baroclinic—The condition and type of motion when isobaric surfaces of a fluid are not parallel with isopycnal surfaces. barotropic—The condition and type of motion when isobaric surfaces of a fluid are parallel with isopycnal surfaces.

barycenter—The common center of mass of the Sun-Earth System or the Moon-Earth System. The distance from the center of the Sun to the Sun-Earth barycenter is about 280  miles. The distance from the center of the Earth to the Moon-Earth barycenter is about 2,895 miles.

bench mark (BM)—A fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal bench mark is one near a tide station to which the tide staff and tidal datums are referred. A primary bench mark is the principal mark of a group of tidal bench marks to which the tide staff and tidal datums are referred. The standard tidal bench mark of the National Ocean Service is a brass, bronze, or aluminum alloy disk 3-½ inches in diameter containing the inscription NATIONAL OCEAN SERVICE together with other individual identifying information. A geodetic bench mark identifies a surveyed point in the National Spatial Reference System. Bench mark disks of either type may, on occasion, serve simultaneously to reference both tidal and geodetic datums. Numerous bench marks of predecessor organizations to NOS, or parts of other organizations absorbed into NOS, still bear the inscriptions: U.S. COAST & GEODETIC SURVEY, NATIONAL GEODETIC SURVEY, NATIONAL OCEAN SURVEY, U.S. LAKE SURVEY, CORPS OF ENGINEERS, and U.S. ENGINEER OFFICE.

Benguela Current—A South Atlantic Ocean current setting northward along the southwest coast of Africa.

bore—Same as tidal bore.

Brazil Current—A South Atlantic Ocean current setting southwestward along the central coast of South America.

bubbler tide (water level) gauge—Same as gas purged pressure gauge.