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
Solar compass as was suggested to him on the faith he would get paid for his patent of such a valuable instrument. However the compensation indicated did not materialize in Burt's lifetime or at any time thereafter. Since there was no patent on Burt's solar compass after 1850, instrument makers sold "Burt's solar compass" to surveyors. In the preface to his " Key to the Solar Compass and Surveyor's Companion " published in 1858 by his associate William S. Young, Burt refers to the many requests for such a book on how to use his solar compass. He explains a magnetic compass