horizontal

A direction parallel to the horizon, or what is commonly termed lying flat. One of the greatest inconveniences navigators have to struggle with is the frequent want of a distinct sight of the horizon. To obviate this a horizontal spinning speculum was adopted by Mr. Lerson, who was lost in the Victory man-of-war, in which ship he was sent out to make trial of his instrument. This was afterwards improved by Smeaton, and consists of a well-polished metal speculum about 3-1/2 inches in diameter, inclosed within a circular rim of brass, so fitted that the centre of gravity of the whole shall fall near the point on which it spins. This is the end of a steel axis running through the centre of the speculum, above which it finishes in a square for the convenience of fitting a roller on it, bearing a piece of tape wound round it. The cup in which it spins is made of agate flint, or other hard substance. Sextants, with spirit-levels attached, have latterly been used, as well as Becher's horizon; but great dexterity is demanded for anything like an approximation to the truth; wherefore this continues to be a great desideratum in navigation.