A marine chronometer numbered 4584, by A. Johannsen & Co. I has a two-day full-plate fusee movement with four pillars, all fixed with blued screws. The movement is marked with the broad arrow. The fusee, with stop-work and Harrison's maintaining power, has a pipe round the winding square. There is an Earnshaw-type spring detent escapement and two-arm balance with a helical balance spring.

The movement is mounted in a three-tier brass-bound mahogany box with a push-catch for the lid and lock for the upper half. The upper half has a strut limiting opening to 90°. On the front of the upper half is screwed a rectangular tablet inscribed ‘A. JOHANNSEN & Co / TWO DAYS / 4584’ with a broad arrow. The box fittings are standard, with flush handles on the sides, brass gimbals and a ratchet-winding key. The push-catch and lock are surrounded by shaped brass inserts. There is a shield-shaped brass insert on the box lid. The underside of the box is covered with green baize. Inside the upper lid is a brass label-holder (empty). Inside the upper box half is an applied black label reading ‘MINISTRY OF DEFENCE / HYDROGRAPHIC DEPT. / CHRONOMETER MARINE / Stores Ref. H. S. 527-3634 / Serial No. 4584’. The box glass is missing.

A silvered-brass dial has Roman numerals and a large seconds dial has Arabic ten-second figures with straight batons at alternate five-second intervals. The dial is inscribed in the centre ‘A. Johannsen & Co., / MAKERS TO THE ADMIRALTY, / THE INDIAN GOVt & ROYAL NAVIES OF / Italy, Spain, & Portugal. / 149 Minories, London.’ In the seconds dial is the number 4584 and the broad arrow. The up-and-down dial, below XII, is marked with Arabic numerals in eight-hour intervals up to 56 and is marked 'UP' above zero and 'DOWN' above 56. The chronometer is in very good condition except for the missing glass.

Johannsen & Co. was established in London in circa 1859. Asmus Johannsen was Danish, and made chronometers for the navies of India, Italy, Spain, Portugal, Austria and China. His work was frequently submitted to the Greenwich Trials where it was awarded high marks. It was also submitted to Kew Trials.