Compass has a half-life of only about 12 years, so a compass that contains 120 mCi of tritium when new will contain only 60 when it is 12 years old, 30 when it is 24 years old, and so on. Consequently, the illumination of the display will fade. Mariners' compasses can have two or more magnets permanently attached to a compass card, which moves freely on a pivot. A "lubber line", which can be a marking on the compass bowl or a small fixed needle, indicates the ship's heading on the compass card. Traditionally the card is divided into thirty-two points
Swirl How spot height for Swirl How, although it is included on the 1:50,000 and the height is stated as 802m. Birkett and Wainwright also quote 2,630 ft (802 m) whilst Richards and the British Mountaineering Council 1:40,000 map quote 2,637 ft. (804 m) Swirl How sends out ridges to the four points of the compass, each leading to further fells. Consequently, it also feeds the headwaters of four valleys. The ridge northward to Great Carrs is named Top of Broad Slack, Broad Slack being a ferociously steep grass slope climbing out of the Greenburn valley between neighbouring crags. The ridge is