DICTIONARY OF

Aqualung 20 Arctic Circle
Aqualung. Apparatus consisting of bottles of compressed air, reducing valve, and face-mask
which enables a swimmer to breathe under water.
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.).
Arbitration. Judging of a matter under dispute by a person, or persons, mutually agreed
upon by contending parties, and whose decision the contending parties agree to accept.
Rules concerning arbitration are laid down in Arbitration Act, 1889.
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.
Arched. Hogged.
Archimedean Screw. Name given to screw propeller when first introduced.
Archimedes' Theorem. A body partially or wholly immersed in a fluid suffers an apparent
loss of weight equal to weight of fluid displaced.
Arch Type. System of cargo ship construction developed by North-East coast shipbuilders.
Outboard sides of holds were curved inward and upward towards hatchways, so that
trimming of bulkhead cargoes was reduced.
Arc of Excess. That small portion of graduated arc of a sextant in which graduations from
zero read in a direction opposite to that of the main graduations; thus allowing small angles
below the horizontal to be measured.
Arc Proper. Arc of sextant graduated from 0° to 120° (about), and excluding the arc of
excess.
Arctic. Pertaining to area of Earth's surface enclosed by parallel of latitude 66° 33' N.
Arctic Air. Atmospheric air that has come directly from north polar regions and arrives,
eventually, as maritime polar air.
Arctic Circle. Parallel of 66° 33' N. Marks limit of Sun's visibility when in maximum
southerly declination.