Marine chronometer · Two-day marine chronometer

Thomas Mercer Two-Day Survey Chronometer Model S.C. 2

Partially verified

This two-day survey chronometer, Model S.C. 2, was produced around 1955 by Thomas Mercer in St. Albans, England. Developed for surveying tasks requiring sidereal time, it features a four-pillar, single-chain fusee movement with Harrison's maintaining power, an Earnshaw-style spring detent escapement, a helical palladium balance spring, and a split bimetallic balance. Its four-inch silvered dial presents a 24-hour display, subsidiary seconds with an aperture revealing an electrical contact wheel, and an up/down reserve indicator. The movement is mounted in an aluminium casing fitted with external electrical contact terminals and a start/stop control.

Top-down view of the Thomas Mercer survey chronometer drum, dial, and detached bezels — Dreweatts. Rights pending — permission not yet requested. Original source

Catalogue identity

Source: auctions.dreweatts.com
Category: Marine chronometer · Two-day marine chronometer
Form: Survey chronometer
Maker / attribution: Thomas Mercer
Reference: Model S.C. 2
Catalogue / lot identifier: 629
Source-reported date: circa 1955
Source-reported place: United Kingdom
Source-reported serial: 20867

Maker profile: Thomas Mercer · Thomas Mercer — history, key models and current watchmaking.

Source-reported attribution and specifications are not independent authentication. A listing does not establish present ownership, current display, availability or permission to reproduce.

Object description

Description and Components

The instrument is identified as a Thomas Mercer two-day survey chronometer, Model S.C. 2, dating to circa 1955. It features a four-pillar, single-chain fusee movement with Harrison's maintaining power and an Earnshaw-type spring detent escapement. The regulation is provided by a split bimetallic balance with cylindrical compensating weights and a helical palladium balance spring.

The dial is a 4-inch circular silvered display showing Arabic numerals in a 24-hour 'sidereal' format. It includes a subsidiary seconds dial with an open center revealing a ratchet-tooth wheel for electrical contact, and a power reserve dial (0-56) marked 'DOWN/UP'. The assembly is housed in a cylindrical aluminium case with external contact terminals.

Evidence for this section

“THOMAS MERCER, ST. ALBANS, MODEL S.C. 2 CIRCA 1955”
“The circular four pillar single chain fusee movement with Harrison's maintaining power”
“Earnshaw type spring detent escapement regulated by split bimetallic balance with cylindrical compensating weights, helical palladium balance spring”
“4 inch circular silvered Arabic numeral twenty-four hour 'sidereal' dial”
“subsidiary seconds dial incorporating open centre revealing ratchet-tooth wheel”
“DOWN/UP, 0-56 power reserve dial”

Military Provenance and Inscription

The instrument bears specific military identification markings. A printed metal label on the associated outer canvas field case identifies it as property of the Ministry of Defence, Hydrographic Department.

The equipment is marked with the stores reference H.S.527-3635 and serial number 20867, which is also noted on the dial.

Evidence for this section

“applied printed metal label inscribed MINISTRY OF DEFENCE, HYDROGRAPHIC DEPT., CHRONOMETER SURVEY, Stores Ref. H.S.527-3635, Serial No. 20867 to front.”
“DOWN/UP, 0-56 power reserve dial numbered 20867”

Condition and Observations

The movement is reported to be in working condition. Observations note slight discoloration or oxidation to the movement plates and mild oxidation to the dial silvering. The cylindrical case shows minimal scratching to the underside paint.

The outer canvas field case and leather pouch containing the typsy winding key are present. A wooden frame member within the case features a 'scribbled-over' inscription, the nature of which is not fully described.

Evidence for this section

“Movement is in working condition”
“The mechanism has some slight discolouration/oxidation to the plates”
“The drum containing the movement and dial is in fine condition with only minimal scratching to the paint to the underside.”
“faults limited to scribbled-over single word inscription to a wooden frame member adjacent to the movement.”

Open research questions

What is the specific nature of the 'scribbled-over' text on the wooden frame member?
What specific maintenance or service history exists for this chronometer beyond the provided source description?

Source evidence

Recorded claims and source quotations for Thomas Mercer Two-Day Survey Chronometer Model S.C. 2
Field	Source-reported value	Source quotation
Maker and Model	Thomas Mercer, Model S.C. 2	“THOMAS MERCER, ST. ALBANS, MODEL S.C. 2 CIRCA 1955”
Escapement and Balance	Earnshaw type spring detent escapement with helical palladium balance spring	“Earnshaw type spring detent escapement regulated by split bimetallic balance with cylindrical compensating weights, helical palladium balance spring”
Dial Configuration	Four-inch silvered Arabic 24-hour sidereal dial with power reserve	“4 inch circular silvered Arabic numeral twenty-four hour 'sidereal' dial with subsidiary seconds dial”
Serial Number	20867	“DOWN/UP, 0-56 power reserve dial numbered 20867”
Military Inscription	Hydrographic Department stores reference H.S.527-3635	“MINISTRY OF DEFENCE, HYDROGRAPHIC DEPT., CHRONOMETER SURVEY, Stores Ref. H.S.527-3635, Serial No. 20867”
Dimensions	17cm high, 28cm wide, 21cm deep	“The box 17cm (6.75ins) high, 28cm (11ins) wide, 21cm (8.25ins) deep.”

Visible details

24-hour silvered dial with blued steel spade hands and an open aperture at six o'clock showing contact mechanism parts
Flanged cylindrical aluminium casing fitted with two external terminal posts and an adjustment screw
Flanked by two detached circular metal bezels/covers, one with a central circular fitting and one glazed

Research qualifications

The outer green canvas field case, leather pouch, and typsy winding key described in the source are not depicted in this photograph.
Internal escapement and fusee mechanism details are source-reported and not visible in this dial-side view.

Photograph identity and source facts were screened before import; rights clearance remains pending.