DICTIONARY OF AVIATION 133
hotftube ignition, ignition in an internal^combustion en- gin by means of compressing the explosiv gaseous mixture thru a tube which is kept hot by an external flame.
jumpfspark ignition, a frm of electric ignition, in an mternal^combustion engin, which utilizes a spark which is made to jump between the gap^terminals of the secondary circuit of an induction-coil.
makezandfbreak ignition, a form of electric ignition, in an internaUcombustion engin, which utilizes a spark in a primary circuit which is made and broken mechanically or magnetically.
ignition-point ,ig'ni$9n,p0int n. the point of time at which the ignition of the explosiv gaseous mixture takes place in an internal-combustion engin; that peint in the stroke of a gasoline-engin at which the charge is ignited.
ignition-tube ,ig'm$3n,tguub n. the tube in an internal^ combustion engin in which the ignition of the explosiv gaseous mixture takes place, as in the het-tube system.
impeller im'pelaz n. a device for impelling or driving for- ward; a propeller: as, Gordon's fumifi2 *impeller.
impermeable im'pBiz-mi-a-bal adj. not permeable; net per- mitting the passage of gas or other fluid thru its substance, as a gas-tight balloon-envelop.
impulsion im'pAlJan n. the act of impelling or forcing for- ward ; impelling force or action ; propulsion.
incidence 'msi-dns n. the falling of a line, or anything moving in a line, upon a surface; also, the manner of such falling.
angle of incidence, see under angle.
index 'm,deks n. a numerical quantity which mesures some property of a particular substance or structure, for which it is constant; an exponent; a coefficient.
Indian 'indi-an adj.
Indian summer, a hazy condition of the air known as dry fog, caused by dust high up in the air, occurring sometimes in November or early December in some parts of the United States and known also in Central Europe.
indirigible ^n'dizidsa-ba! adj. not dirigible; incapable of being steerd.
indraft ,in'dra:ft n. [also spelt indraught] an inward draft or current, as of air; a drawing or flowing inward.
inflate m'fteet tr. blow out or distend, as with wind, air, or gas; fill (a balloon, gasbag, tire) with air or gas.
inflated m'fLee-tid adj. puft out or swollen by air or gas.