The Mariner's Chronometer

Just to be clear, this blog is mainly about the marine chronometer, sometimes called a box chronometer. Some of them were made for use in the air and for use by surveyors, but usually do not differ fundamentally from the basic marine chronometer. I will not be writing about what is nowadays often called a chronometer: a mechanical wrist watch made to standards that meet the requirements of certain Swiss manufacturers’ bodies, nor will I be including what is properly called a  chronograph : a  watch with a stop watch function. However, I will cover a certain class of large watches designed for navigation, as I come across them.

There is very little difference between various makers and you can recognise a marine chronometer because it is in the form of a mechanical (as opposed to quartz) clock that sits in a heavy case face up in gimbals in a fine wooden case with a glass top. A clock is essentially a clockwork motor that is permitted to run down step by equal step by some sort of vibrating mechanism, a pendulum or balance wheel, that in return receives a little energy from the motor to keep it vibrating. The intermediate part that takes care of this exchange is called the escapement and it is this (and the quality of manufacture) that mainly distinguishes the chronometer from a common clock, as it has ben devised to interfere as little as possible with the balance wheel (makers soon learned that a pendulum clock was useless at sea).

Supposing you find a chronometer or are offered one for sale. How do you tell whether it is worth buying? A useful question to ask is “Does it go?” If answered in the negative, it may be because a former owner has carefully prepared it for storage by wedging the balance wheel so that it cannot vibrate and be damaged by careless handling. Hamilton Model 21 chronometers mostly have a special mechanism to do this. It may be that someone has attempted to wind it clockwise with the key provided and found that it just turns round and round without anything happening inside or it may be that it has been wound anticlockwise (correctly) and that nothing has happened on completing the wind. This is because chronometers, unlike lever clocks and watches, are not self starting.

If it does not go, ask the owner if you may wind it and try to start it by giving a brisk turn through about thirty degrees with the face up. If this starts it and the second hand moves half a second at a time with two ticks, a loud one and a much softer one per half second, you have a running chronometer. If it jumps through full seconds, or ticks irregularly you still have  a running instrument but one that needs attention and that should be stopped as soon as you can.

If you cannot start it, (assuming that it is not a Hamilton Mod.21, which will need another post) ask if you may look inside. To do this, you will have to remove it from its gimbals. Tilt the gimbal ring forward so that the 12 is uppermost and rotate the case through 90 degrees. This will allow you to loosen a circular locknut and remove a large screw that acts as a pivot at the top of the ring (Figure 1) and you can then lift the chronometer off the other pivot and manoeuvre it out of the box. You can enlarge the photos by clicking on them, returning to normal by using the back arrow at top left.

Figure 1 : Removing the chronometer from the gimbals

It is best at this point for you to be wearing close-fitting light rubber gloves to avoid marring the internal finish of the instrument by sweat or dirt. Unscrew the heavy glass top and place it aside and then invert the movement into the fingers of one hand placed around the perihpery of the dial, well away from the hands. With any luck it will fall out of the case on to your fingers (Figure 2) and you can then place it back into its case face downwards. There is a little peg at 12 o-clock that fits into a slot in the edge of the case.

Figure 2 : Removing movement from case.

Most chronometers will look something like in Figure 3, unless it is an Ulysse-Nardin or Hamilton Mod 21, when it may look something like in Figure 4. In the latter’s case, you can see the Y-shaped locking fork.  The balance is the ring shaped thing with a helical spring in its centre.

Figure 3 : Movement out if the case.

Figure 4: Hamilton Model 21.

If there are slips of cork underneath the balance, ask the owner if s/he would remove them (carefully, using tweezers) and then try again to see if the instrument will start. If it will not and it is fully wound, you are faced with a bill to overhaul it of US$250 at the least and probably quite a lot more if there is a broken part to be renewed or repaired. This may be well worth while and add value to a chronometer dating from before 1940 and you may well feel that you would like to own a nineteenth  century chronometer, whatever its condition. If it is a mess of rust and verdigris it is probably best left with its present owner unless offered at a price that cannot be refused.

“The Mariner’s Chronometer” will take you safely through the overhaul of most chronometers, which show remarkably little variation between makers from about 1840 onwards, but of course, you embark on this  at your own risk. I well remember that it took me a few days to pluck up the courage to  start an overhaul that at the end of the day may have converted a going instrument, worth perhaps US$2000 on a good day, into an expensive ornament. However, there are really only three delicate components that require more than ordinary care: the balance staff, the detent and the escape wheel. Buy the book and learn more about them. If you have pots of money and bags of time, entrust it to someone with experience of chronometers (a diminishing bunch) and ask if you may watch him or her at work.