The Construction and use of the sea quadrant, commonly called Hadley's quadrant. Containing such directions, as are necessary to render the use of this noble instrument both easy and certain, particularly with respect to the back observation, which has been hitherto neglected for want of proper instructions. To which is added, an introduction, explaining the theory of the instrument in a plain and familiar manner, with correct tables of the sun's declination, which being calculated for four years are thereby rendered perpetual.