Straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bearing no relationship to the degrees upon a circle.

DENSITY. Closeness of texture. When two bodies are equal in bulk, that which weighs the most, has the greatest density.

DIAGONAL. A line drawn so as to connect two remote angles of a square, or other four-sided figure.

DILATATION. The act of increasing in size. Bodies in general, dilate when heated, and contract by cooling.

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 ma.s.s 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 pa.s.sing 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 b.a.l.l.s, 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 s.p.a.ce, 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 r.e.t.a.r.ded. Friction may be lessened, but cannot be destroyed.

FRIGID ZONES. The s.p.a.ces 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.

GAS. Any kind of air; of these there are several. The atmosphere consists of two kinds, mixed, or combined with each other.

GEOMETRY. That branch of the mathematics, which treats of lines, of surfaces, and of solids; and investigates their properties, and proportions.

GLOBE. A sphere, or ball. It has a point in its centre of magnitude, from which its surface is every where equally distant.

GRAVITY. That species of attraction which appears to be common to matter, existing in its particles, and giving to them, and of course to the ma.s.ses which they compose, a tendency to approach each other. By gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other.

HARMONY. A combination of musical sounds, produced by vibrations which bear a certain ratio to each other; and which thence affect the mind agreeably, when heard at the same time. Sounds not so related, produce discord.

HEMISPHERE. Half a sphere or globe. A plane pa.s.sing through the centre of a globe, will divide it into hemispheres.

HORIZON. This is generally divided into _sensible_, and _rational_. The sensible horizon is that portion of the surface of the earth, to which our vision extends. Our rational horizon is that circle in the heavens which bounds our vision, when on the ocean, an extended plane, or any elevated situation. In the heavens our sensible, and our rational horizon are the same; its plane would divide the earth into hemispheres at 90 degrees from us; and a person standing on that part of the earth which is directly opposite to us, would, at the same moment, see in his horizon, the same heavenly bodies, which would be seen in ours.

HORIZONTAL. Level; not inclined, or sloping. A perfectly round ball, placed upon a flat surface, which is placed horizontally, will remain at rest.

HYDRAULICS. That science which treats of water in motion, and the means of raising, conducting, and using it for moving machinery, or other purposes.

HYDROSTATICS. Treats of the weight, pressure, and equilibrium of fluids, when in a state of rest.

HYDROMETER. An instrument used to ascertain the specific gravity of different fluids, which it does, by the depth to which it sinks when floating on them.

IMAGE. The picture of any object which we perceive either by reflected or refracted light. All objects which are visible, become so by forming images on the retina.

IMPENETRABILITY. That property of matter, by which it excludes all other matter from occupying the same s.p.a.ce with itself at the same time. If two particles could exist in the same s.p.a.ce, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point.

INCIDENCE. The direction in which a body, or a ray of light, moves in its approach towards any substance, upon which it strikes.

INCLINED PLANE. One of the six mechanical powers. Any plane surface inclined to the horizon, may be so denominated.

INERTIA. One of the inherent properties of matter. Want of power, or of any active principle within itself, by which it can change its own state, whether of motion, or of rest.

INHERENT PROPERTIES. Those properties which are absolutely necessary to the existence of a body; called also essential properties. All others are denominated accidental. Colour is an accidental--extension, an essential property of matter.

LAt.i.tUDE. Distance from the equator, in a direct line towards either pole. This distance is measured in degrees and minutes. The degree of lat.i.tude cannot exceed ninety, or one quarter of a circle. Places to the south of the equator, are in south lat.i.tude, and those to the north, in north lat.i.tude.

LAt.i.tUDE, PARALLELS OF. Lines drawn upon the globe, parallel to the equator, are so called; every place situated on such a line, has the same lat.i.tude, because equally distant from the equator.

LENS. A gla.s.s, ground so that one or both surfaces form segments of a sphere, serving either to magnify, or diminish objects seen through them. Gla.s.ses used in spectacles are lenses.

LEVER. One of the mechanical powers. An inflexible bar of wood or metal, supported by a fulcrum, or prop; and employed to increase the effect of a given power.