Compass Rose
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.
This compass rose is clearly graduated in degrees (0 to 360) and Mils (0 to 6400).
Where do these scales originate?
I have never found a consistent answer to why there is a 360 degree scale on the compass card. Personally the one that makes the most sense to me is that it was created by the Babylonians who used a Base 60 numerical system. This is the scale that has found universal acceptance. 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, this leads to a 6400 mils scale for military/field use. It is interesting to note that during the French Revolution, it was 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 when survey work is involved.
The Chinese divided their compasses into 24 points instead of the 32 found elsewhere.
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 and is another useful means of dating a compass. 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 Resources page is a useful link that can be used for decoding these marks and therefore establishing the approximate period of a 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, has the degrees (or mils) markings at the edge of the rose. In this case there is a Chapter ring marked every 2 degrees positioned 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 also a means of steadying the compass when a bearing is taken. The bow can also be known as the bale.
The compass rose can vary according to the market the compass was made for. Most of this website describes English compasses, but many other countries used their own markings, generally the change in markings is minimal.
A lot of compasses, particularly military ones used Radium based paint for a few years from around 1906 onwards. Radium is radioactive and an internet search will reveal plenty of information on safety measures to take. You should follow these and handle any such compasses with extreme care. I have been told that one of the issues is that the paint used dries out with time and forms a dust. I personally would not open a compass for this reason. Despite this, the use of Radium paint is a useful way to date a compass.
One question you sometimes end up asking yourself is about the originality of a compass. Today there are many reproduction compasses available on the internet. When you look at these compasses alongside an original compass, it is quite easy to spot any differences. If you are just looking for a display piece and have a limited budget then there will be plenty to choose from. If you are offered a compass and are told it is genuinely original and you are suspicious, then my advice is if in doubt, to leave well alone.