its distance from the centre of the earth increases, and vice versa. But if the body be

within the surface of the earth, the force varies

inversely as the distance from the centre. Gravitation. Sensible gravity.

Grylli. Grasshoppers, crickets, locusts, &c.

Harmonics. The doctrine of musical sound.

Harmonic sounds.

The sympathetic notes heard along with the principal note of a musical string, or other sonorous body.

Harmonic divisions. The parts into which a vibra

ting musical string spontaneously divides itself, each of which gives a distinct note, besides the principal note arising from the vibration of the whole string.

Harmonic colours. Tints which become visible upon

looking steadfastly at a bright coloured light, supposed to be analogous to the sympathetic notes of a musical string.

Helix. A curve like a corkscrew, whose turnings may either be circular or elliptical.

Homogeneous light. Rays of the same colour.

Homogeneous spheroid.

density.

A spheriod of uniform

Horizontal plane. An imaginary plane touching

the surface of the earth in one point, and terminating on all sides in the horizon.

Horoscope. The relative positions of the planets at the time of a person's birth.

Hyperbola. One of the conic sections. An open curve, having two infinite branches, a b, a »,

(fig. 4.) and a focus F, to which every point in the curve has a fixed relation.

Hypothesis. A system upon supposition. An assumption.

Iceland spar. A transparent and colourless carbonate of lime, consisting of fifty-six parts of lime, and forty-four of carbonic acid. It splits or cleaves into solids called rhombs (fig. 14.) which are bounded by six similar surfaces, whose sides are parallel, but the angles are not right angles : it possesses the property of double refraction in an eminent degree.

Impetus. A force whose intensity is measured by the mass of a body and the square of its velocity conjointly.

Incidence of light. The angle which rays make with a perpendicular to the surface upon which they fall. Let A B, fig. 9, be the reflecting surface,

then s I is the incident and I R the refracted ray,

the angle s I

being equal to the angle R I P.

Inclination of an orbit. The angle in fig. 8. which the plane of an orbit P NAN makes with the plane of the ecliptic C N E n.

Indigenous. Native to a particular spot or country. Inertia. The disposition of matter to remain in its state of rest or motion.

Interference of undulations. The combination of two series of waves in a fluid so as to augment, diminish, or destroy each other.

Isochronous. In equal times.

Isothermal lines. Imaginary lines passing through such places as have the same mean annual temperature.

Isogeothermal lines. Imaginary lines passing through all those places within the surface of the earth, where the mean internal temperature is the

same.

Kepler's laws. Three laws in the planetary motions discovered by Kepler, which furnish the data from whence the principle of gravitation is established: they are, First, that the radii vectores of the planets and comets describe areas proportional to the time: Second, that the orbits of the planets and comets are conic sections, having the sun in one of their foci; and third, that the squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun. These laws extend also to the satellites.

Latent heat. Caloric existing in all bodies, which is not sensible, and cannot be detected by the thermometer.

Latitude.

Terrestrial latitude is the angular distance between the vertical or plumb-line at any place and the plane of the equator. Celestial latitude is the height of a heavenly body above or below the plane of the ecliptic, as m s p, fig. 8; when above, it has north, and when below that plane, it has south latitude.

Length of a wave. The distance between two particles of an undulating fluid similarly displaced and moving similarly, consequently the length is the distance between two consecutive hollows or elevations.

Lens. A transparent substance with curved surfaces.

The glasses of a telescope and of spectacles are lenses. A lens may be convex on both sides, or it may have both sides concave; one side may be convex and the other concave; one side plane and the other convex; or lastly, one side may be plane and the other concave.

Libration. A balancing motion.

Lines of the second order. The circle, ellipse, parabola, hyperbola, and generally such as are expressed algebraically by a quadratic equation, See Curves of the second order.

Lines of no variation. Imaginary lines passing through all places where the needle of the mariner's compass points to the true north, that is, to the pole of the earth's rotation.

Lines of perpetual snow. Imaginary lines passing through the limits of perpetual snow from the equator to the poles.

Longitude. Terrestrial longitude is the angular distance of a place from a meridian arbitrarily chosen, such as that of Greenwich.

Longitude of a heavenly body. The true longitude of a planet, as of m, fig. 8. is its angular distance sm from the vernal equinox, estimated on its elliptical orbit; its mean longitude is its angular distance from the same point, supposing the planet to move equably in a circle whose radius is equal to the mean distance of the body from the sun. The difference between the two is the equation of the centre.

Longitude of the perihelion. The angular distance

of the perihelion of an orbit from the vernal equinox, ass P, fig. 8.

Longitude of the node. The angular distance of the node of an orbit from the vernal equinox as, fig. 8.

SN,

Longitude of the epoch. The angular distance of a

celestial body from the vernal equinox at the instant assumed as the origin of time whence all its subsequent and antecedent longitudes are estimated.

Lunar distance. The angular distance of the centre of a celestial object from the centre of the moon.

Magnetic equator. The imaginary line passing through those places where there is no dip, that