for those plants that bear Legumes, or pods, such as peas and beans; the common English term is pulse. They constitute an order in the botanical system of Jussieu.

LEVEL.-See Glossary I. LIBRATION OF THE MOON. Though the moon always presents nearly the same face to the earth, yet we sometimes see more of the eastern hemisphere, and at other times, more of the western. The same variation is occasionally observable in the northern and southern hemispheres, and this oscillation is called her Libration. See Astronomy, pages 83 and 84. LIGHT.-See Glossary I. LIMB, with Astronomers, designates the curved edge of a circle, such as the divided limb of a quadrant, and the outermost border of the sun or moon. In their observations, for example, they speak of the moon's lower or upper limb, and even of her eastern or western limb; and especially in the case of an eclipse, when a portion of her disc is obscured. LINE OF THE NODES. See Orbit. LOG-LINE. The Log, in sea-language, is the name of a piece of wood in the form of the sector (usually a quadrant) of a circle of five or six inches radius. It is about a quarter of an inch thick, and so balanced by means of a plate of lead nailed upon the circular part, as to swim perpendicularly in the water with about two-thirds immersed under the surface. The Log-line is a small cord of about one hundred fathoms in length, one end of which is fastened (by means of two legs) to the centre and to the arched part of the Log, while the other is wound round a reel in the gallery of the ship. The Log thus poised keeps its place in the water while the line is unwound from the reel by the ship's sailing; and the length of line unwound in a given time gives the rate of the ship's course. This is calcu lated by knots made on the line at between forty and fifty feet distance, while the time is measured by a sand-glass of a certain number of seconds. The length between the knots is so proportioned to the time of the glass, that the number of knots unwound shows the number of miles which the ship is sailing in the hour. LONGITUDE. The Longitude of any heavenly body is measured on an arc of the ecliptic, intercepted between the vernal equinoctial point and a great circle passing through the body, and perpendicular to the ecliptic. Longitude on the earth is measured in a similar manner upon an arc of the equator, and counted to the east or west, from a certain meridian.-See Equinor. LONGITUDE, GEOCENTRIC.-See Geocentric.

LOXODROMIC CURVE.-See Rhumbline.

MAGNETIC AZIMUTH.-See Glossary II. Horizon and Mariner's Compass.

MERIDIAN.-See Glossary

II. Horizon and Mariner's Compass. MAMMALIA, in the Linnæan System, the denomination of that class of animals which suckle their young. See Physical Geography, page 53.

MAMMIFEROUS ANIMALS. Such as have teats (Latin mamma) for nourishing their young.-See Physical Geography, page 53.

MARINER'S COMPASS.-See Glossary II.; also Navigation, pages 15 and 16. MARSUPIAL, (Greek marsupos, a purse) the designation given by naturalists to a tribe of the class mammalia, of which the mamma and young of most of the species

are,

for a time, inclosed within an external pouch, or second womb, the pouch being supported by two marsupial bones.-See Physical Geography, page 54. MEAN DAY.-See Day. MEAN MOTION.-See Motion. MEAN TIME.-See Day. MERCATOR'S CHART.-See Chart. MERIDIAN LINE.-See Glossary II. Horizon and Astronomy, pages 4 and 16. METONIC CYCLE.-See Epact. MOLLUSCA, the name of one of the orders of the Linnæan Class of Vermes, or worms. They are simple animals, furnished with limbs; some are naked and others testaceous, that is, covered with shells. They are chiefly inhabitants of the sea. MOMENTUM.-See Glossary II. A dif ferent view of the Momentum, or impetus, of a moving body is taken when the motion is supposed to be accelerated. See Astronomy, page 12. MONOCOTYLEDONOUS.-See Cotyledon. MONSOONS are periodical winds in the Fast Indies, blowing constantly the same way during six months of the year, and the contrary way during the remaining six. They are a species of Trade-winds, which, in some quarters, blow constantly in one direction throughout the whole year. For the causes of this difference, see Physical Geography, page 31. MONTH.-See Calendar. MOTION. For a general definition, see Glossary II. Mean motion is understood to be a calculated average of a series of known variable motions.-See Acceleration and Astronomy, page 31.

NADIR. The Zenith and the Nadir (two Arabic words) are scientific names for two opposite points of direction in space. They are the poles of an interminably extended straight line, passing upright or downright, from the feet to the head, or from the head to the feet, of the person whose zenith and nadir they are: the former ending in a point of the sky above, and the latter (through the centre of the earth) in a point of the sky below. The

all, the lines of observation from the centre and from the surface of the earth coinciding. The fixed stars have no parallax. This may be easily accounted for from their immense distance compared with the semi-diameter of the earth; but even when seen at opposite points of the earth's orbit, where what is called the Annual Parallax might have been expected, none has been satisfactorily observed. For further particulars on this subject, see Astronomy, pages 54-61, and 146. PARALLELOGRAM.-See Glossary II. PARALLELS OF DECLINATION AND OF LATITUDE.-See Declination and Latitude.

PENUMBRA.-See Glossary II. PERIGEE.-See Glossary II. and Astronomy, page 19.

PERIHELION.-See Glossary II. and Astronomy, page 127.

PERPENDICULAR.-See Normal, and Glossary I. Angle. PHANEROGAMOUS.-See Cryptogamous. PHASES (Greek phaino, to shine). A term denoting the several appearances, or shapes, of the illumination of certain heavenly bodies, such as the moon, Venus, Mercury, &c. Of these the lunar phases (except perhaps Saturn, on account of his ring) are the most varied. She appears circular when full, gibbous a few days before and after the full, a semicircle at the quadratures, and horned at a few days before and after the new moon. In the latter situation, when visible, the tips or horns of her Crescent (which are at the extremities of her diameter) are termed Cusps, from the Latin Cuspis, a point. Crescent (Latin crescare) is growing, and the Crescent Moon is, literally, the Growing Moon, when she appears like a bow; but the same appearance is exhibited before her change, when she is said to be waning. For a representation of those several phases, see Astronomy, page 72. PHYSICS. See Glossary II. PLANE CHART.-See Chart. PLANET (Greek planetes, wandering) is a name given to those heavenly bodies which change their position with respect to the fixed stars, and are found to revolve round the sun as a centre. These are properly termed Primary Planets; for other wandering stars circulate about these primaries, and are therefore called Secondary Planets, or Satellites.

PLANETS, INFERIOR and SUPERIOR.

-See Conjunction of the Sun and Planets. PLANETARY MOTIONS.-For an illustration of the law by which they are said to describe equal areas in equal times, see Radius Vector.

PLUVIOMETER.-See Rain-gage.
POLES, NORTH AND SOUTH.-See
Arctic.

PRECESSION OF THE EQUINOXES.
-See Equator.

PREHENSILE. (Latin prehendo, to seize) an epithet given by naturalists to certain animals which are capable of grasping with their tails as with a claw. PRIME VERTICAL-See Vertical Circles. PROLATE SPHEROID. The same as Oblong Spheroid, which see in Glossary I. PROMONTORY.-See Headland. PROPORTION.-See Glossary I.

QUADRANT. A quadrant in Geometry is merely the quarter of a circle, and as such is noticed in Glossary I. under the head Angle. The term is also applied to an instrument for measuring angles, which is a quarter circle of wood or metal, having its circular part, or limb, divided into 90 parts, or degrees, and these again subdivided into minutes, &c., by means of a Nonius or a Vernier. Hadley's quadrant is properly an Octant, or eighth part of a circle, in which the angles are taken by means of the reflexion of light, and when the limb is extended to 60 degrees (the sixth of a circle) the instrument is called a Sextant. These are severally described in the Society's treatise on Optical Instruments.

QUADRATURES.-See Orbit. When the moon is in either of the middle points of her orbit, between her conjunction and opposition (lines from the earth to the moon and to the sun, including a quadrant, or 90 degrees), she is said to be in her quadrature. Her face is then half shown; it is bisected, or dichotomized. The places of her orbit, where she is either in conjunction or opposition, are her Syzygies, a Greek compound signifying conjunction.-See Conjunction.

RADIUS.—See Glossary I. Angle. RADIUS OF CURVATURE. In speaking of the concavity of other curves than the circle, the radius of curvature at a given point is the radius of a circle that has the same curvature as the curve has at that point. It is the varying length of the thread B D, when forming the involute ADE, (Fig. 9) Glossary I. See Articles Curves, Evolutes, and Involutes. RADIUS VECTOR. The radius vector is a right line drawn from the centre of force (in any curve, on which a body is supposed to move by centripetal force) to that point of the curve where the body is supposed to he. It is a general radius to the curve, and has the addition of Vector (Latin, a carrier) because it is imagined to carry forward the body to which it is attached. The earth, for example, moves in an elliptic orbit, of which the sun (the centre of force) is in one of the foci; and, of consequence, the radius vector is continually increasing in length during her course from the perihelion to the aphelion, and decreasing in the same proportion in the progress of her return.

Let the ellipses AMNBOP repre

When the earth is in the aphelion, at A, the radius vector is at its maximum, and equal to A S; but, in moving from A to B, in the curve A M N B, it is regularly shortened until it reaches its minimum B S. The law of the planetary motions, as guessed by Kepler and demonstrated by Newton, is that the radius vector passes over equal areas of the orbit in equal times; and, if we suppose the revolution to be completed in twelve months, the semi-ellipse A M N B being divided into three equal triangular areas, A S M, MSN, and NS B, by the radius vector at M S and N S, will mark three portions (A M, M N, and NB) of the elliptic curve, each corresponding to two months of the time of the revolution. These curvilineal bases of the three equal triangles are, obviously, themselves of very unequal lengths, owing to the varying lengths of the other sides; but, by the law of motion just mentioned, they must each, nevertheless, be run over by the earth in the same period of time; and, consequently, the motion inust be continually accelerated as she approaches her perigee at B. In the progress of her return to A, through the other half of her orbit, BOP A, the velocity of her motion will be continually diminished in a corresponding proportion.

The angle A S M formed at the sun by the line of the apsides and the radius vector, at any point of the orbit, is termed the Anomaly. It increases irregularly through the whole of that semi-ellipsis (decreasing in its opposite, as compared with time. A calculated medium angular increase gives the Mean Anomaly; and the difference between the True Anomaly and the Mean Anomaly is the Equation of the Centre. See Astronomy, pp. 32 and 124. RAIN-GAGE, an instrument for ascertaining the comparative quantity of rain which falls in different places, and in different seasons. Rain-gages, of a simple construction, are common throughout Europe, and are sometimes called Pluviometers, from the Latin pluvia, rain,

RATIO.-See Glossary II.

EXTREME AND MEAN.-See Extreme and Mean Ratio.

RAY OF LIGHT.-See Glossary I. REFLEXION OF LIGHT.-See Glossary I.

REFRACTION.-See Glossary I., and Astronomy, pages 46-54.

RHUMBS are the thirty-two points of the horizon, as marked on the circle of the mariner's compass; and serve to calculate the angle which a ship's course makes with the magnetic meridian.-See Navigation, pages 15 and 16. RHUMB-LINE is a line prolonged from

any point of the compass (in a nautical chart), except the four Cardinal points. It cuts all the meridians under the same

angle; and when delineated on the globe, it forms a curve termed the Loxodromic Curve. See Navigation, page 19. RIGHT ANGLE.-See Glossary I. Angle. RIGHT ASCENSION OF THE SUN OR STAR, is that degree and minute of the equinoctial, counted from the vernal equinox (the first degree of Aries), which comes to the meridian with the sun, star, or other point of the heavens, whose right ascension is required.-See Astronomy, page 16.

SATELLITES, secondary planets which circulate round some primary one, as the moon does about the earth.-See Planets. SECANT.-See Glossary I. Angle. SECONDARY CIRCLES, are such as are in planes that are perpendicular to those circles of which they are the secondaries. SEXTANT.-See Quadrant. SIDEREAL DAY.-See Day. YEAR.-See Year. SINE AND VERSED SINE.-See Glossary I. Angle. SINES, LAW OF THE.—See Glossary I. Refractive Power. SOLAR DAY.-See Day.

YEAR.-See Year. SOLSTICE.-See Ecliptic. SOLSTITIAL COLURE.-See Ecliptic. SOTHIAC PERIOD.-See Canicular Period.

SPECIES.-See Genus. SPHERE, SPHERICAL, AND SPHEROID. See Glossary I. SPRING-TIDES.-See Tides. STARBOARD.-See Larboard. STYLE, NEW AND OLD.-See Calendar and History of Astronomy, pages 47 and 48. SUBTEND.-See Triangle. SUPPLEMENT OF AN ANGLE, OR ARC, is what it wants of a semicircle, or 180 degrees. See Glossary I. Angle. SYNODIC MONTH, a complete Lunation, or the period from one conjunction of the woon with the sun to another, being 29 days, 12 hours, and 44 minutes. The Greek synodus is a meeting or convention. SYZYGY.-See Quadrature.

TIDES.

The rising and falling of the waters of the ocean, which occur twice in twenty-four hours, throughout all parts of the earth, are called the Tides. It is an alternate rise and fall, a flux and a reflux, a flow and an ebb of the water in respect to the land: the rise being called the Flood-tide, and the fall the Ebb-tide. When the flux is at its height, and about to recede at any particular place, it is there High-water; and when at its lowest, and about to rise, it is Low-water. The cause of the tides, as explained in Astronomy, from page 23 to 27, being the attraction of the sun and moon (chiefly the latter), the situations of those luminaries, with respect to one another, have an effect on the height of those swells of the waters. When the moon is in conjunction, or in opposition, the powers cf both bodies being united, the tides are highest, and called Spring-tides; but when the moon is in her quadratures, they are lowest, and called Neap-tides.

TRADE-WINDS.-See Monsoons. TRANSIT (Latin transive, to pass over) in Astronomy, is the passage of one heavenly body over the disc of a larger one.

When the nearer body has a greater apparent diameter, so as to hide the other, the passage is termed an Occultation of the latter. -See Occultation. TRANSLUCENT and TRANSPARENT. See Glossary II.

TRAVERSE. Traverse-sailing, or the working of a traverse, is the method of calculating a ship's place after she has made two or more short courses on different points of the compass.

TRIANGLE. A surface contained under three lines, has necessarily three corners, or angles; and is, therefore, called a Triangle. When these lines are straight, the figure being on a plane, is called a Plane Triangle; but when they are circular, lying on the surface of a globe, it is termed a Spherical Triangle. Each of the sides, in either case, is opposite to an angle, and is said to subtend that angle to which it is opposed. When one of the angles is a Right Angle, the side which subtends it is necessarily the longest of the three, and is called the Hypotenuse, from a Greek verb, signifying to subtend. The science of triangles is Trigonometry.

TROCHOID. The Trochoid and the Cy

The cycloid a aa aa is formed by the point a, while the circle is rolling along the base line a ba; but if, instead of the tracing point being at the extremity of the diameter, it were taken within the cir cle, as at d, it would then describe the dotted curve line d d d, which is called a Trochoid. The right line de d joining the ends of the curve is the base of the Trochoid: it is parallel to the path of the rolling circle, and equal in length to its circumference. The perpendicular de, from the vertex d to the base, is the Axis, It divides the Trochidal Space d d d e into two equal portions; and the axis itself is bisected by the right line ccc, which is the path of the centre of the generating circle. The Trochoid is sometimes called a Protracted Cycloid; and a curve formed by a point without the circle (upon the diameter extended) is termed a Contracted Cycloid: the base of this latter, too, is of the same length; being always equal to the circumference of the generating circle in all the forms of the cycloid. If in the trochoid a circle be drawn round the centre c, having the axis d e as a diameter, the two circles will represent the wheel and its nave, in the famous Aristotelian paradox. -See Epicycloid. TROPICS.-See Ecliptic. TROPICAL YEAR.-See Year.

UMBELLIFEROUS.-See Physical Geography, page 47.

VAPOUR.-See Glossary I.
VARIATION OF THE COMPASS.-
See Glossary II.
VARIETIES.-See Genus.
VELOCITY is the comparative celerity, or
swiftness, of a moving body.
VERNIER.-See Glossary I.
VERSED SINE.-See Glossary I.
VERTICAL CIRCLES-See Glossary II.

Horizon. A vertical circle, passing through the east and west points of the horizon, is called the Prime Vertical. VIVIPAROUS, a general designation for such animals as bring forth their young alive, in opposition to Oviparous, which see. VORTICES.-See Glossary II.