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Harrison H4

John Harrison’s Successful Longitude Timekeeper, H4; a Reconstruction

Concerning H4, John Harrison said, “…I think I may make bold to say that there is neither any other mechanical nor mathematical thing in the world that is more beautiful, or curious in texture than this my watch or timekeeper for the Longitude…”. And “I hereby thank Almighty God that I have lived so long, as in some measure to complete it”.1

Sadly for Derek Pratt, he was not afforded the same luxury as Harrison, of having sufficient time to complete ‘his H4’, so shortly before his untimely death in 2009 he asked Frodshams if we would complete the timekeeper.

Having made a number of individual pocket watches, all with innovative ideas, Derek returned to his interest in Harrison in 1997, and decided to make his own copy of H4. The following years were spent researching every aspect of the timekeeper, and talking with other learned horologists to glean as much information as possible.

Derek had identified four areas in the fabrication that would be particularly challenging; the large (over 5 inches in diameter) silver pair cases, the dial, the pierced and engraved decoration and the diamond pallets. For the first three he inspired other highly skilled craftsmen to help; Martin Matthews who made the superb silver pair cases (London hallmarked for 2002), Jos Houbraken who made the beautiful enamel dial, and Charles Scarr for the exquisite piercing and engraving on the balance bridge, fly guard, and other components.

Harrison H4

Work on the movement began in earnest with the making of the frame, pillars, plates, brass edge, cover plate, balance bridge and winding key. This was followed by the fusee with its complex maintaining work, the mainspring and chain barrels, and the wheel train including the third wheel with its internally cut teeth.

Frodshams work to complete the timekeeper took over four years, and involved making many parts, including the remontoire components, the motion work, the compensation mechanism, the balance and balance spring, the balance staff, and the complicated jewelling, including the diamond pallets which are just 1mm x 2mm in size.


These pallets are very small and potentially very difficult to make in natural diamond. Unfortunately, Harrison did not leave any clues as to how he produced them. The method for the shaping and correct polishing of the pallets in H4 has taxed the minds of many horologists and diamond experts ever since.

The work undertaken by Derek Pratt and Charles Frodsham & Co. on the reconstruction would not have been possible without the kind assistance of a number of others involved in the study of Harrison’s H4, most notably Anthony Randall, Dr. Jonathan Hird and Jonathan Betts, to whom thanks is given.

Harrison H4

DP/CF H4 was included as a loan exhibit in Ships, Clocks and Stars: The Quest for Longitude, at the National Maritime Museum, Greenwich, 11 July 2014 – 4 Jan 2015. The exhibition then travelled to the USA in 2015/16, first at The Folger Shakespeare Library, Washington DC and then at Mystic Seaport, The Museum of America and the Sea, Mystic CT, before being transferred to The Australian National Maritime Museum, Sydney Australia in 2016.

A more in depth study of the construction of Derek Pratt’s H4, is available in various publications:

  • A Detailed Study of H4, a Work in Progress. BHI 150, June 2008, published by The British Horological Institute, 2010.
  • Derek Pratt, Watchmaker, Proceedings of a Memorial Seminar, September 2011, published by The British Horological Institute, 2012.
  • A three-part series on the making of DP/CF H4, The Horological Journal (HJ) February/March/April 2015, published by The British Horological Institute.
  • A Detailed Study of H4. The DVD; filmed and produced by Barbara Darby, available at (all profits go to the charity Prostate Cancer UK).

  1. John Harrison, An Explanation of my Watch or Timekeeper for the Longitude… (7 Apr 1763),
    Museum and Library of the Worshipful Company of Clockmakers, London, MS 3972/1.
Harrison H4.