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  Surveying Instrument Collection 


Thomas Mercer Ltd. (St. Albans, England)



Serial Number



Length 275mm, Width 210mm, Height 155mm


Thomas Mercer Chronometer


This instrument is used for providing time with sub- second accuracy and is commonly referred to as a marine chronometer. The instrument provides time over a period of 12 hours to an accuracy far better than is needed for ordinary astronomical field observations. It is a one-second chronometer consisting of three graduated dials. The largest dial indicates the hours (graduated from 1 to 12) and minutes, the medium dial indicates the seconds and the smallest dial indicates the mechanical spring tension which shows how soon the chronometer will need rewinding. It contains a spring powered clock mechanism regulated by a balance-wheel escapement. A special key is required for winding up the spring and a switch on the top of the box. An interesting feature of this instrument is the fact that the dials have some 'auxiliary' compensation. This is an addition to the standard balance, and is generally 'discontinuous', acting only in certain extremes of heat or cold.

There is a compartment on the left-hand side of the box containing several leads allowing the device to be connected through a suitable battery to a magnet which actuates a pen on a chronograph

History & comments

Thomas Mercer was born in 1822 and died in 1900. John Arnold (1736-1799) and Thomas Earnshaw (1749-1829) were responsible for the earliest production of marine chronometers in large numbers and in a standard form. While Arnold turned out a greater number of instruments, it was Earnshaw who developed the customary features.


In canvas bag


Glass cover is loose and the instrument is not operational

  • This Chronometer is one of two in the collection manufactured by Thomas Mercer Ltd. (See also 0117)
  • Catalogued by T. Ko
  • Updated by F. Pall


Manufactured in 1910 (approx). Catalogued in 1997

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