a new one drawn in lieu. 

30 



THE MAGNETIC COMPASS 

Magnet Block. These are supplied for 
holding the adjusting magnets. They should 
be placed so that their centre is directly under 
the centre of the compass, and care should be 
taken that the magnet holes, of which there 
are two sets at right angles to each other, 
should be set so that they are in line respec- 
tively with the longitudinal and transverse 
axes of the machine. These blocks will not 
be met with in Pattern 255 and later types. 

Effect of Banking. The effect after a 
heavy bank is to make the compass unsteady 
for a short time. It has been found by 
experiment that if on a fast machine steering 
anywhere within 20 of the north point, a 
quick alteration of course will cause the north 
pole of the compass to follow the machine's 
head round. On steadying the machine, the 
north pole of the compass swings back to its 
correct position. 

For a description of the various types of 
compasses used in aircraft, reference should be 
made to the pamphlet entitled ' Compasses 
for Use in Aircraft/ by Captain F. Creagh 
Osborne, R.N. 



CHAPTER III 

THE ANALYSIS AND ADJUSTMENT 
OF DEVIATION 

THE effect of the magnetic qualities in hard 
and soft iron is to deflect the compass needle 
from the magnetic meridian. This deflection, 
or local attraction, as it is otherwise called, 
is known as ' deviation/ 

For purposes of analysis and adjustment, 
this deviation can be split up into five ' co- 
efficients/ as they are called, viz. A, B, C, D, 
and E. 

These coefficients, with the exception of 
A, may be assumed to be acting immediately 
over or under the centre of the compass, 
longitudinally, transversely, or diagonally. 
Coefficients A, D, and E are caused by soft 
iron, and B and C by hard iron. 

Coefficient A. Is due to iron being un- 
symmetrically distributed around the com- 
pass, or is due to the latter being out of the 
middle longitudinal line of the machine. 

32 



COEFFICIENTS 

It is extremely rare in a well -placed compass. 

An ' apparent ' A may be caused by an 
error in the magnetic bearing of an object 
which is being used for swinging. In practice 
it will be found that nearly every aeroplane 
compass has an ' A.' 

It cannot be corrected but can only be 
allowed for. It is called a ' constant ' devia- 
tion, because it is the same in amount and sign 
for all directions of the machine's head. 

It is found by taking the mean of the devia- 
tions on a number of equidistant points, 
calling all easterly deviations + and all 
westerly deviations -. In practice, it is usual 
to take the deviations on the cardinal and 
quadrantal points. 

Coefficient B. Is caused by the hori- 
zontal component of the permanent magnet- 
ism of the machine acting longitudinally. 

It is called -j- if the north end of the needle 
is drawn towards the nose of the machine, 
and - if drawn towards the tail. 

It is maximum on east and west, diminish- 
ing to zero on north and south. 

It is found by taking the mean of the 
deviations on east and west, changing the sign 
on west. 

33 D 



AIR NAVIGATION FOR FLIGHT OFFICERS 

It is corrected by horizontal magnets, 
placed red end to the front for a + B, and red 
end to the rear if B is -. It causes a semi- 
circular deviation, so called because its sign 
changes once only. 



315 

Wly.Devn. 



270 < 

Wly Devn^CX_ 
Max. 



225 

Wly. Devn. 




315* 



360* 



FIG. 13. Diagram for + B. 



Curve for 



_j_ 



It varies inversely as the horizontal force, 

e.g. Hor. Force i. B = 10 + say 
Then with Hor. Force 2. B = \ x 10 

= 5+. 
34 



COEFFICIENTS 

Diagram for + B. (Fig. 13.) 

The shaded rod denotes the permanent 
magnetism acting longitudinally. The pecked 
arrow denotes the direction in which the needle 
is deflected. 



No Devr 



315 
Ely. Devn 




225 
Ely. Devn. 



FIG. 14. Diagram for B. 



Curve for B. 



Diagram for B. (Fig. 14.) 

The above remarks apply here as well. 

Coefficient C. Is caused by the hori- 
zontal component of the machine's permanent 
magnetism acting transversely. 

35 



AIR NAVIGATION FOR FLIGHT OFFICERS 

It is called + if the north end of the needle 
is drawn to the right-hand side of the machine 
and - if drawn to the left-hand side. 

It is maximum on north and south, dimin- 
ishing to zero on east and west. 

It is found by taking the mean of the 
deviations on north and south, changing the 
sign on that of south. It is corrected by 
horizontal magnets placed transversely, red 
end to the right if C is + and red end to the 
left if C is -. 

It is called ' semicircular ' for the same 
reason as B, and like B changes inversely as 
the horizontal force. 

Diagram for + C. (Fig. 15.) 

The shaded rod denotes the permanent 
magnetism acting tiansversely, and the 
pecked arrow the direction the needle is 
deflected to. 

Diagram for -- C. (Fig. 16.) 
See remarks above. 

Coefficient D. Is the effect of induction 
in horizontal soft iron acting longitudinally 
or transversely. 

If the effect of the transverse iron is greater 
than that of the longitudinal it causes a + D, 

36 



COEFFICIENTS 

and if the effect of the longitudinal is greater 
it causes a - D. 

D is maximum on the quadrantal points 
(i.e. 45, 135, 225, 315), diminishing to zero 
on the cardinal points. 



315 
Ely.Devn 




225 

Wly. Oevn 



FIG. 15. Diagram for -f C. 



It is found by taking the mean of the 
deviations on the semicardinal points, chang- 
ing the signs on south-east and north-west. 
That is on 135 and 315. 

It is corrected by soft iron spheres placed 
37 



AIR NAVIGATION FOR FLIGHT OFFICERS 

transversely if D is + and longitudinally if 
Dis-. 

The size of the spheres and the distance of 



315 
Wly. Devn 




225 

Ely. Devn 



Ely. Devn 
ISO Max 



135' 




FIG. 1 6. Diagram for C. 



315* 



Curve for C. 



their centre from the centre of the compass is 
given in tables to be found in the ' Admiralty 
Compass Manual/ 

D is called ' quadrantal ' because it 
changes its sign in each quadrant. It does not 
change on change of position because the 
force acting on the iron is the same as that 

38 



COEFFICIENTS 

acting on the compass needle, and the two are 
therefore always in proportion, as shown 
below. 



(a) Hor. Force 1 

1 
l 



Proportion -}- 



(b) Hor. Force 2 

2 
2 t 2 



Proportion - = 



FIG. 17. 



315 

Wly.Devn 
Max. 




225 

Ely. Devn. 
Max. 



FIG. 1 8. Diagram for + D. 



360' 

Curve lor + D. 



39 



AIR NAVIGATION FOR FLIGHT OFFICERS 



Diagram for + D. (Fig. 18.) 

The white rods denote that the iron is 
non-magnetic. 



315 
Wly.Devn 

Max. 




225 

Wly. Devn. 
Max. 



180 



FIG. 19. Diagram for D. 



The other coefficient is called E and is 
caused by iron running obliquely, but it is 
not proposed to go into it here. 

The Effect of Vertical Iron. Vertical iron 
causes what is known as ' Heeling Error/ which 
comes into action on the machine banking. 

40 



ANALYSIS OF DEVIATION 

This is maximum on north and south, and 
zero on east and west. 

Analysis of a Table of Deviations. By 
this is meant the splitting up of a table of 
deviations into the various coefficients, which 
is done by following the rules given in the 
explanation of the various coefficients. 

A worked example is here given. 

Analyse the following table of deviations : 



Machine's 
Head. 


Deviation. 


Machine's 
Head. 


Deviation. 





3.W- 


180 


5-E. + 


45 


4.E- + 


225 


3-W 


9 


7.E.+ 


270 


8.W. 


135 


IO.E.+ 


315 


6.W. 



Coefficient A. 
4- 

4 3 
7 3 

10 8 

5 6 

26 20 




AIR NAVIGATION FOR FLIGHT OFFICERS 

Coefficient B. Coefficient C. 

+ 7 -3 

+ 8 Original sign 5 Original sign 

2 |+I5 changed. 2 | ^8 changed. 

+ 7 30'. -4 oo' 

Coefficient D. 



T 





-13 


4 


10 Sign 


+ 10 


6 Sign 


changed. 


4l -- 3 


changed. 


3 


-o- 45' 


10 


13 




Coefficient E. 







5 3 4 I +3 



+o 4 5' 
13 10 

Correctors would then be placed as follows : 

B would be corrected by magnets placed 
longitudinally, red end in front. 

C would be corrected by transverse mag- 
nets, red end to left. 

D would be corrected by spheres placed 
longitudinally, the size, etc., being taken from 
the tables. 

But in this case, D being so small, it would 
be best to ignore it altogether. 

42 



CHAPTER IV 

THE PRACTICAL CORRECTION OF A 
COMPASS; METHODS OF SWING- 
ING, ETC. 

BEFORE going into the practical correction of 
a compass, it is proposed to give a description 
of the various methods of swinging. 

This swinging should always be carefully 
carried out, as a well-placed compass whose 
behaviour is good, and whose errors are known 
and can be trusted, is a great relief to a pilot 
making a flight, when objects below are hidden 
by cloud, fog, etc. 

There are five methods of swinging, as 
follows : 

(a) By Sun or Star. 

(b) By ' Reciprocal Bearings/ 

(c) By Distant Object. 

(d) By two objects in line or ' in transit/ 
as it is usually called. 

(e) By using a marked-out flying ground. 

43 



AIR NAVIGATION FOR FLIGHT OFFICERS 

In connection with this, the last-mentioned 
one is the only one possible for the later pat- 
tern compasses, owing to their construction. 

(a) By Sun or Star. The requirements 
for these are a watch whose error on Green- 
wich mean time is known, a shade for use if 
the sun is observed, notebook and tables for 
giving the true bearing of the body at various 
intervals of time. 

The body observed should be fairly low in 
altitude. 

Place the machine's head in the required 
direction by compass, and observe bearing of 
the body, noting the time of doing so by the 
watch. Transfer this watch time into appar- 
ent time at place (this will be explained in 
the chapter on Astronomy), and look up the 
body's declination. 

With these data, and knowing your 
latitude, the true bearing can be looked out 
from the tables. 

Apply the variation to this to get the 
magnetic bearing. The difference between 
the true and magnetic bearings will be the 
deviation for that particular direction in 
which the machine is heading. 

This operation should be repeated for all 

44 



METHODS OF SWINGING 

required directions. In practice it is custom- 
ary to work out a table of times and magnetic 
bearings in advance, as it much facilitates 
operations. 

This method is always used at sea when out 
of sight of land, but is not of much practical 
value on a flying ground. 

(b) By ' Reciprocal Bearings/ For this 
purpose a compass known as the ' Landing 
Compass/ or ' Shore Compass/ is set up in 
some place on the flying ground where it will 
be free from all local attraction in the shape 
of sheds, adjacent machines, etc. 

This ensures it being free from deviation. 

The machine to be swung is wheeled out 
and also placed in a position similar to the 
other compass, and heading in any required 
direction. 

Simultaneous bearings are taken of the 
shore compass by the machine's compass, 
and of the machine's compass by the shore 
compass. 

Either of these bearings should now be 
reversed and the' difference between this 
reversed bearing and the other one will be 
the deviation for that particular direction of 
the machine's head. 

45 



AIR NAVIGATION FOR FLIGHT OFFICERS 

This operation can be repeated on any 
other direction of the machine's head. 

Examples : 

(1) Bearing of shore compass . . 227 
Bearing of machine's compass . 40 

Reversed bearing by shore compass 47 
Bearing by machine's compass . 40 

Deviation "7 E. 

(2) Bearing by shore compass . . 62 
Bearing by machine's compass . 247 

Reversed bearing by shore compass 242 
Bearing by machine's compass . 247 

Deviation . 5 W. 

The shore compass should be set up some 
little distance from the machine, not less than 
fifteen or twenty yards. 

(c) By Distant Object. This is a very easy 
method, as it entails the use of no instru- 
ments, and only one observer is needed. 

The magnetic bearing of some distant 
object having been found beforehand, from a 
particular spot, the machine is wheeled out 
and placed so that the compass is over this 
spot, and heading in the required direction. 
All that has to be done now is to take the 

46 



METHODS OF SWINGING 

bearing of the distant object by the compass, 
and repeat it on any other direction. 

The difference between the compass bear- 
ing of the distant object and the magnetic 
bearing already found, will be the deviation for 
that particular direction of the machine's head. 

Examples : 

Machine's Magnetic Compass -^ 

Head. Bearing. Bearing. Deviation. 

o 309 314 5W. 

45 309 3H 2W. 

90 309 306 3 E. 

(d) By Two Objects in Line. This is the 
same as Case (c). But in place of one object 
there are two in line, the magnetic bearing of 
one, and therefore of both, being known. 

This case is valuable for checking devia- 
tion, as the magnetic bearing can be obtained 
from the chart ; and when flying, as soon as the 
objects come into line, the bearing can betaken. 

This will show at once whether or not the 
deviation has altered. 

(e) By a Marked-out Flying Ground. This 
is the simplest method of all, requiring no 
instruments and no objects, and a machine's 
compass can be adjusted at any hour of the 
day or night, and also in thick weather when 
all distant objects and marks are obscured. 

47 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The spot having been chosen, permanent 
lines are marked out running north, south, 
east, and west. The north-east, south-east, 
south-west, and north-west lines may also 
be drawn in if required. Permanent marks 
should be placed at the ends of these lines and 
also at the central spot. 

All that has to be done is to place the 
machine's head along the desired line and 
note the compass reading. 

The difference between this and the lubber 
point of the compass will be the deviation. 

An explanation of the methods of marking 
out a flying ground will now be given. 

In the working of the following example, 
the explanation of the various terms used will 
be found in the chapter on Astronomy. 

The marking out of a flying ground can 
be done in two ways. 

(a) By means of the shore compass, set up in 
any convenient spot free from local attraction. 

The lines can be got straight away by 
direct observation, and marked in. 

(b) An alternative method, which in- 
volves a little more trouble, but once done 
holds good as long as the first case. It con- 
sists of finding the magnetic bearing of one 
or more conspicuous objects visible from the 

48 



MARKING OUT A FLYING GROUND 

swinging ground, and from this bearing to 
get the magnetic directions required. The 
magnetic bearing of one of the objects is 
obtained by simply taking a horizontal angle 
between the sun's limb and the object 
required. The sun's bearing can now be 
worked out and this angle applied to it. 

The result will be the true bearing of the 
object, so, to get the magnetic bearing, the 
variation must be applied. It is just as well 
to have the bearings of two or three objects 
in case one is done away with, so if angles 
between the first object and one or two others 
be taken, they can be applied to the bearing of 
the first. 

An example of this follows. 

On April 14, 1916, at a certain flying 
ground in Latitude 51 North, Longitude 
3 West, it was desired to lay out magnetic 
lines for compass adjustment. 

The following observations were made. 

Rough time about 5.45 A.M. 

The watch, which was slow on Greenwich 
mean time o hrs. 2 min. 15 sec., showed 
5 hrs. 55 min. 33 sec. At the same time 
the observed horizontal angle between an 
object A to the right of the sun and the 
sun's near limb, was 97 50'. 

49 E 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The following angles were also observed. 
Right of A. Left of A. 

Object B 64 40' Object C 37 50' 

Variation 16 W. Required the magnetic 
bearings of A, B, and C. 

N. B. In this example the working is not 
rigorously exact, but is near enough for 
practical purposes. 



h. m. s. 

Time by watch . 5 55 33 

SlowonG.M.T. .' ' 2 15 

G.M.T. 5 57 48 

Long, in time "..*. 12 oo 

Mean time at place . 5 45 48 
Equation of time 

from Nautical 

Almanac . ,,... 



-{- 20 



Apparent 
place 



time at 



5 46 08 



Right of A. 
Magnetic bearing of 

A . -,;, 

Angle to B . ,, . 



195 23' 
64 40' 



Magnetic bearing of 

B . 260 03' 



True bearing sun's 
limb from table 

Sun's semidiame- 
ter from Nau- 
tical Almanac . 

True bearing sun's 

centre 
Angle to A right 

of sun 

True bearing of A 
Variation . 



81 if 



16' 



81 33' 



97 50' 

179 23' 
i6oo'W. 



Magnetic bearing 

of A , . 195 23' 

Left of A. 
Magnetic bearing of 

A . .'..* 195 23' 
Angle to C . 37 50' 

Magnetic bearing of 

C . 157 33 



MARKING OUT A FLYING GROUND 

To get the sun's true bearing from the 
tables, we require to know three things : 
the latitude, the sun's declination, and the 
sun's hour angle. 

The latitude we know already, the hour 
angle will be the apparent time, since we keep 
our time from the sun, and the declination 
can be taken out of the Nautical Almanac for 
that day at sight. The declination is given 
for noon each day, but as its total change 
for twenty-four hours is comparatively small, 
this can be neglected, as it is near enough 
for compass work. 

The bearing given in the tables is that of 
the sun's centre, so to get the bearing of 
the sun's limb, the semidiameter must be 
applied. Whether to add or subtract it can 
easily be ascertained from a figure ; the one 
on p. 52 is the one for the example given. 

NOA is the angle given in the tables, and 
NOB is the angle required. As A is to the 
right of the sun, the angle AOB is additive 
to the angle NOA. 

It should be remembered that the observer 
is at O facing the sun. 

Therefore, in this case, the semidiameter 
must be added to the bearing from the 
tables. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The semidiameter, in the case of compass 
work, may be taken as a constant of 16'. 
Having found the magnetic bearing of A 




270 



80 



To A 

FIG. 20. 



to be 195 23', it follows that the magnetic 
north must lie either 195 23' to the left of A, 
or 360 oo x 195 23', i.e. 164 37' to the right 
of A as shown in the following sketch. 

To lay out the ground, the following 
procedure should be adopted. 

52 



MARKING OUT A FLYING GROUND 

Set the vernier of the verge plate to zero, 
and having placed the landing compass over 
the central position on the swinging ground, 
turn the whole bowl of the compass round 
until the object A is seen in the prism slit 
in line with the sight wire. 




Now set the vernier either 195 23' to the 
left of A or 164 37' to the right of A. The 
sight wire will now be pointing direct to 
the magnetic north. 

Get someone to walk slowly across your 
line of sight with a peg, and stop him when he 
comes in line with the sight wire. Drive this 

53 



AIR NAVIGATION FOR FLIGHT OFFICERS 

peg into the ground, this will represent the 
north point from the position of the compass. 
Taking this north point as the starting point, 
the remaining points of the compass can now 
be pegged out in turn and the lines painted 
in if required, the pegs being left standing or 
replaced by small base plates flush with the 
ground. 

The advantage of having two or more 
marks whose bearing is known, lies in the fact 
that some of them may be destroyed in course 
of time, in which case bearings to new marks 
would have to be found. 

If it is desired to lay out more than one 
swinging ground, the bearing may be found 
from one position and calculated for the 
others from the chart or map on the largest 
scale possible. 

When taking the horizontal angle between 
the sun and an object, always have the sun as 
low in altitude as possible. 

The Practical Correction of a Compass. 
By this is meant the actual placing of the 
adjusting magnets to neutralise the effect of 
the iron surrounding the compass. 

If the machine is a new one, it should be 
swung for its natural deviations on the eight 

54 



PRACTICAL CORRECTION 

principal points of the compass by one of the 
methods already described. 

By ' natural ' deviations is meant the 
deviations of the compass before any of the 
correctors are applied. 

These deviations having been ascertained, 
the coefficients can be worked out and 
the various correctors placed in position 
roughly. 

Coefficient D should always be corrected 
first, if intended to correct it, its amount and 
requisite size of spheres being obtained from 
the published tables. 

This can be done with the machine head- 
ing in any direction, and when once done, 
holds good for any place the machine may 
be in. 

Now place the nose of the machine north 
or south, or east or west ; and correct the 
coefficients C and B by adjusting the trans- 
verse and longitudinal magnets respectively 
as necessary. 

This is done, in the case of taking a distant 
object, by so adjusting the magnets as to 
make the compass bearing of the object agree 
as nearly as possible with the magnetic 
bearing previously found. 

If, however, the machine is being swung on 
55 



AIR NAVIGATION FOR FLIGHT OFFICERS 

a marked-out flying ground, it is only neces- 
sary to place the machine heading along the 
lines on the ground, and make the compass 
point accurately by altering the position of 
the magnets as requisite. 

Having done this, all that remains to be 
done is to again swing the machine and 
tabulate the remaining deviations, which 
will be the deviations to be used when flying. 

If the compass has been swung before, it 
will only be necessary to readjust the magnets, 
if required, by placing the nose of the machine 
in the requisite directions and making the 
compass bearing agree as nearly as possible 
with the magnetic bearing. 

The machine should then be swung as 
before, to get the remaining deviations. 



CHAPTER V 

CORRECTING COURSES. NAMING 
DEVIATION RULES FOR GETTING 
THE CORRECT ANGLE FROM THE 
BEARING TABLES. FINAL NOTES 

On the Correction of Courses. A knowledge 
of how to apply the variation and deviation 
to different courses in a correct manner is of 
great importance, as by doing so wrongly in the 
case of the variation only, the pilot may find 
himself flying on a course about 30 from 
his right direction if the variation is 15. 

If, however, the following rules be learnt 
and attended to, he need never get into that 
position. 

Rules. The following rules apply to varia- 
tion and deviation alike. 

(a) Given compass course or magnetic 
course to find true course. 

Add easterly. Subtract westerly. 

57 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Examples : 

Compass course 228. Variation 17 E. 
Deviation 4 W. Find true course. 

Compass Course . . . 228 
Variation . . . .4-17 

245 

Deviation . . . . 4 
True Course . . . 241 

Magnetic course 163. Variation 14 W. 
Find true course. 

Magnetic Course . ., . 163 
Variation J . . 14 

True Course. . . . 149 

(b) Given true course to find magnetic or 
compass course. 

Add westerly. Subtract easterly. 

True course 117. Variation 14 W. 
Deviation 6 E. Find magnetic and compass 
courses. 

True Course. . . . 117 
Variation . . . . +I4'W. 

Magnetic Course . . . 131 
58 



CORRECTING COURSES 

True Course. . . .117 
Variation . . . . +14 



131 



Deviation . . . . 6 

Compass Course . . . 125 

Should the result, after correction, be 
found to exceed 360, the latter amount must 
be subtracted from the total. 

Example : 

Compass course 355. Variation 15 E 
Deviation 5 E. Find true course. 

Compass Course . . . 355 
Variation . . . . +15 



370 



Deviation . . . +5 

True Course. . . ,-. 375 

-360 

True Course. . . . 15 

Notes on How to Name Deviation. Naming 
deviation is, to a novice, at first a little diffi- 
cult, but if Figs. 22 and 23 be studied, it 

59 



AIR NAVIGATION FOR FLIGHT OFFICERS 



270 




180 

Deviation 5E. 



FIG. 22. Showing easterly "deviation. 




180 
Deviation 6W. 



FIG. 23. Showing westerly deviation. 



NAMING DEVIATION 

will at once become apparent, and after a 
little practice it can be done without any 
writing down. 

In either figure the circle is supposed to 
represent the compass card, and AB the line 
joining the distant object to the centre of the 
card. 

This line AB should be considered as 
absolutely fixed, and in Fig. 22 suppose it to 
run in the direction of, say, 235 magnetic. 

The compass card is free to revolve about 
its centre, at B, and in this case the degree 
230 is found by observation to be lying under 
the line AB. 

Therefore the 235th degree must have 
moved in the direction shown by the black 
arrow. 

Now, if one part of the card moves, the 
whole must move in the same direction ; hence, 
if we follow the card round to the north 
point, the latter must clearly move in the 
direction shown by the pecked arrow. 

As the direction of the pecked arrow is 
eastward, the deviation must be easterly. 

In Fig. 23, suppose the line AB to run, say, 
132. From observation we find the degree 
138 to be under this line. The card's motion 
must have been in the direction of the black 

61 



AIR NAVIGATION FOR FLIGHT OFFICERS 

arrow, and following its motion round, we 
see the north point of the card must move in 
the direction of the pecked arrow. Hence the 
deviation must be westerly. Therefore, for 
any card graduated according to the new 
style, i.e. from o to 360, the rule is as 
follows : 

// the compass bearing is less than the 
magnetic bearing, the deviation is easterly ; 
if greater than the magnetic bearing, the de- 
viation is westerly. 

Notes on the True Bearings taken from the 
Tables. With reference to the bearings taken 
from the tables, it must be remembered that 
these tables were made out for the old pattern 
graduation of the card, and therefore require 
some manipulation before the bearing by the 
new style of card can be written down. 

The following rules should be well learnt : 

(a) In north latitude. 

If the time is A.M., the bearing may be taken 
straight out of the tables and written down. 

If the time is P.M., the bearing given in the 
tables must be subtracted from 360 and the 
result written down as the bearing to be used. 

(b) In south latitude. 

If the time is A.M., subtract the bearing 
62 



NOTES ON TRUE BEARINGS 

given in the tables from 180, and use the 
result. 

If the time is P.M., add 180 to the bearing 
given in the tables, and use the result. 

N. B. The bearings in the tables are 
always given from the pole of the observer's 
hemisphere. 




FIG. 24. 

These rules will now be illustrated graphic- 
ally by figures. 

Fig. 24. NORTH LATITUDE. A.M. Time. 

NBC is the angle given in the tables and 
is the one required. 

63 



AIR NAVIGATION FOR FLIGHT OFFICERS 

N 





NOTES ON TRUE BEARINGS 

Fig. 25. NORTH LATITUDE. P.M. Time. 

NBC is the angle given in the tables, but 
the angle NESC is the one required, i.e. 
360 - NBC. 

Fig. 26. SOUTH LATITUDE. A.M. Time. 




SBC is the angle given in the tables, and 
NBC the angle required, so 180 - SBC = 
NBC. 

Fig. 27. SOUTH LATITUDE. P.M. Time. 

SBC is the angle given in the tables, and 
65 F 



AIR NAVIGATION FOR FLIGHT OFFICERS 

NESC the angle required, so that 180 + 
SBC = NESC. 

To Test a Compass. This should be done 
now and again to see if the cap and pivot are 
in good working order, as they are liable to 
damage from shocks in landing, etc. 

This can be done in two different ways. 

(1) By comparing it with another compass 
which is known to be accurate. 

The two compasses should be placed as 
near to one another as possible without inter- 
fering with each other's field. Bearings of a 
distant object as far away as possible should 
be taken by both compasses on various direc- 
tions of the aeroplane's head. 

The bearings taken by the machine's 
compass corrected for the known deviation 
should be practically the same as the bearing 
shown by the other compass. 

Should they differ by any moderate 
amount, the cap and pivot should be examined. 

(2) By deflecting the card about a point 
from its normal position of rest, and noting 
if it returns to its old position. If not, it is 
probable that something is wrong. 



66 



CHAPTER VI 
METEOROLOGY 

IN this chapter a few notes will be given of 
the relation of wind and weather, and from a 
study of these it is hoped that the pilot may 
be able to deduce, from his own observations, 
the type of weather he is likely to encounter. 

He must remember, however, that even the 
best observatories, equipped as they are with 
every improved type of instrument and with 
all their telegraphic facilities, are sometimes 
very much out in their forecasts, so that he 
need not wonder at the very frequent apparent 
failures of his attempts. 

Wind, which is simply the atmosphere in 
motion, is of two kinds, called cyclonic and 
anti-cyclonic. 

The following remarks on cyclones and 
anti-cyclones are written for the Northern 
Hemisphere, and to apply them to the Southern 
Hemisphere, all directions of the wind round 
its centre should be reversed. 

67 



AIR NAVIGATION FOR FLIGHT OFFICERS 

A cyclonic wind is one that either brings 
rain or is associated with bad weather. 

It blows spirally round a centre or core of 
low pressure in a direction contrary to the 
hands of a watch in the Northern Hemisphere. 
The sequence of wind and weather in a cyclone 
are everywhere the same, and they differ in 
intensity only according to the steepness and 
closeness together of the isobars. 

The following definitions should be learnt : 

Path of a Storm. Is the direction that the 
whole storm is travelling in. 

Trough of a Storm. Is the line more or 
less at right angles to the path where the 
barometer has reached its lowest and has 
just turned to the rise. 

Right and Left Hand Semicircles. Are the 
two halves of the storm situated on the right 
and left hand respectively of the observer, 
when he is standing in the centre of the storm 
facing the direction it is travelling in. 

Centre of a Storm. Is the area of lowest 
pressure. Here the wind often drops to a 
flat calm. 

68 



METEOROLOGY 

Isobar. Is a line of equal barometric 
height or pressure. 

Isotherm. Is a line of equal temperature. 

The wind in a cyclonic disturbance does 
not blow tangentially to the isobars, but spir- 
ally inwards at an angle of about io-i5 to 
them, being more incurved in the rear part of 
the storm. 

In the Temperate Zones these depressions 
almost invariably travel eastwards, but their 
paths may be deflected by land or by an area 
of high pressure. 

The centre of a storm can always be 
found by the following rule, known as Buys- 
Ballot's Law. 

Rule. Face the wind, and the centre will 
be found to bear about 135 on the right hand 
until the barometer has fallen three-tenths of 
an inch, about 112 between three-tenths 
and six-tenths, and about 90 after six- 
tenths. 

N. B. In the Southern Hemisphere it will 
be as above, but on the observer's left hand. 

The sketch on p. 70 shows the relation of 
the wind to the isobars, in a cyclonic depres- 
sion, the egg-shaped lines representing lines of 
equal pressure. 

69 



AIR NAVIGATION FOR FLIGHT OFFICERS 

It must be clearly understood that this 
sketch is purely arbitrary, and that a cyclonic 
depression may take any shape or form of 
isobar. 




FIG. 28. 

With reference to the statement made 
before, that the weather sequence in a cyclonic 
depression was always the same but differing 
in intensity only, it must be understood that 
by intensity is meant that whereas in one case 
with a slight and gradual fall, which means 

70 



METEOROLOGY 

that the isobars are spaced wide apart, cnly 
a mild type of wind, rain, and cloud are 
experienced, yet, on the other hand, when the 
isobars are close together, the above men- 
tioned are met with in a much greater and 
stronger form. 

As a general rule, the steeper the fall of the 
barometer the stronger the wind and coming 
weather. 

Sometimes it will be noticed that a big 
fall of the barometer is not attended by 
any drastic change in the weather, but that, 
after a lime, the former recovers itself. This 
is due to what is known as ' Surge/ 

The best explanation of this is to consider a 
general lowering of pressure over a large area, 
which takes some time to fill up again, the 
area being so large that it only fills up com- 
paratively slowly. 

The sketch on p. 72 shows the weather 
sequence in a cyclonic depression. 

The rate at which a storm, as a whole, 
travels is very uncertain, depending on the 
areas of high pressure round it and the 
amount of land about. 

Anti-cyclone. This is a region of high 
pressure associated with fine and mild weather. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

in which the wind blows more or less tangenti- 
ally to the isobars with the hands of a watch 
in the Northern Hemisphere. 



Blue 



Refraction/ 

Detached /Strato- 
-Cumulus 



Hard/Sky 

Cool 

Showers) 
"^r 
Squalls 



Cirro-Stratus. 

Overcast 



OirtySky 



III define 
k x- showers 

* 



Patches 
D^- 
Blue 



Driving Rain 
/ 
Dirty Sky 



terySun 
pped Hills 



Stratus 



Strato-Cdmulus 



DetKM 

Cumulus^ 1 



Mane<tails 



FIG. 29. 

Its force scarcely ever rises above a 
pleasant breeze. Unlike a cyclonic depres- 
sion, an anti-cyclone may remain stationary 
for days on end. 

One great feature of an anti-cyclone is the 
radiation weather in it. 



72 



METEOROLOGY 

The sketch below gives the sequence 
of weather usually experienced in an anti- 
cyclone. 




FIG. 30. 

Formation of Cloud, Fog, and Dew. If 

the barometric pressure at any place falls, 
a current of air rises, carrying with it a large 
amount of water vapour, more especially if 
the low pressure should happen to be situated 
over the sea. 

73 



AIR NAVIGATION FOR FLIGHT OFFICERS 

As this air rises, it expands owing to the 
diminished pressure ; this causes a loss of heat, 
which is further accentuated by the low 
temperature in the upper regions. This loss 
of heat results in the condensation of the 
water vapour, which also mixes with the small 
particles of dust and other matter floating in 
the air. 

The result of this is to cause the familiar 
appearance which we know as cloud. 

There are two theories that have been put 
forward as to the formation of cloud. 

(1) Is known as condensation by cooling. 
This method has been described above. 

(2) Is known as condensation by mixing. 
This is supposed to take place when a mass 
of damp air, on rising, meets another mass 
of damp air at a different temperature. 

There are ten different classes of clouds, 
four of which are known as ' Fundamental 
Clouds/ whilst the other six are made up 
of mixtures of the other four. 

The following tables give the names and 
average heights of these clouds. 

(a) ' Fundamental Clouds/ 

(1) Stratus.. . . . o to 3,500 feet. 

(2) Nimbus, or Rain Cloud . 3,000 ,, 6,400 

74 



MARE'S TAIL 

CIRRUS 

27.0OO to 50.000ft. 



CIRRO-STRATUS 

Average 29.5OOft. 



MACKEREL SKY 

CIRRO-CUMULUS 

10.0OO to 23,000 ft. 



ALTO-CUMULUS 

10.000 to 23.00Oft. 



ANDES 

(ACONCAGUA) 



ALTO-STRATUS 

10,000 to 23.0OO ft 



STRATO-CUMULUS 

About 6,5OOft. 



CUMULUS 

4.5OOto6.0OOft 



STORM CLOUD 

CUMULO-NIMBUS 

4,5 00 to 2 4000ft 



RAIN CLOUD 

NIMBUS 

3OOO to 6,400 ft 



STRATUS 

O to 3,5OOff 



ElFELTOWER 
S T . PAULS 




THE TEN DIFFERENT KINDS OF CLOUDS. 



METEOROLOGY 

(3) Cumulus . . . 4,5oo to 6,000 feet. 

(4) Cirrus, or Mare's Tail . 27,000 ,, 50,000 

(b) ' Composite Clouds/ 

(1) Cumulo Nimbus, or Storm 

Cloud ' . . . 4,500 to 24,000 feet. 

(2) Strato Cumulus . Average 6,500 ,, 

(3) Alto Stratus . . 10,000 to 23,000 

(4) Alto Cumulus . . 10,000 ,, 23,000 ,, 

(5) Cirro Cumulus, or Mac- 

kerel Sky . . 10,000 ,, 23,000 ,, 

(6) Cirro Stratus . . Average 29,500 ,, 

The accompanying illustration has been 
published through the courtesy of Mr. Elliott 
Stock, 7 Paternoster Row, E.G., whose per- 
mission has been obtained. 

Cause of Fog. This may be caused in two 
different ways. 

(1) Warm air saturated with moisture 
passing over a cold surface of water, the 
vapour in the air is chill and condensed, 
forming a white cloud called fog. 

(2) Cold air blowing over warm water 
chills the water vapour rising from the latter, 
with the same result as in the first case. 

A fog bank may be driven a good distance 
from the place where it started, provided that 
the air temperatures are nearly the same; 

75 



AIR NAVIGATION FOR FLIGHT OFFICERS 

but such fogs do not last long, and soon 
disappear. 

It sometimes happens that during a fog 
very large and heavy raindrops come down, 
this is a sure sign that the fog will disappear 
very shortly. 

General Forecasting of Weather. The 
general forecasting of the weather of the 
British Islands is done by two methods : 

(1) By a Synoptic Analysis. 

(2) By Lord Dunboyne's method. 
Taking the two in the sequence mentioned 

above. 

(i) By Synoptic Analysis. At 7 A.M. 
every morning certain information is tele- 
graphed to the headquarters of the Meteoro- 
logical Office from all stations connected 
with it, and also wireless reports are received 
from ships. 

The information thus received is collated 
and placed on the weather chart for the day, 
ready for issue. The information telegraphed 
to the central office is as follows : 

Force and direction of wind. 

Height of barometer, and whether rising 
or falling, 

76 



METEOROLOGY 

Temperature of air and sea, the latter 
only at those stations bordering the coast. 

State of weather prevailing at the time 
at each station. 

State of sea. 

These observations are placed on the chart 
as necessary ready for issue to the general 
public, though this has been modified during 
the war by issue only to official bodies. 

The symbols in use on the synoptic chart 
are given below. 

Isobars. Are denoted by continuous lines. 
Isotherms. Are denoted by pecked lines. 

Wind force is denoted as shown below ; 
the direction of the wind goes with the arrows, 
and is named according to where it comes 
from. 







Force above 10. 
Force 8-10. 
Force 4-7. 
Force 1-3. 
Calm. 



FIG. 31. 

77 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The general state of the weather is shown 
below. 

Rain. 
A Hail. 

Snow. 
~E Fog. 

T Thunder. 

K Thunderstorm 

Rough Sea. 

==: High Sea. 

Wireless Report. 

FIG. 32. 

(2) Dunboyne's Weather Report. This 
report is issued by the Admiralty daily at 
10.30 A.M. It is liable to revision as time 
goes on, and actual observation shows the 
need for it. 

The weather report is for the British 
Islands in general and London in particular. 

It divides the British Islands into three 
parts as follows : 

(1) North of the latitude of the Wash. 

(2) The English Channel and north 
coast of France. 

(3) The southern halves of England 

78 



METEOROLOGY 

and Ireland south of the latitude mentioned 
in (i). 

On the daily sheet is printed an explana- 
of the terms used, as follows : 

The day referred to is a twenty-four hour 
day. 

Fine. The wind moderate in force or 
less, no appreciable rainfall, probably some 
hours of sunshine. 

Fair. The wind fresh in force or less, 
little or no rain, probably cloudy. 

Changeable. Sometimes fair or fine, some- 
times unsettled. 

Unsettled. A high wind alone or heavy 
rain alone, or both wind and rain combined 
in moderation. 

Disturbed. High wind or gale with rain 
more or less. On some occasions the term 
' Very Disturbed ' may be used. 

N.B. Intervals of fog may occur during 
the periods of fair or fine. 

Period. Four days or more. 

Interval. Twelve hours or less. 

Spell. More than twelve hours, less than 
four days. 



79 



CHAPTER VII 

v 

GENERAL WEATHER IN THE 
BRITISH ISLANDS 

THE prevailing wind in the British Islands is 
from some westerly point. 

Two of the principal reasons are as 
follows : 

(1) The British Islands, situated as they 
are in a high northern latitude, are in the 
region of the ' Aati-trades ' or Westerlies. 

(2) There is usually a low pressure 
round about Iceland, and a high pressure 
about the Azores, and, bearing in mind the 
direction of the wind circulation round a 
high and low pressure respectively, the result 
is as shown in the sketch on p. 81. 

Much could be said about the cause of 
wind due to the earth's rotation, but it is 
not proposed to touch on this in these notes. 
(See Appendix.) 

Should the reader require to go further 
into this matter, he should consult the 
Admiralty ' Manual of Navigation/ 

80 



WEATHER ] IN BRITISH ISLANDS 

Westerly gales are very prevalent in the 
winter months, i.e. from October to March 
inclusive ; they are rare from May to July, 
also inclusive, and seldom last lorg. 

In the English Channel, winds from N.N.E. 
to E. cause the land to become covered with 
a thick white fog resembling smoke. 

Iceland 



British Islands 

* 

Azores 
FIG. 33. 

Easterly winds are very common in the 
spring months. A south-easterly wind with 
a falling barometer is an almost infallible sign 
of a coming gale. 

Land and sea breezes may occur during 
a long spell of fine weather, the land breeze 
by night and the sea breeze by day. 

Long-drawn-out calms are suspicious, and 
81 G 




AIR NAVIGATION FOR FLIGHT OFFICERS 

are generally the advance guard of a spell of 
bad weather. 

The paths of storms passing over the 
British Islands are rather erratic, owing to 
their being deflected by the land. They may 
also be deflected by coming up against a high- 
pressure system. 

Storms passing over the British Islands 
almost always have their centres north of 
the English Channel ; from this, reference to 
Fig. 28 will show that the usual dangerous 
wind will be south-easterly, with, of course, 
the barometer falling. 

Storm Signals. These are hoisted by the 
various storm signal stations according to 
orders received from the Meteorological Office, 
from the warnings given by their synoptic 
charts. The new system is known as the 
International Code, but its introduction has 
been delayed by the war. 

It consists of a display of either one or two 
cones hoisted as follows : 

One Cone, point upwards.- Gale commencing with 
wind in the north-west quadrant. 

One Cone, point downwards. Gale commencing 
with wind in the south-west quadrant. 

Two Cones, one above the other, both points 
82 



BEAUFORT'S SCALES 



upwards. Gale commencing with the wind in the 
north-east quadrant. 

Two Cones, one above the other, both points down- 
wards. Gale commencing with the wind in the 
south-east quadrant. 

Two Cones, bases together. Hurricane. Wind 
force, 12 Beaufort Scale. 

Beaufort's System of Weather Notation. 
The following tables have been copied from 
the Admiralty ' Manual of Navigation/ per- 
mission to do so having been given by the 
Controller of H.M. Stationery Office. 

Beaufort's System of Wind Notation 

For Coast Use. 

Calm : smoke rises 
vertically 

Direction of wind 
shown by smoke drift 
but not by wind vane 
Wind felt on face, 
leaves begin to rustle, 
ordinary vane moved 
by wind 

Leaves and twigs 
in constant motion, 
wind extends a light 
flag 

Raises dust and 
loose paper, small 
branches are moved 
Small trees in leaf be- 
gin to sway, crested 
wavelets form on in- 
land waters 

83 



No. 
O 


General Description 

Calm 


I 


Light air 


2 


Slight breeze 


3 


Gentle breeze 


4 


Moderate 
breeze 


5 


Fresh breeze 



Miles per 
Hour. 

Less 
than i 


Metres per 
Second. 

Less than 
0-3 
0*3-1-5 


4-7 


i '6-3-3 


8-12 


3,-5, 


13-18 


5.5-8-0 


19-24 


8-1-10.7 



AIR NAVIGATION FOR FLIGHT OFFICERS 



b 

be 

c 

o 

g 

m 

f 



For Coast Use. 

Large branches in 
motion, whistling 
heard in telegraph 
wires 

Whole trees in mo- 
tion, inconvenience 
felt when walking 
against wind 
Breaks twigs off trees, 
generally impedes 
progress 

Slight structural dam- 
age occurs, chimney- 
pots and slates re- 
moved 

Seldom experienced 
inland, trees up- 
rooted, considerable 
structural damage 
occurs 

Very rarely experi- 
enced, causes wide- 
spread damage 



Beaufort's System of Weather Notation 

Blue sky, i.e. sky not more than J clouded. 

Sky I to | clouded. 

Sky \ to clouded. 

Sky overcast, i.e. more than f clouded. 

Gloomy, 

Mist. 

Fog. 

84 



No. 
6 


General Description. 

Strong breeze 


7 


High wind 


8 


Gale 


9 


Strong gale 


10 


Whole gale 


i 
ii 


Storm 


12 


Hurricane 



Miles per 
Hour. 

25-31 


Metres per 
Second. 
IO-8-I3-8 


32-3 


13-9-17-1 


39-46 


17-2-207 


47-54 


20-8-24-4 


55-63 


24-5-28-4 


64-75 


28-5-33*5 


Above 

75 


33-6 and 
above 



BEAUFORT'S SCALES 

r Rain. 

d Drizzling rain. 

e Wet air without rain falling, 

p Passing showers, 

h Hail. 

s Snow. 

t Thunder. 

1 Lightning, 

tl Thunderstorm. 

tlr Thunderstorm accompanied by rain, 

q Squalls. 

u Ugly threatening sky. 

v More than ordinary visibility, 

w Unusually heavy dew. 

x Hoar frost. 

z Dust haze or smoke. 

Beaufort's Scale for Sea Disturbance 



STo. 


Description. 





Calm. 


I 


Very smooth. 


2 


Smooth. 


3 


Slight. 


4 


Moderate. 


5 


Rather rough. 


6 


Rough. 


7 


High. 


8 


Very high. 


9 


Phenomenal. 




85 



CHAPTER VIII 

FORECASTING BY SOLITARY 
OBSERVER 

IN connection with this, the solitary observer 
has the following information at his disposal. 

(1) The ordinary Daily Weather Notice, 
from which he can obtain the positions of the 
high pressures. This gives him the probable 
path of any low pressure. 

(2) His knowledge from personal observa- 
tion of the present state of the weather. 

(3) The movements of the barometer from 
his record, or from the trace shown by his 
barograph. 

(4) The wireless reports received from 
stations or ships to the westward of him, 
bearing in mind that nearly all depressions, 
with their attendant bad weather, are travel- 
ling to the eastward. 

With reference to the trace shown by the 
barograph, it should be remembered that, 
should the fall of the barometer be at a uni- 
form rate, the trace on the paper will be a 
descending straight line ; if the rate of fall is 

86 



FORECASTING WEATHER 

increasing, the trace becomes convex, and if 
the rate is decreasing, the trace is concave to 
the top of the recording sheet. 

If the rise of the barometer is at a uniform 
rate, the trace is shown by an ascending 
straight line ; if the rate is increasing, the 
trace is concave ; whilst if it is decreasing, the 
trace is convex to the top of the recording 
sheet. 

Thus all we can tell from the movements 
of the barograph is that, with a falling glass, 
a convex trace means that the wind and 
weather will get worse much more rapidly 
than with a concave trace, and with a rising 
glass, a concave trace will indicate that the 
weather will improve more rapidly than with 
a convex trace. 

On the other hand, the quicker the rise or 
fall, the steeper the isobars, and therefore the 
stronger the wind. 

The above is shown by the diagrams on 
p. 88. 

Bad weather, which takes a long time to 
develop, is also long in showing improve- 
ment, and vice versa. 

This can easily be remembered by the old- 
time jingle : 

Long foretold, long last, 
Short notice, soon past. 



I. Steady Rate of Fall or Rise. 

HPS.I n in IY Y 5i fin Yin ix xxixn i 













Barer 
30 


neter 
00 




_2_ 










N 

j 


^ 




29 


50 


o 



















L 29 


00 


x5 


















1 


N 

28 


f ^ 
50 





































II. Increasing Rate of Fall 01 Rise. 
Hrs.I HD1 lYYYIYHYffllXXXI 




Barometer 
30-00 



29 



50 



29 



00 



50 



28 00 



III. Decreasing Rate of Fall or Rise. 
HrsI H ffl 





\ 






Baror 

30J 


icter 
00 
















\ 






29 


50 








r ^- < 








c 


^ 


's- 


29 


00 


V 


^ 
r ^ 


^ e 












V 


r^ 


^ 28 


50 j 


/' 


F 


















28 


00 






































FIG. 34. 
88 



WEATHER RULES 

In connection with the forecasting of the 
coming weather, the rules given by the late 
Admiral Fitzroy are well worth committing 
to memory. 

Taken in conjunction with other instru- 
mental aids, they are of the greatest use in 
foretelling weather. 

These rules are given below. 

Admiral Fitzroy 's Weather Rules. 

Whether clear or otherwise, a rosy sky at 
sunset indicates fine weather ; a sickly green- 
ish hue, wind and rain ; tawny or coppery 
clouds, wind ; a dark or Indian red, rain ; 
a red sky in the morning, bad weather, or 
much wind, perhaps also rain ; a grey sky 
in the morning, fine weather ; a high dawn, 
wind ; a low dawn, fine weather. 

The darker or angrier the colour of the 
red in the morning, the worse the coming bad 
weather will prove to be. Also an opal-tinted 
sky in the morning is a sign of coming bad 
weather. 

A high dawn is when the first indications of 
daylight are seen above a bank of clouds. 

A low dawn is when the day breaks on or 
near the horizon, the first streaks of light being 
very low down. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Soft-looking or delicate clouds foretell 
fine weather, with moderate or light winds ; 
hard-edged oily looking clouds show wind. 
A dark, gloomy blue sky is windy ; but a light, 
bright blue sky indicates fine weather. 

Generally, the softer clouds look, the less 
wind but perhaps more rain may be expected ; 
and the harder, more greasy, rolled, tufted 
or ragged, the stronger the coming wind 
will prove to be. 

A bright yellow sky at sunset foretells 
wind ; a pale yellow, rain ; orange or copper- 
coloured, wind and rain ; and thus, by the 
prevalence of the various tints in the sky, 
the coming weather may be foretold fairly 
accurately, and, if aided with the usual 
instruments, almost exactly. Light delicate 
quiet tints or colours, with soft indefinite 
forms of clouds, indicate and accompany fine 
weather, but gaudy or unusual hues, with 
hard definitely outlined clouds, foretell rain 
and probably strong wind. 

Small inky-looking clouds foretell rain ; 
light scud clouds driving across heavy masses, 
show wind and rain ; but if alone, may 
indicate wind only, the latter proportionate 
to their motion. 

High upper clouds crossing in a direction 
90 



WEATHER RULES 

different from that of the lower clouds, or 
from the surface wind felt below, foretell a 
change toward their direction. 

After fine clear weather, the first signs in 
the sky of a coming change are usually light 
streaks, curls, wisps, or mottled patches of 
distant cloud, which increase and are followed 
by a general overcasting of vapour that grows 
into cloudiness. 

This appearance, more or less oily or 
watery, as rain or wind will predominate, 
is a certain sign. 

Usually, the higher and more distant such 
clouds seem to be, the more gradual but more 
general the coming change of weather will 
prove to be. 

Misty clouds forming or hanging on heights 
show wind and rain approaching; if they 
remain, increase or descend. If they rise or 
disperse, the weather will get better or become 
fine. 

Dew is an indication of fine weather, its 
formation never begins under an overcast 
sky or when there is much wind. Great 
clearness of the air, especially near the 
horizon, distant objects very well defined or 
raised by refraction, also what is called a good 
hearing day, are signs of rain or wind coming. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

A great deal of refraction is a sign of 
easterly wind. 

More than usual twinkling or apparent 
size of the stars, haloes, etc., are more or less 
indications of approaching wind, with or 
without rain. 



92 



CHAPTER IX 
ASTRONOMY 

IN olden days the sky and its stars were 
divided into twelve constellations. 

These constellations were supposed to 
represent human beings and different animals. 

After telescopes were invented, and as the 
power of the latter grew, more and more 
stars became visible, and the original twelve 
constellations outgrew themselves. 

In modern star maps, this number twelve 
has been greatly increased, and in those drawn 
by the late Mr. R. A. Procter, no less than 
eighty-four constellations are given. 

Some of the latter are very small and 
do not contain any stars which would be 
of practical value to the pilot, and in the 
following star maps, twenty-two in number, 
only those constellations are given which 
might be of use to a Flight Officer. 

These drawings only give the principal 
stars in each of the constellations ; of course 

93 



AIR NAVIGATION FOR FLIGHT OFFICERS 

there are many more, but it would serve no 
good purpose by putting them in, and would 
only lead to confusion. 

If any more stars are required by the pilot, 
he cannot do better than consult Procter's 
Star Atlas. 

The stars in the following drawings are 
not put in exactly correct as regards their 
declinations and right ascensions, but they 
are near enough for all practical purposes. 

As the stars in the constellations are 
lettered according to the Greek alphabet, the 
latter is here appended for the benefit of those 
who may not know it. 

a Alpha. v Nu. 

Beta. f Xi. 

ry Gamma. o Omicron. 

8 Delta. TT Pi. 

e Epsilon. p Rho. 

f Zeta. o- Sigma. 

i] Eta. r Tau. 

Theta. v Upsilon. 

1 Iota. < Phi. 
K Kappa. % Chi. 
\ Lambda ^ Psi 

fjL Mu. co Omega. 

These Greek letters are given against "each 
star in the sketches, and also the old Arabic 

94 



ASTRONOMY 

names in the case oi the more important ones 
in each constellation. 

In these drawings the true north should be 
taken as the top of the page. 

Owing to their immense distance away, 
the relative position of the stars to one another 
as seen from the earth seems to be always the 
same, but as a matter of fact they all undergo 
a slight change every year in the same direc- 
tion, known as ' Precession/ This does not, 
of course, alter their relative positions to one 
another. So that, having once picked up a 
star with reference to its relative position to 
another constellation, it will always be found 
in that same place. 

On account of the diurnal motion of the 
earth, the Compass Bearing of any star is 
always changing from the time it rises to the 
time it sets. 

When looking for stars at night, it often 
happens that the constellations they are in 
may be upside down. 

This is due to the apparent rotation of the 
Stellar Sphere, which appears to revolve from 
east to west round the axis of the earth. 

Several of these constellations are what is 
known as ' Circumpolar/ that is to say, they 
never set in these latitudes. 

95 



AIR NAVIGATION FOR FLIGHT OFFICERS 

The Great and Little Bears and Cassiopeia 
are examples of this. 

A sketch is given to illustrate this 
paragraph. 



Hour Angle 

I2 h 00 m 00 s 



Hour Angle 

I8 h 00 m 00 s 



*PoleStar 



HourAnle 



HourAnqle 
oo h oo m oo* 



24 h O0 m 00s 



FIG. 35. 



Note. The hour angle referred to is the 
hour angle of a (Dubhe) Ursa Majoris. 

The pole star or ' Polaris ' is situated very 
nearly at the north pole of the celestial 
concave, revolving round it about i^ degrees 

96 



CONSTELLATIONS 

from it. It can be found by drawing a line 
from a and /3 Ursa Majoris, and continuing 
it towards the pole. 

When looking for a star, it should be 
remembered that if the declination of the 
star is less than (or south of) the observer's 



*AJioth 



*Megrez *Dubhe 



y 

if Phecda 



*Merak 



FIG. 36. Ursa Majoris. (The Great Bear.) 

latitude, it will cross the meridian south of 
him ; if equal to the latitude, it will rise due 
east, pass directly overhead, and set due west ; 
if greater than (or north of) the observer's 
latitude, it will always be north of him. 

For a beginner, the best constellations to 
learn in order to connect up the other big 
stars, are the Great Bear and Orion. 

97 H 



AIR NAVIGATION FOR FLIGHT OFFICERS 




FIG. 37. Cassiopeia. (The Chair.) 



a 

*Mirfack 



y 

*Alrnach 



*Mirach 



*Alpheratz 



*Scheat 



a 

*Markab 



FIG. 38. Square of Pegasus. 

98 



CONSTELLATIONS 



y 

*Tara2ed 



*Altair 
fi I st Mag 

*Alshain 



FIG. 39. Aquila. (The Eagle.) 




FIG. 4 o. Aries. (The Ram.) 

99 



ft fcCapella l s 'Maq. 

*Menkalinam 



FIG. 41. Auriga. (The Charioteer.) 



* Nekkar 



*Muphrid 



fcArcturus 
i^t Ma. 



FIG. 42. Bootes. (The Herdsman.) 
IOO 



CONSTELLATIONS 



HtSinus 
I st Mag 



*Mirzam 



*Adara 



FIG. 43. Canis Major. (The Greater Dog.) 



ft 

*Gomeisa 



% Procyon 
I st Mag. 



FIG. 44. Canis Minor. (The 
Lesser Dog.) 

101 



AIR NAVIGATION FOR FLIGHT OFFICERS 




FIG. 45. Catus. (The Whale.) 




FIG. 46. Corona Borealis. (The 
Northern Crown. 

102 



. 



CONSTELLATIONS 




FIG. 47. Crux. (The Southern Cross.) 



a^Arided or 
Deneb 



*Albireo 



FIG. 48. Cygmis. (The Swan.) 
103 



AIR NAVIGATION FOR FLIGHT OFFICERS 



ft 
* Pollux 



*Mebsuta 



*Wasat 



*Alhena 



FIG. 49. Gemini. (The Twins.) 




FIG. 50. Leo. (The Lion.) 
104 



CONSTELLATIONS 



*Vega 
I st Maq 



*Sheliak 



Sulaphat 



Beteleux 



* Bella tnx 



& 

e *Mmtaka 
*Almlam 



TheGreat Nebula 



*Riqel 



FIG. 51. Lyra. (The Lyre.] 



FIG. 52. Orion. (The Hunter.) 



*Scheat 



* Algenib 



*Markab 



*Homan 



FIG. 53. Pegasus. (The Winged Horse.) 
105 



AIR NAVIGATION FOR FLIGHT OFFICERS 



ft 
*Nath 



The Pleiades 
?** 
Alcyone 



Aldebaran 
I^Mag. 



FIG. 54. Taurus. (The Bull.) 




FIG. 55. Ursa Minor. (The Little Bear.) 

106 




FIG. 56. Scorpio. (The Scorpion.) 




Fici 57. Corvus. (The Crow.) 
107 



AIR NAVIGATION FOR FLIGHT OFFICERS 



* * Square of Pegasus 



Schedar 

* Cassiopeia 



*Veqa 



* Pole Star 



^Menkalman 
^Capella 



Little Bear 



Corona Borealis * 



* Great Bear 



Arcturus 



* Regulus 



F IG . 5 s.^The Great Bear and the Stars it leads to. 

108 



CONSTELLATIONS 



*Castor 
* Pollux 



*Sinus 



*Canopus 



*Nath 



Aldebaran Pleiades 



Alcyone 



FIG. 59. Orion and the Stars it leads to. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Before entering into the problems con- 
nected with the sun and moon, it would be 
as well to give some explanation of the various 
terms used. 

The reader should make himself acquainted 
with the following definitions in Nautical 
Astronomy : 

Definitions. A Sphere. A sphere is a 
solid bounded by a surface, every point of 
which is equally distant from a fixed point 
called the centre. 

A Great Circle. A great circle is a section 
of the surface of a sphere, made by a plane 
passing through the centre. 

A Small Circle. A small circle is a section 
of the surface of a sphere, made by a plane not 
passing through the centre. 

Earth's Axis. The axis of the earth is 
the diameter about which it revolves with a 
uniform motion from west to east. 

Earth's Poles. The poles of the earth are 
the extremities of its axis. 

Equator. Is the great circle whose axis 
and poles are the axis and poles of the 
earth. 

Meridians. Are great circles whose planes 
pass through the poles of the earth. 

no 



DEFINITIONS 

Meridian of a Place. Is that meridian 
which passes through the place. 

Prime Meridian. Is that fixed meridian 
by reference to which the longitudes of all 
other places on the earth are measured. 

Parallels of Latitude. Are small circles 
whose planes are parallel to the plane of the 
equator. 

Longitude of a Place. Is the smaller arc of 
the equator, intercepted between the prime 
meridian and the meridian passing through 
the place. 

Latitude of a Place. Is the arc of a 
meridian intercepted behind the equator 
and the place. 

Difference of Latitude between two Places. 
Is the arc of a meridian intercepted between 
their parallels. 

Difference of Longitude between two Places. 
Is the smaller arc of the equator intercepted 
between their meridians. 

Celestial Concave. Is the interior surface 
of a globe bounded by the blue of space, and 
on which all the heavenly bodies appear to be 
situated. 

Poles of the Heavens. Are the points 
where the earth's axis produced, cuts the 
celestial concave. 

in 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Ecliptic. Is the apparent path of the sun 
during the year on the celestial concave. 

Equinoctial or Celestial Equator. Is the 
great circle formed by the plane of the earth's 
equator produced, cutting the celestial 
concave. 

The Equinoctial Points. Are the two 
points on the celestial concave where the 
ecliptic and the equinoctial cut one another. 
One is known as the First Point of Aries 
(the point on the ecliptic where the sun's 
declination changes from south to north), 
the other as the First Point of Libra (the 
point on the ecliptic where the sun's declina- 
tion changes from north to south). 

Circles of Declination. Are great circles 
which pass through the poles of the heavens ; 
they correspond to terrestrial meridians. 

Parallels of Declination. Are small circles 
whose planes are parallel to the plane of the 
equinoctial. 

Declination. Is the arc of a circle of 
decimation intercepted between the equi- 
noctial and the place of the body. It is 
thus similar to latitude on the earth. It is 
measured north and south of the equinoctial 
from o at the equinoctial to 90 at each 
celestial pole. 

112 



DEFINITIONS AND TIME 

Polar Distance of a Heavenly Body. Is the 
arc of a circle of declination through the body 
intercepted between the elevated pole and 
the body, and is therefore (90 - dec.) or 
(90 + dec.) according as the declination is 
of the same or opposite name to the latitude. 

N.B. The elevated pole is that one 
situated in the same latitude as the observer. 

Right Ascension. Is the arc of the equi- 
noctial intercepted between the First Point of 
Aries and the Circle of Declination which 
passes through the body, measured anti- 
clockwise from o h to 24 h . 

Notes on Time. As time plays a very 
important role in the sun and moon problems, 
a few notes on the subject are given here 
before going into the problems. 

This should be thoroughly studied and 
understood ; by doing so, half the difficulty of 
working out the problems is done away with 
in fact more than half. 

Time may be divided into two sorts Civil 
and Astronomical. 

Civil Time is divided into two periods 
called A.M. (ante meridiem], and P.M. (post 
meridiem). 

Each of these is a period of twelve hours, 
113 i 



AIR NAVIGATION FOR FLIGHT OFFICERS 

the A.M. time being from midnight to noon, 
and the P.M. time from noon to midnight. 




FIG. 60. The Celestial Concave. 

ECKQ is the equinoctial. 

FBHG is a parallel of declination. 

ABCD and AHKD are circles of declination. 

LYM is the ecliptic Y the First Point of Aries. 

HK is the declination of the body H. 

YK is the right ascension of the body H. 

114 






TIME 

The civil day and date commences at 
midnight and ends the following midnight. 

Astronomical Time is reckoned in one 
period of twenty-four hours, the day and date 
commencing at noon and changing the follow- 
ing noon. 

From this it will be seen that the civil date 
is always twelve hours ahead of the astro- 
nomical date, i.e. the former begins at mid- 
night and the latter the following noon. 

When working problems in time, it must 
be remembered that twenty-four hours can 
always be added to any time, provided that 
the date is placed one day back. 

Examples : 

4h oom oos on May 4, can, if necessary, be 
shown as 28 h oo m oo on May 3. 

2o h oo oo s on June 17, can be shown as 
44 h oo m oos on June 16. 

Civil time can always be converted into 
astronomical time, and vice versa, remembering 
that civil date is always twelve hours ahead 
of astronomical date. 

Examples : 

Civil time 4 A.M. March 30. 

Astronomical time, i6 h oom oo March 29. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Civil time, n P.M. March 30. 
Astronomical time, n h oo m oos March 30. 

Civil time, n^ A.M. March 30. 
Astronomical time, 23 h oo m oos March 29. 

From No. 2 of the above examples it will be 
noticed that civil time and astronomical time 
are identical in date during P.M. civil time, but 
whilst the civil date changes at midnight, the 
astronomical date goes on for another twelve 
hours. 

Examples : 

Astronomical time, 15^ oom oos July 12. 

Civil time, 3^ A.M. July 13. 

Astronomical time, 10^ oo m oo 8 July 12. 
Civil time, io h P.M. July 12. 

Time is divided into two kinds Apparent 
Solar Time and Mean Solar Time. 

Apparent Time. Is the actual time shown by 
the sun, but owing to the elliptical shape of the 
earth's orbit, the apparent proper motion of the sun 
is not uniform, so that the apparent solar day, hour, 
minute, and second are not quite of constant length. 

Mean Sun. Is an imaginary sun which moves 
in the equinoctial with the apparent sun's mean 
motion in R.A. 

Mean Time. This is the time shown by an 
116 



EFFECT OF LONGITUDE ON TIME 

imaginary sun whose motion is uniform in velocity, 
along the ecliptic, and to which our clocks are set. 

The velocity of the apparent sun not 
being uniform, it follows that it will be some- 
times ahead of the mean sun and sometimes 
behind it. 

This difference is called the ' Equation of 
Time/ and is given in the Nautical Almanac 
for every two hours of the day throughout the 
year. In problems connected with the sun's 
bearing, the times given in the true bearing 
or azimuth tables are all apparent times, 
so that it is necessary to change the time by 
watch into apparent time. 

The Effect of Longitude. The way in 

which the longitude of a place on the earth's 
surface affects the time of that place, should 
be clearly understood, as it helps to give one 
a firm grasp on the problems later on. 

The earth revolves from west to east in 
twenty-four hours, but it is more convenient 
to imagine the earth as stationary, and the 
celestial concave revolving from east to west 
about its own axis. 

This comes to the same thing. 

Consequently, as we measure our noon by 
the sun's passage across the meridian of our 

117 



AIR NAVIGATION FOR FLIGHT OFFICERS 

place, it follows that the sun must have 
already crossed the meridian of any place to 
the eastward of our position, and not yet 
crossed the meridian of those places to the 
westward of us. 

In all our charts, the meridian passing 
through the transit instrument at Greenwich 
Observatory is taken as the prime meridian, 
from which all our measurements for the 
longitude of other places are made ; hence the 
mean time of all places to the eastward of 
Greenwich is ahead of Greenwich mean time 
(or G.M.T. as it is usually called), and the 
mean time of all places to the westward of 
Greenwich is behind G.M.T. 

As the revolution of the earth from noon 
to noon, at any place, occupies twenty-four 
hours for an angular value of the circumfer- 
ence of a circle, or, in other words, 360, it 
follows that longitude may also be expressed 
in time. 

Example : 

Arc. Time. 

h. m. s. 

360 %' . . . <, . 24 oo oo 

180 . . . ; . . 12 oo oo 

90 . ' <j : .". . V 6 oo oo 

15 . ,"' , ; . i oo oo 

i . . . . . o 04 oo 

118 



EFFECT OF LONGITUDE ON TIME 

As the meridian of every place is different, 
local time must differ at any place from 
any other place, and would cause endless 
trouble as regards setting clocks. 

Consequently, different countries adopt 
what are known as ' Standard Meridians/ 
and all clocks in that country are set to 
the time of that standard meridian. 

In the United Kingdom, except Ireland, 
the standard meridian is that of Greenwich 
Observatory, as mentioned before, and all 
clocks are kept set to it. N.B. Ireland 
now keeps G.M.T. 

N.B. Since writing this, summer time 
has been introduced by Act of Parliament. 
By this, clocks are put on one hour on May i 
at midnight, and are put back at midnight 
on September 30. 

An easy rule to remember how to apply 
longitude in time is given in the old rhyme : 

Longitude west, Greenwich time best, 
Longitude east, Greenwich time least. 

In connection with time, it is interesting 
to understand what happens to the day and 
date when crossing the i8oth meridian. 

This is explained below. 

On leaving the prime meridian, and 
119 



AIR NAVIGATION FOR FLIGHT OFFICERS 

steering east, the ship's local time gradually 
gets ahead of Greenwich time, until in 180 
she is just twelve hours in front. 

Continuing to the eastward, she imme- 
diately enters a longitude which is twelve 
hours behind Greenwich, so that she must 
count that day and date over again. 

For instance, supposing she crossed the 
iSoth meridian at 9 P.M. on August 14, going 
east. 

When she got to the iSoth meridian it 
would only be 9 A.M. on August 14 at Green- 
wich, and, continuing her course, she would 
at once be twelve hours more behind Green- 
wich, i.e. 9 P.M. on August 13. 

Hence her next day and date must again 
be reckoned as August 14. 

On the other hand, suppose she sailed from 
the meridian of Greenwich, going west about. 

She would gradually get more and more 
behind Greenwich time, until at the i8oth 
meridian she would be twelve hours late. 
On crossing to the westward of this meridian, 
she would at once get twelve hours ahead of 
Greenwich time, therefore she must skip a 
day altogether. 

For instance, supposing she crossed the 
iSoth meridian, going west, at 9 P.M. August 14. 

120 



HOUR BANGLES 

When she was there, it would be 9 A.M. 
August 15 at Greenwich, and, on going 
farther west, she would be twelve hours ahead 
of this latter date, so that it would be 9 P.M. 
on August 15. 

Hence she must skip the i5th altogether, 
and call the next day the i6th. 

Hour Angles. By the term ' Hour Angle/ 
is meant the angular distance of a body from 
the observer's meridian expressed in time, 
either before or after its meridian passage. 

By ' Meridian Passage ' is meant the cross- 
ing of the body over the meridian of the 
observer. 

All heavenly bodies rise to the eastward 
of the observer, and after a certain time attain 
their greatest altitude above the horizon this 
occurs when the body is on the observer's 
meridian ; they then decline in altitude, and 
finally set in the westward. 

This meridian passage is known as the 
' Upper Meridian Passage/ 

Their lower meridian passage takes place 
twelve hours later in the case of the sun ; 
slightly under (3 56* ) twelve hours in the 
case of a star ; and an average of I2 h 24 in 
the case of the moon. 

121 



AIR NAVIGATION FOR FLIGHT OFFICERS 

When we talk of a body being so many 
hours away from, the meridian, this does not 
mean any A.M. or P.M. time : it is simply a 
measure of time from its meridian passage. 
If we want to know the local time when a body 
is, say, three hours from its meridian passage, 
we must find out the sun time of the body 
crossing the meridian and apply these three 
hours to this latter time. 

The reason for this is because the meridian 
passage of a body is reckoned by mean sun 
time, and so, to get the time of rising or setting 
of any body other than the sun, we must first 
find the sun time* of the body's meridian 
passage and then apply its hour angle from 
the meridian when on the horizon. 

This hour angle must, of course, be sub- 
tracted from the time of meridian passage for 
rising, because it must rise before it comes to 
the meridian, and be added to the time of 
meridian passage for setting, as it sets after 
crossing the meridian. 

With reference to the times shown in the 
sun's true bearing tables of sunrise and 
sunset, it must be remembered that as we 
count our civil day as beginning at midnight, 
so the actual A.M. time of sunrise, as given 
in the tables, is counted from the inferior 

122 



EXPLANATION OF NAUTICAL TABLES 

meridian ; so that, to get the actual hour 
angle of the sun from the superior meridian, we 
must subtract the A.M. time from twelve hours. 
This is, of course, not necessary in the 
P.M. time, as our afternoon time is measured 
from the superior meridian. The sun's hour 
angle, both from the inferior meridian to the 
superior meridian (A.M. time), and from the 
superior meridian to the inferior meridian 
(P.M. time) is, of course, apparent time. 

Explanation of the Various Tables. (i) 

Nautical Almanac. This is a work published, 
giving all the data necessary for navigation 
by the sun, moon, planets, and stars. These 
data are given for every day in the year. It is 
published in two forms an extended form, 
and an abridged form for the use of seamen. 

The latter should be used in all problems 
of rising and setting, and is also sufficient for 
all problems in navigation. 

On the first two pages of every month are 
given all the data for the sun and moon. 

Each column indicates the contents of 
that column, so that there should be no 
difficulty in taking out what is wanted. 

The only column that needs any explana- 
tion is the one headed 'Equation of Time/ 

123 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Here it gives, at the top of the page, 
instructions as to whether the equation of 
time is to be added to, or subtracted from, 
apparent time. In either case, if the equa- 
tion of time is to be applied to mean time, 
the instructions must be reversed e.g., sup- 
pose the instructions say the equation of time 
is to be added to apparent time, then it 
must be subtracted from mean time. 

Again, it sometimes happens that there 
is a black line drawn both in the instructions 
and in the column giving the values. 

This simply means that all the values in the 
column above the black line follow the instruc- 
tions above the upper black line, and those 
below follow the instructions below the upper 
black line. 

The next few pages give the data for the 
sun for every day of the month at two-hour 
intervals of Greenwich mean time (G.M.T.). 

After that comes the same thing for the 
moon, and these must be made use of when 
getting moonrise or moonset. In the case of 
finding sunrise or sunset, it will be near enough 
to take the declination out for noon at Green- 
wich, as it does not alter enough in twenty- 
four hours to have any practical effect on the 
accuracy of the problem ; but in the case of 

124 



EXPLANATION OF NAUTICAL TABLES. 

the moon, her declination changes so rapidly, 
that a few hours may make an appreciable 
difference in the result. 

On p. 160 of the Abridged Nautical 
Almanac will be found a table giving the 
hour angle of a body from the meridian when 
rising or setting. 

Running right across the top of the two 
pages are the degrees of declination from 
o to 30. 

Down the left hand side of each page, and 
printed in thick type, are the degrees of lati- 
tude from o to 60. In the body of the table 
are the hour angles. 

To look out an hour angle, all that has 
to be done is to enter the table with the 
latitude of the place and the declination of 
the body. 

Under the latter, and opposite the former, 
will be found the required hour angle. 

The following rule is important and 
should be well learnt : 

' If the latitude and declination are the 
same names, i.e. both north or both south, 
the hour angle can be taken straight from the 
tables ; but if they are different names, i.e. one 
north and the other south, the hour angle 
found in the tables must be subtracted 

125 



AIR NAVIGATION FOR FLIGHT OFFICERS 

from twelve hours, and the result used 
instead/ 

On p. 170 is a table of proportional parts 

which must be applied to the hour angle as a 

final correction. Across the top of the page 

are certain numbers, in the case of the moon 

these correspond with the daily difference as 

given in the column after the upper meridian 

passage. Running down the right hand side 

of each page are times ranging to twelve hours. 

The final correction is simply a rule-of- 

three sum, which is given in this table. 

1 If the difference in twenty-four hours is 
so much, what will it be for an hour angle of 
so much ? ' This hour angle being the one 
just found. 

This final correction is always additive 
to the hour angle, as the moon crosses the 
meridian of any place later every day. 

(2) Inman's Tables. On p. 116 will be 
found a table for the correction of the moon's 
meridian passage, depending on the ]ongitude. 
The rule for adding or subtracting it is given 
on the top of the page. 

Just below it, and running right across the 
page, is a row of thick figures, which represent 
the daily difference of the moon's meridian 
passage which is given in the Nautical 

126 



EXPLANATION OF NAUTICAL TABLES 

Almanac in the column next to the moon's 
upper meridian passage. 

Running down either side of the page is a 
column showing the longitude of the place, 
and in the body of the table is the correc- 
tion to be applied. This correction is given 
in minutes of time. 

This table is merely a worked out rule-of- 
three sum. 

'If the daily difference for 360 is that 
given in the Nautical Almanac, what is it for 
the longitude of the place ? ' Instructions 
whether to add or subtract it are given at 
the top of the table. 

Haversine Table. This table is of great 
use in giving the longitude in time any- 
where. All that has to be done is to look 
up the longitude and take out the correspond- 
ing time shown at the top of the page and 
also down the sides. 

Sun's True Bearing or Azimuth Tables 
(Davis and Burdwocd). These are printed 
for a limit* of latitude of 60 north and south, 
and a limit of declination of 23 north and 
south, this latter being approximately the 
farthest limits of the sun's apparent motion 
north or south. 

The times shown are, as the sun itself 
127 



AIR NAVIGATION FOR FLIGHT OFFICERS 

is actually observed, apparent time : the 
intervals as given in the tables are four 
minute ones. 

It should be noticed that the A.M. times 
run up the left hand side of each page, and the 
P.M. times run down each page on the right 
hand side. 

With reference to the A.M. time of rising, 
it should be remembered that the time given 
for rising is counted from the inferior or mid- 
night meridian, and therefore, to get the hour 
angle from the noon or superior meridian, the 
value given in the tables must be subtracted 
from twelve hours. This is not necessary for 
the P.M. hour angle, as P.M. is counted from 
the time that the sun crosses the superior 
meridian. 

All the bearings given in the tables are 
for the sun's centre. 

In both sun and star tables, the rules for 
naming the bearing are the same in principle, 
as the statement ' When apparent time is A.M.' 
means exactly the same thing as ' When the 
body is rising or east of the meridian/ and 
similarly for P.M. 

In the sun tables, each degree of latitude 
appears over two separate headings, one 
when latitude and declination are the same 

128 



SUNRISE PROBLEM 

name, i.e. both north or both south, and the 
other when they are opposite names, i.e. one 
north and the other south. Care should be 
taken not to confuse the two. 

At the end of every degree of declination 
is given the apparent time of rising and 
setting, and the true bearing of the body. 

The star tables (Davis') are the same in 
principle as the sun tables, except that instead 
of the apparent time being given, the hour 
angle of the star is shown. 

We now come to the examples of sunrise 
and sunset, and moonrise and moonset, 
which are appended. In practice it is not 
necessary to work rigorously, so the declina- 
tion and equation of time may be taken out 
at sight. 1 The elements, if taken out exactly, 
only add to the time in working out without 
any compensating advantages, and make no 
practical difference to the answer. 

Sunrise Problem 

Example i. Find the Greenwich mean time of 
sunrise and sunset and the true bearing at each 
time, in Latitude 50 N., Longitude 8 E., on 
May 7, 1916. 

From Abridged Nautical Almanac, p. 50, on 

1 In the case of the sun to the nearest noon, but with the 
moon to the nearest hour. 

129 K 



AIR NAVIGATION FOR FLIGHT OFFICERS 



May 7, we get : ' Sun's declination i6f north. 
Same name as latitude.' 

P. 207, pt. ii. of sun's true bearing 
tables, with Lat. 50 N. (same name as 
declination), under the columns headed 16 
and 17, we get by interpolation as follows : 



Rising. 



h. m. s. 



Apparent time of 

rising at place . 4 36 oo 
Equation of time, 

p. 50, Naut. Aim. . 3 30 



Mean time of rising 

at place . . 4 32 30 
Longitude in time . o 32 ooE. 



G.M.T. rising A.M. 4 oo 30 

Bearing. 
From p. 207, 
sun's true bear- 
ing tables, for 
Lat.5oN.,Dec. 
i6f N we get 
by interpolation 63 23' oo* 



Setting. 



h. m. s. 



Apparent time of 

setting at place. 7 24 oo 
Equation of time, 

p. 50, Naut. Aim. 3 30 



Mean time of set- 
ting at place . 7 20 30 
Longitude in time . o 32 ooE. 



G.M.T. setting P.M. 6 8 30 

Bearing. 
From p. 207, 
sun's true bear- 
ing tables, for 
Lat.50N.,Dec. 
i6|^N. we get 



63 23' oo' 
360 oo' oo' 



296 37' 



Therefore the answer to the problem is 
that the 

Sun rises at 4 h oo m 30 s A.M., bearing 
63 23' o". 

130 



SUNRISE PROBLEM 



Sun sets at 6 h 48 30 s P.M., bearing 
296 37' oo". 

These of course are true bearings ; should 
magnetic bearings be required, the variation 
must be applied. 

If local mean time be required, of course 
the longitude in time would not be applied. 

Example 2. Find G.M.T. of sunrise and sunset 
and the true bearing at each time in Latitude 40 S., 
Longitude 10 W., on August 5, 1916. 

From Nautical Almanac : ' Sun's declination is 
17 north, i.e. contrary name to latitude/ 



h. m. s. 



Rising. 

Apparent time at 

place . . 6 56 oo 
Equation of time -J- o 06 oo 



Mean time at place 7 02 oo 



Long, in time W-{- o 40 < 

G.M.T. rising A.M. 7 42 oo G.M.T. setting P.M. 5 50 oo 



h. m. s. 



Setting. 

Apparent time at 

place . . 5 04 oo 
Equation of time . + o 06 oo 



Mean time at place 5 10 oo 
Long, in time W-j- o 40 oo 



Bearing. 

S. in 05' E. 

i.e. 68 55' from rule given 
before. 



Bearing. 
S. in 05' W. 

i.e. 291 05' from rule given 
bef ore. 



Examples on Moonrise Problem 
Example I. Find the time of moonrise and 



AIR NAVIGATION FOR FLIGHT OFFICERS 



moonset and the moon's true bearing at each in 
Latitude 40 N., Longitude 70 W., on June 5, 1916. 



From Naut. Aim., 
p. 63, moon's mer. 
pass at upper 
transit on June 5 

Correction in In- 
man's tables, 



h. m. s. 

3 43 oo 



p. 116 



. -}- o 09 oo 



Corrected local 

mean time of 

passage . . 3 52 oo 
Longitude in time . -j- 4 4 

Rough G.M.T. of 

passage, June 5 . 8 32 oo 
Moon's hour angle 6 25 oo 

Rough G.M.T. of 

rising, June 5 . 2 07 oo 
Rough G.M.T. of 

setting, June 5 . 14 57 oo 



Daily difference in 
next column to 
upper transit 



44 1 



Moon's declination for 

8 h 32 m G.M.T. on 

June 5 . . 7i N. 

Same name as Latitude. 

From p. 160, Naut. Aim., 
with Lat. 40 N., Dec. 7^ N. 
Moon's hour angle is 6 h 25 oo s 
This means that the moon, 
when rising and setting, is 
6 h 25 m away from the time of 
her passage over the observer's 
meridian as shown by sun mean 
time; 



Owing to the rapid change in the moon's 
decimation, the example must now be re- 
worked, using the rough G.M.T. times of rising 
and setting to get the declinations. 



Rising. 

Moon's dec. for 2 h 07, 
G.M.T., Junes . i8N. 



Setting. 
Moon's dec. for i4 h 57, 

G.M.T., June 5 . i5fN. 



Both same name as latitude. 
132 



MOONRISE PROBLEM 



Rising. 
Hour angle from 

p.i6o,Naut.Alm., 

with Lat. 40 N., h. m. s. 

Dec. 18 N. . 7 03 oo 
Correction p. 170, 

Naut. Aim., with 

daily difference 

44 m and hour 

angle y h 03 -j- 13 oo 

Corrected hour 

angle . . 7 16 oo 

Corrected local 

mean time of 

passage, June 4 . 27 52 oo 



Moon rises June 4 . 20 36 oo 
I.e.moon rises Junes, 

civil time . A.M. 8 36 oo 



* Setting. 

Hour angle from 
p.i6o,Naut.Alm., 
with Lat. 40 N., 
Dec. I5f N. 

Correction p. 170, 
Naut. Aim., with 
daily difference 
44 m and hour 
angle 6 h 55 

Corrected hour 
angle 

Corrected local 

mean time of 
passage, June 5 . 



h. m. 

6 55 



+ 13 oo 



7 08 oo 



3 52 oo 



II 00 OO 



Moon sets June 5 
Moon sets June 5, 

civil time P.M. n oo oo 



Moon's Bearing. 
With Lat. 40 N., 
Dec. i8N., same 
name as lat. 
true bearing from 
tables . . 66 13' oo" 



Moon's Bearing. 
WithLat. 4 oN., 

Dec. I5f N., 69 15' oo" 
same name as 360 oo' oo" 

lat. true bear- 

ing from tables 290 45' oo" 



Note. In the second part of the foregoing 
problem, it should be noticed that under the rising 
heading the corrected local mean time of passage 

133 



AIR NAVIGATION FOR FLIGHT OFFICERS 



has been given as 2j h 52 oo s . This simply means 
that 2/|. h has been added on to the original 
3 h 52 m oo s ., as 7 h i6 m oo s has to be subtracted 
from it. By adding 24 h to it, the date has, of 
course, to be placed one day back. 

Example 2. Find the time of moonrise and 
moonset and the true bearing at each time in 
Latitude 50 S., Longitude 100 E., on October 12, 
1916. 



From Naut. Aim., 

p.ui,moon'smer. 

passage at upper h. m. s. 

transit on Oct. 12 13 04 oo 
Correction in In- 

man'stables,p.n6 oo 14 oo 



Corrected local 

mean time of 

passage, Oct. 12 . 12 50 oo 
Longitude in time . 6 40 oo 



Rough G.M.T. of 

passage, Oct. 12 . 6 10 oo 
Moon's hour angle. 4 26 oo 

Rough G.M.T of 

rising, Oct. 12 . i 44 oo 
Rough G.M.T. of 

setting, Oct. 12 . 10 36 oo 



Daily difference 
next column 
upper mer. pass 



in 
to 



52 ] 



Moon's declination 
for G. M. T. of 
6 h io m oo s Oct. 12 



N- 



Opposite name to latitude. 

From p. 160, Naut. Aim., 
with Lat. 50 S., Dec. i8 N. 
Hour angle is y h 34. 

' Subtract this from i2 h , 
because lat. and dec. are 
opposite names. 

Moon's hour angle from 
meridian is therefore 4 h 24 oo s . 



Re-working the problem as in Example i, 
we get as follows : 



MOONRISE PROBLEM 



Rising. 



Moon's dec. for i 1 
G.M.T. Oct. 12 



44* 



N. 



Setting. 

Moon's dec. for io h 36' 
G.M.T. Oct. 12 



Both opposite name to latitude. 



Hour angle from 
p.i6o, Naut.Alm., 
with Lat. 30 E., 
Dec. i8N. 

Subtract from I2 h . 

Moon's hour angle . 

Correction p. 170, 
Naut. Aim., with 
daily difference 
52, and hour angle 



h. m. s. 

7 31 oo 

12 OO OO 

4 29 oo 



4 h 29' 



oo s . 



IO OO 



Corrected hour 

angle of moon . 4 39 oo 
Local mean time of 

passage, Oct. 12 . 12 50 oo 



Moon rises Oct. 12. 8 n oo 
I.e. civil time 

Oct. 12 . P.M. 8 ii oo 

Moon's Bearing. 

With Lat. 50 S., 
Dec. 1 8 N., op- 
posite name to 
lat. bearing from 
tables . ".' 1 1 8 44' 

Subtract fro mi 80 . 180 oo' 

Moon's true bearing 61 16' 



Hour angle from 
p.i6o, Naut.Alm., 
with Lat. 50 S., 
Dec. I9i N. 

Subtract from I2 h . 

Moon's hour angle . 

Correction p. 170, 
Naut. Aim., with 
daily difference 
52, and hour angle 

h 20 m OO 3 . 



h. m. s. 

7 40 oo 

12 OO OO 

4 20 oo 



-j- 10 00 



Corrected hour 

angle of moon . 4 30 oo 
Local mean time of 

passage, Oct. 12. 12 50 oo 



Moon sets Oct. 12 . 17 20 oo 
I.e. civil time 

Oct. 13 . A.M. 5 20 oo 

Moon's Bearing. 

With Lat., 50 S., 
Dec. 19^ N., op- 
posite name to 
lat. bearing from 
tables . '. 121 17' 

Add 180* . , 180 oo' 

Moon's true bearing 301 17' 



135 



AIR NAVIGATION FOR FLIGHT OFFICERS 

It cannot be too often stated that if the 
latitude and declination are of opposite names, 
the hour angle found on pp. 160-1 of the 
Abridged Nautical Almanac must be sub- 
tracted from twelve hours, and the result 
substituted. 

The declination of the moon may go up as 
high as 29 on either side of the equator, so 
that after 23 the sun tables are not available ; 
in this case the star tables may be used, using 
vols. i or 2 according to the latitude. 

The principle of looking out the bearings 
is exactly the same as in the sun tables, or 
the amplitude tables in Inman's may be used. 

In the left-hand column of the star tables 
will be found the body's hour angle that 
is, its angular distance from the meridian 
expressed in time. 

Opposite the hour angle and under the 
declination of the body, will be found the 
true bearing. 

These bearings are, however, only given 
for when the body is some degrees above the 
horizon, consequently interpolation will be 
necessary. 

As the rate of change of the bearing of 
a body varies with its altitude, declination, 
and position of the observer, this interpola- 

136 



STAR TABLES 

tion will not be rigorously exact, but for 
compass work it will be quite near enough. 

The rule for naming the bearing is, like 
in the sun tables, given at the foot of each 
page. 

Example. Lat. 40 N., Dec. 30 N. Required 
true bearing at rising and setting. 

By table on pp. 160-1 of Nautical Almanac 
the body's hour angle when rising and setting is 
7 h 56 m oo s . 

On p. 33 of Davis' star tables, for 40 
lat. and under 30 dec., same name, the first 
bearing given after the body is above the 
horizon, is for an hour angle of 7 h 50 s , and 
is 5o'2. 

Now the change between the bearing for 
7 h 40 and 7 h 50 is i'6, and bearing 
increasing. 

Therefore the change for six minutes is 
A of i-6. 

That is to say, the change is -96 of a degree, 
say i degree. Therefore the bearing at rising 
and setting will be 5o'2 i*o., i.e. 49'2, 
named according to the rule at the foot of the 
page. So that bearing at rising will be 49'2, 
and at setting 36o-oo 49*2., i.e. 3io'8. 

As stated before, these bearings are not 



AIR NAVIGATION FOR FLIGHT OFFICERS 

absolutely exact, but are near enough for 
compass work. 

In connection with these bearings, there 
is another method of looking out the true 
bearings at rising and setting. This can be 
done by means of the ' Amplitude Table ' 
given on pp. 138-41 of Inman's tables. 

Before explaining the tables, it may be 
as well to state that an amplitude is merely 
the bearing of the body when rising or setting, 
reckoned from the east or west point according 
as to whether the body is rising or setting. 

It differs from an azimuth, inasmuch as 
the latter is reckoned from the north point, and 
the amplitude only applies to a body when on 
the horizon. 

Running across the top of the pages are 
the degrees of declination, and down the sides 
are the degrees of latitude from i to 64. 

Under each degree of declination are two 
columns, one headed ' Time Amp/ and the 
other ' Bearing Amp/ 

The time amplitude is merely an interval 
of time to be added to or subtracted from 
6 h oo m oo s , which will give the hour angle of 
the body from the superior meridian expressed 
in time. 

Whether it should be added or subtracted 
from 6 h oo m oo s depends on whether the 

138 



AMPLITUDES 

latitude and declination are of the same or 
opposite names. 

If they are of the same name, the hour 
angle of rising must be greater than 6 h oo m oo s ; 
and if they are of opposite names, the hour 
angle of rising must be less than 6 h oo m oo s . 

Similarly in the case of setting. 

Therefore, if the latitude and declination 
are of the same name, the time amplitude 
found in the tables must be added to 6 h oo m oo s ; 
and if they are of different names, it must be 
subtracted from 6 h oo m oo s . 

With regard to the bearing amplitude, if 
latitude and declination are of the same name, 
the body must rise north of the east and west 
line, and also set north of it. If they are of 
opposite names, the body must rise south 
of the east and west line and set south of 
it. This will at once show which way the 
bearing amplitude should be applied to the 
east or west point. 

This paragraph refers to north latitude; 
for south latitude, if latitude and declination 
are the same names, the body will rise 
south of the east and west line and also 
set south of it, whilst if latitude and declina- 
tion are of opposite names, the body will 
rise north of the east and west point and set 
north of it. 

139 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Taking the example given for the star 
tables, it is proposed to work it out by the 
amplitude table as well. 

Example. Latitude 40 N., Declination 30 N. 
Find hour angle of body when rising and setting, 
also true bearing at each time. 

h. m. s. 

P. 141, Inman's tables, under 30 and opposite 

40, time amplitude is . . . i 56 oo 

Lat. and dec. being same names, add 6 h oo m oo s -{- 6 oo oo 



Time amp. or hour angle of body from superior 

meridian on rising 7 56 oo 

Which agrees with the hour angle given 

in the last example. 
P. 141, Inman's tables, under 30 and opposite 

40, bearing amp. is . . . . 40 - 8 

[As lat. and dec. are same names, this 

will be north of the east point.] 
East point ... . . . . 90 -o 



Bearing of body . . . . . 49 -2 

Which agrees with bearing given in the last 
example. 

Similarly, the setting hour angle will be 
6 h oo m oo s + i h 56 oo s , which gives 7 h 56 oo 
from the meridian. And the bearing will be 
4O*8 north of the west point, which gives a 
bearing of 270 oo' oo" -f 40'8 or 3io'8. 
Or 90 0< o 40-8 = 49% west of the north point 

140 



AMPLITUDES 

and36o-o 49*2 = 3io'8. This agrees with 
the bearing given in the first example. 

The east and west points are reckoned as 
being 6 h oo m oo s in time, and 90 in arc away 
from the north and south points. 

The following figures may be of assistance 
for amplitude work. 




FIG. 61. North Lat. Lat. 
and Dec. same name. 

Body rises at X and 
sets at. X', XZE and 
X'ZW are the ampli- 
tudes from the tables, 
and X and X' must be 
north of the east and 
west line, so that the 
hour angles SZX and 
SZK' are greater than 
90, i.e. greater than 
6hoo m oos, so that amps, 
must be added. 




FIG. 62. North Lat. Lat 
and Dec. opposite names. 

Body rises at X and 
sets at X', XZE and 
X'ZW are the ampli- 
tudes from the tables, 
and X and X' must be 
south of the east and 
west line, so that the 
hour angles SZX and 
SZX' are less than 90, 
i.e. less than 6 h oo m oo, 
so that amps, must be 
subtracted. 



141 



AIR NAVIGATION FOR FLIGHT OFFICERS 

N N 




FIG. 63. South Lat. Lat. 
and Dec. same name. 

Body rises at X and 
sets at X'. Same re- 
marks as in Fig. 61 apply. 



FIG. 64. South Lat. Lat. 
and Dec. opposite names. 

Body rises at X and 
sets at X'. Same re- 
marks as in Fig. 62 apply. 



142 



CHAPTER X 
CHART WORK 

Admiralty Charts. These are of two kinds 
the ' Gnomonic ' and the ' Mercator's/ 

The gnomonic projection is used for plans 
of harbours, where the scale of the chart 
exceeds two inches to the mile, for charts 
above the latitude of about 70 north and 
south, and for polar charts. 

It is constructed on the following principle : 




AIR NAVIGATION FOR FLIGHT OFFICERS 

The observer is supposed to be situated 
at C, the centre of the earth, which is sup- 
posed to be transparent so that he can see 
the surface. 

A is the central point of the part to be 
surveyed, and from this point a tangent DAB 
is drawn to the earth's surface. This point 
A is known as the ' Point of Tangency/ 

The arc GAE of the earth's surface is the 
part to be surveyed, and lines CG, CA, CB 
are drawn, produced if necessary, to cut the 
tangent to the earth's surface at D, A, and B 
respectively. 

Hence the arc GAE will be represented by 
the straight line DAB. 

Reference to the figure will show that CA 
being at right angles to the line DAB, the 
observer is looking directly at A, and at any 
other point on this line he will be looking 
more and more obliquely as D and B, the 
extremities, are approached, the maximum 
being at the points D and B. 

Hence at A there will be no distortion, but 
this will increase all round on leaving A, 
reaching a maximum at the edges of the chart. 

The amount of distortion of the arc GAE 
will be represented approximately by the 
amount DH-BF. 

144 



MERCATOR'S CHART 

The plan of a harbour, representing as it 
does such a very small portion of the earth's 
surface, has practically no distortion ; but in 
a polar chart, embracing as it does a big 
area, may have a considerable amount. 

The Mercator's Chart. This principle is 
used for general charts, coasting sheets 
and between the limits of about 70 north 
and south. After about 70 the distortion 
becomes so rapid and excessive that its 
use is prohibitive. 

The principle of construction is as follows ; 




FIG. 66. 



\ 



FIG. 67. 



In Fig. 66 imagine a cylinder of paper 
ABCD to be wrapped round a flexible globe 

145 L 



AIR NAVIGATION FOR FLIGHT OFFICERS 

marked with the meridians and parallels, so 
that it is touching along every point round 
EQ, the globe's equator. 

If the globe be now blown out until every 
point on its surface touches the cylinder, and 
the latter be then removed and laid out flat, 
it will be found that all the meridians and 
parallels are represented by straight lines at 
right angles to one another. 

Fig. 67 gives a section of the earth from 
pole to equator. 

It will be readily seen that along the line 
EQ, or, in other words, along the equator, there 
has been no distortion, as the cylinder was 
already touching the globe. 

As one goes towards either pole, it will be 
seen from the figure that the parallel of lati- 
tude DE has been expanded to the length 
CF, and the parallel HK to the length GL. 
As the parallel HK is less than DE, and as GL 
and CF are equal to one another, it follows 
that HK must have been expanded a greater 
amount than DE. 

Hence, as the poles are approached, the 
expansion must get greater and greater. 

As AB, GL, CF, and EQ are all equal, the 
degrees of longitude on a Mercator's chart 
must be represented by parallel straight 

146 



MERCATOR'S CHART 

lines, and therefore they must be all equal to 
one another. 

The degrees of longitude having been 
expanded on an increasing scale as the poles 
are approached, the proportion of the chart 
must be preserved by expanding the degrees 
of latitude in the same proportion as the 
degrees of longitude have been. 

And as this expansion becomes greater 
as the latitude is increased, the degrees of 
latitude will become larger and larger from 
the equator to the north and south. 

For this reason, when measuring distance 
on a Mercator's chart, the latitude scale 
should always be used, and if the two places 
are far apart in latitude, the mean of middle 
latitude must be taken as the measuring 
point. 

Theoretically, a Mercator's chart can be 
constructed nearly up to the pole itself, but 
the construction fails here, because the pole, 
being a point, has, according to Euclid, no 
parts and no magnitude, and would therefore 
have to be expanded to infinity. 

In practice, Mercator's charts are not 
constructed for a higher latitude than about 
70 north or south, as after that the distortion 
increases very rapidly, and the degrees of 

147 



AIR NAVIGATION FOR FLIGHT OFFICERS 

latitude get so very long, that the chart would 
become unwieldy owing to its size. 

A Mercator's chart is constructed accord- 
ing to the following method, of which an 
example is now given. 

Supposing it is required to construct a 
Mercator's chart on a scale of ' x ' inches to 
a degree of longitude, between certain limits 
of latitude and longitude. 

The only table required is the one in 
Inman's tables, called ' Meridional Parts/ 

This table merely gives the distance 
represented on a Mercator's projection, of any 
distance from the equator, instead of the 
true one. 

For instance : 

Latitude 50, 50 x 60' = 3000', i.e. the 
parallel of 50 is 3000' from the equator. 
The table of meridional parts gives for latitude 
50 3474*47 miles ; this means that, according 
to the Mercator's projection, the parallel of 
latitude 50 would be drawn in 3474*47 miles 
from the equator. 

Example. Construct a Mercator's chart 
between the parallels of 50 and 54 north 
latitude, and between the meridians of 3 
and 7 east longitude, on a scale of two 
inches to one degree of longitude. 

148 



CONSTRUCTION OF MERCATOR'S CHART 

The rule is : 

Length of a degree of latitude equals 
Difference between its limiting meridian 
parts multiplied by scale of longitude and 
divided by 60'. 



D<+ 

3"-363 










53 










3"-256 








i 


R? 










\J. 

3"-212 










51 










3"-145 


















s 



3 2" 4 2" 5 2" 6 2" 7 
FIG. 68. 

Draw in the lower horizontal line, and 
mark it off in equal spaces of two inches each 
to the limits of the longitude required. 

149 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Draw perpendiculars to each of the ends 
of this line. It is now required to measure off 
along these perpendiculars the length of each 
degree of latitude. 

Lat. 50 Mer. Parts .... 3474-47 
Lat. 51 Mer. Parts . . . 3568*81 

Difference ..... 94*34 

X2 



188-68 



60 



Therefore the length of the degree of 
latitude between 50 and 51 is 3*145 inches. 

Lat. 51 Mer. Parts . . . 3568*81 

Lat. 52 Mer. Parts . , . 3665*10 



Difference . . 96-38 

X2 



60 



192-76 



3*212 

Therefore the length of the degree of 
latitude between 51 and 52 is 3*212 inches. 

Lat. 52 Mer. Parts . " . . 3665*19 

Lat. 53 Mer. Parts . . 3763*76 



Difference r, : }j;-jj ' .- a 9^*57 

X2 



60 
150 3*286 



CONSTRUCTION OF MERCATOR'S CHART 

Therefore the length of the degree of 
latitude between 52 and 53 is 3-286 inches- 

Lat. 53 Her. Parts . . . 37 6 376 

Lat. 54 Her. Parts . . . 3864-64 



Difference . . iocr88 

X2 



60 20176 



3*363 

Therefore the length of this degree will be 
3*363 inches. 

These distances can now be measured off 
along the perpendicular lines and the remain- 
ing necessary meridians and parallels put in. 

One great advantage of a Mercator's 
chart is, that the course between any two 
places can be found by joining the two, 
placing a parallel ruler along this line, and 
transferring it to one of the compasses en- 
graved on the chart. Where the ruler cuts 
the graduated circle on the compass will be 
the course required. 



CHAPTER XI 

INFORMATION GIVEN ON AN ADMIR- 
ALTY CHART, CONVENTIONAL SIGNS 
AND SYMBOLS 

THE information given on an Admiralty 
chart is expressed by means of certain signs 
and symbols, which should be carefully 
studied, as by knowing them thoroughly the 
various markings can be read at a glance 
like the print in a book. 

On the seaward part of the chart are given 
the soundings or depth of water at a certain 
standard state of the tide, the various banks 
and shoals with the depths over them, arrows 
showing the direction of the tidal streams, 
the various harbours, lights, light vessels, 
buoys, etc. 

Soundings on banks which are underlined 
may mean two things : either the amount they 
uncover, or the depth on them at high water. 
This can always be ascertained by looking at 
the title of the chart. 

On the land part of the chart are given 
152 



INFORMATION GIVEN ON CHARTS 

the general topography of the coast, the 
nature of the coast line, whether rocky, cliffy, 
sandy, etc. ; the various lighthouses, towns, 
harbours, hills, roads, villages, railways, etc. 
The topography is, however, not given in such 
detail for any distance inland as it is in an 
ordnance map, as it is not so much required 
by the seaman. 

On one side of the chart is engraved 
what is known as the ' Title of the Chart ' ; 
the information contained in this is im- 
portant, and should be carefully studied for 
each chart. 

The date of printing is given on the lower 
margin of the chart. 

When using an Admiralty chart, it must 
be remembered that the nautical mile is used 
as a unit, which is equivalent to 6000 feet 
in length, and this nautical mile is sub- 
divided into ten ' cables ' of 600 feet each. 

In the Admiralty chart drawn on the 
Mercator's principle, the latitude scale will 
be found running up and down the sides of 
the chart, and the longitude scale along the 
top and bottom. 

This longitude scale must only be used 
for measuring the difference of longitude 
between two places, and never for distance. 

i53 



AIR NAVIGATION FOR FLIGHT OFFICERS 

In an Admiralty plan a scale of latitude 
and distance is always given, and usually a 
scale of longitude. 

Should the latter not be shown, it is easy 
to construct one if required, and the method 
of doing this will be given later. 

Conventional Signs and Symbols in Use on 
Admiralty Charts. 



o 



(240) 



Steep coast. 
Islands and rocks. 




Cliffs. 



Sandy shore. 



Shingle or stony shore, 



Breakers. 



CHART SYMBOLS 




Stones, shingle, or 
gravel, dry at 
L.W.O.S. 



Mud, dry at L.W.O.S. 




Sand, gravel, or stones, 
dry at L.W.O.S. 



Rocky edges which 
cover and uncover. 



Sandy beach. 

Sand banks, dry at 
L.W.O.S. Figures on 
banks denote either 
amount they un- 
cover at L.W.O.S. or 
depth at H.W.O.S. 
This information is 
always given on title 
of chart. 

Sand hills 



155 



AIR NAVIGATION FOR FLIGHT OFFICERS 



*? T? Trees. 




Cultivated land. 




Towns. 




Villages. 



2*s ^ jE*aistT Swamp or marsh. 

A-iT jflU - ' * 



Church or chapel. 

Beacon, flagstaff, or 
chimney. 

156 



CHART SYMBOLS 

Windmill. 

Roads : 
ist ^Class. 

2nd Class. 
Track. 



Railway. 



Tramway. 



jjj_ Wreck 
"^^ (1910) 



Rock awash atL. W.O.S. 

Rock with less than 
six feet of water at 
L.W.O.S 

Wreck submerged. 



o c> 
a 



Rocks with limiting 
danger lines. 




Kelp. 



AIR NAVIGATION FOR. FLIGHT OFFICERS 



iiiniliiiiiiiijiliim lllllllltlllliilliilimiMlUii ~P*j c Vi 1 Tl Cf S'l'JllcPS 



Beacons. 

Light vessels, 

Fathom Lines 

1 fathom. 

2 fathoms. 

3 fathoms. 



4 fathoms. 

J5 fathoms. 
158 



ABBREVIATIONS ON CHARTS 



6 fathoms. 

so on till 10 ,, 
I0 fathoms. 



20 fathoms. 

so on till 90 ,, 

100 fathoms. 



Quality of the Bottom of the Sea 



b 


Blue. 


blk 


Black. 


br 


Brown. 


brk 


Broken. 


c 


Coarse. 


chk 


Chalk. 


cl 


Clay. 


crl 


Coral. 


d 


Dark. 


f 


Fine. 


g 


Gravel. 


gn 


Green. 


grd 


Ground. 


gy 


Grey. 


h 


Hard. 



m 


Mud. 


mus 


Mussels. 


oz 


Ooze. 


peb 


Pebbles. 


r 


Rock. 


s 


Sand. 


sft 


Soft. 


sh 


Shells. 


shin 


Shingle. 


spk 


Speckled 


st 


Stones. 


w 


White. 


wd 


Weed. 


y 


Yellow. 



159 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Tidal Abbreviations 
Equin 1 . Equinoctial. L.W.O.S. Low Water 



Fl. 


Flood. 




Ordinary 


H.W. 


High Water. 




Springs. 


H.W.O.S. 


High Water 


m. 


Minute-s. 




Ordinary 


Np. 


Neaps. 




Springs. 


ord. 


Ordinary. 


h. 


Hour-s. 


Q'. 


Quarter. 


kn. 


Knot-s. 


Sp. or Spr. 


Springs. 


L.W. 


Low Water. 







<) 



Eddies. 



-^=^=^- ^m^i^- Overfalls. 



Abbreviations for Buoys 



B., Blk. Black. 
Cheq. Chequered. 
G. Green. 

Gy. Grey. 
H.S. Horizontal 
Stripes. 

No. Number. 
R. Red. 



S.B. , Submarine Fog 
Bell (sounded by 
wave action). 

S.F.B. Submarine Fog 
Bell (mechanic- 
ally sounded). 

V.S. Vertical Stripes. 

W. Wh. White. 

Y. Yellow. 



160 



ABBREVIATIONS ON CHARTS 



Abbreviations for Lights 



V., L ts . Light-s. 

V Alt. * Light Alternat- 
ing. 

L* F. *Light Fixed. 

V Fl. *Light Flashing. 

V Occ. * Light Occult- 
ing. 

V Rev. * Light Revolv- 
ing. 

L'F.FL^Light Fixed and 
Flashing. 

L* Gp. * Light Group 
Fl.(2) Flashing. 

L'F.Gp^Light Fixed 
Fl.(3) and Group 
Flashing. 

L* Gp. *Light Group 

Occ. (3) Occulting. 

* Position of lights. 

N.B. Figures in parenthesis after the de- 
scription of a light, denote the number of 
flashes or occupations in its cycle or phase. 

General Abbreviations 
B. Bay. 

Bat y . Battery. 
Bk., Bks. Bank-s. 
Bn., Bns. Beacon-s 
Br. Bridge. 



Alt. 

ev. 

fl., fl s . 

G., Gn. 

Gp 

hor 1 . 

irreg. 

m. 

min. 

obsc d . 

occas 1 . 

R. 

sec. 

(U) 

vert 1 . 

vis. 
W., Wh. 



Alternating. 

Every. 

Flash-es. 

Green. 

Group. 

Horizontal. 

Irregular. 

Miles. 

Minute-s. 

Obscured. 

Occasional. 

Red. 

Second-s. 

Unwatched. 

Vertical. 

Visible. 

White. 



c. 

Cas. 


Cape. 
Castle. 


Cath. 


Cathedral. 


C.G. 


Coast Guard 


Ch y . 
161 


Chimney. 

M 



AIR NAVIGATION FOR FLIGHT OFFICERS 



Conspic. 


Conspicuous. 


L*Ho. 


Lighthouse. 


Cr. 


Creek. 


L*. Vess. 


Light Vessel. 


D. 


Doubtful. 


m. 


Mile-s. 


dist. 


Distant. 


Mag 2 . 


Magazine. 


Estab*. 


Establishment. 


Mon*. 


Monument. 


Fm., Fms 


.Fathom-s. 


Mon y . 


Monastery. 


F.S. 


Flagstaff. 


M*. 


Mountain. 


P, ft. 


Foot, Feet. 


Obs y . 


Observatory. 


F. 


Fort. 


Ord. 


Ordinary. 


h., hrs. 


Hour-s. 


Pass. 


Passage. 


H d . 


Head. 


P.D. 


Position 


H n . 


Haven. 




Doubtful. 


Ho. 


House. 


Pen la . 


Peninsula. 


H r . 


Harbour. 


P k . 


Peak. 


I., I 4 . 


Island, Islet. 


Pos n . 


Position. 


P. 


Islands. 


Prom y . 


Promontory. 


in. 


Inch-es. 


R. 


River. 


L. 


Lake. 


R f . 


Reef. 


Lit. 


Little. 


R d .,R ds . 


Road-s. 


L, La, Lag n Lagoon. R k .,R ks . 


Rock-s. 


Lat. 


Latitude. 


R.S. 


Rocket 


L.B. 


Life Boat. 




Station. 


L.B.S. 


Life Boat 


Ru. 


Ruin. 




Station. 


R y . 


Railway. 


L^. 


Leading. 


s. 


Second-s. 


L., L 


Ledge-s. 


S d . 


Sound. 


L.S.S. 


Life Saving 


Sem. 


Semaphore. 




Station. 


Sh. 


Shoal. 


162 



SYSTEM OF LIGHTS 



Sig.' 


Signal. 


Uncov. 


Uncovers. 


St n . 


Station. 


Vil. 


Village. 


Str. 


Strait. 


W.T. 


Wireless Tele- 


Tel. 


Telegraph. 




graphy Stn. 


Temp y . 


Temporary. 


Y<is 


Yard-s. 


Tr. 


Tree. 







System of Lighting 

Lights may be divided into two classes, as 
follows : 

(1) Those whose colour does not change in 
its entire system. 

(2) Those whose colour does change. 
The following table gives the various 

descriptions of the different lights. 



Lights whose Colour 
does not Change. 

Fixed 
Flashing 



Group Flash- 
ing 



Characteristic Phase. 

A continuous steady light 

(1) A single flash at regular 
intervals, the period of light 
being less than the period of 
darkness 

(2) A steady light varied at 
regular intervals with a 
sudden and total eclipse, of 
greater duration than the 
light 

Shows a group of two or more 
flashes at regular intervals 

163 



Lights whose Coloui 
does Change. 

Alternating 

Alternating 

Flashing 



Alternating 
Group 
Flashing 



AIR NAVIGATION FOR FLIGHT OFFICERS 



Lights whose Colour 
does not Change. 

Occulting 



Group Occult- 
ing 

Fixed and 
Flashing 



Fixed and 
Group Flash- 
ing 

Revolving 



Characteristic Phase. 

A steady light varied at regu- 
lar intervals by a sudden 
and total eclipse, the period 
of light being equal to or 
greater than the period of 
darkness 

A steady light varied at regu- 
lar intervals by a group of 
two or more occultations 

A steady light varied at regu- 
lar intervals by a single flash 
of relatively greater brilli- 
ancy : this flash may or 
may not be preceded by a 
short eclipse 

As above, but with a group of 
two or more flashes 



Light gradually increasing to 
full, then decreasing to 
eclipse 



Lights whose Colour 
does Change. 

Alternating 
Occulting 



Alternating 
Group Oc- 
culting 

Alternating 
Fixed and 
Flashing 



Alternating 
Fixed and 
Group 
Flashing 

Alternating 
Revolving 



The letter (U) against a light denotes that 
it is unwatched, and too much reliance must 
not, therefore, be placed on seeing it. 

Certain details of the lights are given 
opposite them on the charts ; should a fuller 
description be required, all details will be 
found in the Admiralty Light Lists, which 
are published every year. 

The height stated against a light is the 
height of the centre of the lantern above 
high water springs. 

164 



LIGHT VESSELS 

The distance of visibility given in the light 
lists and against the light on the chart, is 
calculated for a height of eye of 15 feet above 
the sea level. 

Light vessels are painted red in England 
and Scotland, and black in Ireland, with their 
name in white letters on each side. These 
latter are not shown during the war. They 
carry a distinguishing mark by day, and their 
light by night. 

Should they be out of position, they 
strike their day mark by day ; and at night, 
instead of showing their light, they show a 
red light at each end of the vessel, and a red 
flare up. 

The following problems all come under the 
heading of chart work, and will be found 
useful at times. 



To Construct a Scale of Longitude if none 
is given on Chart : 

Draw a straight line AB and divide it into 
a number of convenient units according to the 
scale of latitude of the chart. 

From the point A, draw a line AC making 
with the line AB an angle BAG equal to the 
latitude of the place. 

165 



AIR NAVIGATION FOR FLIGHT OFFICERS 

From each of the divisions a, b, c, d, etc., on 
the line AB, draw perpendiculars to the line 
AC, cutting it at the points a', b', c', d', etc. 

The divisions A0', a'b', b'c', c'd', etc., will 
be the scale of longitude required. 



a b c d e f 




Since the triangle aAa' is a right-angled 
triangle, having its right angle at a', the scale 
of longitude can be found as follows : 

- = Cosine aAa'. 
Aa 

i.e. Aa'= Aa x Cosine aAa f 
or scale of longitude = scale of latitude x 
cosine latitude. 

To Lay Off a Course. This is a compara- 
166 



LAYING OFF A COURSE 

lively easy matter, and can be done in two 
ways. 

(a) By Parallel Ruler. 

Join the points of departure and arrival 
by a straight line. Place the parallel ruler on 
this straight line, and transfer its direction 
to one of the compasses engraved on the chart 
so that the edge of the ruler is over the centre 
of the compass. 

The reading on the outer edge of the 
compass card will give the course to be 
steered. 

Care should be taken to take the side of 
the compass card nearest to the point of 
arrival. 

(b) By Transparent Protractor. 

Place the centre of the protractor on the 
point of departure, taking care that its sides 
are pointing true north and south. 

Draw the string tightly along until it is 
over the point of arrival. 

The degree on the protractor over which 
the string passes will be the true course to be 
steered. 

Variation must be applied if the magnetic 
course is required. 

To Allow for Drift Due to Wind. It must 
167 



AIR NAVIGATION FOR FLIGHT OFFICERS 

be remembered that the compass only gives 
the direction of the machine through the air, 
and to get the direction of the actual course 
made good over the land, an allowance for drift 
will have to be made. 

The direction of this allowance must, of 
course, be always into the wind, the amount 
depending on the speeds of the machine and 
wind, and the relative angle between the 
course of the aeroplane and the direction of 
the wind. 

The method of finding the allowance for 
the drift is as follows : 

Example : 

It is required to fly from A to B. The wind is 
blowing in the direction shown by the arrow at 
10 units (miles, knots, kilometres, etc.) per hour. 
The speed of the machine is 86 units per hour. What 
is the course to steer, and what will be the distance 
made good over the land in one hour ? 

Join AB. 

From A lay off a line AC parallel to, and with 
the wind's direction, and mark off along it a distance 
AC equal to, say, one hour's effect, i.e. 10 units. 

With centre C and radius equal to 86 units 
(i hour's machine speed), sweep an arc cutting 
AB at D. 

168 



INTERCEPTING HOSTILE AIRCRAFT 

Join CD and draw AE parallel to CD. 

AE referred to the compass is the course to steer, 
and AD is the distance in units made good over the 
land in one hour. 




FIG. 70. 

Intercepting Hostile Aircraft. Three cases 
come under this heading as follows : 

(a) When the enemy is in sight of the pilot. 

(b) When they are out of sight of one 
another, but in a wind of the same direction 
and strength. 

(c) When they are out of sight of one 
another, and in winds of different direction 
and strength. 

N. B. In case (c) it is presumed that the 
force and direction of the wind at the place 

169 



AIR NAVIGATION FOR FLIGHT OFFICERS 

where the enemy passed over has been tele- 
phoned to the air station. 

Case (a). When the pilot is in sight of 
the enemy. 

Upon all these occasions endeavour to 
steer a converging course whilst keeping the 
compass bearing of the enemy constant. By 
doing this, you are approaching him in the 
quickest possible way. 

If observation shows that the compass 
bearing of the enemy is changing towards the 
nose of your machine, it means that he will 
pass ahead of you. If the compass bearing 
changes towards the tail of your machine, it 
means that he will pass behind you. 

In the first case, the course should be altered 
away from the enemy ; and in the second case, 
the course should be altered towards him. 

This, of course, is only the principle of the 
problem ; the two machines may be flying at 
different altitudes, one may be faster than the 
other, the enemy may alter course, etc., so 
that much must be left to the pilot's discre- 
tion ; but if he acts on the above principle, he 
will be doing all he can to close the enemy. 

Example : 

A pilot at A sights an enemy machine at B, 
170 



INTERCEPTING HOSTILE AIRCRAFT 

bearing 100, and steering approximately in the 
direction BM. 

A steers in the direction AC at first, and on 



100* 




FIG. 71. 

arriving at C, finds the bearing of the enemy machine 
to be still 100. He therefore keeps on his course. 

At E he finds the bearing of the enemy to be 92, 
showing him that he is going ahead of the enemy. 

He therefore alters his course to EG. 
171 



AIR NAVIGATION FOR FLIGHT OFFICERS 

At G he finds that the enemy is bearing 100, 
showing him that the latter is going ahead of him. 

He then alters course to the direction GJ, and 
on arrival at J, finds the bearing is now 95. 

He then steers the course JL, and finding that 
the bearing remains constant at 95, knows that 
he is closing as fast as possible. 

Case (b). When they are out of sight of 
one another, but in a wind of the same direc- 
tion and strength. 

This is quite a simple problem, as both 
being affected by the same wind force, the 
latter may be neglected, and the only thing 
to do is to consider it as a case of closing 
preserving the bearing. 

In the figure,^ C is the position of the 
enemy when reported, and A the aerodrome 
you are stationed at, situated east 60 miles 
from the former. He is reported as steering 
north at 45 miles an hour, and the speed of 
your machine is 85 miles an hour. 

Firstly. To find the course necessary 
to steer, the following procedure should be 
adopted. 

Join AC. 

From C lay off the enemy's course CE, 
and mark off along this line a part CB equal to 
the enemy's speed for one hour, i.e. 45 miles. 

172 



INTERCEPTING HOSTILE AIRCRAFT 

With centre B and a radius equal to your 
speed for one hour, i.e. 85 miles, describe an 
arc cutting CA, produced if necessary, at D. 

Join BD. 




FIG. 72. 

BD will be the course to steer from A. 

Secondly. To find the rate of closing, or, 
in other words, to find how long you will be 
before you will catch him. 

Measure the number of units contained in 
173 



AIR NAVIGATION FOR FLIGHT OFFICERS 

the line CD. This will be the rate of closing 
in one hour, and in this case is 71 units. 

So that the time taken to catch the enemy 
will be : 



~. This equals = 0*84 of an hour 

or 50*4 minutes. 

N.B. This problem can be worked out 
either on a chart or on a mooring board, 
whichever is found most convenient. 

Case (c). When they are out of sight of 
one another, and flying in winds of different 
strength. 

Example. Information is received at your aero- 
drome that a hostile machine has passed over a 
station A, making good 290 at the rate of 40 miles 
per hour. 

Your aerodrome is 190 42 miles from this point, 
and you have a machine capable of a speed of 70 
miles per hour. The wind at your station is north- 
east (45) at 12 miles per hour. 

What course must you steer to intercept the 
enemy, and how long will you be getting there ? 

Note. It should be remembered that the enemy, as 
reported, is making good course and speed given. If 
his course and speed and direction and force of the 
wind are signalled, you will have to work out first 

174 



INTERCEPTING HOSTILE AIRCRAFT 

what he is making good, and then proceed as given 
below. 

A is the position of the enemy, and B your 
aerodrome. From A lay off AC equal to one 




Scale 
o 



30 



FIG. 73. 

hour's course and distance made good by the 
enemy. 

From C lay off CD in the direction the 
wind is coming from, equal to the speed of 
your wind for one hour. 

With centre D and a radius equal to one 



AIR NAVIGATION FOR FLIGHT OFFICERS 

hour's speed of your machine, sweep an arc 
cutting AB, produced if necessary, at E. 

Join DE and EC. 

From B draw BF parallel to EC. 

DE will be the course to steer, and EC 
will be the course and distance made good 
by your machine in one hour. 

BF is the course and distance made good 
by steering a course parallel to DE, and the 
two machines will meet at F. The time taken 

AF 12 
will be -p-' i.e.- = 0*8 hrs. or 54 mins. 



These distances can be actually measured 
on the chart or mooring board, and the time 
ascertained from that. 



176 



CHAPTER XII 
FIXING POSITIONS 

IN an aeroplane, one of the best methods of 
fixing one's position is to be able to read a 
chart or map thoroughly so that, if flying over 
the land, one can tell just what spot is verti- 
cally under the machine. 

As, however, this is not always possible in 
a seaplane, it is proposed to explain one or 
two methods of fixing. The last method 
given will be more suitable for airships or 
observation balloons, where there is a great 
deal more room than on an aeroplane. 

(a) Fixing by l Cross Bearings.' Choose 
two objects that are marked on the chart as 
nearly 90 apart as possible, as this will give 
a very definite cut. 

Correct these bearings for variation and 
deviation, and you are ready to lay off on the 
chart. 

Place the parallel rulers over the centre of 
the compass engraved on the chart, and turn it 

177 N 



AIR NAVIGATION FOR FLIGHT OFFICERS 

round until the edge of the ruler is cutting the 
corrected bearing on the edge of the compass. 

Transfer this line to the first object, and 
draw a line through it in the opposite direction 
to your bearing. Do exactly the same with 
your second bearing. 

The intersection on the chart of these 
two lines will be your position. 

Example : 




FIG. 74. 

After correction the church bears 336 true and 
the flagstaff 57 true. 

Draw your lines in the direction 156 and 237^ 
i.e. from the objects, and the intersection at the 
circle will be your fix. 

(b) Fixing by .Doubling the Angle on the 

178 



FIXING POSITIONS 

Bow.' This is a very simple method, and 
merely consists of taking a bearing of an 
object ' x ' degrees on the bow of your machine 
and noting the time, and again taking the 
bearing when it is ' 2 x ' on the bow with, of 
course, the time again. Knowing your engine 
speed, or your speed over the land, you get 
your distance run in the interval of time 
between the two bearings and : 

Distance run in the interval = Distance off 
at second bearing. 

Example : 




Course East 



'True) 



FIG. 75. 

Speed, 60 miles per hour. Course, east (true). 
9 A.M. Tower bore, 54 (true). 
9.10 A.M. Tower bore, 27 (true). 
Distance run in 10 minutes is 10 miles. 
Therefore position at second bearing is, with 
tower bearing 27 (true), distant 10 miles. 

179 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Fixing by Station Pointer. To understand 
this method of fixing, it will be necessary to 
go into the theory a little. 

Fixing by station pointer does not call 
for the use of a compass : all that is required 
is a sextant and an instrument known as a 
station pointer. 

The station pointer fix depends on a 
certain theorem in Euclid (iv. 5), which states 
that a circle can be drawn through any three 
points. 

If, therefore, three points on the chart be 
chosen, and taking our position as the fourth 
point, it is obvious that we can draw two 
circles as follows : 

One circle passing through the left-hand 
object, the middle object, and our position. 

The other circle passing through the right- 
hand object, the middle object, and our 
position. 

From this we see that these two circles will 
intersect at two common points, viz. at the 
centre object and at our position, and as we 
cannot be at the former, the second inter- 
section must be our fix. 

Another theorem that the station pointer 
fix depends on is Euclid (iii. 21), which states 
that the angles on the circumference of a 

180 



STATION POINTER FIXES 

circle, subtended by the same chord, and on 
the same side of the chord, are equal to one 
another. So that all we have to do is to ob- 
serve two angles to our three chosen objects, 
and place these angles on the station pointer 
and fit them in on the chart. This does away 
with the necessity of actually "drawing in the 
circles. The size of the circles is, of course, 
governed by the dimensions of the observed 
angles. 

The following figure shows why we must 
be at the second intersection of the two circles. 




FIG. 76. 

The angles ADB and BDC are the angles 
actually observed. Now D is the only point 
we can be at, for, supposing we were at E, 

181 



AIR NAVIGATION FOR FLIGHT OFFICERS 

although the angle AEB is equal to the angle 
ADB, yet the angle BEC is not equal to the 
angle BDC, which latter was the one taken with 
the sextant. Hence there can be only one 
place that will fit in with our observed angles, 
and that is the point D which is common to 
both circles. 

In practice, all that has to be done is to take 
two angles between the three objects chosen, 
place these angles on the station pointer, fit 
its three legs over the three points on the 
chart, and the small nick in the centre leg 
indicates your position. 

A certain amount of care is necessary in 
the selection of the objects. The following 
examples are worth remembering : 

(i) The objects may lie in the same 
straight line. 




STATION POINTER FIXES 

(2) The objects may lie in a curve, with 
the middle object nearest to the observer. 




FIG. 78. 



(3) The objects may lie in a curve, concave 
to the observer, provided the latter is on or 
within a line joining the right and left hand 
objects. 




AIR NAVIGATION FOR FLIGHT OFFICERS 

(4) The objects may lie in a curve, concave 
to the observer, provided the latter is well 
outside the circle passing through the three 
objects. 




FIG. 80. 



(5) Two of the objects may be in transit 
with the observer. In this case one angle to the 
third object is all that it is necessary to take. 




STATION POINTER FIXES 

(6) If two of the objects are much nearer 
to the observer than the third, and seem about 
equidistant from the observer, at whose 
position they subtend an angle of between 
60 and 120, the fix is a good one. 




FIG. 82. 



185 



CHAPTER XIII 
ORDNANCE MAPS 

THESE maps are to the pilot flying over the 
land, what a chart is to a seaman navigating a 
ship, with the advantage that, given a clear 
day, the pilot can always see the land below 
him, which is impossible in a ship. 

There is a much greater wealth of detail, 
as fegards the land, in an ordnance map 
than in a chart, as obviously a navigator at 
sea does not require the topography for any 
distance inland. 

Ordnance maps are constructed on the 
gnomonic projection, and are not provided 
with any magnetic compass, so that all 
courses have to be referred to the true north 
and south, which direction is given on the 
inner border of the map. The sides of an 
ordnance map are not graduated like a 
Mercator's chart, but a scale is provided at 
the bottom of each map in whatever unit of 
length it is drawn to, i.e. miles, yards, or feet. 

186 



MARKINGS ON ORDNANCE MAPS 

The statute mile of 5280 feet is used in 
ordnance maps, unlike the Admiralty chart, 
where the unit is a sea mile. 

In using an ordnance map which is not 
squared, it is convenient to draw a series 
of parallel lines to the true north and south 
lines to facilitate laying of courses. It is also 
better to cross these lines with east and west 
ones. 

Conventional Markings. These are as 
follows : 

Hills are shown in brown, their heights 
being those above a certain level, which is 
given at the foot of each map. 

Rivers and canals are coloured blue. 

First class roads are coloured red. 

Second class roads are left uncoloured. 

Railways are denoted by thick black 
lines. 

If the map shows any part of the sea, this 
is coloured blue. 

Towns and villages are represented by 
black blocks of rectangular or other shape, 
with the streets running through them, the 
amount of detail shown depending on the 
scale of the map. 

Woods are coloured green. 



AIR NAVIGATION FOR FLIGHT OFFICERS 

Lakes are coloured blue. 

Isolated houses are denoted by black 
dots. 

All other symbols conform to those given 
on Admiralty charts. 

To Lay Off a Course. A celluloid pro- 
tractor is supplied, marked from o to 360 in 
the same way as a compass card. 

It is pierced in the centre, and a string is 
let through the hole. 

To lay off a course, the centre of the 
protractor is placed on the starting-point, with 
its sides parallel to the true north and south 
line. 

Place the string over the point it is desired 
to go to, and draw it tight. 

The degree on the protractor that the 
string passes over will be the true course 
required. 

To Measure a Distance. This is done by 
means of a pair of dividers. Place one point 
of the latter on the starting-place and the 
other point on the place you wish to go to. 
Transfer this distance to the scale at the 
bottom of the map. 

Should the distance be too big for 
the dividers, put a certain or convenient 

188 



SQUARED MAPS 

distance on the latter from the scale, and run 
this distance along a straight line joining the 
two points, noting how many times it goes 
into the total distance. 

Squared Maps. These are ordnance maps 
divided into large rectangles, each named by 
a letter. 

These rectangles are divided into thirty 
or thirty-six squares, each of whose sides are 
1000 yards long. 

The squares are numbered from I to 30, 
or i to 36, starting at the top left-hand corner 
and running across to the right. 

Each of these squares is divided into four 
squares, each of whose sides is 500 yards long. 

These squares are lettered as shown 
below. 



FIG. 83. 



In any report sent in, the centre of the 
189 



AIR NAVIGATION FOR FLIGHT OFFICERS 



small square is taken as the spot mentioned ; 
but if more accuracy is required, a cardinal 
or semi cardinal point can be introduced, 
giving the direction of the object from the 
centre of the small square. 

If great accuracy is required, each side of 
the small square can be divided into ten equal 
parts, each 50 yards long, always starting from 
the south-western end of the square. In this 
case, the number along the east and west line 
is always mentioned first. 

Example : 



r. 



FIG. 84. 

Supposing there was a windmill in square ' c ' 
as shown, and it was required to report its position 
accurately. 

It was located in rectangle B. 26 c., but to be 
absolutely accurate, it should be reported as Wind- 
rnill B. 26 c. 2, 4. 

190 



SQUARED MAPS 

A sketch of the whole rectangle is given 
below. 













a b 












c d 


7 


S 


9 


10 


II 


12 


13 


14 


15 


16 
> 


17 


IS 


19 


20 


21 


3^ 

22 


23 


24 


25 


26 


27 


28 


29 


30 


31 


32 


33 


34 


35 


36 

^i 



FIG. 85. 

Only one of the squares of each rectangle 
is marked a, b, c, d. This is to prevent over- 
crowding, but all the others follow the 
same law. 

A scale is provided at the bottom of each 
squared map, and a magnetic compass is 

191 



AIR NAVIGATION FOR FLIGHT OFFICERS 

printed on the north-west corner. The topo- 
graphy is the same as on an ordinary ordnance 
map. 

Selection of Suitable Landmarks, etc. 

When flying from one place to another, it is 
desirable to check the position as frequently 
as possible. 

This can easily be done in clear weather, 
provided the pilot can read his map 
thoroughly. 

If possible before a flight, the pilot should 
look over his map, and note what he would 
expect to pass over on his way. During the 
flight he should endeavour to pick up each of 
these marks as he passes them. 

Roads, rivers, canals, railways, bridges, 
lakes, woods, villages, and towns are all good 
marks, as are tall chimneys, churches, clumps 
of trees on hills, etc. 

Very often a distant mountain peak or 
other conspicuous object will give him a good 
mark for direction, either by steering straight 
for it, or keeping it a little on one side of the 
machine. 



192 



APPENDIX 



Variation of Wind Velocity with Height, page 206. 
The Gradient Wind, page 208. 



These Tables have been included here by the kind 
Permission of the National Physical Laboratory, 
Teddington, Middlesex. 



315 

No Devn. 



225 

No Devn 



45 

No Devn. 




Ely Devrv 
180 Max. 



135 

No Devn. 



315 
No Devn 




225 > 

NoOevn. 



Ely Devn 
Max 



135 

No Devn. 



Wly Devn, 
180 Max. 



Coefficient E 



APPENDIX 

Coefficient E. This is due to the effect of induc- 
tion in horizontal soft iron running diagonally. 

If it runs from left front to right rear, ' E ' is -}- ; 
if from right front to left rear, ' E ' is 

It is maximum on the cardinal points, diminish- 
ing to zero on the quadrantal points. 

It is found by taking the mean of the deviations 
on the cardinal points, changing the signs of those 
on east and west. 

It is called ' Quadrantal ' because it changes its 
sign in each quadrant. 

It is corrected in conjunction with Coefficient 
' D ' by placing the spheres at an angle with the 
transverse line, if ' D ' is -f ; and with the longitudinal 
line, if ' D ' is ; so that 

jr 
Tangent 26 = 

If ' E ' is -f- the left-hand sphere goes in front : 
vice versa for E. 

The amount to be corrected is 



+ E 2 

See Diagram on opposite page. 

Composition of the Air. Air is an invisible 
193 



APPENDIX 

gas largely composed of nitrogen and oxygen in 
the following proportions : 

Nitrogen . . . .77-11 per cent. 
Oxygen .... 20-65 
Water Vapour . . . 1-40 
Argon . . . 0-79 

Carbonic Acid . . . 0-04 

Height of the Air. From various observations, 
the most important of which is that of meteors, it 
is estimated that the major portion of the atmos- 
phere extends about one hundred miles above the 
earth's surface, also that it exists from there to a 
height of 400-500 miles, but of course in a very much 
thinner form. 

Density of the Air. The atmosphere is densest 
at the surface of the earth, and gets gradually 
more and more attenuated until its confines are 
reached. At a height of about seven miles it has 
only one-quarter of the surface density ; about 
fourteen miles, one-sixteenth ; whilst at twenty-one 
miles merely one-sixtieth. 

The Meteorological Elements. Under this head- 
ing come the following : 

Pressure, Temperature, Humidity, Wind, and 
Cloud. The last has already been dealt with in the 
body of the book, and wind partly dealt with. 

(i) Pressure. By this is meant the capability 
of the density of the air at sea level to support a 
column of mercury enclosed in a glass tube. 

This pressure is nearly always changing, hence 
the reading of the barometer scale indicating the 

194 



APPENDIX 

height of this column is scarcely ever the same from 
hour to hour. 

Pressure is measured by a barometer, which 
is merely a glass tube filled with mercury, which 
is then boiled to expel any particles of air or 
water vapour, and then inverted into a cup 
mercury. 

The mercury will fall in the tube until the pres- 
sure of the outside air balances its fall and prevents 
any further drop in the tube. The space between 
the top of the enclosed column of mercury and the 
top of the tube is the nearest known approach to a 
perfect vacuum, and is known as a ' Torricellian 
Vacuum.' 

If now the pressure of the air increases, it will 
press more heavily on the mercury in the cup. This 
will be communicated to the mercurial column, 
causing it to rise in the tube. Conversely, if the 
atmospheric pressure decreases, it will, by not press- 
ing so heavily, cause the column to fall in the tube. 
This is known as the rise or fall of the barometer, 
and its amount is measured by a fixed and also a 
movable scale at the side of the tube ; the latter is 
known as the Vernier. 

Owing to friction at the sides of the glass tube, 
the top of the mercury assumes a convex form, as 
shown below. 

When reading the barometer, the bottom of 
the pointer of the vernier plate should be brought 
down by the milled screw at the side so as to 
touch the top of the mercury, as seen in the sketch. 

Owing to what is known as the ' Vertical Pres- 
sure Gradient/ barometer readings, when sending in 

195 



APPENDIX 

reports, are corrected for their height above sea 
level to reduce them to the latter, this being the 



FIG. 86. 

common level used on meteorological charts. The 
barometer has also to be corrected for the tempera- 



Vernier Plate 
(graduated) 



^ Pointer of 
Vernier Plate 



APPENDIX 

ture, owing to the column of mercury in the tube 
expanding or contracting according to the rise or 
fall of the temperature. 

(2) Temperature is the thermal condition of a 
body which determines the exchange of heat be- 
tween it and some other substance. Heat may be 
imparted in three ways : 

(i) Radiation. (2) Conduction. (3) Convection. 

(3) Humidity. Interspersed between the mole- 
cules of nitrogen and oxygen, which are the chief 
constituents of air, are also molecules of water 
vapour invisible because of their transparency. 

This water vapour is caused by the continued 
evaporation which is always taking place from 
water, ice, snow, or any moist surface. This quantity 
of water vapour is constantly changing owing to 
the evaporation from the earth's surface becoming 
faster or slower. As the temperature rises, the 
capacity of dry air for holding moisture increases, 
so that the warmer the air, the greater quantity of 
water vapour it can sustain in an invisible state. 
Now any given volume of dry air can only take up 
a certain invisible quantity of water vapour, and 
when this amount is exceeded the latter becomes 
visible as cloud mist or fog. The humidity, or 
in other words, the amount of moisture in the air, 
can be gauged by means of the wet and dry bulb 
thermometer. 

Heating and Cooling of the Atmosphere. The air 
receives its heat from the sun, but being a bad 

197 



APPENDIX 

conductor, only gets a very little of it by conduction. 
The sun's rays pass through the air and strike the 
earth, the amount of heat the latter received de- 
pending on the obliquity of the rays. The earth 
radiates this heat received, which warms the layer 
of air in immediate contact with it ; this warm air 
rises and cold air fills its place. This latter is known 
as convection, so that the air is chiefly warmed 
by radiation and convection and only slightly by 
conduction. 

Measurement of Temperature. Temperature is 
measured by means of a thermometer, an instru- 
ment consisting of a glass bulb and tube, the latter 
partly filled with mercury or alcohol, the latter for 
use in very cold climates. In graduating the ther- 
mometer we know of two fixed points which are 
always the same at sea level, viz. the boiling and 
freezing points of distilled water. 

The thermometer being placed in each, marks 
are made showing the level which the mercury 
attains, and the space between is divided into a 
convenient number of divisions called degrees. 

Two kinds of thermometers are in use : 
The Fahrenheit thermometer. 
The Centigrade thermometer. 

(i) Fahrenheit Thermometer. The boiling and 
freezing points having been marked, the space 
between them is divided into 180 equal parts. 

When this thermometer was first invented, it 
was also put into a mixture of ice and salt, which 
produced the lowest known cold in those days. The 

108 



APPENDIX 

point to which the mercury descended was taken 
as the zero of the scale, and was thirty-two divisions 
below the freezing point of distilled water. Hence, 
in a Fahrenheit thermometer, freezing point is 
represented by 32 and boiling point by 212. 

(2) Centigrade Thermometer. In this thermo- 
meter the space between the freezing and boiling 
points of distilled water is divided into 100 parts, so 
that freezing point is represented by o and boiling 
point by 100. 

The Absolute Zero. By this is meant the tempera- 
ture at which gases would have no volume and exert 
no pressure if they went on contracting with cooling 
as at ordinary temperatures. 

This temperature is about 459 below zero of 
Fahrenheit. 

Measurement of Pressure. Pressure is measured 
by the barometer or aneroid, whose scale is marked 
in inches or millibars. 

The latter is about the thousandth part of the 
ordinary atmospheric pressure at sea level, and is 
also known as a ' pressure limit.' 

A table giving the equivalents of mercury inches, 
millimetres, and millibars is given on p. 9 of the 
' Handbook of Meteorology/ 29-92 mercury inches, 
which is the normal pressure in the British Islands = 
1013 '2 millibars ; 10 millibars = 0-03 mercury 
inches. 

The Vertical Pressure Gradient. This is the 
decrease in the height of the mercury in the baro- 

199 



APPENDIX 

meter owing to the rarefied air being unable to 
support the same column that it could on the sea 
level. This fall amounts to i inch of mercury in 
about 900 feet. 

Deflection of Wind due to the Earth's Rotation. 
The maximum velocity of the rotary motion of the 
earth occurs at the equator and diminishes to zero 
at either pole. 

In consequence of this, a mass of air flowing from 
a high to a lower latitude, i.e. towards the equator, 
will be deflected to the westward, owing to the 
increased velocity of the earth. On the other hand, 
a mass of air flowing from a low to a higher latitude, 
will be deflected to the eastward, owing to the earth's 
decreasing velocity. 

For example, a southerly wind in the Northern 
Hemisphere, i.e. a wind blowing from the equator 
towards the pole, will be deflected to the right and 
becomes south-westerly ; and a northerly wind, i.e. 
setting from the pole towards the equator, is also 
deflected to the right and becomes north-easterly. 

The direction right or left is obtained by standing 
with your back to the wind. 

The reverse holds good in the Southern Hemi- 
sphere, the northerly wind being deflected to the 
left and becoming north-westerly, and the southerly 
wind being deflected to the left and becoming 
south-easterly. 

From this we see that when an air current sets 
towards an area of low pressure, from the surround- 
ing high pressure, it is deflected to the right and left 

200 



APPENDIX 



Example : 

(i) Northern Hemisphere. 
N 





FIG. 88. Northern Hemisphere. FIG. 89. Northern Hemisphere. 
Southerly wind. Northerly wind. 

(2) Southern Hemisphere. 
N 





S 

FIG. 90. Southern'Hemisphere. Fic.gi. Southern Hemisphere. 
Southerly wind. Northerly wind. 

in the Northern and Southern Hemispheres respec- 
tively. This air current does not set directly towards 
the low pressure, but acquires a motion round it, 
but inclined inwards towards the centre of the low 

201 



APPENDIX 

pressure. This circular motion is against the hands 
of a watch in the Northern Hemisphere, and with 
the hands of a watch in the Southern Hemisphere. 

Again, when the air from an area of high pressure 
flows towards an area of low pressure, it is deflected 
to the right or left according to its hemisphere, and 
acquires a motion round the high pressure area 
inclined outwards. 

This motion is with clock hands in the Northern 
Hemisphere, and against clock hands in the Southern 
Hemisphere. 

The Different Forms of Isobars. Isobars are 
divided into seven different groups, of which the 
cyclonic and anti-cyclonic types have already been 
given ; the remainder, together with the weather 
encountered in them, are given below. 

(3) Secondary Cyclone. 

Ueiacned Cloud 



29 90 




Irregular Cumulus^ 



30-00 

Cirrus 



FIG. 92. 

A secondary cyclone is usually found on the 
edge of a cyclone, but very often on that of an 
anti-cyclone. 

202 



APPENDIX 



(4) The Wedge. 



Cyclone 



29-50 



29-80 



3010 



\ I Cyclone 
Radiation \ Blue 




29-50 
FRONT 



Visibility 



30 30 



FIG. 93- 
(5) The Straight Isobar. 



29 50 




29-9 



APPENDIX 

The wedge is an area of high pressure interposed 
between two cyclonic depressions. 

The gradients are slight and the wind never 
strong. 

The isobars may run in any direction. The 
wind is generally strong or gusty but does not 
attain gale force. 

(6) The V Depression. 




FIG. 95^ 

The point of the V is generally directed towards 
the equator, and in the Northern Hemisphere the 
convex side of the trough is usually facing to the 
eastward. 

204 



APPENDIX 

The wind does not veer in the usual manner, 
but the passage of the trough is marked by a sudden 
shift of wind and a violent squall. 

(7) The Col. 

This is an area of low pressure between two 
areas of high pressure. No typical weather is met 
with, but the presence of a col indicates unsettled 
conditions. 

N.B. Arrows show direction air currents are 
flowing towards. 

Land and Sea Breezes. These are met with in 
the tropics and also in the temperate regions during 
fine settled weather. 

They are caused by the unequal heating of land 
and water. 

After sunrise the land gets heated quicker than 
the sea, consequently the air above the former rises, 
and the cool air over the latter flows in to take its 
place, causing the ' sea breeze/ 

After sunset, the land parts with its heat quicker 
than the sea, so that the warm air above the latter 
rises and the cooler air from the land flows out to 
take its place, causing the ' land breeze/ 

Variation of Wind Velocity with Height. It has 
been found by experiment that the velocity of the 
wind increases with the height, and tends to 
gradually become parallel to the isobars. 

The veering of the wind with height may be 
roughly estimated in degrees from a certain formula. 

205 



APPENDIX 

Where ' V ' is the veering and ' H ' the height : 



jcooo 

H 

4-2 



1000 

Height. Veering. 

O 

1000 10 

2OOO 15 

3000 18 

40OO 20 

500O 21 

6000 22^ 

70OO 23^ 

8000 24J 

9000 24^ 

10000 25 

nooo 25^ 

I20OO 2 5f 

Fluctuation and Gustiness. The velocity of the 
wind is seldom uniform, but varies in gusts and lulls. 

The difference between the average maximum 
velocity of the gusts and the average minimum 
velocity of the lulls is known as the ' fluctuation 
of the wind.' 

The gustiness of the wind is found as follows : 

Fluctuation 

Gustiness = - - 

Average velocity 

Let V be the maximum and v the minimum 
velocity. 

V -v 
Then Gustiness = 



206 



APPENDIX 

It has been found that the gustiness of the 
wind at any particular place for a given direction is 
practically constant. 

Twilight. This is caused by the air reflecting a 
certain quantity of light from the sun when the 
latter is below the horizon. 

Thus, twilight occurs twice a day, in the morning 
and evening. There are two kinds of twilight 
Astronomical and Civil. Astronomical twilight 
begins and ends when the sun's centre is 18 below 
the horizon, when only first magnitude stars are 
visible. It will last all night if the latitude and 
declination are of the same name, and their sum is 
not less than 72. 

Civil twilight begins and ends when the sun's 
centre is 6 below the horizon, when stars of the first 
magnitude are not visible. 

The Gradient Wind. Observation has shown 
that a wind due to a difference of pressure between 
two places is greater the bigger the difference of 
pressure and the closer the isobars. 

If the differences of pressure over a certain area 
are marked on a chart by means of isobars, it is 
possible to calculate the force of the wind by means 
of a formula. 

This wind is known as the ' Gradient Wind/ 
but the formula does not take friction into account. 
The gradient direction should be regarded as along 
the isobars. 

207 p 



APPENDIX 

The following was table issued for finding the 
gradient velocity of the wind : * 

V is the velocity of the wind, and D the distance 
apart of the isobars in nautical miles. 

Then V =- 2 at a height of about 2000 feet, 

where D is the distance between the isobars corres- 
ponding to YO f an i ncn f mercury. 

D V 

280 I0 4200 

140 20 V = .^ when D = distance 

100 28 , ,, . , 

apart of the isobars corres- 

' ponding to c 5 of a centibar. 

40 70 

If the standard distance apart of the isobars, 
i.e. fifteen miles, is used, the following table gives 
the velocity of the gradient wind, assuming ordinary 
conditions of pressure and temperature and making 
no allowance for the curvature of the path : 

Barometric Pressure Difference VpWitv 

per 15 Nautical Miles. 

o-oi inch 19 miles per hour. 

0-02 ,, 39 ,, 

0-03 ,, 58 

0-04 77 

0-05 97 

The observed velocity is seldom the same as the 
theoretical velocity, the latter being usually con- 
siderably in excess of the former. 

* Permission to reprint above table has been given by the 
Controller of H.M. Stationery Office. 

208 



APPENDIX 

High and Low Pressure Areas. On account of 
the circulation of the air, the latter in high latitudes 
is moving faster than the earth's surface. This 
increases its centrifugal force, making it press on 
the air in low latitudes. The expansion of the air 
over the tropics, due to the heat, causes it to press 
on that in higher latitudes. 

This combined effect causes a distribution of 
pressure as shown roughly in the figure below. 




High 



Low 



High 







FIG. 96. 

The land being more quickly affected by change 
of temperature than water, bigger changes are 
experienced, due to change of seasons, on land than 
on sea. 

Triangle of Velocities. Velocity not only signi- 
fies the rate of pace but embraces the quarter from 
which any force travels. The velocity of a point 
may be represented by a straight line, the speed being 

209 



APPENDIX 

measured by scale, and the direction it is moving 
in by the direction of the line. 

If a point A has two velocities AB and AC, the 
resultant velocity is represented by the line AD, 
which is the diagonal of the parallelogram ABCD. 

In practice, it is only usual to draw the two lines 
AC and CD, of which the third side AD is the 
resultant. 

Example : 




FIG. 97- 



Speed of Machine 
,. Wind 



North, 60 knots. 
N.E., 20 knots. 



Required, resultant velocity both in magnitude 
and direction. Scale 10 knots = i inch (see Fig. 98). 

Radius of Action. The radius of action in a 
particular direction is the farthest distance in that 
direction that a machine can go and return. The 
area of action is the area to every point of the peri- 
meter of which an aeroplane can just go and return. 

Distance = Speed X time 

Distance 
Speed = 



Time = 



Time 

Distance 

Speed 



210 



APPENDIX 



Example : 



Then 




FIG. 98. 

Wind ahead, 20 knots. 
Speed of machine, 80 knots. 
Fuel for 5 hours. 

Let R = Radius of action. 

K + ^- = 5 . 

60 ioo 



R = 187-5. 

A machine flies i mile in 60 sees, with wind, 
i.e. 60 M.P.H. 

htn A-=aM&hme flies i mile in ioo sees, against wind, 
i.e. 36 M.P.H. 

211 



APPENDIX 

Find its speed in still air. 

Let V and v be velocities in still air and in the 
wind respectively. 

V + v = 60 
V - v = 36 

2 ' 



Radii of Action. These can be worked out 
graphically, knowing radius of action with and 
against the wind together with fuel hours, and the 
results plotted on squared paper, the resulting 
radii being afterwards drawn in. 



212 



INDEX 



The letter '/' after a page number indicates 'following page 
or pages.' 

PAGE 

ABBREVIATIONS, general ..... i6if. 

light .... .161 

tidal and buoy .... .160 

ABRIDGED NAUTICAL ALMANAC . . . 125, 136 

ABSOLUTE ZERO ... . 199 

ACTION OF AN AEROPLANE, area and radius of . 2ioff. 
ADJUSTMENT OF DEVIATION .... 32ff 

ADMIRALTY CHARTS ... 7, 143*. 

fathom lines used on . .1581. 

light lists . .164 

' Manual of Navigation ' 80, 83 

signs and symbols used on . . . . 154^. 

variation chart ...... 7 

AERO COMPASS . . 24f. 

allowance for heeling . . .; . 29 

essential features in an . ...... . ... . . 28f. 

expansion and contraction . .29 

lighting of . . * ; 29 

making of the card . , ,*.__, 29f . 

steadiness . . . '.-- - - - 2 ^ 

AIR, composition of . . I93 1 - 

density of . .' . , . 194 

height of the . , . 194 

humidity in the . . - 197 

pressure of the . . ,. 1946*. 

AIRCRAFT, intercepting hostile . . . . i6gft. 

213 



INDEX 



ALLOY USED IN MAKING MAGNETS ... 14 
AMPLITUDE TABLES . . . . . .138 

ANALYSIS or DEVIATION .... 32*!., 4 if. 

ANGLE OF DEVIATION ..... 8 

hour ....... 121 

of variation ...... yf. 

to find hour ...... i4of. 

ANTI-CYCLONE . . . . . . 7 iff. 

APPARENT DEVIATION . . . . -33 

AQUILA 99 

AREA OF ACTION OF AN AEROPLANE . . . 2ioff. 
AREAS, high and low pressure .... 209 
ARIES ........ 99 

ARTIFICIAL MAGNETS 2 

ASCENSION, right . . . . . .113 

ASTRONOMICAL TIME 115 

twilight ....... 207 

ASTRONOMY, notes on ..... 93^- 

ATMOSPHERE, heating and cooling of the . . ig7f. 
AURIGA ........ 100 

Axis OF THE EARTH no 

AZIMUTH TABLES i27f. 

BANKING ON A COMPASS, effect of . . .31 

BAROMETRIC SURGE . . . . . . 71 

BEARING AMPLITUDE . . . .138 

rule for naming the 137 

BEARINGS, swinging a compass by reciprocal . 45 

fixing position by cross .... iyyf. 

taken from the tables, true . . . . 62ff. 

BEAUFORT'S SCALE FOR SEA DISTURBANCE . 85 

system of weather notation . < . . 84f. 

system of wind notation . . . . 83f. 

BLOCK, magnet .* .', * . . . . . , 31 

BOOTES . . '..;' . . . . * 100 

BREEZES, land and sea . . . . . 205 

BRITISH ISLES, weather in the . . .. . * Soft. 

' BROADSIDE-ON ' MAGNET . . . . . 2off . 

214 



INDEX 

PAGE 

BUBBLE FROM COMPASS, removal of . .26 
BUOY ABBREVIATIONS . . . . .160 
BURDWOOD, DAVIS AND .... 1 27, I2Q 
BUYS-BALLOT'S LAW 69 

CANIS MAJOR ....... 101 

CANIS MINOR 101 

CASSIOPEIA ...... 96, 98 

CATUS 102 

CELESTIAL CONCAVE in, 114 

equator . . . . . . .112 

CENTIGRADE THERMOMETER .... 199 

CENTRE OF A STORM 68 

CHART, gnomonic ...... 

Mercator's 

CHARTS, Admiralty 7, 143!. 

CIRCLES, definition of great and small . . no 

of declination . . . . . .112 

CIRCUMPOLAR CONSTELLATIONS .... 95 

CIVIL TIME . . . . . . .113 

twilight ....... 207 

CLOUD, formation of a . . . . 73!!. 

CLOUDS, composite ...... 75 

fundamental ...... 74 

COEFFICIENTS OF DEVIATION . . . 32ff., 193 

COL ISOBAR, the 205 

COMPASS. See Aero Compass. 

correction of a .... 43ff., 54!!. 

effect of banking on a . . . .31 

error ........ 9 

essential features in an aero . . 28f. 

landing ....... 45f. 

liquid used in a . . . . . .25 

magnetic 23ff. 

methods of swinging 43ff. 

needle, earth's effect on .... 4f. 

placing a . . . . . , . 27 

removal of bubble from . . . 26 

215 



INDEX 

PAGE 

COMPASS, shore ....... A^ 

to test a 66 

' COMPASSES FOR USE IN AIRCRAFT ' . . -31 
COMPOSITE CLOUDS . . . . . -75 
COMPOSITION OF THE AIR ..... i 93 f 

CONCAVE, celestial in, 114 

CONDUCTION OF HEAT 198 

CONSTANT DEVIATION 33 

CONSTELLATIONS, circumpolar .... 95 
CONTRACTION IN AERO COMPASS ... 29 

CONVECTION OF HEAT 198 

COOLING OF THE ATMOSPHERE . . . i97f 

CORONA BOREALIS . . . . . . 102 

CORRECTION OF A COMPASS . . . 43!!, 

of courses. . . . . 

CORVUS . . . . . . . 107 

COURSE, to lay off a . . . ; . 167, 1 88 
COURSES, correction of ..... 57ff 

CRUX . . '. . . . , - -. . 103 

CYCLONES . . . . . .68 

secondary , - , . . . 202 

CYGNUS . . . . . ; >. -; . 103 

DAVIS AND BURDWOOD . . . . 127, 129 

DECLINATION, circles of . v i V '' , - . .112 
parallels of . . . . . .112 

DEFLECTION OF WIND DUE TO THE EARTH'S 

ROTATION ....... 2oof. 

DENSITY OF THE AIR . . . . . 194 

DEVIATION, angle of . . * . ... . 8 

analysis and adjustment of . . - . 32ff., 4if . 
apparent or constant . ..-.-. 33 
coefficients of . . . , '. . . '- . 32fL, 193 
effect of vertical iron on . ..... 4of . 

how to name '. f -. ,' , *- .""' '.-. ; '- 59ff. 
natural . ,- ; , . .' . .- * . 55 
quadrantal iijfV ' * ; J 6, 38 
semicircular 1 6, 34, 36 

2l6 



INDEX 

PAGE 

DEVIATIONS, analysis of a table of . 4 if. 

DIP MAGNETIC ANGLE . . . . 5, 9 

DISTANCE, to measure . . . . 151, 188 
DIVIDED TOUCH IN MAKING MAGNETS . . 12 

DRIFT DUE TO WIND, to allow for . . . i6yf. 
DUNBOYNE'S WEATHER REPORT .... ySf. 

EARTH, axis of the no 

effect on compass needle ^i. 

poles of the . . . . . .no 

ECLIPTIC PATH 112 

ELECTRO MAGNET IN MAKING MAGNETS, use of . 13 
ELEMENTS, meteorological .... 194-9 

' END-ON ' MAGNET i8f., 22 

ENGLISH CHANNEL, weather in . . . . 8 if. 

EQUATION OF TIME 117, 1231. 

EQUATOR . . . . . . . .no 

equinoctial or celestial . . . .112 

magnetic ....... 8 

EQUINOCTIAL POINTS . . . . .112 

ERROR, compass ...... 9 

EXPANSION IN AERO COMPASS .... 29 

FAHRENHEIT THERMOMETER .... ig8i. 

FATHOM LINES USED ON ADMIRALTY CHARTS . i58f. 

FITZROY'S WEATHER RULES, ADMIRAL . . . 

FLYING GROUND, methods of marking out a . 

swinging a compass by marked-out . . 47 

FOG, cause of . . . . . . -75 

FORCE, horizontal . . . - . . . . 9 

lines of 2f., 8 

method of denoting wind ' .. ^ . 77 

vertical . . . . ' ' .- . . 10 

FORECASTING OF WEATHER . ','-.' 76, 86ff. 

FORMATION OF CLOUD, FOG, AND DEW ' . ' . 73fL 

FUNDAMENTAL CLOUDS . - . ' . ~ 1 ' . 74 

GEMINI . . . . . .... 104 

GEOGRAPHICAL POLES . ' . * . . 2 

217 



INDEX 

PAGE 

GNOMONIC CHART 143!. 

G.M.T. u8f. 

GRADIENT, vertical pressure . . . 195!, 1991. 

wind 2oyf. 

GREENWICH OBSERVATORY n8f. 



' HANDBOOK OF METEOROLOGY ' ... 199 

HAVERSINE TABLE . . . . . -. 127 

HEAT, radiation, conduction and convection of . 197!. 

HEATING OF THE ATMOSPHERE .... 1971. 

HEAVENS, poles of the ..... in 

HEIGHT OF THE AIR ..... 194 

variation of wind velocity with . . . 2O5if. 

HIGH PRESSURE AREAS ..... 209 

HORIZONTAL FORCE ..... 9 

HOSTILE AIRCRAFT, intercepting .... i6g&. 
HOUR ANGLE ..... . .121 

to find ....... i4of. 

HUMIDITY IN THE AIR ..... 197 



INMAN'S TABLES . . . . i26f., 138, 
IRIDIUM POINTS, use of ..... 24 
IRON ON DEVIATION, effect of vertical . . 4of. 

effect of magnetism on hard and soft . . I5f. 
ISOBAR LINE . . . . v . 69, 77 

ISOBARS, different forms of ... 202-5 

ISOTHERMAL LINE . . . . 69, 77 

LAND AND SEA BREEZES ..... 205 

LANDING COMPASS . ... i . 45f. 

LANDMARKS, selection of suitable . . . 192 

LATITUDE, magnetic . . . . . . 9 

of a place . ' v - . in 

parallels of . . . . , - in 

LAW OF MAGNETISM, first . ... 3 

LEO . ; ,' V . . ... 104 

LIGHT ABBREVIATIONS . . . .1 . 161 

218 



INDEX 

PAGE 

LIGHTING OF AERO COMPASS .... 29 

system of . . . . . . . i6$i. 

LINES OF FORCE ..... 2f., 8 

LIQUID USED IN A COMPASS .... 25 

LOCAL TIME . . . . . . .119 

LODESTONE /....... i 

LONGITUDE OF A PLACE . . . . . in 

on time, effect of . . . . .117 

to construct a scale of .... 165! 

Low PRESSURE AREAS 209 

LYRA ........ 105 

MAGNET BLOCK 31 

' broadside-on ' . . . . . . 2off . 

electro ....... 13 

' end-on ' . . . . i8f., 22 

MAGNETS, artificial ...... 2 

divided touch in making . . . .12 

effect of temperature on . . . i5f. 

methods of making . . .. . . nf. 

natural ....... if. 

percussion in making . . . . .11 

single touch in making . . . . nf. 

MAGNETIC ALLOY . . . . . .14 

compass 23fL 

dip angle . .... 5. 9 

equator ....... 8 

latitude 9 

meridian ....... yf. 

poles . . . . . . 2, 8f. 

MAGNETISM, elementary . . . . . iff, 

first law of ...... 3 

on hard and soft iron, effect of . . i5f. 

sub-permanent ...... i6ff. 

' MANUAL OF NAVIGATION, ' ADMIRALTY . . 80, 83 

MAPS, Ordnance ...... i86ff. 

use of squared . . . . . iSgft. 

MEAN SOLAR TIME . . . .. ... ". 116 

219 



INDEX 



MEASUREMENT OF PRESSURE OF AIR . . . 199 
of temperature . . . . . .198 

MEASUREMENTS, distance . . . . 151, 1 88 

MERCATOR'S CHART ...... i45ff. 

MERIDIAN, magnetic ...... yf. 

passage . ... . . . 121 

prime . . .-> . . . . in 

MERIDIANS ....... nof. 

standard . 119 

METEOROLOGY GyfL, 194-9 

' METEOROLOGY, HANDBOOK OF ' . . . 199 
MOONRISE AND MOONSET, how to find time of . i32ff. 

NATURAL DEVIATION 55 

magnets ....... if. 

NAUTICAL ALMANAC, Abridged . . . 125, 136 

' NAUTICAL ALMANAC TABLES ' .... 123!?. 

NEEDLE OF COMPASS, earth's effect on. . . 4f. 

ORDNANCE MAPS ...... i86ff. 

ORION . .... 97, 105, 109 

OSBORNE, R.N., CAPTAIN F. CREAGH . . .31 

PARALLELS OF DECLINATION . . . .112 

of latitude . . . . . . . in 

PASSAGE, meridian ';. . . . .121 

PATH OF A STORM 68 

PEGASUS .... ' 98, 105 

PERCUSSION IN MAKING MAGNETS . . .11 

PIVOTS, broken . .'.. . . . . 30 

PQLAR DISTANCE OF A HEAVENLY BODY . 113 

POLE STAR ,.'. > . t . . . 96 

POLES, angle of variation . . . . y 

geographical V ; * . , . 2 

magnetic . . . . . 2, 8f. 

of the earth : '. ; . . . . - no 

of the heavens . . . . ." .. in 

of the magnet . . . . . . :0 2, 8f. 

220 



INDEX 



POSITION BY CROSS BEARINGS, fixing . . . 

by doubling the angle on the bow . .179 

by station pointer ..... iSoff. 
PRECESSION OF STARS ..... 95 
PRESSURE AREAS, high and low .... 209 

gradient, vertical .... i95f., 1991. 

of the air . . r . . . . 194^:. 

measurement of, of the air . . . .199 

PRIME MERIDIAN . . . . . . in 

PRISMS . . ...... 30 

PROCTER, R. A. . . . . . -93 

QUADRANTAL DEVIATION . . . . 16, 38, 193 

RADIATION OF HEAT ..... I97 

RADIUS OF ACTION OF AN AEROPLANE . . 2ioff 
REFLECTORS ....... 30 

RIGHT ASCENSION . . , . . .113 
ROTATION OF STELLAR SPHERE ... 95 
RUBY POINTS, use of. . . . . .24 

SAPPHIRE POINTS, use of . . . . .24 

SCORPIO ........ 107 

SEA BREEZES ....... 205 

Disturbance Scale, Beaufort's . . . 85 
nature of . . . . . . . 159 

SECONDARY CYCLONES ..... 202 

SEMICIRCLES OF A STORM ..... 68 

SEMICIRCULAR DEVIATION . . . 16, 34, 36 
SHORE COMPASS ...... 4^. 

SIGNALS, storm ....... 82f. 

SIGNS USED ON ADMIRALTY CHARTS . . . i54ff. 
SINGLE TOUCH IN MAKING MAGNETS . . .. nf. 

SOLAR TIME, apparent . . ..... 116 

mean . . . ,. . . , 116 

SPHERE, definition of a . . . ~ -. .110 
SQUARED MAPS, use of . . m . .-..< f - 



STANDARD MERIDIANS . . . . . 119 

221 



INDEX 

PAGE 

STAR PRECESSION ...... 95 

swinging a compass by a . . .44 

STAR ATLAS, PROCTER'S ..... 93!. 

STATION POINTER, fixing position by . . . iSoff. 
STELLAR SPHERE, rotation of . . '95 

STORM, centre of a 68 

path of a . . . . . . .68 

semicircles of a 68 

signals ....... 82f. 

trough of a .68 

STRAIGHT ISOBARS 203 

SUB-PERMANENT MAGNETISM .... i6ff. 

SUMMER TIME 119 

SUN, mean . . . . . . .116 

swinging a compass by the .... 44 

SUN'S TRUE BEARING TABLES . . . i2yf. 

SUNRISE AND SUNSET, how to find time of . . 1 2gft 
SURGE IN THE BAROMETER . . . .71 

SWINGING A COMPASS, methods of ... 43ff. 
by distant object ., . . .46 

by marked-out flying ground ... 47 
by reciprocal bearings ..... 45 

by sun or star 44 

by two objects in line ..... 47 
SYMBOLS USED ON ADMIRALTY CHARTS . . i54*f. 

TABLE, HAVERSINE . . . . . .127 

of deviations, analysis of a . . . . 4 if. 

TABLES, amplitude 138 

explanation of various . . . .1235. 

Inman's ..... i26f., 138, 1481!. 

sun's true bearing or Azimuth . . . i27f. 

TAURUS * , ; . i . . . 106 

TEMPERATURE . . ; . 197 

measurement of ... . . . . 198 

on magnets, effect of . . . . . i5f. 

TESTING A COMPASS . . ... . . 66 

222 



INDEX 



THERMOMETERS ...... 

TIDAL ABBREVIATIONS ..... 160 

TIME AMPLITUDE ...... 138 

astronomical . . . . . .115 

effect of longitude on . . . . .117 

equation of ..... II 7> I2 3 J ' 

local . . . . . . . .119 

notes on ....... 113^. 

solar . . . . . . . .116 

summer . . . . . . .119 

TORRICELLIAN VACUUM ..... 195 

TOUCH IN MAKING MAGNETS, single and divided . uf. 

TRIANGLE OF VELOCITIES ..... 209f. 

TROUGH OF A STORM ...... 68 

TRUE BEARING TABLES, SUN'S . . . -. i2yf. 

TRUE BEARINGS TAKEN FROM THE TABLES . . 62ff. 

TWILIGHT, ASTRONOMICAL AND CIVIL . . . 207 

UPPER MERIDIAN PASSAGE . . . .121 

URSA MAJOR ...... 96, 108 

URSA MINOR ...... 96, 106 

V DEPRESSION ISOBAR ..... 204 

VACUUM, TORRICELLIAN ..... 195 
VARIATION, angle of (magnetic and geographical) . 7! 

of wind velocity with height . . . 2056. 

VELOCITY WITH HEIGHT, variation of wind . 2051!. 

VELOCITIES, triangle of . . . . . 2O9f. 

VERNIER PLATE . '. .". . . . 196 

tube . . . . ." . . 195 

VERTICAL FORCE . . . . . . 10 

pressure gradient . , . . i95f., i99f. 

WEATHER, forecasting of .... 76, 86ff. 

in the British Isles . . r . . 8off. 

in the English Channel . . . . 8 if. 

method of denoting . ' . . . . 78 

notation, Beaufort's system of . . . 84! 

223 Q 



INDEX 



PAGE 



WEATHER report, Dunboyne's . . . . j8i. 

rules, Admiral Fitzroy's .... Sgff. 

WEDGE ISOBAR ...... 203 

WIND, cyclonic . . . . . . .68 

due to the earth's rotation, deflection of . aooff. 

force, method of denoting . . . 77 

gradient ....... 2oyf. 

notation, Beaufort's system of ... 83f. 

to allow for drift due to .... i6yf. 

velocity . ... . . . 



ZERO, absolute . . . . . .399 



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