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.
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.
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.
LATITUDE. Distance from the equator, in a direct line towards either
pole. This distance is measured in degrees and minutes. The degree of
latitude cannot exceed ninety, or one quarter of a circle. Places to the
south of the equator, are in south latitude, and those to the north, in
north latitude.
LATITUDE, PARALLELS OF. Lines drawn upon the globe, parallel to the
equator, are so called; every place situated on such a line, has the
same latitude, because equally distant from the equator.
LENS. A glass, ground so that one or both surfaces form segments of a
sphere, serving either to magnify, or diminish objects seen through
them. Glasses 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.
LIBRA. One of the twelve signs of the zodiac. That into which the sun
enters, at the autumnal equinox.
LIGHT. That principle, by the aid of which we are able to discern all
visible objects. It is generally believed to be a substance emitted by
luminous bodies, and, exciting vision by passing into the eye.
LONGITUDE. Distance measured in degrees and minutes, either in an
eastern, or a western direction, from any given point either on the
equator, or on a parallel of latitude. Degrees of longitude may amount
to 180, or half a circle. A degree of longitude measured upon the
equator, is of the same length with a degree of latitude; but as the
poles are approached, the degrees of longitude diminish in length,
because the circles upon which they are measured, become less.
LUNAR. Relating to _Luna_, the moon.
LUNATION. The time in which the moon completes its circuit. A lunar
month.
LUMINOUS BODIES. Those which emit light from their own substance; not
shining by borrowed, or reflected light.
MACHINE. Any instrument, either simple or compound, by which any
mechanical effect is produced. A needle, and a clock, are both machines.
MAGIC LANTHORN, OR LANTERN. An optical instrument, by which transparent
pictures, painted upon glass, are magnified and exhibited on a white
wall or screen, in a darkened room. The phantasmagoria, is a species of
magic lanthorn.
MATHEMATICS. The science of numbers and of extension. Common arithmetic,
is a lower branch of the mathematics. In its higher departments, it
extends to every thing which is capable of being either numbered or
measured.
MATTER. Substance. Every thing with which we become acquainted by the
aid of the senses; every thing however large, or however minute, which
has length, breadth, and thickness.
MECHANICS. That science which investigates the principles, upon which
the action of every machine depends; and teaches their proper
application in overcoming resistance, and in producing motion, in all
the useful purposes to which they are applied.
MEDIUM. In optics, is any body which transmits light. Air, water, glass,
and all other transparent bodies, are media. Medium also denotes that in
which any body moves. Air is the medium which conveys sound, and which
enables birds to fly.
MELODY. A succession of such single musical sounds, as form a simple air
or tune.
MERCURY. That planet which is nearest to the sun. Quicksilver, a metal,
which remains fluid at the common temperature of the atmosphere. It is
capable of being rendered solid, by intense cold.
MERIDIAN. Midday. A meridian line, is one which extends directly from
one pole of the earth to the other; crossing the equator at right
angles. It is therefore half of a great circle. The hour of the day is
the same at every place situated on the same meridian. Longitude is
measured from any given meridian, to the opposite meridian. Places at
the same distance in degrees, to the east or west of any meridian, have
the same longitude.
MICROSCOPE. An optical instrument, by which minute objects, are
magnified, so as to enable us to perceive and examine such as could not
be seen by the naked eye.
MINERAL. Earths, stones, metals, salts, and in general all substances
dug out of the earth, are denominated minerals.
MINUTE. In time, the sixtieth part of an hour. In length, the sixtieth
part of a degree. A minute of time, is an unvarying period; but a minute
in length varies in extent, with the degree of which it forms a part.
The degrees and minutes are equal in number, upon a common ring, upon
the equator of the earth, or, on any circle of the heavens.
MIRRORS. Polished surfaces of metal, or of glass coated with metal, for
the purpose of reflecting the rays of light, and the images of objects.
Common looking-glasses, are mirrors. Those used in reflecting
telescopes, are made of metal.
MOBILITY. Capable of being moved from one place to another. This is
accounted one of the essential properties of matter, because we cannot
conceive of its existence without this capacity.
MOMENTUM. The force, or power, with which a body in motion acts upon any
other body, or tends to preserve its own quantity of motion. The
momentum of a body, is compounded of its quantity of matter, and its
velocity. A body weighing one pound, moving with a velocity of two miles
in a minute, will possess the same momentum with one weighing two
pounds, moving with a velocity of one mile in a minute.
MOTION. A continued and successive change of place, either of a whole
body, or of the particles of which a body is composed; the earth in
revolving upon its axis only, would not change its place as a body, but
all the particles of which it is composed, would revolve round a common
axis of motion. In revolving in its orbit, its whole mass is constantly
occupying a new portion of space.
NATURAL PHILOSOPHY. That science which enquires into the laws which
govern all the natural bodies in the universe, in all their changes of
place, or of state.
NEAP TIDES. Those tides which occur when the moon is in her quadratures,
or half way between new, and full moon; at these periods the tides are
the lowest.
NODES. Those points in the orbit of the moon, or of a planet, where it
crosses the ecliptic or plane of the earth's orbit. When passing to the
north of the ecliptic, it is called the ascending node; when to the
south of it, the descending node.
OBLATE. See SPHEROID.
OCTAGON. A figure with eight sides, and consequently with eight angles.
OPAQUE. Not transparent; refusing a passage to the rays of light.
OPTICS. That branch of science which treats of light, and vision. It is
generally divided into two parts. _Catoptrics_, which treats of the
reflection of light, and _Dioptrics_, which treats of its refraction.
ORBIT. The line in which a primary planet moves in its revolution round
the sun; or a secondary planet, in its revolution round its primary.
These orbits are all elliptical, or oval.
PARABOLA. A particular kind of curve; that which a body describes in
rising and in falling, when thrown upwards, in any direction not
perpendicular to the horizon.
PARALLELOGRAM. A figure with four sides, having those which are
opposite, parallel to each other. A square, an oblong square, and the
figure usually called a diamond, are Parallelograms.
PARALLEL LINES. All lines, whether straight or curved, which are every
where at an equal distance from each other, are parallel lines.
PARALLEL OF LATITUDE. See LATITUDE.
PERIHELION. That part of the orbit of a planet, in which it approaches
the sun most nearly.
PENDULUM. A body suspended by a rod, or line, so that it may vibrate, or
oscillate, backwards and forwards. Pendulums of the same length, perform
their vibrations in the same time, whatever may be their weight, and
whether the arc of vibration, be long or short.
PERCUSSION. The striking of bodies against each other. The force of
this, depends upon the momentum of the striking body.
PERIOD. The time required for the revolution of one of the heavenly
bodies in its orbit.
PERPENDICULAR. Making an angle of 90 degrees with the horizon. When two
lines which meet, make an angle of 90 degrees, they are perpendicular to
each other.
PHASES. The various appearances of the disc, or face of the moon, and of
the planets; that portion of them which we see illuminated by the rays
of the sun.
PHENOMENON. Any natural appearance is properly so called; the term,
however, is usually applied to extraordinary appearances, as eclipses,
transits, &c.
PISTON. That part of a pump, or other engine which is made to fit into a
hollow cylinder, or barrel; and to move up and down in it, in order to
raise water, or for any other purpose.
PLANE. A perfectly flat surface. The plane of the orbit of a planet, is
an imaginary flat surface, extending to every part of the orbit.
PLANET. Those bodies which revolve round the sun, in orbits nearly
circular. They are divided into _primary_, and _secondary_; these latter
are also called satellites, or moons; they revolve round the primary
planets, and accompany them in their courses round the sun.
PLUMB-LINE. A string, or cord, by which a weight is suspended; it is
used for the purpose of finding a line perpendicular to the horizon; the
weight being always attracted towards the centre of the earth.
PNEUMATICS. That branch of natural philosophy, which treats of the
mechanical properties of the atmosphere, or of air in general.
POLES. The extremities of the axis of motion either of our earth, or of
any other revolving sphere. The poles of the earth have never been
visited; the regions by which they are surrounded, being obstructed by
impassable barriers of ice.
POWER. That force which we apply to any mechanical instrument, to effect
a given purpose, is denominated power, from whatever source it may be
derived. We have the power of weights, of springs, of horses, of men, of
steam, &c.
PRISM. The instrument usually so called, is employed in optics to
decompose the solar ray: it consists of a piece of solid glass, several
inches in length, and having three flat sides; the ends are equal in
size, and are of course triangular.
PRECESSION OF THE EQUINOXES. Every equinox takes place a few seconds of
a degree, before the earth arrives at that part of the ecliptic in which
the preceding equinox occurred. This phenomenon is called the precession
of the equinoxes. There is consequently a gradual change of the places
of the signs of the zodiac: a fact, the discovery of which has thrown
much light on ancient chronology.
PROJECTION. That force by which motion is given to a body, by some power
acting upon it, independently of gravity.
PULLEY. One of the six mechanical powers. A wheel turning upon an axis,
with a line passing over it. It is the moveable pulley only, which gives
any mechanical advantage.
PUMP. An hydraulic, or pneumatic instrument, for the purpose of raising
water, or exhausting air.
QUADRANT. A quarter of a circle. An instrument used to measure the
elevation of a body in degrees above the horizon.
QUADRATURES OF THE MOON. That period in which she appears in the form of
a semicircle. She is then either in her first, or her last quarter; and
exactly half way, between the places of new, and of full moon.
RADIATION. The passage of light or heat in rays, or straight lines;
these being projected from every luminous, or heated point, in all
directions.
RADIUS. The distance from the centre of a circle, to its circumference;
or one half of its diameter. In the plural denominated radii.
RAINBOW. An appearance in the atmosphere, occasioned by the
decomposition of solar light, in its refraction, and reflection, in
passing through drops of rain. The bow can be seen, only when the sun is
near the horizon, when the back is turned towards it, and there is a
shower in the opposite direction.
RAY. A single line of light, emitted in one direction, from any luminous
point.
REACTION. Every body, whether in a state of motion, or at rest, tends to
remain in such state, and resists the action of any other body upon it,
with a force equal to that action. This resistance, is called its
_reaction_.
RECEIVER. This name is applied to glass vessels of various kinds,
appertaining to the air pump, and from which the air may be exhausted.
They are made to contain, or receive, any article upon which an effect
is to be produced, by taking off the pressure of the atmosphere.
REFRACTION, of the rays of light, is the bending of those rays, when
they pass obliquely from one medium into another of different density. A
stick held obliquely in water, appears bent or broken at the surface of
the fluid.
REFRANGIBILITY. Capacity of being refracted. Light is decomposed by the
prism, because its component parts are refrangible in different
degrees, by the same refracting medium.
REPULSION. The reverse of attraction. A tendency in particles, or in
masses of matter, to recede from each other. The matter of heat within a
body, appears to counteract the attraction of its particles, so as to
prevent absolute contact.
RETINA. That part of the ball of the eye, upon which the images of
visible objects are formed; and from which, the idea of such forms, is
conveyed to the mind.
REVOLUTION, of a planet; is either diurnal, or annual; the former, is
its turning upon its own axis; the latter, is its passage in its orbit.
SATELLITES. Moons, secondary planets.
SEGMENT OF A CIRCLE. A portion, or part of a circle; called also, an arc
of a circle.
SEMI-DIAMETER. Half the diameter. The semi-diameter of the earth, is the
distance from its surface, to its centre.
SIDERIAL. Belonging to the stars. A siderial day, is the time required
for a star to reappear on a given meridian. A siderial year, the period
in which the sun appears to have travelled round the ecliptic, so as to
have arrived opposite to any particular star, from which his course was
calculated.
SIGNS, or CONSTELLATIONS. Collections, or groups, of stars. Those of the
zodiac are twelve, corresponding with the twelve months in the year. In
the centre of these the ecliptic is situated. The sun appears to pass in
succession through these signs; entering the first degree of Aries,
which is accounted the first sign, about the 21st of March.
SKY. That vast expanse, or space, in which the heavenly bodies are
situated. Its blue appearance is supposed to arise from the particles of
which the atmosphere is composed, possessing the property of reflecting
the blue rays, in greatest abundance.
SOLAR. Appertaining to, or governed by, the sun: as the solar system,
the solar year, solar eclipses.
SOLID. Not fluid. Having its parts connected so as to form a mass. Solid
bodies, are not absolutely so, all undoubtedly containing pores, or
spaces void of matter.
SOLSTICES. The middle of summer and the middle of winter; those two
points in the orbit of the earth, in which its poles point most directly
towards the sun.
SONOROUS BODIES. Those bodies which are capable of being put into a
state of vibration, so as to emit sounds.
SPECIFIC GRAVITY. The relative weight of bodies of different species,
when the same bulk of each is taken. Water has been chosen as the
standard for comparison. If we say that the specific gravity of a body
is 6, we mean, that its weight is six times as great as that of a
portion of water, exactly equal to it in bulk.
SPECTRUM. That appearance of differently coloured rays, which is
produced by the refraction of the solar ray, by means of a prism, is
called the prismatic spectrum; it exhibits most distinctly, and
beautifully, all the colours seen in the rainbow.
SPHERE. A globe, or ball.
SPHEROID. Spherical; a body approaching nearly to a sphere in its
figure. The earth, is denominated an _oblate spheroid_; it not being an
exact sphere, but flattened at the poles, so as to cause the polar
diameter to be upwards of thirty miles less than the equatorial. Oblate,
is the reverse of oblong, and means shorter in one direction, than in
another.
SPRING TIDES. Those tides which occur at the time of new, or of full
moon. The tides then rise to a greater height than at any other period.
SQUARE. A figure having four sides of equal length, and its angles all
right angles.
In numbers; the product of a number multiplied into itself; thus, the
square of 3 is 9, and the square of 8 is 64.
STAR. The _fixed_ stars are so called, because they retain their
relative situations; while the planets, by revolving in their orbits,
appear to wander amongst the fixed stars.
SUBTEND. This term is applied to the measurement of an angle; when the
lines by which it is bounded recede but little from each other, they are
said to subtend; that is, to be contained under, a small angle.
SUPERFICIES. The surface of any figure. Space extended in length and
width.
SYSTEM. The mutual connexion, and dependance of things, upon each other.
The solar, or Copernican system, includes the sun, the planets, with
their moons, and the comets.
TANGENT. A straight line touching the circumference of a circle; but
which would not cut off any portion of it, were it extended beyond the
touching point, in both directions.
TELESCOPE. An instrument by which distant objects may be distinctly
seen; the images of objects being brought near to the eye, and greatly
magnified.
TEMPERATE ZONES. Those portions of the surface of the earth situated
between 23-1/2 and 66-1/2 degrees of latitude. Within these boundaries,
the sun is never vertical; nor does he ever remain, during a whole day,
below the horizon.
THERMOMETER. An instrument for measuring the temperature of the
atmosphere, or of other bodies.
TORRID ZONE. That portion of the earth which extends 23-1/2 degrees on
each side of the equator, to the tropical circles; within this limit,
the sun is vertical, twice in the year.
TRANSIT. Mercury or Venus, are said to transit the sun, when they pass
between the earth and that luminary. They then appear like dark spots,
upon the face of the sun.
TRANSPARENT. Allowing the rays of light to pass freely through. The
reverse of opaque. Glass, water, air, &c. are transparent bodies.