DICTIONARY OF

Opposed Cylinder Engine 242 Oropesa Sweep
Opposed Cylinder Engine. Internal combustion engine having cylinders on opposite sides
of each crank. Impulses are applied at twice the usual rate.
Opposed Piston Engine. Internal combustion engine having two pistons in each cylinder.
Charging, compression and ignition take place between the pistons, each of which operates
its own connecting rod.
Opposition. Relative positions of two heavenly bodies, particularly solar system bodies,
when their celestial longitudes differ by 180°, or their R.A.'s differ by 12 hours.
Optional Pilotage. Pilotage service that is available but is not compulsory for specified
vessels.
Orbit. Path of planet around Sun, or of a satellite around its primary.
Ordinary.* Formerly, 'in ordinary" denoted H.M. ships that were not in commission but had
care and maintenance crews on board.
Ordinary Seaman. Seaman aged 18, or more, who has not qualified to be rated able
seaman.
Ordnance Datum. Level to which heights and depths are referred in English surveying.
Formerly, the level was a rather inaccurate approximation of mean sea-level at Liverpool. Is
now mean sea level at Newlyn, Cornwall.
Orient. The Far East. East point. To define a direction by its relation to the east point of
compass-a relic of the days when the east point of compass was the prime cardinal point, and
was marked by a fleur-de-lys.
Original Bill. The first of a set of bills of exchange to be presented. Prior to presentation,
No. 1 is the original bill.
Orion. One of the most notable constellations. Lies between R.A.'s 5h and 6h and Dec.
N10° to S10°. Name is that of a mythical giant hunter who was supposed to be the son of a
peasant. Other myths make him a prince of Tanagra. As Lecky says, "Orion moves in the
highest stellar society," being surrounded by important constellations and stars.
Orionids. Meteor shower through which Earth passes in autumn, and which appears to
radiate from constellation Orion.
Orlop Beams. Beams supporting an orlop deck.
Orlop Deck. Was the lowest deck in a line of battleship, being a platform laid over timbers
of the hold.
Ormonde Beacon. Dan buoy used by surveying ships. Buoyancy is supplied by three 25gallon
oil-drums. Is ballasted with a 3-cwt. sinker, its bamboo mast carries a flag, 16 ft. by
10 ft.
Orographic. Pertaining to mountains.
Orographic Rain. Deposit of rain by a saturated air-stream coming into contact with rising
land.
Oropesa Sweep. Towed wire used for sweeping sea bottom when surveying for shoals, or
locating wrecks. Consists of two kite 'otters', one at each end of a 450-ft. sweep wire, and a
third otter-close astern-to keep both sweep wires down.