An exceptionally rare early 19th Century English spring detent chronometer in a purpose made silver case. Full plate gilt keywind fusee movement. Harrison’s maintaining power. Chased and engraved cock, a bust depicted on the table, diamond endstone. Arnold “Z” type compensation balance, freesprung helical blue steel hairspring. Adjustable polished steel stud fixed by two polished steel screws and blue steel washers to a gilt cock. Very rare form of spring detent chronometer escapement combining features of Arnold’s and Earnshaw’s escapements. The detent in a slot unlocks towards the centre of the escape wheel as in a normal Arnold type, jewelled locking stone. Impulse is given by the flat face of the escape wheel teeth as in an Earnshaw type. Very unusually the impulse is given to a solid sapphire roller by a large delicate polished steel escape wheel. Screwed in jewelling, escape pivots with endstones. Signed white enamel dial with subsidiary seconds, Roman and Arabic numerals, gold hands with heart shaped tips, the seconds hand with a slotted steel arbor. Substantial purpose made plain silver Irish open face case, the back engraved with the Scottish Pollock clan crest and motto. Dublin Hallmarks for 1820 and maker’s mark “AON” in a rectangle. Fine three tier mahogany deck box with brass furniture, locking key. A very rare, perhaps unique, English chronometer. An insight to its design is given by the maker. Listed simply as "Robert Best, London, Clockmaker's Company 1783 - 1828, Bankrupt 1796". Robert Best is mentioned a number of times by Earnshaw in his Appeal of 1808 claiming to be the inventor of the spring detent. In the Appeal he states that on the way to display the components of the first spring detent watch in early 1782 to Thomas Wright he showed it to John Brockbanks and Robert Best. He refers to Robert Best as both John Brockbank's "shopman" and "foreman". Thomas Wright applied for the patent on Earnshaw's behalf. Earnshaw lists watchmakers who were supplied with movements stamped "Wright's Patent". Robert Best was among those listed which included Vulliamy, Barraud, Frodsham and Margetts. In December 1803 Robert Best gave evidence to the Board of Longitude giving his account of the meeting and other technical details. It contains the following:- "Examination of Mr. Robert Best, before the Board of Longitude, at the Admiralty, Dec. I, 1803, relative to Mr. Earnshaw's Timekeepers. ---- Mr. Best had been to Constantinople, where he was employed in selling watches. He was formerly a watchmaker at the Royal Exchange. He knew both Mr. Earnshaw and Mr. Arnold; he lived with Mr. Brookbank as shopman, or foreman, from about 1778 to 1782: he may have been four or five years there: he saw nothing (while he was there) of Mr. Brookbank's like a Detached Escapement." Best was also one of five watchmakers called upon to compare the technical differences between Arnold and Earnshaw chronometers. It is clear is that Robert Best was actively involved in the inception and manufacture of the earliest spring detent chronometers. This pocket chronometer appears to incorporate the most advantageous features of both Arnold and Earnshaw. The detent spring itself is in tension allowing it to be thinner. Impulse is given in a straight line thus avoiding the need for oil. The balance appears to be constructed after Earnshaw's method, turned from a solid piece rather than the laminae being rivetted. The affixes have then been well made and attached by screws to the arms. Neither Arnold or Earnshaw used steel for the escape wheel or a solid sapphire impulse roller. It may be doubtful either convey any benefit but does reflect that every endeavour has been made to ensure the chronometer is accurate and reliable over a long period. Two pinions in the train have been replanted at some time by making new brass bushes with eccentric pivots. The movement closely resembles those of Arnold. If from Arnold the serial number, 2056, would indicate a date of about 1808 (no Arnold watch is known with this number). The Irish casemaker is Arthur O'Neill of Dublin.