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 Pn A N (fig. 8) be the orbit of a planet, s the Sun, C N En the plane of the ecliptic, and the first point of Aries, then the six

elements are the major axis P A,

the excentricity

so, the longitude s P of P the perihelion, the longitudes N of N, the ascending node, the inclination of the orbit n A N on the plane of the ecliptic n E N, and 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, F and ƒ (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 between the two points. The points F and ƒ are called the foci, and the distance F C is the excentricity, c being the centre of the ellipse; it is evident that the less r c is, the nearer does the ellipse approach the form of a circle. A B is the major axis, a b the minor axis, and F A the focal distance. From the construction, the length of the string, Fm f, is equal to the major axis. If r t be a tangent to any point m, and F m, ƒ m lines from the foci, the angle Fm T is equal to the angle fm 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.

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 circular or spherical form.

Elongation. The angular distance of a celestial body from the sun, as it would be seen from the centre of the earth.

Epoch. The assumed instant from whence all the subsequent and antecedent periods of a celestial body are estimated.

Equation of time. The difference between the time shown by a watch, and that given by a dial, or the difference of mean and true time.

Equation of the centre. The difference between the true and mean motion of a planet or satellite. At its maximum, it is equal to the excentricity of the orbit, since it is the difference of the motion of a body in an ellipse, and in a circle whose diameter is equal to the major axis of the ellipse. Equator. The terrestrial equator is the equinoctial

line. The celestial equator is the great circle traced in the starry heavens by the imaginary extension of the plane of the terrestrial equator.

Equinoxes. The vernal and autumnal equinoxes are two points in the heavens diametrically opposite to one another, that is 180° apart. The

line 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 retrograde 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 motion 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 FA in the conic sections,

(fig. 2, 3, and 4.)

Foci of an ellipse. Two points F and ƒ (fig. 2.) in the major axis, such, that the sum of the two lines drawn from them to any point m in the ellipse is equal to the major axis a B.

Fossils, organic. The remains of ancient animals and plants embodied in the strata of the earth, Fundamental note. The natural note of any sonorous body, as of a string or organ-pipe.

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 families, 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 increasing 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,,,, &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 observations.

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