Discord. When the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is produced, which is called discord.
Divergent Rays. Those which proceed from the same point, but are continually receding from each other.
Divisibility. Capability of being divided, or of having the parts separated from each other. This is called one of the essential properties of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &c. as the largest mass of matter.
Echo. A sound reflected back, by some substance, so situated as to produce this effect.
Eclipse. The interruption of the light of the sun, or of some other heavenly body, by the intervention of an opaque body. The moon passing between the earth and the sun, causes an eclipse of the latter.
Ecliptic. A circle in the heavens. The apparent path of the sun, through the twelve signs of the zodiac. This is caused by the actual revolution of the earth, round the sun. It is called the ecliptic, because eclipses always happen in the direction of that line, from the earth.
Elasticity. That property of bodies, by which they resume their dimensions and form, when the force which changed them is removed. Air is eminently elastic. Two ivory balls, struck together, become flattened at the point of contact; but immediately resuming their form, they react upon each other.
Ellipsis. An oval. This figure differs from a circle, in being unequal in its diameters, and in having two centres, or points, called its foci. The orbits of the planets are all elliptical.
Equator. That imaginary line which divides the earth into northern and southern hemispheres, and which is equally distant from each pole.
Equilibrium. When two articles exactly balance each other, they are in equilibrium. They may, notwithstanding, be very unequal in weight, but they must be so situated, that, if set in motion, their momentums would be equal.