The Kamal is a simple Arabic instrument to measure the hight of the pole star from the horizon. The navigator keeps the exact knot between his teeth, for one known latitude ( for example his homeport), and he looks at the pole star above, and the horizon at the bottom of this horizontally kept Kamal.
When he sails southward the pole star will drop. The moment the pole star is at the required hight, or at the ship is at the required latitude, the navigator starts sailing straigth west of eastward to his known port of initial departure, his homeport.