AQUE. Wall-sided flat-floored boats, which navigate the Rhine. AQUEDUCT. Conduits or canals built for the conveyance of water. AQUILA. The constellation Aquila, in which α Aquilæ is an important star of the first magnitude: used by seamen in determining the latitude and longitude; also in lunar distances. (_See_ ALTAIR.) AQUILON. The north-east wind, formerly much dreaded by mariners. ARAMECH. The Arabic name for the star Arcturus. ARBALIST [from _arcus_ and _balista_]. An engine to throw stones, or the cross-bow used for bullets, darts, arrows, &c.; formerly arbalisters formed part of a naval force. ARBITER. The judge to whom two persons refer their differences; not always judicial, but the arbiter, in his own person, of the fate of empires and peoples. ARBITRAGE. The referring commercial disputes to the arbitration of two or more indifferent persons. ARBITRATION. The settlement of disputes out of court. ARBOR. In chronometry, a shaft, spindle, or axis. ARBY. A northern name for the thrift or sea-lavender. ARC, OR ARCH. The segment of a circle or any curved line, by which all angles are measured. ARC DIURNAL. _See_ DIURNAL ARC. ARC NOCTURNAL. _See_ NOCTURNAL ARC. ARC OF DIRECTION OR PROGRESSION. The arc which a planet appears to describe when its motion is direct or progressive in the order of the signs. ARC OF VISION. The sun's depth below the horizon when the planets and stars begin to appear. ARCH-BOARD. The part of the stern over the counter, immediately under the knuckles of the stern-timbers. ARCH OF THE COVE. An elliptical moulding sprung over the cove of a ship, at the lower part of the taffrail. ARCHED SQUALL. A violent gust of wind, usually distinguished by the arched form of the clouds near the horizon, whence they rise rapidly towards the zenith, leaving the sky visible through it. ARCHEL, ARCHIL, ORCHILL. _Rocella tinctorum fucus_, a lichen found on the rocks of the Canary and Cape de Verde groups; it yields a rich purple. Litmus, largely used in chemistry, is derived from it. ARCHES. A common term among seamen for the Archipelago. (_See_ also GALLEY-ARCHES.) ARCHI-GUBERNUS. The commander of the imperial ship in ancient times. ARCHIMEDES' SCREW. An ingenious spiral pump for draining docks or raising water to any proposed height,--the invention of that wonderful man. It is also used to remove grain in breweries from a lower to a higher level. The name has been recently applied to the very important introduction in steam navigation--the propelling screw. (_See_ SCREW-PROPELLER.) ARCHING. When a vessel is not strongly built there is always a tendency in the greater section to lift, and the lower sections to fall; hence the fore and after ends droop, producing arching, or _hogging_ (which see).