Aquarius. Constellation lying, approximately, between R.A.s 21h and 23h. Has many discernible stars, but all are small: α Aquarii is Mag. 3-2. 2. Eleventh sign of Zodiac, extending from 300° to 330° celestial longitude. Sun is in this sign from January 20 to February 20. Name is Latin for 'Water Bearer'.
Aquila.Constellation lying between R.A.s 19h-20h. Dec. 0°- 10° N. Contains important star Altair, α Aquikæ.
Arbalest.* Olden instrument for measuring star altitudes. Also called 'Jacob's Staff' or 'Cross Staff' (q.v.).
Arc. Part of circumference of a circle. Of sextant, is that part on which the graduations are carried.
Arcform. Method of ship construction introduced by Isherwood. Sharp bilge of box form was done away with and replaced by an arc form from keel to deck line. To regain displacement lost through removing angle at bilge, beam in region of water line was increased. This results in a definite reduction of immersed midship girth and, due to easy sweep of bilge, improved water flow to propeller. Sea kindliness and economical consumption follow.