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. EQUINOX. The two periods of time at which the nights and days are every where of equal length. The _vernal_ equinox is in March, when the sun enters the sign _Aries_; the _autumnal_ equinox in September, when the sun enters _Libra_. At these periods, the sun is vertical at the equator. EXHALATIONS. All those articles which arise from the earth, and mixing with the atmosphere, form vapour. EXPANSION. The same as dilatation, which see. EXTENSION. One of the essential properties of matter; that by which it occupies some space, to the exclusion of all other matter. FIGURE. All matter must exist in some form, or shape; hence figure is deemed an essential property of matter. FLUID. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. Liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force. FOCUS. That point in which converging rays unite. FORCE. That power which acts upon a body, either tending to create, or to stop motion. FOUNTAIN. A jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure. FRICTION. The rubbing of bodies together, by which their motion is retarded. Friction may be lessened, but cannot be destroyed. FRIGID ZONES. The spaces or areas, contained within the arctic and antarctic circles. FULCRUM. A prop. The point or axis, by which a body is supported, and about which it is susceptible of motion.