Useful, easy, directions for seamen, who use Hadley's quadrant. [electronic resource] : Shewing, to the meanest capacity, how to hold the quadrant to take the Fore and the Back Observations of the Sun and of a Star; and the Meaning of the Observations. How to examine whether the Index-Glass, and the Fore and Back Horizon-Glasses, stand in a right Posture; and how to set them right when they do not stand right. The Meaning of what is called the Dip of the Horizon, or the Height of the Eye above the Water; and how to find it by the Quadrant. The Meaning of what is called the Resraction of the Air, and how to prove it. Many other very necessary Things, which every Seaman, who uses the Quadrant, should understand, if he be desirous of knowing the Meaning of what he is about. A Book of this Sort has been much desired by some Seamen, and it may be very useful to many.