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Contents

Introduction

Market Report

Find out what sort of prices you can expect with the market reports.

 Article

Read more about these popular compasses in the article on WWI Pocket watch compasses.

Article

Intrigued to know what all that compass jargon means.

Article

In my view Singer's Patent was an iconic design of the 1860's.  So who was Singer?

Article

World War 1 prismatic compasses prove very popular at auction, but what were their origins?

Article

World War 1 prismatic compasses prove very popular at auction, but what where were their origins?

Article

Old Italian compass

How compasses were a few hundred years ago

Book Reviews

Books on compasses that are invaluable reference material

Useful Links

And finally some useful links to compass sites around the world.

Now for some foreign compass roses......

Chinese

Chinese compass markings

French

French compass markings

German ↓

German compass markings

Hungarian ↓

Hungarian compass markings

Swedish ↓

Danish compass markings

Anatomy of a pocket compass

 
   
  To start I want to described some basic types of compass.  The number of times I have seen a compass described as something it is not is quite amazing.  There are many types of pocket compass and I want to categorise them into their main classes.  In some instances the compass is a combination of elements from  two different classes of compass.  
  Closed face compass

Quite simply a compass with a lid that covers and protects the crystal.  One of the best known examples is the WWI pocket watch compasses also referred to as Hunter cased (after the pocket watch design).  Sometimes the lid is hinged, other times it lifts off completely.

Closed face pocket compass

 
  Open face compass

Full marks, you are catching on quickly, in this type of compass there is no lid.

Open face pocket compass  
  Prismatic Compass

To qualify as a prismatic compass there must be a prism, usually mounted in a protective case for reading the scale when taking a bearing.  This is easily spotted by the presence of  a triangular hinged prism case at the rear of the compass.  A good example of these compasses is the military compasses commonly available in the UK.

Military Prismatic compass  
  Lensatic Compass

In this compass there is at least one lens mounted in the rear site and sometimes a second as an integral part of the crystal for reading the scale.  The American Lensatic used by the US military is the obvious example.  Lensatic compasses were available from the English manufacturer F Barker & Son as early as 1910.

lensatic compass  
  Transit compass

For me a transit compass has to have a front and rear transit sight.  There are compasses where the rear sight is a single prism and these are sometimes categorised as transit compasses  If you examine most prismatic and lensatic compasses you will see they have a front sight. 

Maybe Prismatic and lensatic compasses should be a subclass of Transit compass?

 

Transit compass  
  Baseplate compass

This is a very simple design, where the compass is mounted on a transparent baseplate.

baseplate compass  
  This has covered the main generic versions, there are invariably combinations of design so feel free to tell me about them for inclusion in an update of this article. 

 

 
  Next we look at the compass markings and then the working parts of the compass.  
 

The Compass Rose

Compass rose showing cardinal points

 

The compass rose evolved from the main points of a map where in the very earliest days they were used to identify the direction of the wind.  The Fleur de Lys denotes the North of the compass and will often be found on older compasses.

The Cardinal points are N,E,S and W and the Half Cardinals NE, SE, SW and NW.  From there the points were broken down into 8 False points (e.g. NNE, WSW etc) and 16 By points (e.g. W by WSW).

Very early compasses required less precision and so had fewer points marked and this is often a good indicator of the age of a compass.

 
 

 

As greater precision was required so compasses were marked in degrees and later mils.

degrees and mil rose 

This compass rose is clearly graduated in degrees (0 to 360) and Mils (0 to 6400).

 
  Where do these scales originate?

The 360 degree scale developed for nautical use and the 6400 mils for military/field use.  The use of mils is frequently found on military or more accurately artillery compasses where 1 mil represents 1 yard at 1000 yards or 1 meter at 1000 meters.  It is worth noting that the French Government, during the French Revolution, mandated that the circle would have 400 grade as part of their decimalisation process [1], where a right angle was divided into 100 grade.  This marking, though not common, is still in use today. and Swedish and Swiss compasses can be purchased graduated in 400 gon.  You will find examples of French and Northern European compasses from the 19th and 20th Century that are graduated 0 to 400.  The French even introduced decimal time during the French Revolution, but this failed to catch on! The use of a 400 degree scale is unusual and requires the the recalculation of trigonometric tables where survey work is involved.

The Chinese divided their compasses into 24 points instead of the 32 found elsewhere,  The term points comes from the points on a Pelorus.

 
  The Compass parts

Having established the the markings we can now concentrate on the parts of a compass.  I have chosen a French open faced pocket compass since it clearly illustrates the main parts.

The Variation marking is often only found on older compasses.  This mark is specific to a particular location and point in time and is the difference between magnetic north and true north. In America it is also known as Declination, but to be technically precise Declination includes both the variation and dip of the magnetic field.  In many modern digital compasses it is possible to set the Variation value, as is the case in some modern baseplate type compasses.  The National Geophysical Data Center link on the Useful Links page is a resource that can be used for decoding these marks and therefore establishing the approximate period the compass.  Variation should not be confused with the term Deviation which is a marine term that refers to the effect that ferrous metal in a ships hull has on the ships compass.

The Needle (or card in the case of a floating card compass) is mounted on a bearing, typically either jewelled, as in this case, or brass or some other suitable material.  This bearing allows the needle (or card) to rotate freely and point North.  Within the case the needle (or card) can be either air damped or liquid damped.  In this case we have air damping and this has limited effect on the needle as it settles at North.  Liquid damped compasses by comparison settle more quickly and have a different response.

The Transit lock when fitted lifts the needle off the pivot and presses it against the crystal.  This both prevents wear to the pivot and provides a means of fixing a bearing that has been taken.  For obvious reasons liquid filled compasses do not have transit locks.  In some cases there are two locks, one for transporting the compass which might be operated as in this case by a sliding button at the side, or by  a button that operates when say a lid or one of the sights is closed.  A second lock is in the form of a button that momentarily locks the bearing whilst it is read.

The Compass Rose, in this case, marked on the base with the various Cardinal marks can have the degrees (or mils) markings at the edge of the rose.  In this case there is a Chapter ring marked every 2 degrees raised above the compass rose.  A floating card compass would have the compass rose marked on the card itself.

The glass covering the compass is called the Crystal and can be either bevelled (as in this case) or flat (where it is flush with the top edge of the case).  In modern compasses the use of glass is frequently replaced by plastic where it might become an integral part of a bezel moulding.

The Bow, where present, is used so that a lanyard of some description can be attached.  In some cases (e.g. the lensatic compass) it is a means of steadying the compass when a bearing is taken. The bow can also be known as the bale.

 
  References

[1] -The Measure of All Things - Ken Alder p150 (ISBN '0-316-85989-3)

Acknowledgments

My thanks to David James, a British Army Navigation Instructor who has clarified and corrected my original notes.

 

 
 

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