DICTIONARY OF AVIATION 77
density- of the air; the rate of variation of atmosfenc density.
departure di'paet$az n. (seaiterm) the distance (in nauti- cal miles) by tuhich a ship in sailing departs or moves east or west from a given meridian; change of longitude; also, the bearing of an object on the coast, taken at the com- mencement of a voyage, from which the ded-reckonmg be- gins.
depression di'psejan n. condition or action of being de- prest or lowerd, as a balloon in the air; a low place; a condition or region of deprest or lowerd barometric pres- sure, as indicated by an isobar; a low-pressure line or area; a low.
barometric depression, a depression or lowness of the bar- ometric readings, due to diminution of atmosfenc pres- sure.
depression of the dew-point, see under dew-point, secondary depression, a depression in a barometric pres- sure adjacent to a larger depression; a kink or loop in one or more of the isobars of a low, often indicativ of rain, and in summer, especially of thunderstorms.
V-shaped depression, an area of depression of barometric pressure whose isobars intersect like the lines of the letter V, as on the side of the trough of a cyclone.
derecho di'e:ee-t$D n. a strong wind or squall blowing in a straight course, with no apparent rotatory movement.
derrick 'dezik n. a hoisting-apparatus for operating the falling weight which is sometimes used in starting an aero- plane flying-machine; a launching-derrick; a starting-der- rick; a catapult; a pylon.
descend di'send intr. go or come down; move downward toward the land or water underneath; fall gradually and easily to or toward the surface of the earth.
descending-position di'sendin-p3,zi$3n n. the position of the rudder of a flying-machine appropriate to descending or sinking toward the ground.
descensional di'sen$9-nf adj. of descension or descent : as, ascensional and *descensional movements of the atmosfere.
descent di'sent n. the action of descending to or toward the surface of the earth; a going or coming down to the ground; downward motion towards the land or water under- neath.
gliding descent, a descent, as of a flying-machine to the earth's surface, made by means of gliding; a downward glide or volplane, -see glide.
deviation /divi'ee-^n n, the deflection of the needle of a