astrolabe—An instrument of the late Middle Ages with which mariners attempted to learn their latitude. The instrument was very imperfect in its workings.
astronomical ring—An instrument that was meant to improve on the astrolabe, but which was just as inaccurate.
athwart, athwartships—Across. Hence the rowers’ seats in an open boat are called thwarts because they lie athwart, or across the boat. To drop athwart anything—To come across it; to find it.
auxiliary—A sailing ship equipped with an engine for use in emergency or in crowded waters is said to be an auxiliary. Sometimes sails are carried on power-driven vessels for use in case it is desirable not to use the engine or in case of breakdown. In this case also the ship is an auxiliary.
avast—The order to stop or pause in any exercise; as, “Avast heaving.”
aweather—Toward the weather side; i. e., the side upon which the wind blows.
aweigh—Spoken of an anchor when it has been lifted from the bottom.
aye (adv., perhaps from ajo, Lat. (defective verb), to say yes)—Yes; always used in lieu thereof at sea, with a repetition, “Aye, aye, sir,” meaning, “I understand; and will execute the order.”
back—With sailing ships: To back a square sail is to haul it over to windward so that the wind blows it against the mast. With steam vessels: Back her is an order to reverse engines, so that the ship may be suddenly stopped or made to go astern.
back-stays—Ropes stretched from a mast to the sides of a vessel, some way aft of the mast, to give extra support to the masts against falling forward.