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Glossary - M

Magnetic north The north direction shown on a compass. The compass needle is attracted by the earth's internal magnetic field. The true north pole and the magnetic north pole are about 1500km apart.
Manifold A device that connects two cylinders of a twin set together. It may contain its own 'isolation' valve in the middle.
Maritime Any activity connected with seafaring.
Marlinspike A tool for opening the strands of a rope so that they can be spliced.
Maximum Depth The deepest depth reached during a dive.
MCA Maritime and Coastguard Agency.
MCS  Marine Conservation Society.
MDT Maximum Dive Time. The length of time that may be spent at a given depth without being required to make a mandatory decompression stop. Also referred to as No-Stop Time and Non-Decompression Time.
Mediastinal emphysema The condition that exists when air from an over expansion injury escapes into the chest area near the heart and behind the sternum (breast bone).
Meridian A line of longitude.
MGR Mixed Gas Rebreather.
Mixed Gases Any mixture of gas that deviates from normal (normoxic) atmospheric air.
Mooring Securing a vessel at a pier or elsewhere by several lines so as to limit its movement.
MSA The Merchant Shipping Act 1995. The MSA chapter 21 deals with wreck and salvage and is administered by the Receiver of Wreck.
Multi Level Dive A type of a dive that will not conform to a maximum depth and time profile. Ideally, such dives are conducted with the deepest sections first and then the diver ascends progressively to various shallower depths.










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