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The Marine Chronometer: Its History and Development, John Cronin

The Marine Chronometer: Its History and Development, John Cronin


Maritime chronometers are rare and precious items, crafted with care and precision, to provide an accurate means of measuring time and determining longitude at sea. Developed in the eighteenth century, these beautiful instruments were produced for the next two hundred years to the same design, and played a significant role in the growth of maritime trade, ultimately helping to shape the world as we know it today. In Marine Chronometers, the author explains the workings of the chronometer and highlights some of the most significant makers and provides hints for maintenance. Topics covered include: The problem of longitude The early sea clocks Developments in France and England The mechanism of the chronometer Caring for chronometers.


  • 出版者 ‏ : ‎ The Crowood Press; Reprint版 (30 六月 2010)

  • 語言 ‏ : ‎ English

  • Hardcover ‏ : ‎ 112 頁

  • ISBN-10 ‏ : ‎ 1847971857

  • ISBN-13 ‏ : ‎ 978-1847971852

  • 商品重量 ‏ : ‎ 455 盎司

  • 尺寸 ‏ : ‎ 16.51 x 1.27 x 24.13 cm


The Marine Chronometer: Its History and Development, John Cronin
The Marine Chronometer: Its History and Development, John Cronin


Marine chronometers are rare and precious items, crafted with care and precision, to provide an accurate means of measuring time and determining longitude at sea. Developed in the eighteenth century, these beautiful instruments were produced for the next two hundred years to the same design, and played a significant role in the growth of maritime trade, ultimately helping to shape the world as we know it today.

Maritime history enthusiasts and clock collectors alike regard these instruments as prized possessions and will find this book an invaluable reference work and a welcome addition to their bookshelves.















11. Chronometer Maintenance

Winding

A two-day chronometer should be wound, if possible, every day (naval practice was that they should be wound at the same time by the same person). Simply invert the bowl in the gimbals to expose the base; the dust cover must now be rotated to expose the keyhole and the key inserted. Wind anti-clockwise; a day's winding will be about seven and a half turns. Wind until the key locks.


Starting and setting the hands

If the chronometer has been allowed to run down and stop, first wind it; if you avoid sudden movements the mechanism will not start. Unscrew the bezel to expose the hands; if you have an accurate time source such as a radio controlled clock wait until the two seconds hands match and give the whole instrument a sharp twist, this will start the chronometer. The hands can now be set with the winding key which will fit the square at the centre of the minute hand; turn the hands forwards to set the time.


Transporting a chronometer

If you are carrying a chronometer over a short distance, simply lock the gimbals and keep the box more or less level; if you are careful there is no need to stop it going. The following special precautions must be taken to protect the balance and escapement if you are shipping a chronometer:


  • First unscrew the rear gimbal screw.

  • Next the movement is removed from the case. Unscrew the bezel and invert the case.

  • The complete movement with the dial should drop out of the case. It may stick slightly-if so insert the winding key in the back to push the movement forward.

  • Now cut three wedges from cork and push them under the balance rim.

  • The chronometer may now be assembled and packed for shipping.

Note the small pin on the edge of the dial (usually at 12 o'clock). This must be located in the slot on the edge of the case. If it is being sent for servicing, just send the assembled case with the bezel firmly screwed down. Use a strong box with plenty of bubble wrap rather than polystyrene chips as these can compress and allow the instrument to move about. If the complete chronometer is being shipped, pack the box and movement separately.



Maintenance

Even with modern synthetic lubricants, which last longer than traditional oils, it is not advisable to leave a chronometer more than five years between servicing. The move- ment must be completely dismantled to clean the old oil off, it is then assembled, lubricated, tested and rated. Re-silvering discoloured dials will not devalue the instru- ment; the lacquer deteriorates over time and re-silvering has always been a routine process. Choose your repairer carefully as even very experienced clock or watchmakers find dismantling and assembling these instru- ments difficult. The detent and escape wheel are very delicate and easily damaged. In the UK the British Horological Institute has a website that lists qualified repairers who are experienced in this kind of work.


Regulating a chronometer

It is possible, with care, to regulate a chronometer without special skills. Traditionally, after servicing, chronometers were regulated to keep time within a second or two per day, then the rate was established (i.e. how much the instrument was gaining or losing per day). This was noted on a certifi- cate when it was returned to the owner and the navigator calculated the time by adding or subtracting the rate each day. Chronometers are always 'free sprung' - they do not have a regulating lever like a typical watch. Regulating is carried out by adjusting the timing screws on the balance rim. Most balances have two of these; they are the large split nuts located outside the rim at each end of the balance arm. Some balances also have two small timing screws adjacent to these for fine adjustment. The screws must be adjusted in pairs by exactly the same amount each to retain the poise of the balance.

Remove the movement carefully from the case as above (page 105) and rest it upside down on the upturned bezel (this will protect the hands and dial). To make the chron- ometer go slower the screws are turned anti-clockwise, and vice versa to make it gain. It is best to file up a small brass screw- driver with an indent in the centre of the blade to allow for the point of the screw protruding from the nut. The adjustment is very sensitive, so move the nuts by the smallest amount you can, the thickness of the slot will make several seconds per day difference. Remember to move both screws by the

same amount.


As you have the movement out of the case it is easy to set the instrument to the exact time: hold the balance wheel and wait until the seconds hand agrees with the time stan- dard, then release it. You can now adjust the hour and minute hands.


A word of warning - never wind the chronometer when it is out of the case. The movement has two winding squares; if you turn the wrong one you will release the mainspring and possibly cause a great deal of damage wait until it is safely in the case before winding in the usual way.

With a little patience most chronometers in good condition can be regulated to within a second or two per day.



John Cronin began his working life as an apprentice watchmaker and studied horology at Bradford Technical College, where he was awarded a gold medal and a British Horological Institute prize for the national exams, before being elected to Fellowship. of the Institute. He later went on to gain a history degree with the Open University and teaching qualifications at Cambridge; he taught history for some twenty years before returning to horological conservation. John was winner of the 1994 Artist Craftsman competition of the Worshipful Company of Clockmakers and is now closely involved with ICON (the Institute of Conservation). He is one of just ten horologists who have gained accreditation with the institute as professional conservators, and is the only member specializing in chronometer work.


Cover design: Maggie Mellett

Front and back cover photographs: John Cronin Inside back flap photograph: Richard Walton LPRS


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作者簡介 John Cronin studied horology at Bradford Technical College, where he was awarded a gold medal and a British Horological Institute prize for the national exams, before being elected to Fellowship of the Institute. John was the winner of the 1994 Artist Craftsman competition of the Worshipful Company of Clockmakers and is now closely involved with ICON [the Institute of Conservation]. He is one of just ten horologists who have gained accreditation with the institute as professional conservators, and is the ony member specializing in chronometer work. Resident - Cambridgeshire



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