ASTROLABE. An armillary sphere.--_Sea-astrolabe_, a useful graduated brass ring, with a movable index, for taking the altitude of stars and planets: it derived its name from the armillary sphere of Hipparchus, at Alexandria. ASTROMETRY. The numerical expression of the apparent magnitudes of the so-called fixed stars. ASTRONOMICAL CLOCK. A capital bit of horology, the pendulum of which is usually compensated to sidereal time, for astronomical purposes. (_See_ SIDEREAL TIME.) ASTRONOMICAL HOURS. Those which are reckoned from noon or midnight of one natural day, to noon or midnight of another. ASTRONOMICAL OBSERVATIONS. There have been occasional slight records of celestial phenomena from the remotest times, but the most useful ones are those collected and preserved by Ptolemy. Since 1672, science has been enriched with a continued series of astronomical observations of accuracy and value never dreamed of by the ancients. ASTRONOMICAL PLACE OF A STAR OR PLANET. Its longitude or place in the ecliptic, reckoned from the first point of Aries, according to the natural order of the signs. ASTRONOMICAL TABLES. Tables for facilitating the calculation of the apparent places of the sun, moon, and planets. ASTRONOMICALS. The sexagesimal fractions. ASTRONOMY. The splendid department of the mixed sciences which teaches the laws and phenomena of the universal system. It is _practical_ when it treats of the magnitudes, periods, and distances of the heavenly bodies; and _physical_ when it investigates the causes. In the first division the more useful adaptation _nautical_ is included (which see). ASTROSCOPIA. Skill in examining the nature and properties of stars with a telescope. ASTRUM, OR ASTRON. Sirius, or the Dog-star. Sometimes applied to a cluster of stars. ASWIM. Afloat, borne on the waters. ASYLUM. A sanctuary or refuge; a name given to a benevolent institution at Greenwich, for 800 boys and 200 girls, orphans of seamen and marines. The Royal Military Asylum is also an excellent establishment of a similar nature at Chelsea, besides numerous others. ASYMMETRY. A mathematical disproportion. The relation of two quantities which have no measure in common. ASYMPTOTES. Lines which continually approximate each other, but can never meet. ATABAL. A Moorish kettle-drum. ATAGHAN. _See_ YATAGHAN. AT ANCHOR. The situation of a vessel riding in a road or port by her anchor. ATAR. A perfume of commerce, well known as atar-of-roses; atar being the Arabic word for fragrance, corrupted into _otto_. A'TAUNTO, OR ALL-A-TAUNT-O. Every mast an-end and fully rigged. ATEGAR. The old English hand-dart, named from the Saxon _aeton_, to fling, and _gar_, a weapon. ATHERINE. A silvery fish used in the manufacture of artificial pearls; it is 4 or 5 inches long, inhabits various seas, but is taken in great numbers in the Mediterranean. It is also called _argentine_. ATHILLEDA. The rule and sights of an astrolabe.