Magnetic dial for latitude 52° North. The dial is printed on painted and varnished compass card (red around 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, North by a fleur-de-lys, and the remaining eight marked by stars. Underneath the hour scale is a portrayal of Father Time, winged and with a scythe and hourglass, at the South point. A snake biting its tail surrounds the hour-circle. A simple brass gnomon, which is partially corroded, sits on the compass card. On the dial-plate, it is signed 'S. Porter Fecit.'. The dial is described on the compass card as a 'Magnetic Sun Dial' and has a domed glass cover.

Set in the inside of the box lid is a paper circle (discoloured by age) adorned with the royal coat of arms and an inscription. 'Patronized by the KING. PORTER'S MAGNETIC SUNDIAL.' This dial is similar to AST0421, with the same compass card/dial-plate. However, it is probably slightly later as Porter has added the declaration that his instrument is 'Patronized by the King'. The inclusion of the description 'floating on an agate cap' indicates that a piece of agate has been used on the underside of the bar magnet, to lessen the friction between the magnet and the pin, allowing the dial-plate to turn more freely.

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