There is a curious Thomas Mercer Chronometer on EBay:

http://www.ebay.com/itm/160775...id=p3984.m1423.l2649

It has a 24 second break circuit timer on it. It looks as though it may be set up more of scientific than navigational work; however, most of the break circuits I’m aware of are in the 1 – 2 second range and used for super accurate sidereal measurements like astronomical tracking. It could have a military use, but 24 seconds is a really curious increment; does anyone have any idea what it might be for?

Whether scientific or military is was made for the British naval folks, but have no idea as to timeframe.

Marty

not necessarily!! THEY made chronometers for the Admiralty but THIS may not be one,although it probably is !!! . It appears similar to a surveying sidereal chronometer Konrad Knirim posted on IHC a few years back Walter Mason Stewart guessed close to the mark in his preface

https://ihc185.infopop.cc/eve/f...0103944/m/1673958367

Good memory, Tom! That Mercer does indeed look nearly identical. Interestingly, the little write up mentions Mercer SN# 15032 built to observe a 1948 eclips; the one on EBay is SN 15030. Break circuits do (or did) have a lot of uses including being used to synchrononise all clocks on a ship or network, but it doesn't take a 24 second clock to do that. 24 seconds is just a strange increment; i think too short for targetting & too long for star tracking. I'm sure it has some specific use. It is one-fith of a minute, but so what?

My guess is that it is civilian. The marking "Maker to the Admiralty" on the dial does not mean that this was made "for" the Admiralty, it is just an adverising thing to tell potential buyers that the company was chosen by the Admiralty to make some timepieces for them and in exchange the company is allowed to mention this on everything they produce to increase their sales and prices. I see no military markings on this example which you would expect for pretty much any timepiece, and especially for a higher retail cost one like this one.