A magnetic dry-card compass of a type patented by Henry Jennings in 1818 (no. 4259).

The compass bowl is made of brass. The card is mica covered with paper, and is marked in points, with a fleur-de-lys at north. The unusual graduations resemble those which used to be pasted into the bottoms of bowls when just a needle was suspended on the pivot. The bowl has no gimbals, although the patent drawings show two gimbal rings with four planes of gimballing. The compass is now held in a wooden, canvas-covered case with a leather strap (now broken).

The innovation of the design, Jennings claimed, was that its needle was insulated ‘from all action by iron in its neighbourhood, to which all other compasses are liable’. By this time, compasses were well known to be influenced by any iron placed near them, causing what is known as magnetic deviation, which causes errors in the heading indicated by a compass. In Jennings' design, this protection was provided by a layer of iron filings placed between the inner and outer skins of a double bowl, and by fitting semicircular strips of thin iron to the underside of the card at either end of the needle. The strips were also coated with a sprinkling of iron filings.

Jennings also submitted the design to the Board of Longitude (along with other ideas), hoping for a reward. And although later research showed that this was not an effective means of eliminating magnetic deviation, Jennings' compasses were successfully used on a number of occasions. One was used by captain John Ross on HMS ‘Isabella’ during the 1818 Arctic expedition, and is listed in the published account of the voyage. When tested at the Royal Institution, it appeared ‘to resist the action of iron not magnetical, while a number of other mariners also praised the device, including Admiral Penrose, who apparently considered it ‘the most important invention that he had ever seen’ and Edward Sabine, who found that the ‘instrument answered the purpose for which it was intended, and completely obviated the effect of local attraction; but the card being heavy, and the needle short and not very powerfully magnetized, it ceased to act when the variation was great’. John Ross also reported favourably on the compass a second time in the account of his 1839-42 voyage to the Antarctic, where he wrote that the compass ‘continued to act when no other compass would’.