Magnetic dial for latitude 51° North, set in a round, turned ivory case with a screw-fit lid. The dial is printed on painted and varnished compass card (red around the edge, the remainder yellow), on the underside of which is a bar magnet, set on a point, to orient the dial correctly. Around the edge of the card is a compass ring with sixteen points marked, seven by English initials and North by a fleur-de-lys. The hour ring is numbered clockwise IIII-XII, I-VIII, and at the South point is a portrayal of a winged Father Time, with a scythe and hourglass. A snake biting its tail surrounds the hour-circle and a simple brass gnomon sits on the compass card.

The dial is enclosed under a domed glass cover, held in place with a twisted red string circle. Set in the inside of the box lid is a pink paper circle with the maker's inscription on it: 'S. Porter Fecit Feby, 16. 1824. Entered at Stationer's Hall.'. Samuel Porter seems to have been the first English maker of the magnetic dial. The images from his dial-plates often appear on later instruments. Father Time, with his scythe and hourglass, is a reminder of mortality, while the snake biting its tail is a symbol of eternity.

For more information regarding this dial please refer to the OUP & NMM catalogue, 'Sundials at Greenwich'.