A RARE TWO-DAY ORDINANCE SURVEY CHRONOMETERTHOMAS MERCER, ST. ALBANS, MODEL S. C. 2, CIRCA 1949The circular four pillar single chain fusee movement with Harrison's maintaining power, jewelled pivots for the escape and seconds wheels, Earnshaw type spring detent escapement regulated by split bimetallic balance with cylindrical compensating weights, helical palladium balance spring and jewelled endstone, the spotted backplate with spring set-up ratchet and blued-steel pillar and backcock securing screws, the 4 inch circular silvered Arabic numeral twenty-four hour 'sidereal' dial with subsidiary seconds dial incorporating open centre revealing ratchet-tooth wheel for opening and closing electrical contacts with the beat of the escapement at six o'clock, within annotation in red CAUTION: HANDS, FORWARD ONLY below signature THOMAS MERCER LTD, ST. ALBANS, ENGLAND across the centre and DOWN/UP, 0-56 power reserve dial numbered 18384 beneath twelve o'clock, with blued steel spade hands and red Arabic five minutes beyond the outer minute track, set beneath a screw-down chrome-plated bevel-glazed bezel incorporating dust-cap sealed winding square aperture to glass, mounted onto a disc collar stamped 18384 to underside incorporating START/STOP lever and insulated electrical contact screws engaging with sprung contact posts fitted to the exterior of the grey-speckle japanned cylindrical aluminium case, complete with typsy winding and hand setting keys, rating certificate dated 22nd March, 1949 and Mercer information sheet C 6/61, set in a metal angle reinforced baize-lined canvas covered field case hand-inscribed O.S. 821 A over M.O.D. arrow device to the lid and fitted with leather carry strap; with a ticker-tape recording chronograph, Thomas Mercer, model T. P. A., mid 20th century, the rectangular thick-gauge brushed aluminium box-type frame incorporating brass bushed pivots for the single going barrel movement regulated by an adjustable centrifugal governor, driving a milled aluminium wheel positioned beneath a tensioned jockey wheel to pull through ticker tape fed via a large spool across the top plate, past a pair of solenoids each independently operating a pivoted armature fitted with sharp 'pricker' to mark the ticker tape as it passes beneath, the front with WIND square, 2 SPEEDS, IN FAST and OUT SLOW selection plunger, ON/OFF lever and inscribed MERCER, ST. ALBANS. ENGLAND to centre over number 185 to lower right-hand corner, secured in its original mahogany-framed drop-front box with electrical feeds from paired external contact plugs labelled BATTERY, KEY and CHRONOMETER positioned on the right-hand end, the exterior lined in green painted canvas and with metal reinforced angles, the top indistinctly hand-inscribed W10/W 0002 over M.O.D. arrow device, the front with printed metal plate MERCER, MODEL T. P. A. / 2, MADE BY THOMAS MERCER LTD. ST. ALBANS. ENGLAND, with chrome winding key, contact feed cables, and Mercer information sheet C 7/61, (2).The chronometer case 17cm (6.75ins) high, 28cm (11ins) wide, 21cm (8.25ins) deep; the ticker-tape chronograph case 19.5cm (7.75ins) high, 49cm (19.25ins) wide, 17.5cm (7ins) deep. The chronometer making firm Thomas Mercer of St. Albans was established by Thomas Mercer (1822-60) at North Road, London in 1858 before moving to 45 Spencer Street, Clerkenwell in 1860. Mercer became an active member of the British Horological Institute (established in 1858) where he lectured on the need to modernise the industry and served as honorary treasurer from 1875-95. After occupying several addresses in Clerkenwell Mercer relocated the core of the business to St. Albans in 1874 setting up a factory behind his house at 14-15 Prospect Road. Although the bulk of the businesses work was undertaken in St. Albans Mercer personally made weekly trips to Clerkenwell to deliver/collect parts finished by specialist outworkers as well as visit the Horological Institute. In order to maintain a presence in London Thomas Mercer opened an outlet 18 Westmoreland Road, City Road in 1890. Succuss at the Greenwich trials was also always considered important to Mercer who entered instruments every year; he was placed second in 1881 and his son, Frank, came first in 1911. In 1900 Thomas Mercer accepted an invitation to judge at the Universal Exhibition in Paris but unfortunately caught a cold on the ferry crossing and died soon after his return. The business was subsequently continued by his successors becoming of the most prolific suppliers of chronometers and other timepiece for industrial, maritime and domestic purposes and continued trading in St Albans until 1984. The current chronometer was developed by Mercer for surveying purposes. Fitted with electrical contacts and rated to sidereal time the timepiece is designed to be used in conjunction with the ticker-tape chronograph and an Admiralty pattern 45 degree prismatic Astrolabe or theodolite (to allow accurate measurement of angular distance, or 'zenith distance' between a given star and the zenith). The very accurate timing of the zenith distance of a given star can provide precise 'local time', and, through comparison with Greenwich meantime, allow accurate longitude and latitude of the observer to be established.In practice, once the apparatus has been set-up, the seconds beats of the marine chronometer (synchronised with Greenwich meantime) will be recorded on the ticker tape at up to 1 inch intervals (depending-on the speed setting of the paper feed) through electrical connection to one of the solenoid-operated prickers. The second pricker is connected to a hand-switch (or key) operated by the surveyor observing the star through a theodolite (or prismatic Astrolabe). At the moment the given star reaches its maximum (or chosen) zenith angle the surveyor presses the switch which records this moment, alongside the regular marks of the chronometer, on the ticker tape. By knowing the time at which the chronometer feed was activated the respective time of the star reaching its zenith can be established by simply counting the pricks (for each second) up to the point of the mark created by the observation. Indeed, by taking a ruler and measuring the distance the observed mark occurs between two marks created by the chronometer feed, the time of the mark (in relation to Greenwich meantime) can be ascertained up to an accuracy of around one twentieth of a second.In order to create accurate maps, a location within 50 feet would need to be obtained; this corresponds to a timed observation within 0.03 of a second. With this in mind several observations would need to be taken and any errors in the rate of the chronometer ascertained and corrections factored-in before suitably accurate readings for location can be established. In practice the rate chronometer was often independently checked in the field (both before and after the observations) through comparison with against radio Time Signals transmitted from Greenwich. Markings to the lid of the chronometer box indicates that it was owned by the Ordinance Survey. As such it was likely to have been used alongside aerial photography for small scale maps, particularly in undeveloped or difficult locations, where the location data is governed by a series of points whose positions are fixed by radio Astro Observations.
