31: An eight day chronometer

14 02 2018

Some weeks ago, I received an e-mail asking about an eight day chronometer that the writer had recently inherited. He was able to tell me that it carried the name of John Carter of Cornhill, London and had been sold to the Admiralty in 1872. Its history between then and 1936 was not known, but in that year, it was sold by the Admiralty and in 1946 it had travelled to India with Ron’s uncle where it had remained on a bookshelf in a hot and humid climate until late in 2017.

After an exchange of e-mails, Ron sent me some photographs, which he has agreed I could share with others in my blog. Most of the photos can be enlarged by clicking on them. From Figure 1,  we can see that Carter did not actually make the movement. It was very common for retailers and even other makers to buy in finished movements and put their own names on the face. Makers would also buy from other makers if they had an order that they could not meet at the time. Parkinson and Frodsham as a company were active between 1801 and 1890. I cannot see a number anywhere on the movement that might give a clue to date of manufacture.

7 John Carter 738

Figure 1: Top plate inscription.

The form, size and layout of movements changed remarkably little from about 1800 until manufacture ceased in the 1980s and the same can be said for the bowls and cases. Two day chronometers of whatever make usually had three-tier cases forming a cube with sides of  185 mm, and heavy brass bowls of about 105 mm diameter. Almost always, the cases were brass bound with brass corners, presumably to hold them together in the marine environment if the glue and pins that held them gave way. Curiously, sextant cases, which would be a good deal more exposed to the elements than chronometer’s were almost never brass bound and the only one I have ever seen that was, other than so-called “reproductions”, is in my possession. Straight away, we see from Figures 2 and three, that my correspondent’s chronometer is an exception in the other direction.

1 john carter 738

Figure 2: Front of case.

2 John Carter 738

Figure 3: Side of case.

The front of the case has an interesting little escutcheon around the key hole, a bone or ivory lozenge, perhaps for the Admiralty to add their accession number to, and a broken button catch for the top lid. It is quite hard to find replacements for these little catches. The side view shows that the corners have rebate joints, another good reason for having the case brass bound, as they are less secure than dovetail or comb joints. We also see the two screws that secure the gimbals lock, the screw and rectangular washer that holds one side of the gimbals bearing and a standard drawer pull type of handle.

3 John Carter 738

Figure 4: Bowl in case.

On opening the case, we see a tipsy key  and gimbals lock whose forms did not change over time and between different makers of various nationalities. Unusual, however, is the way the tip of the gimbals lock engages with a brass band that nearly encircles the bowl, whereas usually it engages with a slot in a simple bracket that is attached directly to the bowl. The 15 mm thick walls of the case have dust seals of a darker wood, possibly ebony, let into the top of the bottom tier, except where the box lock and the hinges intervene.

4 John Carter 738

Figure 5: Face.

In an otherwise unremarkable 19th and early 20th century face is a clue that reveals the main interest of this chronometer (which is not to say that this venerable instrument is not desirable for other reasons). The state of wind indicator shows that the chronometer has a movement that runs for eight days. It is in a fully-wound state, while the minute hand is out of step with the seconds hand, whereas it should indicate a full minute when the seconds hand shows 60 (or zero, which amounts to the same thing).

Eight day chronometers are very uncommon and it is a little difficult to understand why any were made, since they would in practice be wound daily at the same time of the day to maintain a constant rate. The famous  and influential Captain Lecky, in his “Wrinkles in Practical Navigation“(ninth edition, London, 1884), writes “…it has been found that eight-day chronometers do not preserve altogether the same rate throughout the week; that is to say, that (though other conditions may be the same) their daily rate towards the end of the week will not agree with their daily rate at the commencement of it; notwithstanding which, the mean  rates of two consecutive weeks  may agree exactly.” He also adds “On account also of the lightness of the balance, the eight day chronometers do not go so well on board steamers which suffer much vibration from their machinery.”

Rupert Gould in his authoritative “The Marine Chronometer” confirms this when he writes (p.218) “...there is absolutely no advantage gained by making a chronometer go for more than two days between windings, and such machines are inferior both in principle and detail to the ordinary two-day pattern, although, if well-made, they may be found quite satisfactory in use at sea.

I reassured my new internet friend that it would not hurt to unscrew the bezel and tip out the movement. Figure 6 shows more details of the exterior of the bowl and the strange provision made for the tongue of the gimbals lock and the dust cover for the key hole. Note that the disc that houses it, attached with three screws, has the letters “H.S.”, followed by a broad arrow indicating government property, and the letter “I (or number 1). Perhaps we may tentatively wonder whether these letters refer to the hydrographic service of one of Britain’s former colonies.

6 John Carter 738

Figure 6: Exterior of bowl and GA of movement.

Turning to the general view of the movement, the most obvious difference from a two day chronometer is its exceptional height, necessary to accommodate a taller spring barrel and fusee, and that the rest of the movement, except for the centre wheel arbor, is sited between a sub-plate and the bottom plate. Figure 7 shows some more details of the spring barrel and fusee.

9 John Carter 738

Figure 7: The power source.

The the tall, large-diameter spring barrel accommodates about eight turns of  chain, while the fusee has about 16 turns, nearly twice as many as a two day chronometer, while the great or fusee wheel seems to have many more than the standard 90 teeth.

Another view of the power source (Figure 8) shows at lower left the elongated click of the maintaining power ratchet wheel attached to one of the pillars and, at top right beneath the top plate, the short pillar for  the fusee iron. This latter proved rather hard to photograph, so a verbal description will have to do. The fusee iron is shaped somewhat like a fork with two prongs. Its base is hinged to the short pillar and held off the top plate by a leaf spring. As the chain mounts the fusee during winding, it comes into contact with the underside of the fusee iron and raises it parallel to the top plate At about this point, a projection, called the snail, which is screwed on to the top of the fusee, enters the fork and brings winding to a halt. In the absence of  stop work, there is nothing to stop the heavy-handed from breaking the chain, with often disastrous consequences for the movement when the large amount of energy in the oversized spring is suddenly released. Also in Figure 8, the very large wheel for the state of wind indicator is seen at bottom right.

11 John Carter 738

Figure 8: Maintaining power.

This leaves the escapement, shown below in Figure 9. I have been able to show only the balance wheel and its associated parts The bi-metallic rim of the wheel is of course split adjacent to each timing weight which are situated as usual at the ends of the spokes, while large, wedge-shaped weights for temperature compensation are at roughly right angles to the spokes. The upper end stone seems to be of diamond mounted in a two-part balance cock. It is just possible to see the ruby locking jewel to the right of the spoke at seven o’clock and the ruby end stone of the escape wheel arbor to the left of the spoke. While in most modern chronometers the upper balance spring collet was attached to the top of the cock, in this instrument it has its own little pillar, and I wonder whether this might have been some form of compensation for middle temperature error, if the horizontal strip turns out on further examination to be bi-metallic.

Copy of 10 John Carter 738

Figure 9: Balance wheel.

I hope that this account may be of interest to those who, like me, have never seen an eight day chronometer “in the metal” as it were. Many more details of the structure of chronometers may be found in my book “The Mariner’s Chronometer”, available via amazon.com







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14 02 2018

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