Electro-dynamics. The science of the motion and

reciprocal action of electric. currents. Electro-dynamic cylinder. A hollow coil of copper wire, (fig. 7,) in the form of a corkscrew, the

Fig. 7.

extreme parts of the wires of which are passed
back through the centre of the coil, and being
bent at right angles are brought out through
its middle. There are several forms of this
instrument, all of which have the same properties
as magnets, when a galvanic current is passing

through them.
Elements of an orbit. In an elliptical orbit there are

six elements. Let P N A N (fig. 8) be the orbit
of a planet, s the Sun, C NE n the plane of the
ecliptic, and op the first point of Aries, then the six

Fig. 8.

elements are the major axis P A, the excentricity so, the longitude op s p of p the perihelion, the longitude ops N of n, the ascending node, the inclination of the orbit n a n on the plane of the ecliptic n E N, and P s m the longitude of the body m, at any given instant called the longitude of the epoch. In a parabolic orbit, there are only

five elements, since the major axis is infinite. Ellipse. One of the conic sections. An ellipse may

be drawn, by fixing the ends of a string to two
points, r and f (fig. 2.) in a sheet of paper, and
then carrying the point of a pencil round in the
loop of the string kept stretched, the length of
the string being greater than the distance be-
tween the two points. The points F and f are
called the foci, and the distance F c is the ex-
centricity, c being the centre of the ellipse; it
is evident that the less F c is, the nearer does
the ellipse approach the form of a circle.
the major axis, a b the minor axis, and F A the
focal distance. From the construction, the length
of the string, F m f, is equal to the major axis.
If r t be a tangent to any point m, and F m, f m
lines from the foci, the angle F m r is equal to
the angle f m t; and as this is true for every
point of the ellipse, it follows that, in an elliptical
reflecting surface, rays of light or sound coming
from the focus F will be reflected by the surface
to the other focus f, since the angle of incidence
is equal to the angle of reflection, by the theory
of optics and acoustics.

a

A B is

F

Ellipsoid of revolution. A solid formed by the re

volution of an ellipse about its axis. If the ellipse revolves about its minor axis, the ellipsoid will be oblate, or flattened at the poles, like an orange : if the revolution be about the major axis, the ellipsoid will be drawn out at the poles,

or prolate, like an egg. Ellipticity. Excentricity, or deviation from the cirline in which the planes of the equator and ecliptic intersect passes through them. The vernal equinox is the point from whence the longitudes or angular distances of the celestial bodies are estimated; it is generally called the first point of Aries, though these two points have not coincided since the early ages of astronomy, about two thousand two hundred and thirty-two years ago, on account of the precession or retro

grade motion of the equinoctial points. Etherial medium. The ether or highly elastic fluid

with which space is filled. Evection. A certain periodic inequality in the mo

tion of the moon. Excentricity. The distances between the centre and

focus of an ellipse, or c F, (fig. 2.) Extraordinary refraction. See Refraction. Extraordinary ray. See Refraction.

Focus. A point where converging rays or lines meet. Focal distance. The line F A in the conic sections,

(fig. 2, 3, and 4.) Foci of an ellipse. Two points F and f (fig. 2.) in Galvanism. Electricity perpetually in motion, and

produced by chemical action. Galvanic battery. An instrument for producing

galvanic electricity, constructed of alternate

layers of two metals and a fluid. Galvanic circuit. Three substances in contact,

generating a stream of electricity, which flows in

a perpetual circuit through them. Galvanometer. An instrument for measuring the

intensity of the galvanic force. Genera of plants. The divisions of plants into fami

lies, each of which contains a variety of species. General analytical expression. The representation

in symbols of a series of reasoning, including

every particular case of the subject in question. Geometrical progression. A series of quantities in

creasing or diminishing by a continual multiplication or division by the same quantity, as the numbers 1, 2, 4, 8, 16, &c., which are constantly multiplied by 2, or the series 1, d, 1, $, &c.,

which decreases by the continual division by 2. Graphical construction of an orbit. The drawing

of an orbit by ruler and compass from given ob

servations. Gravity. The attraction of matter, weight. Gravitating force. The force with which matter

attracts; its intensity varies inversely as the square of the distance; that is, the weight of a body decreases in proportion as the square of