Part 94

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.

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 masses 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 passing 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 space with itself at the same time. If two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point.