DEPRESSION (Fr. depression, from Lat. deprimerc, from de, down + prcmcre, to press), or DIP OF TILE Honizox. The angle through which the sea-horizon appears depressed in consequence of the elevation of the spectator above the surface of the water.
Let A he a point on the surface of the earth, B a point situated in a vertical line from A. Let BH he a tangent to the earth's surface drawn from B, Bit a line in the same vertical plane perpendicular to AB. The angle hB11 is the true dip of the horizon to a spectator at B.
The true dip measured in minutes is equal to the distance in nautical miles of the visible horizon. Let C be the centre of curvature of the surface; then, since CHB is a right angle, the angle 1/13f1 = IICA ; and the minutes in this angle are the nautical miles in the arc AH.
The true dip of the horizon. however. is not exactly the same as its apparent depression. The apparent sea-horizon is raised above its true place by atmospheric refract ion through an angle which varies according to the state of the atmosphere and the relative temperatures of the air and water, the variation ranging from one Ibird to one-twenty-third of the amount of the true dip. The rule commonly employed is to
diminish the true dip by about one-fourteenth of its amount, to find the apparent dip.
If S he a star or the sun in the same vertical plane with AWL anti a II 14-erVat if flu' altj. lade above the .ea 11411i7011 he Made by of a sextant from the point U (as from the deck of a vessel), the apparent dip of the horizon must be subtracted from the observed angle. in order to find the altitude of the sun. owing to the uncertainty of the amount of refraction, the nearest minute to the dip given in the tables is usually taken. The table a notion of the amount of the apparent dip under ordinary state of the atmosphere and equal tem perature of air and water: