observations on board ship,-to find the altitude of the Sun or of a star; that is, the angle which the elevation of such heavenly body makes with a horizontal line, or the boundary line of the sea, as seen in the distance. This angle can never be more than 90°, or the distance from the zenith to the horizon; and the altitude of the Sun reaches this extent only at mid-day, and in certain latitudes. The Quadrant, therefore, whether with relation to the Sun or a star, is always sufficient for taking the latitude; but, as a larger arc is often necessary to be measured in finding the longitude by lunar distances, the mariner usually has recourse to the Sextant.

In the absence of the Sun-dial, the Quadrant or Sextant is useful on land for the regulation of time by equal altitudes of the Sun. If we wish to regulate a watch or clock by this mode, we observe by the timepiece at a certain moment before noon, say one hour, the precise altitude of the Sun. We then wait till the Sun has passed the meridian, and endeavour to notice the precise moment when the altitude of the Sun is the same as we took it before noon. It is clear that, as the Sun continually rises in the heaven till mid-day and then begins to sink, the arc comprised between the two altitudes must be equally divided by the meridian of the place on which the Sun is at twelve o'clock. If the times of the semi-arcs be equal, the time-piece is right by the Sun. If, however, the times of the semi-arcs be unequal, the clock or watch must be adjusted thus:— At eleven o'clock, we suppose that we observed the Sun to have a certain altitude, by our time-piece, which we wish to correct. At twenty minutes past one, we observe the Sun to have precisely the same altitude. The time from eleven o'clock till twenty minutes past one

being divided, gives us ten minutes past twelve, by our time-piece, as the moment of the Sun's crossing the meridian; thereby showing that our time-keeper was ten minutes too fast by the Sun. If we rectify according to this amount, and allow for the equation of time, we have obtained the true clock time for the place of observation.

It is, however, necessary to observe that, in using the Quadrant, Sextant, or Reflecting Circle by land, to take the altitude of celestial objects, the visible horizon, owing to its irregularity, is of no use to us. We therefore use an artificial horizon, which consists of a surface of quicksilver in a wooden trough, with a smooth piece of glass laid on the top of it. The whole is covered with two panes of flat glass placed in the form of a roof, to prevent the quicksilver being agitated by the wind.

A B is the box and roof: c is the pane of glass within, floating on the mercury, which of course lies horizontally. A ray of light coming from the Sun at s, falls on the glass at c, and is reflected at an equal angle to the eye at E. The angle is found by making the image in the

X

horizon-glass of the instrument coincide with the image reflected from c, and seen through the unsilvered part of the same glass. The arc passed over by making these two images coincide, is thus double of the real angle; for which reason, the quadrant will not measure by land an angular quantity of more than 45°

Thus far have we explained the nature and uses of such primitive astronomical instruments, as are concerned in the investigation of time, and as may be conveniently used by the general reader of Astronomy. More searching and finished practical means for pursuing the study of astronomical science, are seldom to be met with beyond the walls of an observatory.

The following Glossary is intended to assist the reader, both in the comprehension of the astronomical terms and allusions used in the present volume, and in his proceeding to the study of more elaborate works on the subject of the heavens.

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GLOSSARY.

ABERRATION OF LIGHT. (Latin, from ab, from, erro, to wander.) The deviation of the direction in which a spectator sees a planet, star, &c., from the true one, as expressed by a straight line, supposed to be drawn from his eye to the object. This deviation is caused by the change of place in space of the Earth, and therefore of the spectator, effected during the time taken by light to travel from the object to him. In consequence of this motion, the axis of the telescope through which we see any body as distant as the planets and stars, at any moment, is not coincident with the line that would really pass through the object at that moment. ACCELERATION, of the Moon's mean Motion. A secular variation in that mean motion, occasioned by one in the eccentricity of the Earth's orbit. The Moon is quickened in her motion round the Earth, in consequence of the diminution of the eccentricity of the Earth's orbit, causing her motions to be less influenced by solar attraction. This acceleration will go on, for countless ages to come, till the eccentricity has reached a minimum ; when it begins again to increase, the acceleration will become a retardation. ACCELERATION, of Nebulous Comets, which produces a diminution in their periodic revolution, is occasioned by the resistance experienced by such highly attenuated bodies, in moving through an ethereal medium, which appears to fill space.

ACHRONICAL Rising or Setting of a Planet or Star, is when it rises at sun-set, or sets at sun-rise.

ÆRA. A certain period of time, from which chronologists. and astronomers begin their computations.

ALGOL. The (Arabic) name given to the fixed star, ẞ Persei. It is a variable star, remaining of the brightness designated the second magnitude for about two days, and then suddenly diminishing to the fourth magnitude, and regaining its splendour in about seven hours; the period of this alternation is 2d 20h 48'.

ALMACANTERS. Certain imaginary circles which, in every position of the globe, are supposed to be drawn parallel to the horizon. They are parallels of altitude.

ALTITUDE. (Latin, height.) The arc of an azimuth circle passing through a star, planet, &c., intercepted between the star and the horizon.

ALTITUDE, MERIDIONAL. The altitude of a star, &c., when on the meridian.

AMPHISCII. (Greek, from aμpis, on both sides, and σkıɑ, a shadow.) The inhabitants of the torrid zone;

SO

called, because their shadows fall at one time of the year towards the north, and at another southward. AMPLITUDE. (Latin, extensiveness.) The arc of the horizon comprised between the east or west points, and that at which a planet, star, &c., either rises or sets.

ANGLE OF POSITION OF A STAR, is an angle formed by two great circles intersecting each other in the place of the star, the one passing through the pole of the equinoctial, the other through the pole of the ecliptic.

ANOMALY. (Greek, from a, without, óuados, equal.) The angular deviation from the mean place of a planet, &c., as conceived to move in a circular orbit; in consequence of its real unequal motion in an elliptic orbit.

ANOMALY, MEAN. The angular distance between the perihelion and the mean place of a planet at any period of its revolution.

ANOMALY, TRUE. The angular distance between the perihelion and the true place of a planet, &c., at any period of its revolution.

ANOMALY, ECCENTRIC. The angle at the centre of the imagi